Boiler Learning Module
Content
Learning modules material- Engineering (FB)
Learning Modules material
For Engineer Trainees
Engineering /Fossil Boilers
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Learning modules material- Engineering (FB)
Index
1. Boiler - Design 1.1 Steam Generation 1.2 1.3 1.4 Types of boilers Boiler Circulation system Boiler Design – Specification & Parameter
2. Engineering processes 2.1) Boiler Performance and proposals 2.2) Product Engineering 2.2.1) Boiler layouts 2.2.2) Pressure parts, pressure parts arrangement and Stress analysis 2.2.3) Boiler Mountings 2.2.4) Ducts and Dampers 2.2.5) Fuel Systems 2.2.6) Lining and Insulation 2.2.7) SS and Buckstay 2.3) ITS &S 2.4) Controls and Instrumentation 2.5) FES and R&M
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3. Boiler Materials 1. Materials used in boiler 4. Boiler auxiliaries 1. Pulverisers 2. Fans 3. Air Pre-heater 4. Dust Collector 5. Environmental Pollution 1. Indian Pollution control board guidelines 6. Codes and Regulations: 1. Material testing codes 2. Coal Analysis Standards 3. Boiler efficiency 7. Destructive and non-destructive testing 8. Water Chemistry 9. Boiler operation, Availability and Reliability and Boiler Tube failure mechanisms.
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1.1
Steam Generation
Modern Thermal power plants operate on the Rankine cycle . In a typical thermal power plant, the heat released during combustion of the fossil fuel is transferred through the walls of the boiler ( waterwalls formed of tubes) to water that flows through the tubes, thus generating steam. In drum type boilers, normally used for subcritical pressure application, a mixture of steam and water leaves the waterwalls. The steam is separated from the water in the boiler drum, superheated in the superheaters and is sent to a steam turbine. In the case of once thru boilers, all the water is converted to steam ( in once thru mode of operation) . The rotor of the steam turbine rotates as the steam passes over the turbine blades, which in turn rotates the generator, connected to the rotor, thus producing electric power. The steam after expansion in the turbine is condensed in the condenser and is pumped back to the boiler.
Steam generation Process Boiler is thus a very vital component of the thermal power plant. Pulverized coal fired boilers today form the backbone of thermal power generation in almost all countries due to the abundant availability and low cost of coal. The increasing concerns on the atmospheric pollution warrant that power be generated with minimal pollution. It is therefore of paramount importance that coal utilisation is done in an environmental friendly manner, which can be attained by efficient processes, technologies and equipments. Increase of plant efficiency is one of the important ways of reducing the fuel consumption and consequently reducing the plant emissions and conserving energy resources. Adopting any one
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Learning modules material- Engineering (FB) or more of the following measures can increase the overall efficiency of pulverised coal fired power plants: 1.2 Increasing main steam pressure Increasing Superheat and Reheat steam temperature Adopting double reheat cycle. Increasing the vacuum of condenser Increasing final feed water temperature Reducing boiler flue gas exit temperature Reducing excess air Boiler Types:
Boilers can be broadly classified on the following basis: Use/ Application: Drum arrangement Heating surface arrangement: Circulation: Operating Pressure: 1.2.1 Boiler application Industrial boilers are mainly for use in process industries and are normally non-reheat units and have partial steam generation in boiler bank tubes. Utility boilers are large capacity boilers used for electric power generation 1.2.2 Drum Arrangement Industrial / Utility application Single drum/ bi-drum/ once thru (no drum) Tower type/ two pass type/ box type/ close coupled type Natural Circulation, Forced/ Controlled circulation Sub critical, supercritical
Drum type boilers employ either Natural circulation or Forced circulation . In drum type boilers steam generation takes place in the furnace water walls and has a fixed evaporation end point - the drum. Steam -water separation takes place in the drum. Separated water is mixed with incoming feed water and flows back to the waterwalls. Bi-drum boilers: In lower pressure ranges it is common to incorporate a boiler bank for heat transfer in the evaporator circuit. The boiler bank is arranged between two drums and hence the name bidrum boilers.
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Learning modules material- Engineering (FB) Single drum boiler: With increase in operating pressures, the drum plate thickness increases considerably and the bi-drum arrangement is not preferred because of the large thickness. At steam pressures above 100 kg/cm2, single drum boilers are hence normally employed. 1.2.3 Heating Surface Arrangement: The heating surfaces like Superheater, Reheater and economizer are dispositioned in a boiler to achieve the most optimum heat transfer. There are different arrangements that could be adopted. Tower type designs have all heat transfer surfaces arranged as horizontal sections above the furnace. Two pass designs employ a combination of pendant and horizontal sections. Box type and close coupled are compact designs normally used with oil and gas fuels. 1.3 Boiler Circulation System:
Choice of Circulating system depends on operating Pressure. The density difference between water and steam provides the driving force for the circulating fluid. Higher pressures units warrant pumps to ensure circulation or alternately the components are to be sized bigger to reduce the frictional resistance, so that natural circulation can still be employed. Natural Circulation Boiler Circulation thru water walls by thermo-siphon effect
Forced/ Controlled Circulation Boiler Thermo-siphon effect supplemented by pumps
Once-through Steam Generators: In once through steam generators, the boiler feed pump forces a once-through type flow through the complete system in the boiler (through the economiser, water walls and superheater sections) in one single continuous pass. The concept is shown in Fig.1
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Learning modules material- Engineering (FB)
Fig.1 Concept of Once Through Steam Generation Once-through steam generators can be designed for both sub-critical and super critical pressures. The important advantages of once-through boilers such as short start-up time, load ramp behaviour and flexibility are available with subcritical design parameters also. Once through steam generators are ideally suited for sliding pressure operation due to the absence of thick walled components and lesser storage requirements as compared to drum type steam generator. In sliding pressure operation, the turbine inlet valves are kept full open during normal operation. Hence, the live steam pressure is directly proportional to the steam flow. Increase in the steam parameters, i.e. temperature and pressure, is one of the most effective measures to increase the efficiency. The supercritical cycle (pressure higher than the critical pressure) offers a ‘burn less fuel for the same output’ approach. However higher steam condition is limited by the availability of the materials required to withstand the duty conditions Development of once through technology, new materials and improvement in design has led to adoption of supercritical cycles. In the 80’s, with the development of high temperature steels, number of new units adopted higher temperature of 565 0C and 5800C. In the 90’s, a few utilities adopted advanced steam parameters of 285 bar and 5800C to 6000C, which are termed as ultra supercritical parameters. The current trend in advanced countries is to go in for increase in steam pressure as well as temperature. In countries like Japan, the parameters in new projects are even higher (300 bar / 600OC / 600OC). World-wide R & D efforts are going on to develop power plants with ultra supercritical steam parameters.
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Learning modules material- Engineering (FB)
1.4.
Boiler Design parameters
The following parameters shall be specified as a minimum for designing a boiler: Feed Water & Steam parameters – flow, pressure, temperature, quality Control load – the load range in which superheat and reheat steam temperature are maintained at rated value. Fuel Firing – Fuels to be fired, fuel combinations, fuel properties, fuel parameters at terminal point. Site conditions – seismic data, wind velocity, altitude, rainfall, ambient temperature, cooling water temperature, humidity etc. Layout constraints/space availability Boiler operating modes (Base load, Cycling, Two-shifting, Constant pressure, Sliding pressure etc.) Emission limits Specification of major components/auxiliaries / systems (type, number, sizing criteria etc)
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Learning modules material- Engineering (FB) 2. Engineering Processses: 2.1 Boiler Performance and Proposals:
Boilers are designed to meet the requirements specified by the Customer. Requirements vary in terms of output, inputs, operating requirements, emission norms, space constraints, preferences on equipment type, level of technology, individual equipment specifications etc. Each boiler is unique, as it is designed to meet these varying requirements. In BPP, the thermal and system design of the boiler is developed to meet the desired performance and configuration requirements. Proposals BPP prepares the technical portion of the tenders against enquiries from Customers for Utility, Industrial and Chemical Recovery boilers. Proposal packages include details of the proposed boiler (Technical specification, Technical data sheets, Drawings etc.). Contract Performance Engineering On award of Contract, the Performance Engineering of the boilers is carried out towards ensuring the required boiler performance. Based on this, necessary engineering inputs are furnished to Product engineering sections for further engineering. Field Performance Data Analysis On commissioning of the boiler, the operating data from the unit is collected, compared with the predicted performance data and reasons for variations, if any, are analyzed. Designs are continuously updated to incorporate the feedbacks to obtain better performance.
Renovation & Modernization (R&M) of Boilers In R&M, the performance aspects of the boilers are studied and the required inputs are generated and furnished to the related agencies. Functions of BPP fall under three categories: Proposal Contract FES support for performance
Proposal Phase BPP Proposals come from the three major business sectors: Power sector /Marketing ( utility boilers)
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Industries sector International operations
Inputs Enquiry specification , addendum / clarification by customer from commercial tendering and estimation Boiler parameter from PEM /PED as applicable Scope, terminal points and exclusions from power sector/ marketing
Deliverables Filled up specification review Filled in proposal datasheets PG wise BOM with estimated weight Technical write up and proposal Details required for erection estimate List of information of spares Proposal valve schedule to valves
BPP processes Enquiry registration Receive enquiry specification from commercial Assign proposal number Detailed proposal Budgetary proposal Detailed proposal Prepare the specification review list Finalize scope, exclusion and terminal points between units consortium partners Seek clarification from customer Seek clarification /comments from PEB, QA, BAP, PC, HYD , valves and C and I
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Obtain feedback from the FES for similar running contracts Design processes Customer clarification Finalize design approach Furnace design SH , RH , ECO arrangement Performance calculation Auxiliary selection Prepare technical deviations Presentation to top management Prepare BOM Prepare support documents Performance Calculation General data Proximate analysis to ultimate analysis Slagging characteristics of coal Air and gas weight calculation s Heat duty Efficiency calculations Flue gas analysis Furnace performance Plan area Volume Effective projected radiant surface Arrangement data Heating surface area
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Flue gas area SH , RH , ECO performance IBP runs Mills AH Check for design limiting values Material selection Metal temperature program Tubing list for PP
Headers and piping Pressure drops Fuel firing Mill selection wind box selection Airheaters Tubular Bisector (a) Trisector Fans, ducts and losses Duct area calculation Fan selection Duct draft loss calculation Circulation Controlled and natural circulation Pump selection CIRGEN
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DNB check Auxiliary selection EP selection Ash collection data Chimney selection data Safety valve selection C and I parameters Boiler performance Guarantee schedule Efficiency program Auxiliary power consumption NOX, SOX Wear life of mill rolls Performance curves Prepare support documents Guarantees Technical data sheets Spares list Technical offer Erection input to regions Finalize technical offer Review by management Presentation to top management Prepare and submit technical offer Attend post bid meeting with customer/ consultant and resolve points Revise BOM, if required
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Revise technical offer, if required Finalize technical offer Submit to commercial
2.2 PRODUCT ENGINEERING(Detailed Engineering): Various Section in Product Engineering are Layouts Pressure Parts, Pressure Parts Arrangement and Stress Analysis Boiler Mountings Ducts and Dampers Fuel System Lining and Insulation Supporting Structures and Buckstays
2.2 .1 Layouts
Various types of Layouts used in the power plants are as follows:
(a) Conventional / Front mill layout
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(b) Vijayawada Layout
(c) Rear mill / Panipat Layout
(d) Side mill layout
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. In the Conventional Layout, mill bay is located adjacent to the TG set. The mill reject started accumulating in TG set causing inconvenience to the people working with it. This led to the change in the location of mills in some of the subsequent boilers. Due to this, dust problem to TG set was rectified and sufficient ventilation was also available as compared to the conventional layout. As this was implemented first in Vijayawada, the layout was named as Vijayawada Layout. In Vijayawada Layout, duct carrying the primary air to the mill bay and the pulverized coal to the furnace, covers the entire area around the furnace. This problem was rectified using the Rear Mill layout. As it is first used for Panipat it is also called as Panipat Layout. As the number of mills operating in the 500MW unit is around 10 and occupies more space in the conventional layouts. Mills are arranged along both sides of the furnace in this layout, named as Side Mill Layout.
2.2. 2 Pressure parts,Pressure Parts Arrangement and stress analysis: DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 1 For a 500 MW, To prepare Production & Erection documents for superheater/reheater assembly and header is being done for each project as per the boiler specification based on the design guideline. This project involves the detailed study of each components in the superheater/reheater assembly and header, classification and analysis of their variant drawings, need for these variants and scope for standardization. It also involves collection of all standards required for the design into a single document for easy reference. The superheater/reheater assembly of various Projects have been analyzed to find the variants involved in each component. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 2
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Learning modules material- Engineering (FB)
DESIGN OF SUPERHEATERS & REHEATERS INTRODUCTION: The Superheaters & Reheaters are part of the pressure parts systems of a boiler. The other pressure parts which act as heat absorbing surfaces are waterwalls or furnace walls. The headers, Drum, Downcomer and other connecting lines and links will also be called as pressure parts in a boiler. Superheaters are used to raise the steam temperature above the saturation temperature by absorbing heat from flue gas to increase the cycle efficiency. Due to superheating the useful energy that can be recovered increases, thus the cycle efficiency also increases. For utility boilers which are meant for power generation, the superheater outlet temperature is limited to 540 + 5 C because the maximum temperature is dictated by the metallurgy and economy in initial cost and maintenance cost. Superheating also eliminates the formation of condensate during transporting of steam in pipe lines and inside the early stages of turbines which is harmful to the turbine blades and pipe lines. Reheaters are used to raise the steam temperature to the same superheat temperature but at a lower pressure since the steam flow through reheater takes place after H.P turbine. The reheating of steam improves the cycle efficiency and reduces the damage to the turbine blades due to condensation of steam at turbine ends. In the case of reheaters this method is used only as an emergency purpose and not as a regular means as direct admission of water in reheater adversely affect the system efficiency. Titling the burners up and down is the major method of control used generally for Reheater to control the temperature. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 3 TYPES OF SUPERHEATRS AND REHEATERS: These heating surfaces are in the form of coils which are made by bending the tubes in cold or hot forming. The hot bends are called squeezed bends which are done in a special hydraulic pressing machine called squeezing press. Squeezed bends are used in the coils where the ratio between Radius of bend and diameter of tube is less than or equal to1.5 (R/D 1.5). The inlet and outlet end of circuit or coil is connected to inlet and outlet heaters correspondingly which will act as supplier and receiver for each stage. The Assemblies may have one or more than one circuits depending on the performance requirements of the boiler. Depending on mode of heat transfer the superheaters and reheaters coils are generally classified as Radiation or convection type and depending on location or Arrangement in the boiler this is further classified as Horizontal and Vertical (pendant) types. In both the Radiation and Convective types, the Horizontal or Vertical arrangement is used based on type of boiler. In a conventional type of boiler vertical and Horizontal superheaters are used with vertical or pendant Reheater. But in Box type units all superheaters and Reheaters are arranged Horizontally which will act as Drainable type.
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The vertical arrangement is simpler in supporting and allowing for expansion and this arrangement is called pendant type. Horizontal superheaters or Reheaters needs supporting of the tubes at multipoints to avoid segging and expansion movement should also be permitted with the advantage of draining. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 4 RADIANT SUPERHEATER/REHEATER: The reheaters or superheaters which can view the flame is called radiant type. The Radiant surfaces are kept at the top of combustion chamber and in our conventional units, only one Radiant superheater or Platen superheater are used. Since the projected area only is the design criteria for radiation heat transfer the radiant superheaters have tubes closely pitched along the flue gas flow which is called longitudinal pitching or spacing between tubes SL. The pitches across flue gas flow between coil assemblies is called as Transverse pitch ST which is wider in the case of Radiant superheater to reduce the velocity of gas and the bridging the surfaces by the ash and they are arranged in line fashion. Because the heat absorption of furnace surfaces does not increase in direct proportion to boiler output but at a considerably lesser rate curve of radiant superheat as a junction of load slopes down ward with increase in boiler output. In the case of typical 210 MW boiler the Radiant superheater is composed of 29 assemblies of dia 51 tubes spaced at 457.2 mm centers along the width of the furnance. Since the adjacent superheaters are closely arranged generally squeezed bends are used to form the coils CONVECTION SUPERHEATER/REHEATER: Convection surfaces are located at moderate flue gas temperatures and also kept in the rear pass. Since the total circumferential area (surface area) is the criteria for heat transfer the pitch between tubes SL along the flue gas flow will be wider to allow flue gas will flow around the tubes. The transverse pitch ST (pitch between assemblies) will be closer when compared to radiant superheater. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 5 In a conventional type unit the reheaters will be of pendant convection arrangement and also the final superheater. The superheater which is placed at lower flue gas temperature region in the second pass of the boiler will be of Horizontal convective type and it is generally called as low temperature superheater (LTSH). Since the tubes are very widely pitched along the flue gas flow the convective SH/RH. Since convection heat transfer rates are almost a direct function of output, the total absorption in the superheater increases with increase in boiler output. The control of combination radiant convection superheaters are relatively simple because of their compensating characteristics. The combination of these two superheaters is generally used in all utility units to give flat superheat curves (to maintain constant temperature) over wide ranges in load. The fundamental considerations governing Superheater design also apply to Reheater design. However the pressure drop in reheaters is critical because the gain in heat rate with the reheat cycle can be
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completely nullified by too much pressure drop through the reheater system. Hence steam mass flows are generally somewhat lower in the reheater. In conventional type units the reheater is composed of two stages or sections, the front pendant vertical spaced section and the rear pendant vertical spaced section. The rear pendant vertical spaced section is located above the furnace arch between the water cooled screen wall tubes and rear water wall hanger tubes. The front pendant vertical spaced section is located between the rear water wall hanger tubes and the superheater platen section. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 6 RELATIONSHIP IN SUPERHEATER/REHEATER DESIGN : Effective superheater/reheater design calls for the resolution of several factors. The outstanding considerations are: 1. The steam temperature desired. 2. The superheater surface required togive this steam temperature. 3. The gas temperature zone in which the surface is to be located. 4. The type of steel, alloy, or other material best suited to make up the surface and the supports. 5. The rate of steam flow through the tubes which is limited by the permissible steam pressure drop but which in turn, exerts a dominant control over tube metal temperature. 6. The arrangement of surface to meet the characteristics of the fuels anticipated, with particular reference to the spacing of the tubes to prevent accumulation of ash and slag. 7. The physical design and type of superheater as a structure. A change in any one of the first six items will call for a counter balancing change in all other items. Steam Mass velocity, steam pressure drop, and superheater tube metal temperatures are calculated after the amount of surface is established. The proper type of material is then selected for the component tubes, headers and other parts. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 7 The same general similarity exists between superheater and reheater considerations, but the reheater is limited in ruggedness of design by the permissible steam pressure drop. The outside diameter of reheater tubes will be bigger than that of superheater tubes as more volume is to flow through reheaters, operating at low pressures. Since the superheaters are at high pressure, their thickness will be higher than that of reheaters, superheater tubes normally vary from 44.5 OD to 54 mm OD whereas reheater tubes vary from 47.63 mm OD to as high as 63.5 mm OD. The thickness for superheaters goes as high as 10 mm whereas reheater thickness do not exceed about 5 mm. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 8
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MATERIAL CONSIDERATION: Oxidation resistance, maximum allowable stress and economics determine the choice of materials of materials for superheater and reheater tubes. The use of carbon steel is extended as far as these considerations permit. Beyond this point carefully selected alloy steels are used. The majority of superheaters and reheaters are made of low & high alloy steels. The steels commonly used for this application are shown in Table. Higher chromium content increase the resistance to scaling or oxidation. Stainless steels are also used to a limited extent wherever the skin temperature of superheaters or of wrapper tube which is exposed to furnace is made of stainless steel. Stabilised stainless steel has Niobium, Titanium, Cadmium, Tungsten etc. added in traces to the steel prevents carbide precipitation in the grain boundaries. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 9 1. Carbon SA192 SA106 Gr.B SA515 Gr.70 SA105 SA216 427 C Waterwalls. Fin welded Steel panels, Economiser. LTSH Lower bank (some portion) Waterwalls headers. SH Headers upto SHH9. Suspensions for plates. 2. Carbon SA209 T1 ----- SA204 SA182 F1 SA217 WC1 482 C Tubes are used for SH Radiant ½ % Mo roof, LTSH lower bank, LTSH upper bank, (some Portion), RH inlet. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 10 3. 1 % Cr SA213 T12 SA335 P12 SA387 Gr.12 SA182 F12 SA217 WC6 535 C Pipes are used for SH hdrs. ½ % Mo (Rod) SHH13. Plates for suspensions. Forged rod fot tie rod Suspensions & nozzles. 4. 1 ¼%Cr SA213 T11 ----- ----- SA182 F11 ----- 552 C tubes are used for LTSH ½%Mo upper bank, terminal tubes. 5. 2¼%Cr SA213 T12 SA335 P22 SA387 Gr.22 SA182 F22 SA217 WC9 577 C Used in platen SH, RH front
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1 % Mo & rear, Final SH pipes are used for final SH & RH outlet headers Plates are used For supporting purposes. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 11 6. 9%Cr. SA213 T91 SA 335P91 635 C Used in SH Platen,SH Final 0.25%Mo RH final coils. 7. 18%Cr SA213TP ----- SA240 SA182 ----- 704 C Tubes are used for wrapper 10%Ni &cb,Ta 347H Gr.347 F347 tube portion of platen SH. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 12 SPACERS FOR SUPERHEATER/REHEATER: Spacers are provided to maintain pitches along and across coil assemblies. The spacers must be able to do the function both in cold and hot conditions. The type of spacers generally used are transverse spacers and alignment ties. Transverse spacers are used to maintain pitch between assemblies ST. Fluid cooled spacers are mechanical spacer bar are used as alignment ties. Alignment ties are used to maintain pitch between tubes in the same assembly i.e., SL. Flexible connector and alignment band are used as transverse spacers. Flexible connectors in combination with fluid cooled spacers are used when the maximum average gas temperature exceeds 900 C in coal and gas fired units and 593C in oil fired units. Mechanical spacer bars in combination with alignment band are used at temperatures below these. The pendant spaced sections have lot of offset bends only to accommodate these small size spacers. Unless the spacers are small or cooled by medium they will be burnt at these higher temperature. Flexible connectors have one male and two female connectors welded to second and first tube as shown in Fig.3. These connectors will allow the tube to expand downwards during operation but at the same time they will maintain SL pitch between tubes. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 13 In the case of fluid cooled spacers, steam cooled spacer is generally used in all our natural circulation units. A tube from low temperature SH header (I utility boilers this tube is taken form Front SCW inlet header) will be taken and passed across the coil assemblies along the furnace width. Spacer plates are welded on either side of this tube in between assemblies to maintain ST which is cooled by steam
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flowing inside the tube. The entire tube will be resting on support lugs which are welded to tubes of coil assemblies as shown in Fig. After the spacer passes through all assemblies the same will be routed thro the gap available between assembly and side wall and it will be connected with low temperature SH header. Alignment band is a strip of 5 mm and height of Maximum 100 mm is wound all round the assembly as shown in figure and the ends are welded. Bar strips will be welded over the band in between tubes to maintain SL and the entire band will be resting on support lugs which will be welded with the tubes of coil assembly. Mechanical spacer is a scalloped bar which will be running across the assemblies over the alignment band and the first tube of alternate assemblies will be connected with this bar by U-Rod .The spacers are all made of stainless steel. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 14 SUPPORTS & SUSPENSION OF SUPERHEATER/REHEATER: The vertical or pendant superheaters or reheaters are suspended from the ceiling and horizontal coils are either self supported or supported by hanger tubes. In pendant SH or RH Assemblies, the tangent ties are welded in between tubes at the top row to transfer the load from center to end terminals on either side. The Horizontal superheaters are supported by Economiser hanger tubes thro tube saddles as shown in Figure 1. On either side of Eco Hgr. tube these saddles are welded along the length and the other side of saddles is welded to horizontal tubes of LTSH. These saddles provide very good spacer cooling from close tube contact and permits each horizontal tube to be picked up individually from the hanger tubes. The pendant superheaters and reheaters are separately suspended by high crown supports. High crown plates are welded on either side of the seal band which is already welded on wither side of the seal band which is already welded to the terminals tubes of coil assemblies as shown in Figure.4. The ends of high crown plates will be welded to end plates. So the load available at terminal tubes will be transferred to end plates by high crown plates. The end plates will be suspended from the ceiling by means of tie rod assemblies. The headers which are required for supports will be independently suspended from the ceiling thro tie rod assemblies. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 15 SUPERHEATER WALLS: In power boiler, the horizontal and rear passes are covered by superheater walls just like furnace is covered by superheater walls just like furnace is covered by waterwalls. But the superheater walls are fin welded whereas water walls are fusion welded. Fin welded walls have flats welded in between tubes rigidly and the walls are gas tight. The fin welded walls are an improvement over skin cased enclosures where the walls are peg finned. The radiant roof will be of peg fin welded construction where these small pieces of flats are welded along the length of each tube with gap and there will be gap in between
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adjacent tubes also these gaps are covered by castable refractory. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 16 COMPONENTS OF SUPERHEATER/REHEATER 1) Super Heater / Reheater circuits 2) Roof seal band 3) Heat shields 4) Connectors (i) Flexible Male connectors (ii) Flexible Female connectors 5) Lugs a. Spacer lug b. Support lug c. Lifting lug 6) Flats 7) Welding bifurcate DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 17 INPUT REQUIRED: (A) BPP transmittals: CPT 1301 - SH, RH & Economiser Arrangement Data (Ref :Annex:A) CPT 1501 - Tubing list of RH,SH, Economiser(Ref:Annex:C) (B) PPA drawing: General Arrangement Drg Sectional Side Elevation – Upper/First pass (C) Applicable standards (if any) of Roof seal band Flexible male/female connectors
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Lifting Lug Spacer Lug Support Lug DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 18 VARIANT OF REHEATER: (i) No of stages ( single or Double) (ii) No of elements (iii) Usage of connectors or Alignment bands VARIANT OF SUPERHEATER: In a 500 MW Boiler, Superheating of steam takes place in 3 stages: i) Low Temp Superheater (LTSH) ii) Divisional panellete iii) Final Vertical platen Superheater In a 250 MW Boiler, Superheating of steam takes place in 3 stages: i. Low temp Superheater (LTSH) ii. Pendant platen Superheater iii. Final Vertical Spaced Superheater DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 19 1) SUPERHEATER COILS: The material specification, no of circuits, design temperature and pressure are obtained from the BPP transmittal – CPT 1501 and CPT 1701 gives the rough sketch with welding bifurcation and location with respect to roof. The position and location constraints are obtained from the PPA – GA drg. The design of the individual reheater circuits are done based on the BPP transmittal which gives the tubing list along with the different material specification that should be used, along with the approximate length of each specified tube. The designer shall ensure that material transistion doesn’t occurs at the bend of the tubes. The minimum Bend Radius that is feasible in shop based on the tube diameter. For Choosing Coil Bend Radius use ‘List of Rotary Bending Tools’ (Refer Annex :F).
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2) ROOF SEAL BAND: Roof seal band are provided with the coil assembly for the purpose of transferring the load of the coils to the roof girders. It transfers the entire load of the coil through the Hi-crown plate, which is welded with the End bar. The end bar transfers the load to the ceiling. Dimension of the roof seal band and hi crown support is determined by the load carried by the reheater coil, which is given by the Stress Analysis dept. The distance between the end bar are governed by the overall size of the reheater coils and this is given in the PPA drawings. The roof seal band varies with the diameter of the coil tube, pitch distance between the two circuits. The plate width is governed by the entire assembly load. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 20 Inputs from BPP to LAYOUTS: Drawings Proposal drawing Scheme of water and steam Scheme of air and gas path Scheme of pulverizer system Scheme of oil system Basic scheme of spout cooling water Secondary air fan and tertiary air fan selection Piping for GL/BL system
Transmittals Boiler parameters Special operating conditions Soot blower selection data Furnace and back pass sketch Wind box selection data Mill selection data Feeder selection data
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Fuel pipe data SH, RH and Economizer arrangement data Air heater selection data SH, RH and Economizer arrangement sketch SCAPH selection data Duct design data FD fan specification ID fan specification PA fan specification GR fan specification Header and piping list for SH, RH and Economizer Header and piping list for circulating system Safety valve selection data Pent house cooling fan selection data Flow measurement device characteristics and data Contact data sheet ESP ash tank data Pressure part arrangement group prepares the detailed drawings of the Pressure Parts of boiler. Pressure parts include steam and water circuits starting from the Economiser inlet header to the Main Steam supply line and Reheater Assemblies. These drawings will be useful for manufacturing and erection purposes.
Inputs: BPP Transmittals PPA sketches Layout drawings Contract specification and scope
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Learning modules material- Engineering (FB)
Proposal BOM from BPP
Activity Sequence Preparation of applicable PGMA Preparation of PP specification review list Preliminary strength calculation Preparation of material forecast IBR fees payment based on heating surface area of boiler For NTPC contracts, a pressure parts schedule is prepared DDR as per CCST in theory Inputs : BPP transmittal PPA drawings For new designs Coil arrangement drawings are drawn to scale for verifying the inner tube allowances, interferences, and geometries. For industrial boilers, type of drum internals, screen dryer or turbo separator etc may vary During DDR, design of vertical coils is interlinked with design of suspensions for the same Pressure parts and their corresponding supports are released together Sequence is usually Drum Top header Upper belt of panels Roof panels Middle portion Lower panels including re heater Bottom headers IBR submission Erection welding schedule preparation
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Learning modules material- Engineering (FB)
Unit material diagram preparation
Various Drawings prepared by this group: General arrangement of pressure parts Expansion movement diagram Plan over headers and links of furnace Ceiling loading plan
Various Schemes supplied for the execution of above Scheme of boiler water and steam with valves and fittings and instrumentation Scheme of boiler water circulating pumps with valves fittings and instrumentation Scheme of SH and RH system with valves fittings and Instrumentation Scheme of soot blowing system with valves fittings and Instrumentation
Stress Analysis Stress analysis group carryout the stress analysis for following components Drums and hanger rods Hanger tubes Ceiling loading Pressure parts hanger rods Header lugs Furnace guide loads Horizontal tube support spacing Header nipple flexibility VLH and CLH Roof tubes supports Flexible connections Steam cooled spacer
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Learning modules material- Engineering (FB)
Height of header above roof Strap type support PANTO support Bent tube lugs U-rod supports Collector channel Washer plates Thermal expansion movements MS support lines and links Drum rocker settings Allowable deviations in tube bends Vibration sobers Fin width Fillet weld designations Eco inlet header support
2.2.3 Boiler Mountings Inputs: Contract scheme of water and steam circuit from BPP BPP Transmittals GA drawings Instrumentation required from C and I Contract and tender spec PPA drawings Dished end tappings for Drum
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Learning modules material- Engineering (FB)
This group prepares the following layouts. instrumentation riser tubes. After preparation of the schemes, valve schedule will be provided. Scheme of boiler water circulating pump with valves, fittings and instrumentation Scheme of soot blower piping with valves, fittings and instrumentation. Collection of water and steam mixture raised through water walls and Scheme of water and steam circuit with valves, fittings and instrumentation
Scheme of super heater and re-heater steam circuit with valves, fittings and
Apart from the schemes mentioned above, this group involves in designing, quoting and procurement of sub delivery items like Control valves Circulating pumps
NTPC approved vendors for procurement of circulating pumps are Hayward Tyler, U.K. and KSB, Germany. Apart from the above two BHEL approved Torishima, Japan for circulating pumps. Transmittals will be sent from BPP about pump characteristics, control valves and block valves. Based on the transmittals received this group orders for the pumps and control valves 2.2.4 Ducts and Damper This group deals with Ducts Dampers Guillotine gates Expansion joints Duct support Flow measuring device Guide vanes
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Learning modules material- Engineering (FB)
The ducts are designed for flue gas, secondary air, and primary air circuits. Dampers are used when 100% leak proof is required. It is only a on/off system. Guillotine gates are big in structure so cannot be used inside the furnace. Ducts are made of IS2062 carbon steel. Its thickness will be 4 mm to 8 mm and the expansion rate is 1mm/100°c/1m.To absorb thermal movements, expansion joints are provided. Three types of duct support are there bottom support, top support, and restraints to restrict the expansion movement in the desired direction. Flow measuring devices like Airfoil in secondary air and venturimeter in primary air. Guide vanes are provided to distribute the flow The following loads are considered in the Design the Ducts. Dead load Live load Pressure load Ash load Wind load
Dampers are used for isolation. It gives an efficiency of 98.5%, but by providing Guillotine gate one can achieve 99.98% efficiency. Generally in ID Fan, guillotine gates will be provided. In case of space restriction, dampers will be used. Location of guillotine gate: PA Fan outlet ESP inlet ESP outlet ID Fan inlet ID Fan outlet Hot air mill inlet Cold air mill outlet
Types of dampers Biplane damper Louver damper
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Learning modules material- Engineering (FB)
Location of dampers: FD Fan outlet SAH air inlet SAH air outlet PAH air inlet PAH air outlet AH Gas inlet
Types of expansion joints: Metallic joint Non-metallic joint
2.2.5 Fuel Systems Firing Systems
Coal Firing Systems
Tangential Firing system Wall firing (turbulent/vortex burners) Direct firing/Indirect firing Stoker firing Fluidised Bed combustion
Tangential Firing System:
In tangentially fired boilers, four tall windboxes (combustion air boxes) are arranged, one at each corner of the furnace. The coal burners or coal nozzles are located at different levels or elevations of the windboxes. The number of coal nozzle elevations are equivalent to the number of coal mills. The same elevation of coal nozzles at 4 corners are fed from a single coal mill. In some designs one mill feed two elevations of burners. The coal nozzles are sandwiched between air nozzles or compartments. That is, air nozzles are arranged between coal nozzles, one below the bottom coal nozzle and one
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above the top coal nozzle. If there are 'n' number of coal nozzles per corner there will be (n+1) number of air nozzles per corner. The coal fuel and combustion air streams from these nozzles or compartments are directed tangential to an imaginary circle at the centre of the furnace. This creates a turbulent vortex motion of the fuel, air and hot gases which promotes mixing, ignition energy availability and thus combustion efficiency. The air nozzles in between coal nozzles are termed as „Auxiliary Air nozzles‟, and the top most and bottom most air nozzles as „End Air Nozzles‟. For a boiler equipped with 10 mills, (or 5 double ended tube mills), the coal nozzle elevations are generally designated as A, B, C, D, E, F,G,H,J and K from bottom to top, the bottom end air nozzles as AA and the top end air nozzle as KK. The auxiliary air nozzles are designated by the adjacent coal nozzles, like AB, BC, CD, DE , EF, FG, GH, HJ and JK from bottom to top. The four furnace corners are designated as 1, 2, 3 and 4 in clockwise direction looking from top, and counting front water wall left corner as '1'. Each pair of coal nozzle elevations is served by one elevation of oil burners located in between the auxiliary air nozzles.ach oil gun is associated with an ignitor arranged at the side. Combustion Air Distribution: Of the total combustion air, a portion is supplied by primary air fans that goes to coal mill for drying and carrying the pulverised coal to the coal nozzles. This primary air flow quantity is decided by the coal mill load and the number of coal mills in service. The primary air flow rate is controlled at the air inlet to the individual mills by dampers.
The balance of the combustion air, referred as Secondary Air, is provided from FD Fans. A portion of secondary air (normally 30% to 40%) called 'Fuel Air', is admitted immediately around the coal fuel nozzles (annular space around the casting insert) into the furnace. The rest of the secondary air called 'Auxiliary Air' is admitted through the auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD Fan inlet guide vane regulation. The proportioning of air flow between the various coal fuel nozzles and auxiliary air nozzles is done based on boiler load, individual burner load, and the coal oil burners in service, by a series of air dampers. Each of the coal fuel nozzles and auxiliary and end air nozzles is provided with a louver type regulating damper at the air entry to individual nozzle or compartment.
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Wall firing The desired intensity and completeness of pulverized fuel combustion in the furnace space can be achieved through the proper supply and intermixing of pulverized fuel with
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Learning modules material- Engineering (FB)
secondary air in a burner assembly. The intermixing in the furnace space is ensured mainly by an appropriate arrangement of burners on the furnace walls and by providing a particular aerodynamic pattern of jets in the furnace space. There are two main types of burners, Straight flow burners: Burners of this type turbulise the air flow less substantially and produce a long ranging jet with a low expansion angle and weak intermixing of the primary and secondary flow. It may be fixed or tiltable burners which facilitate combustion control. They are mainly employed with high reactive fuels.- is this necessary? Turbulent Burners: In this burner, dust air mixture and secondary air are fed as whirled (turbulized) jets which form a cone shaped expanding flame in the furnace space. It has circular cross section. It can be used with any kind of solid fuel but are used mostly widely for low volatile grades.
Stoker Firing: Among renewable energy sources bio - mass fuel occupy an important place. Bagasse, the bio-mass fuels from Sugar mills, is being utilised for generating steam and power. As sugar cane cultivation is seasonal, availability of bagasse is not ensured throughout the year. To overcome this , the generating units are operated using bagasse whenever available and for the rest of the periods the unit can be operated with other fossil fuels like coal or with other bio-mass fuels like bark, peanut husk, coconut shell, wood chips etc These bio-mass fuels are efficiently burnt in stoker grates, which release heat for steam generation. There are a variety of stoker grates viz., inclined grates, vibrating grates, roller grates, dump grates and travelling grates . The choice is based on their capacity and application. Travelling grate stoker is unique in its design and is capable of burning variety of bio-mass fuels as well as burning coal . The travelling grate stoker has a firing bed which moves continuously between two sprockets . Fuel starts burning at one end, and is discharged at the other end as ash. Hence the travelling grate stoker is otherwise called as continuous ash discharge stoker (CAD) There are two types of travelling grate stokers viz.,
Non -catenary type Catenary type
Oil Firing Fuel Oil Atomisation 35
Learning modules material- Engineering (FB) Atomisation is the process of spraying the fuel oil into fine mist, for better mixing of the fuel oil with the combustion air for efficient combustion. While passing through the spray nozzles of the oil gun, the pressure energy of the fuel oil is converted into velocity energy, which breaks up the oil stream into fine particles. Poorly atomised fuel oil would mean bigger spray particles which takes longer burning time resulting in carry over and makes the flame unstable due to low rate of heat liberation and incomplete combustion. Other than pressure, viscosity of the oil is the major parameter which decides upon the atomisation level. For satisfactory atomisation the viscosity shall be within 15 to 20 cst. Fuel Oil Pump & Heaters
Pumping the oil is a major preparatory work on fuel oils for atomization and burning. The fuel pumps used in the fuel pumping house are of positive displacement type. A positive displacement pump causes a liquid to move by trapping a fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge pipe. Normally screw pump is used for pumping the fuel oil up to required pressure. The screw pump is compact in design and more silent in operation compared to the gear pump and vane pump. The screw pump can be operated at high speeds (around 4500 RPM) compared to the gear pump and vane pump. So, it can be directly coupled to the driving motor. Also it can develop very high pressure i.e. 150000 kpa, whereas a gear pump can develop pressure upto 1000 kpa and the vane pump can develop pressure up to 70000 kpa. The main advantage of screw pump is that it is vibration free and hence a smooth pulsation free delivery of oil is possible. Heaters: The viscosity of fuel oil at atmospheric temperature is very high. In other words the pouring temperature of fuel oil is higher than the atmospheric temperature. So to maintain the flow ability fuel oil heaters are used. There are two types of heaters namely steam coil heater and electrical heater. In steam coil heater steam is allowed to pass through the coil to heat the fuel oil up to the required temperature. In electrical heaters the heat energy induced by the electrical supply to the coils is used for heating the fuel oil.
Fuel oil strainers Fuel oil strainers or filters are essential to prevent the mechanical impurities reaching the small clearance and intricate passages in the screw pump. HFO suction strainers are provided with SS mesh of 500 microns filtration and LFO suction and HFO discharge strainers are provided with SS mesh of 250 microns filtration.
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Learning modules material- Engineering (FB) When one of the strainers is in line, the other is serviceable; when the pressure drop across the operating element exceeds the set value, the changeover may be effected by opening and closing the isolation valves. Suitable alarms are provided to indicate such clogged filters. Before such changeovers it is necessary that the standby strainer is filled with oil to avoid air locking and for smooth continued running of the pump. The clogged strainer should be immediately cleaned in solvent, wiped and air blown and replaced in position for ready availability. Strainer and strainer baskets must always be clean. Air Cooled Oil Gun: The atomiser assembly of an operating oil gun is protected from the hot furnace radiation by the flowing fuel oil and steam, which keeps it relatively cool. Once the burner is stopped, there is no further flow of oil or steam. Under such situation, it is required to withdraw the oil gun from firing position or provide some other means provided, in order to protect the atomizers from damages due to overheating. Certain designs employ stationary oil guns and utilize cooling air to keep the guns protected from heat. Before stopping the oil burner, the oil gun is scavenged with steam to keep the small intricate passages of the atomiser parts clean. Oil gun is always scavenged with associated igniter in service, to burn out the oil.
This group involves in Material Handling Coal firing Oil system Feeders and Handling equipment Air system
Inputs required: Scheme of pulveriser system and instrumentation Scheme of oil system GA of boiler Fuel pipe diameter Mill selection data from BPP
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Learning modules material- Engineering (FB)
Windbox selection Furnace sketch Key plan of boiler GA of mill Vertical bracing arrangement
There are two types of fuel systems used for Fossil boilers Gravimetric Feeder Volumetric Feeder
In the case of gravimetric feeders weighed coal will be sent to the boiler. In the case of volumetric feeder, rather than belt conveyer and weight measuring system, volume-measuring system will be adopted. The schematic diagram of the gravimetric feeder is as shown in figure followed by its working. 25mm crushed coal will be supplied to raw coal bunker Raw coal bunker outlet will be connected to feeder by means of coal valves and downspout Two no. of coal valves will be used, one for isolation and other for control Isolation valve is controlled manually, whereas flow regulating valve is motor controlled. Height of the bunker above the feeder will be such that seal air sent to feeder must not escape through the downspout. Coal reaching the feeder will be conveyed using belt drive Coal weight measurement system will be provided in between to weigh the coal. Weighed coal will be sent to the mill through the feed pipe.
The schematic diagram of the gravimetric feeder is as shown in figure.
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Learning modules material- Engineering (FB)
The schematic diagram of the Volumetric Feeder is as shown in figure.
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Learning modules material- Engineering (FB)
Disadvantage associated with volumetric feeder against the cost advantage is, the heat-input measurement in terms of volume for solid fuels is inaccurate, as there will be improper filling in specific volume. For handling chemical ash in the case of chemical recovery boilers, drag link chain feeder will be used, as the chemical ash is highly corrosive. In case of industrial boilers, drum type feeders will be used. This is because of the high bulk density, high moisture content and fibrous nature of biomass-based fuel.
Oil Firing scheme For the start-up of the boiler LDO and HFO will be used. Oil support to the boiler will be continued till the load reaches 30% and above this oil support will be withdrawn. Typical arrangement diagram for oil system is as shown in the following diagram.
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Learning modules material- Engineering (FB)
As the heavy fuel oil is highly viscous, there is need for heating. This results in fewer chances of choking of fuel line. Apart from above, this group also involves in handling equipments like Fan Circulating pumps Air heater and SCAPH Mills Pressure parts (LTSH and ECO) Furnace maintenance platform
2.2.6 Lining and Insulation Wherever the temperature of components is more than 60oC, insulation needs to be provided in all those areas in order to prevent the accidental damage to men and material working in the area. Various components in which insulation is provided are as follows Insulation: Insulation is the material which resists the heat flow from one medium to other. Types of Insulation Materials: Furnace bottom Rear arch Roof tubes Second pass header Furnace first pass & second pass Tubes and pipes >60°c Fuel lines Down comers, economiser coils, oil lines Main steam line Enclosure area
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Learning modules material- Engineering (FB)
Lightly Bonded Wool Mattress Rock wool Slag Wool Glass Wool
This material is used on water walls, enclosures, ducts, pipes, oil lines and EP. Service Temperature: 550 C & Density: 100 Kg / m3. Pourable Insulation This material is used in pent house roof deck and water wall Buckstays. Service temperature 650oC & density 650 Kg /m3. Calcium Silicate Slab
This material is used on water walls, Ducts, and Pipe lines. Service temperature 600oC & density 350 Kg /m3. The other insulation Materials are Lining : Lining is the material which can withstand high temperature. Different types of lining, their usage, temperature upto which they may withstand the heat and their densities are as follows. Insulating Bricks. Asbestos Mill Board Asbestos Rope.
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Learning modules material- Engineering (FB)
.
2.2.7 Boiler Supporting Structure Introduction A boiler is made up of complex masses either hanging or ground / floor supported. The hanging mass is primarily pressure part system consisting of components like Drum, Water wall, Super heaters, Repeaters, Economizers and these masses will be supported at the top most ceiling level of boiler through a ceiling grid consisting of main girders, cross girders and intermediate beams. The boiler structure also supports other equipments like Air heaters, Hot and Cold air ducting, Fuel pipes, Soot blowers, Critical pipes and also takes care of loads due to platforms provided for access and maintenance purpose. Both pressure part and non pressure part system contribute vertical load due to mass and horizontal load due to pressure, expansion etc. Further a boiler structure is subjected to dynamic loads like wind/seismic forces. A boiler supporting structure primarily does the function of supporting all the above masses for vertical as well as horizontal forces and safely transfers the load to foundation. The boiler structure is analyzed for all the above loads and sized using advanced software packages.
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Learning modules material- Engineering (FB)
Structural Arrangement and Load Transfer Mechanism: A boiler supporting structure is a steel structure of complicated arrangement consisting of girders, cross girders, intermediate beams, columns, beams, vertical bracing in both longitudinal and transverse directions along with horizontal floors and bracing system at different levels. The boiler structure primarily does the function of supporting the loads due to all the boiler equipments and platforms /floors (vertical) as well as wind and seismic loads (horizontal) and transfers the loads to the foundation. It acts in a composite form/ integrated form to resist all the induced forces on the structure. Since the basic pressure part system is hanging and subjected to downward thermal expansion the horizontal load transfer from pressure part to main boiler structure is affected by means of boiler guides. These guide forces either due to stability or due to wind/seismic will be transferred to boiler main braced levels and horizontal bracings at these levels will transfer these guide forces further to the vertical bracing planes, which in turn will transfer the force to ground. For seismic analysis of boiler structure, dynamic analysis using response spectrum method as per code IS 1893:2002 is performed. In short, all vertical loads will be transferred to columns through beams/ girders or directly like Airheater loads and the vertical load from columns are transferred to foundations through suitable base plates. The horizontal load acting on the boiler at various locations will be picked up locally by horizontal bracing of main braced levels, which will be transferred to vertical bracing nodal points at those elevations. These vertical bracings will transfer the horizontal load in X-X (along the axis of boiler/longitudinal) and Y-Y (transverse/perpendicular to boiler axis) directions to the column base. Counter balancing couples of reactions at column bases will resist the external moment created by the horizontal forces at the column base. The base shear will be transferred to the foundation through suitable shear lug provision at the bottom of base plate. All the mainframe analysis, ceiling analysis, Main Brace Level (MBL) / Horizontal Floor analysis etc., are accomplished with the help of computer programs “like in house program Anchoring System, MAIN, FLAT and bought out program like STAAD PRO”. Analysis of main boiler structure can be done in two ways namely 2-D or Plane Frame Analysis, 3D or Space Frame Analysis. The complete boiler structure is analyzed in 3 to 4 separate 3D structural models i.e., Main Boiler structural Frame, Boiler ID system frames like Before ESP, After ESP and Near Chimney. The boiler structural frame will consist of columns, beam, vertical bracing and sometimes horizontal bracing are also included for the analysis.
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Learning modules material- Engineering (FB)
Methods of Design: The column / bracing sizes taken from the STAAD output file “Member Selection” are altered to suit the available raw material inventory using program like “ALDESIGN” and “Opt-Plus”. The beam sizes will be designed by taking the forces from the STAAD output file “Member forces “and also the force actually coming in the beam like Floor & Duct load etc,. For Ceiling Structures separate IN HOUSE DEVELOPED PROGRAM will be used to prepare the input file and analyzed in the STAAD.
Engineering transmittals and material forecast: The material forecast will be prepared based on the above and send to Material Planning for the purpose material procurements in advance. The Engg Transmittals are prepared incorporating the final sizes of the members. Based on the transmittal the fabrication, the erection drawings and documents are prepared using the IN HOUSE DEVELOPED DETAILING PROGRAM (SCAD) & AUTOCAD. All foundation analysis, sizing of members for boiler supporting structure, ID structure and lift structure is done through computer programs which enables an accurate and optimum design in the least possible time with error free calculation. Timely release of foundation drawings and design documents are possible due to implement of software in each stage of analysis and design.
Sequence of Design Arrangement of vertical bracings and MBL Preparation of foundation loading table and anchoring detail for main boiler Design of Buckstays and preparation of transmittal DDR for Buckstays through program DDR for galleries and stairs Modeling in STAAD, PRO and analysis Submission of foundation drawings to customer Collection of boiler GA drawings, key plan, floor extension sketches, and vertical bracing arrangement drawings from layout.
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Learning modules material- Engineering (FB)
Collection of vertical chunk load data from sections of PE Calculation of live and dead load Calculation of the wind load Transfer of PP furnace guide loading to MBL Modeling of column , beams, vertical bracings, member releases, member properties and support Addition of vertical load to the model Stabilization of model with wind loading Analysis of model of various load combinations Perform dynamic analysis Design of interconnecting platforms Lift structures Economizer handling structures Mill platform Mill handling structures Machine room structure
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Learning modules material- Engineering (FB)
Flow chart representing the input and output of Structural Design
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Learning modules material- Engineering (FB)
Weight Breakup of Boiler Structural 42% Pressure Parts 26% Ducts 14% L&I 10% Fuel Firing 6% C&I 1% Handling system 1%
Comparison of Boilers (Typical) Capacity (MW) Flow (TPH) Weight (MT) PGMA (No.) No of DU 500 1675 31000 820 25000 250 810 14700 640 18000 210 700 12000 620 16000 120 390 7800 550 10000 60 160 4100 300 5000
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Learning modules material- Engineering (FB) 2.3 Information Technology Systems And Services
The main functions of the department are Develop programs for Engineering cycle time reduction Engineering process automation BCR monitoring system Erection drawing viewing system Collaborator documents transfer system Development of Dolphin drawing management system Re- Engineering technical engineering design fortran programs Operations and Maintenance manual management system Plant modeling
The works dealt till now are Developed a comprehensive O&M Generation and Management System covering aspects from preparation to despatch featuring online viewing of O&M Manuals and easy status tracking Automated PDF document generation and successful deployment in CPT and BCR systems Enhanced security features in online viewing of Dolphin drawings Implemented BCR monitoring system Developed CPT workflow system with extensions to general engineering transmittals Deployed Design Directives, Technical Information and CIP‐Contract Information package in quest Automated generation of 25 CPTs Enhanced Windows‐based Integrated IBP/AH/Mill programs and link programs for selection of Auxiliaries Completed IBP Input & Output storage to database for integration with CPT generation systems Developed programs for ESP Selection, Chimney Selection, Duct Sizing, Drum Length calculation, OCF calculation Heating Surface Area calculation for 500MW boilers and Fan Selection
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Learning modules material- Engineering (FB)
Implemented Single‐stroke multiple load IBP Input Generation system Developed C&I proposal document generation system that generates all the 8 documents Implemented AIX‐based systems, NOWA/WANSYS, RHBP, Metals, Filmx and Supernova for super‐critical boilers on temporary server Quest, an enhanced Technical Information Gateway to access all online engineering applications, including technical reference information with built‐in access control Provided facility for viewing and downloading of Erection Drawings Tender Documents online reference facility was developed & Implemented Developed an online PIR management system to manage developmental projects Implemented CIB System for preserving and accessing CIB documents online. ASTM, BIS, BS, IEC standards made online Facilitated Tour Management System for FES General Arrangement drawing extracted from PDMS model of Budge Budge project: Boiler sectional side elevation Pressure parts arrangement Layout of cold air ducting plan and elevation Marks a major milestone in plant modeling
2.4 Controls and Instrumentation Department of Controls and Instrumentation (C&I) has 5 groups as follows Project management (PM/C&I) Product engineering (PE/C&I) Field engineering services and renovation and modernisation (FES, R&M) C&I Centre Quality assurance and control (QAC/C&I)
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Learning modules material- Engineering (FB)
Department of C&I mainly look after all the controls required for the boiler. A few of the controls are designed and fabricated in BHEL, Tiruchirapalli and the rest are outsourced.
C & I Centre Various Instrumentation getting assembled and tested at C&I centre is Electronic water level indicator (BHEL VISION 20M) BHELMHO level switch Flame scanner head assembly Acoustic steam leak detector Gravimetric Control
Electronic water level indicator is the indigenously developed technology to know the drum level. It works on the principle of difference in the electrical resistance of the steam and water. Steam will have the electrical Resistance in Mega Ohms whereas the same for the water in Kilo Ohms. By measuring the resistance signal will be passed to activate either green or red bulb to indicate the water or steam. Logical checks for the steam below water and water above steam also provided to get the correct signal. System which is operating with 24 V DC supply got certified by the Canadian Standards Association and 230 V AC supply system is under the process of certification. BHELMHO Level Switch will be used in HP Heaters and condenser drying parts. Acoustic steam leak detector works on the principle of steam leaking will have some sound with some frequency. Based on this sound level it will be found whether steam leak is occurring or not and if it is, at what location, such that it can be rectified during the maintenance or annual shutdown. This reduces the damage by preventive action Flame scanner head assembly will be used to detect the flame inside the Combustion chamber and to transfer the signal to FSSS. It works on the principle of intensity discrimination and flicker frequency discrimination. Intensity discrimination decides based on the quantity of light signal received and flicker frequency discrimination is based on that there will be flicker if coal burns in the boiler. Flexible fibre optic cable will be used to tilt the head as coal nozzle tilts and to transfer the light signal.
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Learning modules material- Engineering (FB)
C&I -Product Engineering (PE /C&I): Product engineering activities consist of detailed engineering of activities pertaining to C&I area. This group designs the furnace safeguard supervisory system. Furnace safeguard supervisory system is the systems, which consist of control systems for all electrical components such that plant works healthy. It contains the facilities to trip the boiler or particular equipment at any abnormal operation. It also contains the soot blower control system, secondary air damper control system and burner management system. FSSS manufactured by EDN, Bangalore. Apart from the design of FSSS, this group carries out the procurement of Instruments required.
C&I -Field Engineering Services FES (C&I) activities start with the activities of FES (FB). Site action request and commission action request is the ways of giving complaints. The FES (C&I) group will deal complaints pertaining to commission action request. PG Test and pending work, which will be raised by site or customer, will be taken care off. Responsibility of training the customer for operating the plant will go to FES (C&I).
C&I- Quality Control and Quality Assurance Quality assurance prepares the plan for various items prepared at BHEL. Checking or inspection carried out at various stages like raw material inspection, in-process inspection and final product inspection. There are 4 types of quality plans viz., standard quality plan, vendor quality plan, contract quality plan and reference quality plan. Quality control is the post quality activities of the purchase order. Activities performed by C & I group Proposal Activities : Technical offer preparation, bill of material preparation, marking of instruments in p & id’s, filling up of data sheets, engineering support for obtaining budgetary offer for new items, attending proposal meetings with clients Digital controls boiler interlocks Burner management system Soot blower control Analog controls drum level control Main steam pressure control Main steam temperature control Furnace pressure control
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Learning modules material- Engineering (FB)
Cbd tank level control Local instruments Level measurements Direct level gauge ,electronic water level indicator, transmitters Preparation of specifications and Indenting. Pre bid discussions. Evaluation of offers. Technical recommendation. Scrutiny of vendor documents
C&I PRODUCTS & THEIR INTERFACES WITH BOILER CONTROLS” 1) INTRODUCTION 1 2) C&I PRODUCTS 2 GRAVIMETRIC FEEDER 3 BHEL SCAN 12 BHEL VISION 16 BHELSONIC 18 ELECTROMATIC RELIEF VALVE 21 FURNACE TEMPERATURE PROBE 23 INTRODUCTION Every process available with and used by Us were created with an objective and to serve purpose. While doing the same the process should be in our control. Uncontrolled process often end up in creating harms. The process when controlled manually deprive lot of man power and are less accurate in measurements. The solution for providing a efficient control system is to automate the same and having Soft controls to control the same. Every process created by us need to be monitored for the datas provided by the process. When the datas are plenty it becomes less possible to track its
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Learning modules material- Engineering (FB) performance. Automation provides features to have these datas stored and retrived when necessary. When a process is at risk or at saturated level the conditions need to be communicated for further proceedings to be made with respect to the behavior of the process. The speed of communication is a highly necessary factor when the process is most critical. Automation has also provided Indication and Alarm features which reach the process owners in the same instant of risk formation. C&I PRODUCTS The Instruments or systems supplied in the Boiler serve any of the following purposes like Indication, Control, Quantisation, Alarm generation or Interlock. The In-house products developed by C&I like BHELFEED, BHELSONIC, BHELVISION and BHELSCAN also serve these purposes. BHELFEED controls the coal flow, Quantize the amount fed into the boiler, creates alarm and trip interlocks for malfunctioning observed. BHELVISION indicates the drum water level and also used for Drum level control. The same system also creates Alarm and trip conditions for the starvation and over flow of the drum level. BHELSONIC indicates the Boilertube leaks and creates alarm that are useful in preventing damages to adjacent tubes. BHELSCAN indicates the flame availability and the distinguishes the fuel causing the flame. It also creates Boiler trip when the fuel injection is available but the flame is not sensed. ERV, Electromatic relief valve is a supplementary valve to relieve Boiler pressure in addition to Safety valve. It avoides frequent opening of Safety valve. FTP, Furnace temperature probe is used to indicate the boiler furnace tem perature during Start up. GRAVIMETRIC FEEDER GENERAL ARRANGEMENT OF A GRAVIMETRIC FEEDER The path of the coal flow is from the Bunker Outlet to the Mill and the Gravimetric feeder regulates the flow of the coal in the middle. The purpose of Feeder is to control the speed of the belt according to the fuel demand and hence controlling the Coal flow into the Mill. The
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Learning modules material- Engineering (FB) general arrangement of a feeder has various parts both Mechanical and Electrical for the efficient functioning and data transmission purpose.
FEEDER GENERAL ARRANGEMENT The Gravimetric Feeder weighing system consists of two load cells to measure the mass on belt. Load cells measure the change in resistance in strain gauges to measure force. The control system of gravimetric feeder consists of two electronic assemblies. One mounted along with the feeder and the other a remote power panel at the control room. The electronic assembly mounted to the feeder will have signal conditioning modules for load cell and tachometers, panel mounted rotary and push button switches, lamp indicators for feeder status. The remote power panel will have all necessary power circuits, controller etc.. The controller is a 32 bit embedded processor of Pentium architecture. A 32 switch membrane key panel and two row vacuum fluorescent display of 40 characters each, are provided on the panel door for operator interaction. All the I/O modules are intelligent, EURO standard rack mounted. The controller works on a real time operating system. The system contains the 32- bit microprocessor, memory, digital circuits and a keyboard display etc. All I/Os are optically isolated with respect to processor and its associated circuits. Analog 55
Learning modules material- Engineering (FB) circuits used to amplify and convert the load cell outputs are located separately in the feeder local panel. The display / keyboard located on the Remote panel door provides the means to communicate and receive information from the processor, for parameter setting, mode selection etc. The display / keyboard assembly consists of a vacuum fluorescent display for written communication, in addition to the display of process numerical data. The vacuum fluorescent display consists of two lines with 40 characters each, which indicates totalized weight, feed rate, motor RPM, material density etc. In addition, 9 LEDs serve as system status indicators. The keyboard consists of 32 keys with various functions / operations. Since the feeder control system employs advanced digital processing with 32-Bit microprocessor, the control gives better weighing accuracy and better repeatability as compared to analog electronics based controller.
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OPERATIONAL DETAILS: The MPC based feeder control receives signals from the two load cells, which represent the weight of coal acting on the weigh span. Also the system receive two speed signals from two independent speed sensors (tacho-generators) which represent the speed of the feeder belt. The system is designed to automatically switch to the secondary tacho-generator if the primary tachogenerator fails. The application software computes the feeder delivery as follows: a measurement is taken of the output of one load cell. This signal is converted in to a digital signal with high resolution. This value is compared against parameters stored in temporary memory. The same operation is then performed on the other load cell. The signals are validated and if it is invalid, the feeder is switched to volumetric operation and the controls use a simulated load cell output generated from historical average value stored in the memory. If the signals are valid, the two load cell output signals are summed and the tare is subtracted. The result is multiplied by a calibration factor determined during calibration to arrive at weight of material per unit of belt length. The speed signal from the tacho-generator is multiplied by another calibration factor to arrive at a number representing belt travel speed per second. Subsequently the belt speed and weight signals are multiplied together to arrive at the feed rate. The result is then compared to the demand feed rate to determine the error and to vary the feeder belt speed through the variable frequency drive. The key board with display unit provided on the front door of the remote control panel facilitate user friendly communication which clearly communicate text messages to the operator during parameter setting which are project specific (during initial start-up), calibration, self check and self diagnostics. The display unit has 2 lines of 40 characters each with Vacuum Fluorescent Display. The embedded software is suitable for controlling the gravimetric feeder in real time. The mode of operation and parameters are entered through keyboard and the Vacuum Fluorescent Display unit show the status of operation and indicate the values like Feed rate, total coal delivered, motor speed, coal bulk density etc. The system provides four numbers of 4 - 20 mA DC analog feed rate feedback signals to DDCMIS. In addition, one pulsar unit is provided with two pulse outputs – one for customer use 57
Learning modules material- Engineering (FB) and the other for remote integrator. (For every 100 Kg. of coal delivered, the pulsar unit will give one pulse output) Each feeder controls shall receive federate demand signal from DDCMIS for feeder speed controls. The demand signal provided by DDCMIS shall be galvanically isolated 4-20 mA DC. SALIENT FEATURES : The salient features of the micro Processor based gravimetric feeders are: Automatic digital calibration of feeder to measure and record Tare and Span factors independently requiring no potentiometer adjustments. Providing user interface through Keyboard and display unit by which the feeder controls can be programmed and operated. This includes setting the mode of operation, programming of parameters and display of various status. Automatic changeover from Gravimetric mode to volumetric mode in case of errors in load cell signals. Data logging and storage of process parameter like total coal consumption, historic density and trip details and recovery of data in the event of power interruption
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Automatic change over from main tacho to redundant tacho when main tacho fails. In built diagnostic features, which assist the operator in system trouble shooting, thus resulting in less down time of the feeder. INTERLOCKS RELATED TO FEEDER : TO START A FEEDER : The following conditions are required to start a Gravimetric feeder in the remote mode. i) Adjacent Oil Elevation should be in service to give start permit to feeder. ii) The Primary air capacity also determines the number of feeders that can be put into service. 59
Learning modules material- Engineering (FB) When no PA fans are in service no feeder can be started. When only one PA fan is in service only lower half elevations can be put into service. The Feeders corresponding to the lower half elevations should have been provided with other start permissive. When both the PA fans are in service all the feeders can be put into service provided the start permissive from FSSS is available. During operation while the PA fan trips the boiler load has to be reduced and hence the feeders corresponding to the upper half elevations are taken out of service. Similarly the remaining feeders also stopped while the other fan trips. SIGNALS TO OTHER SYSTEMS : a) The Feeder signal are interfaced with the SADC system. The 4-20 mA output given from the Feeder to the SADC system regulates the Secondary Air Damper. b) Feeder proven signal is given by the feeder when the federate crosses 50%. When the Feeder is proven the Fuel air damper is in regulating mode according to the boiler load. c) When the feeder is OFF the status indicates that coal fuel is nopt available in that elevation and Oil fuel has to be introduced to support the Elevation flame. SIGNALS GIVEN BY FEEDER : i) Feeder Trip : The Feeder Trip is caused by the following conditions. a) Loss of Both tachos. b) Feeder Discharge plugged c) No coal on Belt in Remote d) Material on belt in local/calibration e) Belt motion monitor timed out f) Motor starter fault g) RPM Deviation h) Coal flow monitor fault 60
Learning modules material- Engineering (FB) i) Loss of flow j) AIM/AOM/DIM/DOM/PIM module BAD When the feeder trips due to any of the trip condition the Feeder Off signal is given to the FSSS. If the same happens for all the feeders then All feeders Off signal is generated which is a condition for Loss of all fuel trip. ii) Feeder in Remote : When the feeder is running in remote mode the Feeder remote signal is available and the feeder cannot be operated locally through Local control panel. The operations done locally will be null and void. iii) Feeding Volumetric : When the loadcell difference goes above 12.5% of their count feeder switches to Volumetric mode.The signal given to DCS indicates whether the feeder is currently running in Volumetric or gravimetric mode. iv) Feeder Alarm : The Gravimetric Feeder gives Alarm output for the following Conditions. a) Load cell signals Out of range b) Demand signal out of range c) Loss of any one tacho d) Remote TCI increments too small e) Feed rate Error For all alarm conditions,Feeder Alarm status is communicated to the control system through the Feeder alarm Signal to initiate necessary actions. v) Feeder Running Reverse : The Feeder when taken for maintenance to clean the coal deposited on the belt the feeder will be run in the reverse direction. During this condition feeder cannot be operated from Remote condition. BHELSCAN
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Learning modules material- Engineering (FB) Flame scanner is a digital and reliable way of flame detection. Advancements in flame detection has moved from just identification to discrimination of flame and processing the multiple flames. The advantage of the same is the elimination of individual scanners for different type of fuel to discriminating operation for multiple fuels. PRINCIPLE OF OPERATION : The basic physics of Flame scanner is Light detection. The further processing of the signal is to derive further observations as inputs for other systems. It consists of a Head electronics and a control unit. The Head Assembly of the scanner consists of a Quartz lens for collection of light signal and a fibre optic cable for transmission of light signal to the Head electronics. In the Head electronics a Photodiode is used to convert the light signal to electrical signal.
BHELSCAN SYSTEM CONFIGURATION The electrical output of photo diode is amplified by log amplifier. The amplified signal is then compensated for back ground radiation of the furnace. The flame intensity component and flicker component of the signal is separated and processed for characteristic frequencies of coal and oil flame by the control unit which consists of four signal processing card and 2/4 logic card.
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SIGNAL PROCESSING IN THE BHELSCAN The software ensures discrimination of different flames through proper detection of characteristic flicker frequency range of the flame. For an elevation, all the four corners are centrally monitored by a fifth microcontroller, to give flame out signal by employing 2/4 logic and establishes the connectivity to DCS through RS232C.
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Learning modules material- Engineering (FB) FLAME PROCESSOR MODULE : The individual flame processor module has the following architecture. The Input signal is conditioned and compared with the previous set values done through Dip switch. The display mode can be changed with the Mode Pushbutton. The final output of the availability of flame is given through LED outputs.
FLAME PROCESSOR MODULE INTERFACE FROM BHELSCAN FOR BOILER CONTROLS : 2/4 Flame output from the flame scanner is taken for voting the elevation flame or No flame. The Flame signal along with the open condition of HFO, LDO Trip and Nozzle valves shows that the flame is due to Oil firing.
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Learning modules material- Engineering (FB) The Flame signal along with the Feeder proven(>50%) feed rate shows that the flame is due to Coal firing. These two signals are supporting the discriminating mode of operation of the BHEL SCAN. The No Flame signal is an input toElevation No flame which in turn goes to the Unit Flame failure trip.
The Intensity of the Flame sensed by the Scanner is given as 4– 20 mA signal to DDCMIS for monitoring purpose.
BHELSCAN HEAD PORTION INSTALLATION AT SITE BHEL VISION The BHEL VISION is a Water level Indicator. It works on the Principle of Conductivity. The Conductivity difference between Steam and water is used to distinguish the fluid in the Drum.
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Learning modules material- Engineering (FB) The Drum tappings are connected with a Pressure vessel in which the fluid are almost at the same operating conditions of the Drum. The level in the pressure vessel does not reflect the Drum level directly due to the density difference of the fluid caused due to the pressure difference between the Drum and the pressure vessel. The level difference is corrected in the software. The Pressure vessel is divided in equal parts. The Electrodes are fit one per region. The Steam or the Water in the region induces conductivity as the Electrode is a conductor. The Conductivity for the Water is greater than Steam. The Resistance in the electrode is sensed and measured. Steam region will have the higher resistance due to lower conductivity of the range of 200 K ohm. Water being a good conductor offers less resistance of the order of 100 K ohm. The Resistance observed are compared with the set value for that region. If the Resistance is high the region has been voted for Steam and for lower resistances water is voted. The output has been indicated through Red and Green LED at the display units fixed at the panel door and in UCB. BHEL VISION AT SITE INTERFACES FOR BHELVISION WITH BOILER CONTROLS: The Drum level has been transmitted to DDCMIS as 4 – 20 mA output. Drum Level control is used in Auto control for 2/3 redundancy in evaluating the drum level along with Direct water level gauge and Level transmitters. The Drum level Low alarm and H igh alarm are used as alarm indications to the Drum water level Low or High status.
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Learning modules material- Engineering (FB) The Drum level very low with a 5 seconds time delay is taken as a Low trip signal for Master fuel trip. Similarly, Drum level very high with a 10 seconds delay is ta ken as High trip signal for Master fuel Trip. The probe fault and the System fault along with the Alarm signals for Drum level are taken to the marshalling panel in DDCMIS. There are provisions for disabling the Trip signal also for User’s convenienc e. BHEL SONIC The BHELSONIC is an advancement of the early days manual tube leak detection. This system provides early tube leak detection through acoustic emission.
BHELSONIC SYSTEM ARCHITECTURE BHELSONIC consists of the following operating parts. Sonic tube assembly makes contact with the boiler tube wall and the acoustic waves passes through it to reach the acoustic sensor. 67
Learning modules material- Engineering (FB) Acoustic sensor converts the acoustic signal to electrical signal. The Field amplifier box eliminates the unwanted ba ckground noise through a filter and amplify the signal. The Voltage to Current convertor produces an output of 02.2– 20 mA and it is taken to the panel. The Ascertor modules in the panel convert the signal back to voltage and process them and give to the dB scanner. SIGNAL PROCESSING IN BHELSONIC The scanner scans all the channels and displays the amount of leak in the form of sound decibels. The Scanner displays 0 – 40 dB corresponding to 74 – 114dB in actual sense.
The Alarm preset and tim e delay for alarm are set through manual selection. The PC attached in the panel gives pictorial representation of the Boiler arrangement through mimic and gives the leak pattern of regions as bargraph, Trend and history. 68
Learning modules material- Engineering (FB) INTERFACE FOR BHELSONIC WITH BOILER CONTROL: Sonic tube leak detection has no Trip Interlock attached with it. When the leak is detected after the preset time delay Alarm is generated through the Bhelsonic panel. The Bhelsonic performs the discrimination of Steam leak and Sootblower operation as both the operations are steam flow. The Alarm feature identifies the possible location of steam leak and operator can perform remedial measures to avoid the steam leak causing damage to nearby tubes. ELECTROMATIC RELIEF VALVE The Electromatic relief valve is an automatic, electrically actuated, pressure relief valve which can be set for one percent or less differential between opening and closing by means of a pressure sensitive element that precisely and automatically relieves pressures within very close limits. The application of this valve places at the command of the plant operator a means of instantaneous opening and closing a relief valve on remote header. The electromatic relief valve does not replace the spring-loaded safety valve. Rather, it has been designed as a supplementary operating valve that will conserve power and increase efficiency of a steam generating plant. Regular spring-loaded safety valves rarely pop in service when the electromatic relief valve reduces safety valve maintenance substantially. The relief valve assures more accurate, balanced boiler operation at peak loads, plus a more uniform line pressure. ERV SYSTEM ARCHITECTURE
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Learning modules material- Engineering (FB) ERV PRESSURE SWITCH : The electromatic safety valve is provided with local controller. The controller is provided with Barksdale make pressure switch (B2S-H32SS), having micro switches with dual set points. One micro switch with change over on increasing parameter (PSH) and the other on change over on decreasing parameter (PSL) OPERATION: The UCB mounted control station has a 3-position switch. When the switch is in "OFF" position, the DC supply to the contactor is cut off and the solenoid valve is not energised and hence in closed position. When the switch is in "MANUAL" position, the relay coil of the interposing relay mounted in the relay panel gets energised; and through the contacts of interposing relay, DC power contactor housed in local (in ERV controller) gets 220VDC power supply and gets energised. The DC supply gets connected to the ERV solenoid valve through contacts of this DC contactor and the ERV opens, relieving the steam pressure. The ERV remains open so long the switch is in "manual" position and closes when the switch is turned to the "OFF" position. When the switch is in "AUTO" position, the pressure switch also gets connected in series with the contactor relay coil. When the steam pressure is in normal operating range, the DC supply is not extended to the contactor coil. When the steam pressure reaches a preset relief high level, the PSH is activated, contactor is energised and power supply reaches the solenoid valve energising and opening it. ERV opens and the steam is let off and the pressure is relieved. Subsequently, when the steam pressure reduced to the preset low level, the PSL resets and the control supply to the contactor is cut - off, de-energising the solenoid valve. Solenoid valve closes and ERV closes closing steam line. Hence, normal operation continues without affecting the process even if process pressure overshoots using ERV control INTERFACE WITH BOILER CONTROL : The ERV valve opening and close status are available at the DDCMIS to ensure the action taken against the Boiler pressure relief. FURNACE TEMPERATURE PROBE The furnace temperature probe is intended to measure the temperature in the furnace during the initial start-up of boiler. The probe consisting of: 70
Learning modules material- Engineering (FB) i. A duplex chromel - Alumel thermocouple mounted at the tip of the probe. ii. Limit switches for ‘Probe extended’ and ‘Probe retracted’ positions. iii. Local push-button integral with the probe for local operations. iv. An electrical drive to extend or retract the probe as necessary. v. A helipot potentiometer for remote position indicator. OPERATION : Switch on the power supply switch (PSS). Press the 'EXT' push-button from field or ‘extend’ command from DDCMIS to advance the probe. Power contactor 'EXT' energises and hence the electrical drive is switched ‘ON’ and the probe advances to furnace. Once the probe reaches the fully extended position, the limit switch LS-R changes over to thus cutting off power supply to the 'EXT' power contactor. FTP stops at the forward end. The Cr.Al thermocouple measures the furnace temperature and fed to DDCMIS for remote indication. The probe can be retracted from the forward end position by pressing ‘RET’ pushbutton from the field or by ‘Retract’ command from DDCMIS. The limit switch LS -S is provided to monitor the home position of the probe. Once the probe reaches the home position, LS-S resets tripping ‘RET’ power contactor and tripping the power to probe motor. If intentionally the probe is to be retracted to its home position during the advance / retract cycle of operation, press the retract push button / give retract command from DDCMIS. The contactor RET energises and trips ‘EXT’ contactor, if it is energised already and probe withdraws to home position as described above. Once the probe reaches the ‘home position’ the limit switch ‘LS-S’ stops probe operation. Temperature probe can be stopped inside the furnace at any location within its operating range by pressing ‘STOP’ pushbutton or by giving ‘STOP’ command from DDCMIS. This can be used for measuring the temperature of furnace at any location. From that particular location FTP can be advanced/ retracted using extend/retract commands. FTP INTERLOCKS : i. When the temperature exceeds 540°C, the interposing relay ‘R4’ is energised from DDCMIS, ‘NO’ contact of the same energises the RA contactor, thus bringing the probe to its ‘home position’. ii. When probe motor gets over-loaded FTP will be retracted to home position.
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Learning modules material- Engineering (FB) iii. At any position temperature probe can be stopped by pressing ‘Stop PB’ from local or from DDCMIS, to register steady temperature. iv. Spare potential free contacts for X1, X2, X3, 74, EXT & RET are provided in the starter box for annunciation. v. For remote position indication of temperature probe position, 4-20mA signal will be provided from the local starter box to DDCMIS. INTERFACE WITH BOILER : FTP has no part to play with the Boiler control and it is used only for the Indication purpose. The Operator should ensure fluid flow in the Reheater when the set temperature is acquired. FTP is meant for Reheater Protection. Probe in operation and probe motor overloaded indications are provided in the starter box. 2.5 FES AND R&M:
Field Engineering Services Activities of field engineering services start after the erection of boiler. Key Activities are as follows Project Follow – Up Project Follow–up consist of Advice to customer/site on pre-commissioning checks Support to customer/site in commissioning and trial operation Feed back to the concerned Engineering sections regarding technical problems /equipment performance Recommendations with required documents for repair / modification in case of failure Disposition of CARs and customer complaints Technical directives to site/Region Feed back to similar units for corrective and preventive action Coordination of interface activities between Engineering sections and site during repair/modification
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Performance Testing Performance Testing Includes Functions: Clarification (CL) : Performance Guarantee test Proposal Performance Evaluation test Proposal Performance Guarantee test reports. Performance Evaluation test report Performance test data.
Complaints requiring only Clarifications / advice without involving any material supply / man power are classified under this category. FES activity is complete when the clarification is furnished.
Material supply (MS):
Problems for which materials are to be arranged / supplied are categorized under Material Supply. Phase -I activity of FES is complete when the Work Order is obtained and document is released (if required) by FES and communicated to customer /site.
Phase I is completed with the following activities: Sending of WO Request to Contracts as per format 353-012 Receipt of WO from Contracts Group DDR as applicable (No DDR required for raw material supply or repeat manufacturing of NS-I document) Communicate the proposed action to Contracts / Site / Customer as applicable.
Phase II is completed with the following activities: Contracts will arrange for material supply to BHEL site / Customer and inform FES on despatch of materials.
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On receipt of material at site, FES will further follow-up for implementation and for obtaining feedback. If no complaints are reported within three months of the implementation, it will be construed as the issue is resolved satisfactorily with the customer and treated as closed.
Site work (SW) :
Based on the advice / solution recommended by FES, customer / site has to carry out work / modification. No material supply is involved from the unit. These are categorized under Site Work.
Phase I is completed with the following activities. FES will coordinate with the BHEL site / customer enabling them to arrange material, man-power and work schedule.
Phase II is completed with the following activities: Material supply will be organized by BHEL site / customer as applicable. On receipt of material at site, FES will further follow-up for implementation and for obtaining feedback. If no complaints are reported within three months of the implementation, it will be construed as the issue is resolved satisfactorily with the customer and treated as closed.
Engineering Analysis (EA) :
Complaints received from customer / BHEL site requiring engineering analysis for deciding the solution are classified under this Category. These are referred to concerned Engineering sections whenever required and FES coordinates for solution. Phase I is completed when suitable advice to customer / BHEL site is communicated. However, an interim reply informing the action being taken will be sent to customer / BHEL site. Based on the recommendation, execution will be done by Customer / BHEL site. Follow up of implementation and feedback on performance will be done by FES as Phase II activity. Wherever material supply is needed, WO will be obtained by FES and Document released, if required. Generic problem (GP) : Generic Problems are those which are repetitive in nature for more than three times either in the same boiler or in different boilers.
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The Generic Problems which are reported either by the Project Coordinator or Group Head or HOS are referred to a Select Committee which will review and advise appropriate actions to avoid recurrence / occurrence.
Renovation and Modernisation
Proposals stage: This checklist is designed by incorporating all the details related to the R&M proposal preparation starting from dates, observation and values. The checklist is divided into 6 sections, for easy application and maintenance of data. They are General information about the project R&M requirement in component wise
Proposal Finalisation and Date of Commitment.
Enquiry.
Schemes and BOM Preparation
Proposal document Checklist while forwarding proposal
SECTION 1: GENERAL INFORMATION ABOUT THE PROJECT This section contains general information about the R&M proposal. Some of the main details available here are, Place - from where we received the offer, Type of R&M - which is to be placed and Factors - based on which the R&M is proposed. The enquiry/ tender may be placed by the Spares and service Business Group head quarters or by the corresponding SSBG region. Sometimes the enquiry will come from customer directly to R&M commercial department or R&M engineering department. These are the four ways of getting the R&M proposal from customer. Type of R&M will be decided by following factors,
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Learning modules material- Engineering (FB) Major R&M – Nameplate Rating : To meet the name plate rating of the boiler, which is currently running below the specified rating. Major R&M – with Up rating : Here the R&M is based on increasing the capacity of existing boilers by 5 to 15 MW. C&I R&M : Here the R&M is based on Control and instrumentation requirements and state of art improvements in electrical and electronics field Part R&M – due to requirement from customer for specific replacement of components due to failures and damage.
Requirement of R&M is based on following factors: They are various reasons why R&M of old units is required. To meet the growing demand of power, the R&M of existing old units is a viable option for our government. Sometimes due to some peculiar problem also the customer want the renovation and modernization the unit.
Decrease in Plant Load Factor of the plant Increase in Heat Rate Generic Problem in the thermal station Lost capacity of the power plant Pollution Control regulations Availability / Reliability of the power plant Section 2: R&M requirement in component wise :
The component vise R&M requirement can be broadly classified into Pressure parts, Fuel system, Non- Pressure parts, C&I requirements, Mills, Gates and Dampers and Auxiliaries like Fans, Air pre-heater and Electrostatic Precipitator.
Pressure parts: The R&M of pressure parts may be required due to erosion, clinkering, High gas temperature at AH outlet, second pass modification due to overheating. It may also due to coal property change or modification due to uprating. Fuel system: In fuel system the R&M activity may be carried out due to state of art improvements or coal property change.
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Non Pressure Parts: In Non pressure parts the R&M activity may be based on conditional assessment carried out in the boiler or due to customer requirements. C&I Requirements: The state of art improvements of C&I equipments will create interest in customer minds for R&M of C&I equipments. Fans: The changes in ESP modification and ID system layout change will be sorted out by the corresponding change in fans capacity through R&M activity. ESP : The R&M of Electrostatic Precipitator may be due to state of art improvements in the ESP field or may be due to customer requirements through communication meetings. Gates and Dampers: In gates and dampers, the R&M activity may be based on state of art improvement in the same field or may be due to customer requirements. Mills: Change in coal property change and state of art improvements in mills field will induce the R&M of Mills. Section 3: Proposal Finalisation and Dates of commitments: The important aspect of the checklist is to follow up with dates of commitments, on which certain actions are planned. Because this commitment dates will decide the time taken for the proposal preparation. The first important document, what we get for the proposal preparation is the tender document from the SSBG head quarters. This date is the starting date in proposal preparation schedule. On receipt of this, we will duplicate and distribute the copies to various related departments. Based upon the documents, the next step will be compilation of missing data. After this both the customer and supplier will negotiate between them for the deviation observed in the tender. Then the negotiated deviations have to be discussed with the relevant departments for their concerns and it has to be reported back to SSBG and Customer. Then list of proposal drawings to be submitted will be finalized. After this scope, exclusions, terminal points, codes, standards and division of work among departments will also be finalized. Then list of major sub deliveries will be intimated to corresponding sources. Floating enquires for sub deliveries will issued to the dealers.
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Section 4 : Enquiry This section deals with proposal calculation. The first step is the Verification of Boiler thermal calculation & performance variation from original design condition. If there is no deviation observed then design of fuel analysis will be done and in case if it is found that mill cannot support full load it will be intimated to customer. The boiler parameter will be checked against the margins specified by customer. The Air temperature entering Air- pre Heater is designed accurately based on the calculation. The gas temperature leaving air- heater is another important part of proposal calculation. It mainly depends on the acid dew point and draft loss level at various loads. The excess air required will be fixed, based upon the calculation. After this the air and gas weight calculation will be done. After this, The efficiency calculation and furnace leakage percentage will be calculated. The selection of pulverizer will be based on the spare mill required, mill up rating in future, motor rating to IS, mill reject handling system input and power consumption guarantee given to the pulverizer. Similarly the wind box selection is decided by the Number of oil elevation, maximum oil load, type of scanner, type of ignitor, scanner used, gun cooling air fans, ignitor air fans. In this stage, few data has to be furnished to oil system. They are oil dew point, line tracing, common/ individual system and simple and duplex strainer. Like this some data has to be furnished to C&I for furnace supervisory safe guard system (FSSS). Some of the details including pump house, boiler overall new design, latest design requirement will be given to C&I. The Air Pre- Heater selection will be based on TG min, Sector angle required, modn check op air/ gas parameters. Then Steam coil air preheater (SCAPH) will be designed . The fan selection for Forced draft, Induced Draft, Secondary air and Primary air sectors is dependent on parameter like margin available with existing fans, special drives, cold/ hot PA, checking healthiness of motor by IS, recirculation required by FD and stalling point check.
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Then the parameter for dust collecting plant will be discussed with BHEL Electronics division, Bangalore and BHEL PDX, Bhopal. The Furnace outlet temperature is calculated based on site performance data analysis. Surface effective factor is also calculated based on site performance data analysis. The next step will be performance calculation for upper furnace. The pressure drop calculation is based on margin of drum design pressure and gap between D. O. P and SV closing pressure. Details will be given to stress analysis on pressure parts like check on minimum pressure drop required of coils and SCW. The soot blower selection for repair, replacement, addition of long retractable soot blower is based on the ratio of 100% bypass to the 30% adequacy of steam parameter, additional LRSB layout or interference location of heat flux probe, Location of ASLD, location details of smart soot blowing system, replacement requirement of soot blower central structures, thermal drain system. The data for structural design is based on beam bend second pass modification or change of mill layout. Furnace details will be provided for insulation purpose and the main components will be taken into account are skin temperature required, wind velocity. Similarly furnace details will be provided to stress analysis group for designing buck stays, beam support, eco modification and for column strengthening when change of load transfer. The data sheets for gates and dampers will be issued from BAP, Ranipet. Specific information about Line & insulation and painting can be given to corresponding departments. Section 5 : Schemes and Bill of Material The collection of data for the preparation of schemes and Bill of material starts with oil system, boiler mountings, C&I , structures, L&I, Ducts, Pipe lines, Air Heater and Fans sections. Then the component code wise, bill of material preparation will start. The group wise weights and erection details for PPA will continue after BOM preparation. The suppliers for sub-deliveries will be finalized. Then the BOM and sub deliveries list will be sent to commercial for estimation. After this, list of spares will be finalized. Inputs to erection and commission agency will be furnished along with list of drawings and documents. Section 6: Proposal documents checklist while forwarding the proposal.
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This section deals with options, which have to be checked before forwarding the proposal. Some of the items to be checked are List of tender deviations & clarifications, filling up data sheets, preparation of technical specification and preparation of design summary letter. Then design summary letter will be prepared. After this technical specification and drawings will be sent to SSBG. The list of proposal input to other units will be given. Then erection inputs will be furnished to erection department. Then inter unit scope matrix will be prepared. The sister units inputs will be forwarded to corresponding units. General arrangement drawings including plan and elevation drawings will be issued. Proposal Schemes of steam and water will be prepared followed by proposal schemes for valves and fitting. This will be followed by schemes for air and gas path along with schemes of oil system. Drawing inputs for pressure part arrangement and ID system inputs will be given to respective department. Sometimes specific component designs will be given if applicable. Then the plot plan will be drawn by adding all the changes in the layout details.
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Remaining Life Assessment:
Life Assessment and Life Extension program helps to identify and implement strategies so as to ensure continued running of the unit in a cost - economical way. A team effort is needed between the utility and designer to ensure safe continued operation of steam generators with minimum unscheduled outages. A complete analysis combining the unit‟s operational data with design expertise and problem solving knowledge is basic requirement for success of the exercise. Boiler pressure parts like super heater tubes, Steam pipes and headers operating in the creep range are designed for certain minimum lifetime. As units age, critical Components may distress through mechanism such as oxidation, corrosion, creep, fatigue and interaction of above mechanisms. These components deteriorate continuously during service as a result of the above time dependent material degradation process. In actual practice material damage results from interaction of two or more of these mechanisms causing unanticipated failures. Sometimes such failures may be catastrophic resulting in huge loss. Condition of critical components and the remaining life / service time available before replacement or major repair are the main consideration in life assessment study. Units originally designed for base load require improvements in equipment and controls. Additionally, modification in operating philosophy is also considered during life assessment program. The useful life of components in service may well exceed or fall significantly short of the design life. The reasons for such behaviors are related to design, operational and metallurgical conditions, considerable reserve strength and longevity even at the end of the predicated design life are not uncommon. This happens because of the conservatism built into the original design, over estimation of oxidation effects especially in case of thick walled components and use of large factor of safety and lower bound properties. On the other hand the components may fail prematurely due to unforeseen system, stress concentrations and operating conditions envisaged during design stage. Local conditions and operational factors associated with the particular unit dictate the type and
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extent of damage in a component. Hence even though a station has units of similar type, each unit needs independent study so that exact status of each unit can be established.
TESTS AT SITE:
1. VISUAL EXAMINATION: Visual examination is carried out to assess material wastage due to oxidation, erosion /corrosion problems, fouling conditions of heat transfer surfaces, integrity of attachments in coils and hanger supports in piping. This includes inspection of drum internals to ensure proper steam/water separation. During visual inspection the observations made with reference to discoloration of coils, misalignment is considered in deciding sample tubes removal for metallurgical examination.
Prior evaluation of pressure part condition, based on experience and design knowledge from similar plants makes sample selection more rational Samples from the regions thus determined to be most susceptible to failures and samples depicting the general condition of each component, are selected for an evaluation of the metallurgical condition. 2. DIMENSIONAL MEASUREMENTS: Essentially, thickness and outside diameter measurements form the dimensional
measurements. Thickness measurement at critical areas gives a value of thickness loss over the years, due to erosion and corrosion. Outside diameter measurements are generally employed to determine the swelling (bulging) due to creep. The thickness measurements are made using ultrasonic thickness meters supplied by M/S Kraut Krammer GMBH, West Germany and for diameter measurements, digital vernier calipers are used. 3. NON-DESTRUCTIVE EXAMINATION: The following NDE will be carried out prior to examination by replica technique. A) Liquid Penetrant Inspection:
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This technique is adopted primarily for detection of cracks or crack like discontinuities that are open to the surface of a part, like surface porosity, pitting, pinholes and other weld defects.
In principle, the liquid penetrant is applied to the surface to be examined and allowed to enter into the discontinuities. All excess penetrant is then removed surface dried and the developer applied. The developer serves both as a blotter to absorb the penetrant coming out by capillary action and also as a contrasting background to enhance the visibility of the indication. The testing is as per ASTM-E165-80. B) Magnetic Particle Examination: This technique is adopted for locating surface and sub-surface discontinuities like seams, laps, quenching and grinding cracks and surface rupture occurring on welds. This method is also used for detecting surface fatigue cracks developed during service. Magnetic particle inspection helps to detect cracks and discontinuities on or near the surface in ferromagnetic materials using dry magnetic particle testing equipment. The testing is done by magnetising at least two mutually perpendicular directions to ensure detection of defects in all possible orientations. The magnetic particle testing is carried out as per ASTM-E-709-80 and the adequacy of the magnetic field strength is verified by using octagonal field indicator (ASTM 275). C) ULTRASONIC TESTING: By using high frequency sound waves, the surface and sub-surface flaws can be detected. Cracks, laminations, shrinkages, cavities, flakes, pore and binding faults that act as discontinuities in metal gas interfaces can also be easily detected. 4. SAMPLING FOR LABORATORY ANALYSIS AND STRESS RUPTURE TESTING: Tube samples carefully selected after the visual inspection from super heater / reheaters are analyzed in laboratory for material degradation, extent of oxide scaling and corrosion/erosion. Tube deposit analysis, internal and external surface condition assessments aid in identifying the root cause of failures/degradation noticed.
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4.1. WATERWALLS AND ECONOMISER: Water wall tube samples will be removed from high heat flux zone for evaluating the deposit content and constituents of the deposit. The need for chemical cleaning will be decided on the deposit content and the constituents of the deposit. The metallic constituents of the deposit are determined using atomic absorption
spectrophotometer and the morphology is found using X-ray diffraction analysis. The analysis report may include the need or otherwise for chemical cleaning. Recommendation on solvent for ensuring the effective removal of the deposit will also be included as per requirement.
4.2. METALLURGICAL EXAMINATION OF HIGH TEMPERATURE TUBES: The tube samples removed from super heater will be analyzed for any metallurgical degradation in service. Transverse ring segments from the tubes will be metallographically prepared and examined using light optical microscope up to a magnification of 500X. Carbide morphology and distribution, presence of creep bulging, and tube wall thinning will be evaluated. The oxide scale thickness on steam side surface will be measured and used in estimating the extent of damage as also the general operating temperature for the running hours. The analysis is done according to internationally accepted micro structural criteria as noted below: 5. IN-SITU METALLOGRAPHY BY REPLICA TECHNIQUE: The high temperature components in utilities when subjected to high stress for a long time undergo steady changes in transformation of strengthening carbide phases followed by creep cavitations. This is the beginning of creep or slow plastic deformation leading to gradual bulging of pressure parts. Three distinct stages of creep occur in several alloys. The first stage of creep occurs in a short period which is transient. The second stage or steady state creep occurs over a very long duration of several years. The metallurgical changes like carbide transformation and dispersion occurs. Formation of minute creep voids along the grain boundary surfaces also accompanies creep deformation. In the third stage of creep, the creep voids increase in number and size and get oriented and connected They generate micro cracks, and the micro cracks connect
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themselves resulting in the initiation and growth of macro crack with sudden fracture in some zones depending on the operating stress at that zone. The replication is the technique adopted to obtain the micro structure "in-situ' by non-destructive metallography. The technique is used in areas where sample removal is difficult and not viable on cost economic aspects. 6. REPLICATION TECHNIQUE: The process involves preliminary preparation of the metal surface using polishing equipment. When the spot is ensured free from rust and scale polishing will be done using abrasive paper of varying grits from, 120, 200, 400, and 600 in sequence. Subsequently diamond paste lapping is done followed by etching with 3% Nital to reveal the structure. The surface preparation can also be done by adopting electro polishing. After the preparation of the surface, the micro structure of component is truly transferred to a film. Transparent film with green reflecting foil can be used which can be examined in laboratory with magnification up to 500X to assess the metallurgical damages like creep cavitations. For examination at higher magnification, the microstructure of the components can be transferred to cellulose acetate replicating tape. A cellulose acetate film of 0.1 to 0.15 mm thickness and 20 x 40 mm size is cut from roll or sheet. A few drops of acetone will be applied on one surface for about 5 seconds and this makes the acetate film soft on one side and retains hardness on the reverse side. The soft side is pressed uniformly over the etched surface using clean and plain rubber and exerting the force of the thumb for about 10 seconds. It will be protected against dust and left for some time for drying. The dried film will be lifted up using fine knife and will be kept between parallel glass slides. Silver shadowing or gold sputtering in vacuum can be done on the impression side to improve reflection. This helps in micro structural examination using light optical microscope or scanning electron microscope at higher magnification.
R.L.A. DETERMINATION BASED ON ACCELERATED CREEP RUPTURES TESTING.
Super heater tubes operating at higher temperatures (more than 450 °C) are subjected to a time dependent phenomena known as creep. The sample tubes removed from boiler will be subjected to creep rupture tests at accelerated temperature and at service pressure in the test
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facility available at laboratory. The test parameters can be controlled within close limits. To predict remaining life from the results of rupture tests, the following method is adopted. Specimen from each sample tube will be subjected to a specified stress and temperature. Time to rupture versus temperature will be plotted and the extrapolation will be done for the operating temperature to decide the remaining life. The assumptions made in the above method are; a) Thickness variation is not considered and hence the operating stress is assumed as uniform. b) Metal temperature considered for extrapolation is assumed as constant and metal temperature increase due to building of oxide scale over a period is not accounted.
R.L.A. CALCULATION BASED ON SERVICE TEMPERATURE:
The tube samples removed from boiler will be evaluated for micro structure classification based on which the service temperature can be evaluated taking into consideration the operating hours collected from the plant record. Another method of estimating operating temperature is based on oxide scale measurement. As steam passes through the tubes at high temperature, the metal is oxidized. Knowing the operating hours and oxide thickness measured in mills, the average temperature‟t‟ can be calculated.
CALCULATION OF REMAINING LIFE:
Assuming oxidation rates for a specific period, the average stress can be calculated for the aging duration considered. With average stress value Larsen - Miller parameter can be calculated for the particular material. With the Larsen - Miller parameter rupture life can be calculated using metal temperature values. Fraction of life consumed is the ratio of operating period divided by Rupture life is consumed. The following assumptions are made in this method. 1. Uniform oxidation rate is assumed. 2. Metal wastage is assumed as linear for computing operating stress. 3. Larsen Miller parameter is computed based on lower bound stress.
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3.Boiler Materials
The boilers being built today are demanding in terms of unit sizes and operating requirements ; hence the choice of materials is of prime importance for ensuring satisfactory performance of the boilers.
3.1 VARIOUS STEEL GRADES OF TUBES / PIPES IN BOILERS
Sl. No.
Nominal Composition
MATERIAL SPECIFICATION ASME Section-I DIN 300 – TRD BS 1113
01.
Carbon Steel
SA178 SA192, Gr.A1 & Gr.C
Gr.C, St 35.8 SA210 St 45.8
BS3059 P2 S2 360, 440 BS3602 P1 360, 430, 500 Nb
SA106 Gr.B, Gr.C
02.
½ Mo
SA209 T1
15 Mo3
----
03.
1 Cr ½ Mo
SA335 P12 SA213 T12
13 Cr Mo 44
BS3059 P2 S2 620 BS3604 P1 620 – 440
04.
1¼ Cr ½ Mo
SA213 T11 SA335 P11
----
BS3604 P1, 621
05.
2¼ Cr 1 Mo
SA213 T22 SA335 P22
10 Cr Mo 910
BS3059 P2 S2 622490 BS3604 P1, 622
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06.
9 Cr 1 Mo ¼ V
SA213 T91 SA335 P91
X 10 Cr Mo V ----Nb91
Area Application
of
Material type
Typical spec. for Plates, Tubes, Pipes
Guiding Reason for Upper Limit
07.
12 Cr 1 Mo ¼ V
-----
X 20 Cr Mo V BS3059 P2 S2 762 121 BS3604 P1 762
08.
18 Cr 8 Ni
SA213 TP304 H
-----
BS3059 P2 304 S51 BS3605 – 304 S59 E
09.
18 Cr 10 Ni Cb
SA213 TP347 H
-----
BS3059 P2 347 S51 BS3605 347 S59 E
3.2 Conventional Boiler Materials
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Drum Water walls, Economiser Superheater and Reheater C Steel/ Low Alloy Steel C Steel C ½ Mo steel 1Cr ½ Mo 2 ¼ Cr 1Mo 18 Cr 8 Ni 18 Cr 10 Ni Cb Modified 9Cr 12%Cr SA299 SA192, SA106 A209 T1 SA213T11, SA335P11 SA213T22, SA335P22 SA213 TP304 H SA213 TP347 H SA213T91, SA335P91 X20CrMoV12 1 ASME code German Code SA210, Graphitisation Graphitisation Oxidation/ corrosion, Flue gas Oxidation/ corrosion, Flue gas
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MATERIALS FOR ADVANCED SUPERCRITICAL PLANTS
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3.3
NEW GENERATION STEELS FOR SUPER CRITICAL AND ULTRASUPER CRITICAL BOILERS The changes in boiler design in super critical and ultra super critical boiler calls for advanced
creep resistant steels for the enhanced steam parameters of temperature and pressure. Some of the steels that have been developed and are in service in European and Japan utilities are given below. Ferritic steels of 9Cr-2W-0.5Mo and 2.25 Cr-1Mo-2W steel Austentic stainless steels 17Cr-8Ni-3Cu 9Cr-2W-0.5Mo Steel(T92/P92) The 9 Cr- 0.5 Mo- 2W steel for tube and pipe has been approved for boiler and pressure vessel manufacture and has been included in ASME specification. The tube material (T92) has been listed in SA 213 and the pipe material (P92) in SA 335 of ASME sec IIA. P92 is a modification of P91 steel which is now well established steel in power plants. The P91 steel is modified by reducing the molybdenum content to about 0.5% and adding about 1.7% tungsten plus a few parts per million of boron. Controlled micro alloying in the form of niobium vanadium and nitrogen is retained. This composition modification gives rise to very stable carbides and carbon-nitrides, which improve long term creep strength. The carbon content has been kept low to ensure welding processing characteristics. This steel is designed to operate at temperatures up to 625°C. It is claimed that high temperature rupture strength is 30% more than for P91. For example at 600°C the 100,000hour creep rupture strength of P91 base material is about 95MPa whereas that of P92 is about 123MPa. The steel composition and tensile properties of P92 grades are shown in Table I and II. The steel grade has higher yield and tensile strength compared to P91 grade. Table I Chemical composition of P92 steel C 0.070.13 Mn 0.300.60 S 0.010 P 0.020 Si 0.50 Cr 8.509.50 Mo 0.300.60 Others V 0.15-0.25 ; Cb 0.04-0.09 B 0.001-0.006; N 0.03-0.07 Ni 0.40; Al-0.04 W-1.5-2.0
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Table II Tensile properties of steel for P92 and P91 Steel SA 335 P92 SA 335 P91 *mimnimum 2.25 Cr- 1.7 W steel (T/P23) The steel grade T/P23 was validated and included in the ASTM A 335 in 2001 and incorporated in ASME section II in 2004 .The steel has also been approved for other product forms like fittings, flanges under ASTM A 182 F22. The steel is the modified form of well-known creep resistant steel SA 335 P22 grade. It is a bainitic steel with addition of tungsten upto 1.7% as well as micro alloying with vanadium, columbium (niobium), nitrogen and boron. The carbon content is intentionally lowered to improve the steel welding and processing characteristics. Due to its chemical composition T/P23 developed a bainitic martensite structure and the maximum hardness is only about 350 HV due to low carbon content. It is used in water wall panels in advanced new power boilers such as super critical boilers and in superheater, reheater tubes and water wall panels in conventional power plants and HRSG heat recovery steam generators. The steel has higher yield and tensile strength compared to P22 steel. In addition the long term creep properties are much higher for P23 and they are close to P91 grade. The steel composition and tensile properties are shown in Table III & IV. Table III Comparison of chemical composition of rP22 and P23 grades Grade C Mn 0.300.60 S 0.025 P 0.025 Si 0.50 Cr 1.90-. 2.60 Mo 0.871.83 Others NS *UTS(MPa) 620 585 *YS(MPa) 440 415 *Elong.(%) 20 20 Hardness max.(Hv) 265 265
SA 335 0.05P22 0.15
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SA 335 0.04P23 0.10 0.100.60 0.010 0.030 0.50 1.902.60 0.050.30 V-0.20-0.30; Cb-0.02-0.08 B-0.0010-0.006; N-0.0015 W-1.45-1.75; Ti 0.0050.06
Table - IV Comparison of mechanical properties for P22 and P23 grades Steel *UTS (MPa) SA 335 P22 SA 335 P23 415 510 *YS (MPa) 205 400 *Elong (%) 30 20 *Hardness ( Hv) Not specified 220 HB Allowable stress at 600 :C (MPa)
23.5
57.0
*Maximum limit
Super 304H austentic stainless steels Austenitic stainless steel pipes are primarily used for boilers in thermal power plants and the energy sector. High temperature strength has become one of the most important aspects for the application of boiler tubes. The 18Cr-8Ni steel grade 304 is a conventional austenitic stainless steel. Though the addition of 3wt% of copper to the standard grade, an increased carbon content and certain amounts of niobium and nitrogen, the elevated temperature strength and especially the creep properties are improved in the grade Super 304H. The addition of nitrogen leads to a solid solution strengthening of the material. The steel composition and tensile properties are shown in Table V & VI. This increases the allowable tensile stresses. The allowable stress of this stainless steel is more than 20% higher compared with that of SA-213 grade TP347H. This excellent creep rupture strength is based on the precipitation strengthening effect of fine Cu-rich phase which precipitates coherently in the austenitic matrix during service-exposure. The corrosion resistance of this stainless steel is almost same as that of fine-grained TP 347H. The super 304H tubes have been service exposed as Superheater tubes and reheater tubes, and the 6.5 years service-exposed tubes confirmed that this new stainless steel applicable to the boiler material. Table V CHEMICAL REQUIREMENTS of super 304H C 0.07 – 0.13 Mn 1.00 S 0.010 P 0.040 Si 0.3 Cr 17.00 19.00 Ni Cu Mo 0.871.83 Cb N B 0.001 –0.01 Al 0.0030.03
7.5 – 2.510.50 3.5
0.3- 0.050.6 0.12
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Table VI MECHANICAL PROPERTY REQUIREMENTS of super 304H Tensile strength, min, ksi ( M Pa) Yield Strength , min.,ksi (M Pa) Elongation in 2 in., min, % 85 (590) 34 (235 ) 35
4.0 4.1
Boiler auxiliaries Pulverisers
The pulverizing process is composed of several stages. The first is the feeding system, which must automatically control the fuel-feed rate according to the boiler demand and the air rates required for drying and transporting pulverized fuel to the burner. Because coals have varying quantities of moisture, drying is an integral part of pulverizing process. Part of the air from the steam-generator air preheater, the primary air, is delivered to the pulverizer by the primary air fan. There it is mixed with the coal as it is being circulated and ground.
Grinding is accomplished by impact, attrition, crushing, or combinations of these. There are several commonly used pulverizers, classified by speed:
Types of Pulverisers Low speed (Ball mill/ Tube mill) - Usually rotating between 15 to 25 rpm Medium speed (Vertical Spindle – Bowl / Ball & Race / Roller Mills) – usually rotates between 50 to 100 rpm. High speed (Beater Wheel) - runs at high speed, normally 750-1000 rpm.
The selection of number of mills and capacity of mill shall meet pulverizing requirement for the range of coals specified; Pulveriser selection shall also ensure one spare mill to account for outage for maintenance.
Low speed mills The low-speed ball-tube mill is basically a hollow cylinder with heavy-cast wear-resistant liners, less than half-filled with forged steel balls of mixed size. Pulverization is accomplished by attrition and impact as the balls and coal ascends and falls with cylinder rotation. Primary air is circulated over the charge to carry the pulverized coal to classifiers. The ball tube mill requires low maintenance, but it is larger and heavier in construction and consumes more power than others.
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Medium speed mills The medium-speed ball-and-race and roll-and race pulverizers are the types mostly in use nowadays. They operate on the principles of crushing and attrition. Pulverzation takes place between two surfaces, one rolling on top of the other. The rolling elements may be balls or ring-shaped rolls that roll between two races, in the manner of a ball bearing. The balls are between a top stationary race or ring and a rotating bottom ring, which is driven by the vertical shaft of the pulverizer. Primary air causes coal feed to circulate between the grinding elements, and when it becomes fine enough, it becomes suspended in the air and is carried to the classifier. Grinding pressure is varied for the most efficient grinding of various coals by externally adjustable springs on top of the stationary ring.
Bowl Mill: It is one of the most advanced designs of coal pulveriser. The bowl mill essentially consists of a revolving bowl which is driven by a reduction gear mechanism coupled to an electric motor. It is provided with a set of 3 grinding rollers. The coal fed to the centre of the revolving bowl is forced between the grinding roll and the bowl for getting pulverized. The required grinding pressure is given by means of a set of heavy duty springs. Hot air is sent through an air chamber provided beneath the bowl to dry and transport the pulverized coal. The advantages of Bowl mill are Low power consumption, Reliability, Minimum resistance, Wide capacity, Quiet and vibration less operation and ability to handle wide range of coals. High speed mills High-speed pulverizers use hammer beaters that revolve in a chamber equipped with high-wear resistant liners. They are mostly used with low-rank coals with high-moisture content and use flue gas for drying. Classifier The classifier is located at the pulverizer exit. It is usually a cyclone with adjustable inlet vanes. The classifier separates oversized coal and returns it to the grinders to maintain the proper fineness for the particular application and coal used. Adjustment is obtained by varying the gas-suspension velocity in the classifier by adjusting the inlet vanes.
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4.2 Fans Introduction: Fan is a rotating machine with a bladed impeller which maintains a continuous flow of air or gas. It is continuous because the flow at entry and exit and also through the impeller is steady. Fans may be classified into two major types : Axial flow and radial flow.
Selection of fans for a given application depends on the following parameters. Capacity in m3/sec. Pressure at fan inlet. Total pressure rise required in mbar/mmwc. Specific weight of medium in m3/kg Operating temperature in deg.C.
-
The mechanical design of the fan is governed by the tip speed and the maximum operating temperature. Axial fans may be classified further into Impulse Type and Reaction Type fans. In the Impulse type fans, most of the energy coming out the impeller is Kinetic Energy. It is converted into Pressure Energy in the Outlet Blades and the diffuser. Hence these fans are called Impulse Fans. In the reaction type of Axial Fans, most of the energy coming out of the impeller is in the form of Pressure Energy. Axial fans are best suited for handling large capacities compared to pressure rise with good efficiencies, less floor space and less weight. AN fan has blades particularly suitable for operation with air and dust laden gases. Even with advanced wear due to erosion the performance hardly changes. `AN’ fan is of mechanically simple design. All major components are easily accessible. Where load changes are frequent, Axial reaction fan (`AP’ type) has distinct advantages – because of the highly efficient profiled blades. `AP’ fan is generally offered for clean air application than dust laden gases because the profiles will be affected by erosion.
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Radial fans: Based on the configuration of the blade with respect to the direction of rotation of the impeller, it is called Backward Curved, Forward Curved and Radial Bladed Impeller. Application: Based on the selection parameters, single inlet or double inlet radial fans can be offered. as forced draft, induced draft, primary air and gas recirculation fan in power stations. for refineries, steel, cement, fertilizer, petrochemical, palletizing, sinter plants. for ventilation application including mine ventilation.
Forced Draft fans: Forced draft fans supply air necessary for fuel combustion and this shall deliver stochiometric air plus the excess air needed for proper burning of the specific fuel for which they are designed. These fans supply the total air flow taking in to account the air preheater leakage and some sealing air requirements. Primary Air Fans: These fans supply the air needed to dry and transport the coal either directly from pulverising equipment to a furnace or to an intermediate storage bunker. Induced draft Fans: These fans exhaust combustion products from a boiler to chimney by creating sufficient negative pressure to establish a slight suction in the furnace. Gas Recirculation Fans: These fans draw gas from a point in the flue gas flow path ( normally between economiser outlet and air preheater inlet) and discharge it (for steam temperature control) in to the bottom of the furnace. These fans are used mostly in the oil & gas fired boilers. Seal Air Fans: Seal air fans take suction from FD/ PA fans fans and boost its pressure to deliver the downstream equipments like pulveriser and feeders to maintain the sealing pressure required for them to prevent leakage. Scanner Air Fans: These fans provide air to the flame scanners which are operated at higher temperature zone for cooling of the scanner heads. Normally 1 AC and 1 DC fans are provided to ensure the availability even during power failures. Igniter Air Fans: These fans are provided to supply air to take eddy plate oil/gas igniters and normally suction is taken from FD fans. They ensure the combustion air requirements of the igniters.
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Learning modules material- Engineering (FB) 4.3 Airpreheaters
Schematic diagram of typical coal-fired power plant steam generator highlighting the air preheater (APH) location. An air preheater (APH) is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil. The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).
Types
There are two types of air preheaters for use in steam generators in thermal power stations: One is a tubular type built into the boiler flue gas ducting, and the other is a regenerative air preheater. These may be arranged so the gas flows horizontally or vertically across the axis of rotation.
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Tubular type
Construction features
Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the preheater tubes, transferring heat from the exhaust gas to the air inside the preheater. Ambient air is forced by a fan through ducting at one end of the preheater tubes and at other end the heated air from inside of the tubes emerges into another set of ducting, which carries it to the boiler furnace for combustion.
Regenerative air preheaters
There are two types of regenerative air preheaters: the rotating-plate regenerative air preheaters (RAPH) and the stationary-plate regenerative air preheaters (Rothemuhle).
Rotating-plate regenerative air preheater
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Typical Rotating-plate Regenerative Air Preheater (Bi-sector type)
Principle function for the regenerative preheater. The rotating-plate design (RAPH) consists of a central rotating-plate element installed within a casing that is divided into two (bi-sector type), three (tri-sector type) or four (quad-sector type) sectors containing seals around the element. The seals allow the element to rotate through all the sectors, but keep gas leakage between sectors to a minimum while providing separate gas air and flue gas paths through each sector. Tri-sector types are the most common in modern power generation facilities. In the tri-sector design, the largest sector (usually spanning about half the cross-section of the casing) is connected to the boiler hot gas outlet. The hot exhaust gas flows over the central element, transferring some of its heat to the element, and is then ducted away for further treatment in dust collectors and other equipment before being expelled from the flue gas stack. The second, smaller sector, is fed with ambient air by a fan, which passes over the heated element as it rotates into the sector, and is heated before being carried to the boiler furnace for combustion. The third sector is the smallest one and it heats air which is routed into the pulverizers and used to carry the coal-air mixture to coal boiler burners. Thus, the total air heated in the RAPH provides: heating air to remove the moisture from the pulverised coal dust, carrier air for transporting the pulverised coal to the boiler burners and the primary air for combustion. The rotor itself is the medium of heat transfer in this system, and is usually composed of some form of steel and/or ceramic structure. It rotates quite slowly (around 3-5 RPM) to allow optimum heat transfer first from the hot exhaust gases to the element, then as it rotates, from the element to the cooler air in the other sectors.
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Dust collectors
Types of dust collectors
Five principal types of industrial dust collectors are:
Inertial separators Fabric filters Wet scrubbers Electrostatic precipitators Unit collectors
Inertial separators
Inertial separators separate dust from gas streams using a combination of forces, such as centrifugal, gravitational, and inertial. These forces move the dust to an area where the forces exerted by the gas stream are minimal. The separated dust is moved by gravity into a hopper, where it is temporarily stored. The three primary types of inertial separators are:
Settling chambers Baffle chambers Centrifugal collectors
Neither settling chambers nor baffle chambers are commonly used in the minerals processing industry. However, their principles of operation are often incorporated into the design of more efficient dust collectors.
Fabric filters
Commonly known as baghouses, fabric collectors use filtration to separate dust particulates from dusty gases. They are one of the most efficient and cost effective types of dust collectors available and can achieve a collection efficiency of more than 99% for very fine particulates. Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape. The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags. The fabric primarily provides a surface on which dust particulates collect through the following four mechanisms:
Inertial collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
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Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size. Brownian movement - Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces. Electrostatic forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture.
A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned periodically.
Wet scrubbers
Dust collectors that use liquid are commonly known as wet scrubbers. In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles. The greater the contact of the gas and liquid streams, the higher the dust removal efficiency. There are a large variety of wet scrubbers; however, all have one of three basic operations:
Gas-humidification - The gas-humidification process conditions fine particles to increase their size so they can be collected more easily. Gas-liquid contact - This is one of the most important factors affecting collection efficiency. The particle and droplet come into contact by four primary mechanisms: o Inertial impaction - When water droplets placed in the path of a dust-laden gas stream, the stream separates and flows around them. Due to inertia, the larger dust particles will continue on in a straight path, hit the droplets, and become encapsulated. o Interception - Finer particles moving within a gas stream do not hit droplets directly but brush against them and adhere to them. o Diffusion - When liquid droplets are scattered among dust particles, the particles are deposited on the droplet surfaces by Brownian movement, or diffusion. This is the principal mechanism in the collection of submicrometre dust particles. o Condensation nucleation - If a gas passing through a scrubber is cooled below the dewpoint, condensation of moisture occurs on the dust particles. This increase in particle size makes collection easier. Gas-liquid separation - Regardless of the contact mechanism used, as much liquid and dust as possible must be removed. Once contact is made, dust particulates and water droplets combine to form agglomerates. As the agglomerates grow larger, they settle into a collector.
The "cleaned" gases are normally passed through a mist eliminator (demister pads) to remove water droplets from the gas stream. The dirty water from the scrubber system is either cleaned and discharged or recycled to the scrubber. Dust is removed from the scrubber in a clarification unit or a drag chain tank. In both systems solid material settles on the bottom of the tank. A drag chain system removes the sludge and deposits in into a dumpster or stockpile.
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Learning modules material- Engineering (FB) Electrostatic Precipitator
An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. In contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity).
The plate precipitator
The most basic precipitator contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented vertically, with the plates typically spaced about 1 cm to 18 cm apart, depending on the application. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the stack of plates. A negative voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough an electric (corona) discharge ionizes the gas around the electrodes. Negative ions flow to the plates and charge the gas-flow particles. The ionized particles, following the negative electric field created by the power supply, move to the grounded plates. Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure (given from layer resistivity, electric field, and current flowing in the collected layer).
Unit collectors
Unlike central collectors, unit collectors control contamination at its source. They are small and selfcontained, consisting of a fan and some form of dust collector. They are suitable for isolated, portable, or frequently moved dust-producing operations, such as bins and silos or remote belt-conveyor transfer points. Advantages of unit collectors include small space requirements, the return of collected dust to main material flow, and low initial cost. However, their dust-holding and storage capacities, servicing facilities, and maintenance periods have been sacrificed.
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5.0
Environmental Pollution Control Measures
Major pollutants from a coal fired boiler are particulates, CO2, NOx and Sox. Minimising the emissions calls for provision of dust collectors, Low NOx firing system, and DeNOx system in addition to the inherent reduction achievable by adoption of suitable firing system and by improving boiler efficiencies. Nox emission from a power plant is dictated by the fuel nitrogen (fuel Nox) and the temperature of the flame, which are determined by the operating condition and the load on the unit. Firing system adopted for a particular power plant dictates the Nox emission from the plant. Over fire air system reduces the Nox emission. In coal and oil fired units the SOx emission depends upon the % of sulphur in the fuel and the calorific value of the fuel. Since most of the Indian coals have sulphur content around 0.5 to 0.6 % only, the SOx emission from the power plant is not a problem with the current environmental regulations. However with the use of Imported coals containing high sulfur, Sox emissions need to be focused on. Particulate emissions can be reduced to very low levels by high efficiency Electrostatic precipitators and Bag filters.
5.1 Indian Pollution Control Board Guidelines: EMISSION REGULATION – JULY 1984 - GUIDELINES BOILER SIZE PROTECTED AREA OTHER AREA OLD (BEFORE 1979) < 210 MW > 210MW 150 mg/NM3 150 mg/NM3 600 mg/NM3 ---NEW (AFTER 1979) 350 mg/NM3 150 mg/NM3
MINISTRY OF ENVIRONMENT AND FOREST NOTIFICATION- GUIDELINES. MAY 1993 - Emission 150 mg/NM3 .
MARCH 2003 - to examine feasibility to limit 100 mg/NM3 for existing power plants. 100 mg/Nm3 for new plants. 20-50 mg/NM3 are being followed for most new projects.
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o o
For New Plants PLANT SIZE < 50 MWe < 500 MWe >500 MWe 100 50 50 EMISSION mg/m3
b) Plants in Areas with Degraded or Poor Air Quality ALL 50
6.0
Codes & Regulations
6.1 Material testing codes
List of IS codes applicable for structural design
1. IS:800-2007- Code of practice for General Construction in Steel 2. IS:808-1989- Dimensions for hot rolled steel beams 3. IS:875-Part-1-1987- Dead Loads 4. IS:875-part-2-1987- Live Loads 5. IS:875-part-3-1987- Wind Loads 6. IS:875-part-4-1987- Snow Loads 7. IS:875-part-5-1987- Special Loads and Load combinations 8. IS:1893- Criteria for Earthquake Resistance Design of Structures 9. IS:2062-2006 Hot Rolled Low, Medium and High Tensile Structural Steel 10. IS:456- 2000 Code of Practice for Plain and Reinforced concrete
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6.2 Coal Analysis Standards
Sl No 1 Subject / title BIS number ASTM number D 2234 - 89 and D 2013 - 86
Methods for sampling of coal and IS 436 (Part1/Sec1) coke (manual sampling) Reaffirmed 1996 Methods for sampling of coal and IS 436 (Part1/Sec 2) coke (mechanical sampling) Reaffirmed 1994 Methods of test for coal and coke : Proximate analysis IS 1350 ( part 1 ) Reaffirmed 2001
2
3
D 3302 – 91 D 1412 – 89 D 3172 – 89 D 3173 – 89 D 3174 – 89 D 3175 - 89
4
Determination of calorific value
IS 1350 ( Part II ) Reaffirmed 1994
D 2015 – 91 D 3286 - 91 D 3176 – 89 D 3178 - 89 D 3179 - 89
5
Determination of carbon and Hydrogen
IS 1350 ( Part IV/ Sec 1 ) Reaffirmed 1994 IS 1350 ( Part IV/ Sec2 ) Reaffirmed 2000
6
Determination of nitrogen
7
Determination of sulphur
IS 1350 ( Part III ) Reaffirmed 2000
D 3177 - 89
8
Determination of forms of sulphur
IS 15438 :2004
D 2492 – 90
9
Instrumental determination of carbon, hydrogen & nitrogen in
D 5373 - 93
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coal & coke 10 Grindability IS 4433: Reaffirmed 1994 11 Elemental analysis of ash of coal IS 1355 : 1984 and coke Reaffirmed 2001 D 2795 – 1991 D 3682 – 1991 D 4326 - 92 12 Fusibility of ash of coal, coke, & IS 12891: 1990 lignite Reaffirmed 1995 D 1857 - 87 D 409 - 92
6.3 Boiler efficiency Determining and adjusting the efficiency of a boiler in a power plant or a process industry is essential for energy savings. The main requirements for determining the boiler efficiency is detailed below. Modern boilers of large capacity used in power plants have an efficiency ranging from 80 to 90 %. Elaborate calculation method is given in ASME PTC 4 or BSEN 12952-15:2003 which are the performance test codes for boilers. Input-Output Method: The simple method is to measure quantity of fuel input and the steam energy output. This method is the input output method. Efficiency % = Output / Input X100 = [Steam Flow kg/s x Steam Enthalpy kj/kg] - [Water Flow kg/s×Water Enthalpy kj/kg] / [Coal Flow kg/s x HHV of Coal kj/kg] ×100 In case of reheat units the reheater inlet and outlet enthalpy also has to be considered. One can determine the higher heating value by taking a sample of coal as it enters the boiler and analyzing it in the laboratory. These are normally done on a daily basis in most power plants.
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Fuel flow is more complicated. Gravimetric feeders used in modern power plants can give the coal flows to a certain degree of accuracy. Otherwise this will have to be computed from volumetric flows and bulk density of the fuels. This method, although it looks simple on paper, is not the industry preferred method because •Flow measurements are not accurate nor steady •Good quality flow instruments are costly. •Flow measurements always involve a co-efficient, which can very much alter the results. •Trouble shooting problems for determining the reasons for a lower efficiency is difficult Coal calorific value may change during the course of the day and hence calculation based on one analysis in a day need not be correct. However, this method finds use for quick calculation if the flow measurements are reliable and steady. Losses Method: Another method and a more practical approach is to measure the losses and then calculate the efficiency. Efficiency % = 100 – Losses %. The big advantage is that the calculation is on unit basis i.e.: for 1 kg of coal. This eliminates any inaccuracies in flow measurements. Air and gas quantities are determined on theoretical basis and from laboratory analysis of the fuel. This is more accurate than the field flowmeters. Since each loss is separately calculated it is easy to identify problem areas.
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Learning modules material- Engineering (FB) 7.0 Destructive & Non-destructive Testing NDT is an integral part of manufacturing system for quality control of engineering materials during all stages –raw material, in process and finished condition. The main objective of defect prevention and not just detection. In general most NDT methods in use today indirectly measure the overall quality, strength or serviceability characteristics of the items under test be it a material, component or assembly. There are a number of NDT methods available today. However, it is essential to know the material, process, dimension etc to choose the most appropriate NDE methods. The most widely used NDE techniques in the welding industry are : Surface NDT methods Visual Liquid Penetrate Testing (PT) Magnetic Particle Testing (MT)
Voluminar NDT methods Radiographic Testing (RT) Ultrasonic Testing (UT)
VISUAL INSPECTION: Widely used to detect surface discontinuities, visual inspection is simple, quick and relatively inexpensive. The only aid that might be used to determine the conformity of a weld are a low power magnifier, a boroscope, a dental mirror, or a gage. Visual inspection can and should be done before, during and after welding. Visual inspection is useful for checking the Dimensional accuracy of weldments, Conformity of welds to size and contour requirements, Acceptability of weld appearance with regard to surface rough ness, weld spatter and cleanness, Presence of surface flaws such as unfilled craters, pockmarks, under cuts, overlaps, and cracks etc.
MAGNETIC PARTICLE INSPECTION(MPI):This is used for detecting surface and near surface flaws in ferromagnetic materials. It consists of four basic operations ,viz, Establishing a suitable magnetic field in the material being inspected, Applying magnetic particles to the surface of the material ,Examining the surface of the material for accumulations of the particulars (indications ) and finally evaluating the serviceability of the material.MPI is a particularly suitable for the detection of surface flaws in highly ferromagnetic metals. The types of weld discontinuities normally detected magnetic particle inspection include cracks, LOP, LOF, and
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Learning modules material- Engineering (FB) porosity open to the surface. Linear porosity, slag inclusions, and gas pockets can be detected if large or extensive or if smaller and near the surface.
LIQUID PENETRANT INSPECTION(LPI):It is capable of detecting discontinuities open to the surface in weldments made of either ferromagnetic or non-ferromagnetic alloys, even when the flaws are generally not visible in the unaided eye. Liquid penetrant is applied to the surface of the part, where it remains for a period of time and penetrates into the flaws. After penetrating period, the excess penetrant, remaining on the surface is removed. An absorbent, light – coloured developer is then applied to the surface. This developer acts as a blotter, drawing out a portion of the penetrant that had previously seeped into the surface openings. As the penetrant is drawn out, it diffuses into the developer, forming indications, that are wider than the surface openings. The inspector looks for these coloured or fluorescent indications against the background of the developer.
RADIOGRAPHIC INSPECTION (RT): Radiography is most popular NDT method .In industrial Radiography, the usual procedure for producing a radiograph is to have a source of penetrating radiation (X-rays or gamma rays) on one side of the other side. The energy of the radiation must be chosen so that sufficient radiation is transmitted through to the detector. The detector is usually a sheet of photographic film, held in a light tight envelope or cassette having a very thin front surface which allows X-rays to pass through. Discontinuities detectable by radiography include gas porosity, slag inclusions, cracks, lack of penetration, lack of fusion, Tungsten inclusions etc. However enough care should be given to follow the safety procedures to avoid Radiation hazards.
ULTRASONIC INSPECTION(UT): Ultrasonic inspection is a nondestructive method in which beams of high frequency (0.1 & 25 MHz) sound waves are introduced into materials for the detection of surface and subsurface flaws in the materials. The sound wave travel through the material with loss of energy (attenuation) and are reflected to define the presence and location of flaws or discontinuities.UT equipment generates electric signals which are converted to Ultrasound by piezo electric tranducers and the beams reflected from defects are converted back to electric signals and presented in the Cathods Ray Tubes.UT cab detect almost all the internal defects including Cracks, laminations, shrinkage cavities, bursts, pores, lack of bonds, Inclusions etc.
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Learning modules material- Engineering (FB) EDDY CURRENT TESTING(ECT): Eddy current inspection is one of several NDT methods that use the principal of “electromagnetism” as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle. Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right. ECT can be used to detect Surface Breaking Cracks,SBC using Sliding Probes, Metal thinning(Corrosion),Tube Inspection, Verification of Thickness of Thin wall materials, Thickness of Coatings etc. ACCOUSTIC EMISSION TESTING: Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 -12 m) can be identified. Sources of AE vary from natural events like earthquakes and rockbursts to the initiation and growth of cracks, slip and dislocation movements, melting, twinning, and phase transformations in metals. In composites, matrix cracking and fiber breakage and debonding contribute to acoustic emissions. AE’s have also been measured and recorded in polymers, wood, and concrete, among other materials.
BASIC PRINCIPLES OF NON-DESTRUCTIVE TESTING
Non-destructive inspection is a testing technology based on applied physics and is exactly what the name implies-method of testing materials for cracks or flaws without damaging or altering their physical structure. The complex products and new materials that are developed today and introduced in the market need freedom from defects and assurance for trouble-free performance. Appropriate Non-destructive inspection provides such vital assurance by determining the existing state or quality of materials with a view to find out its acceptance for intended end-use and supplements percentage 'destructive-testing' assuring that all materials employed meet the required quality standards and are reliable. Reason for using NDT:
1. Saves lives & prevents accidents: Reliability to protect human life is essential. Proper NDT assures the axle in a supper fast train does not fail at high speed and prevents collapse of landing gears in an aircraft on touch down. The demand for personal safety is a strong force in the development of non- destructive tests.
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2. Ensures Product Reliability: The public expects a fabricated part to give a trouble free performance for a reasonable period of usefulness, better service year by year and longer life for a given product. 3. Makes profit for user which are tangible and intangible 4. Ensures customer satisfaction: Actual Quality and reputation for quality stands high in customer's mind when choosing among products of competing manufactures. 5. Controls Manufacturing process: The operators need be trained and supervised, the process of fabrication then will be controlled. When any element of manufacturing operation goes out of control quality drops & waste increases. 6. Saves Manufacturing costs: NDT locates undesirable defects of a material or component at an early stage. This saves money that would be spent in further processing or assembly. Using a defective material results in waste of labour and time. 7. Maintains uniform quality level: NDT helps to achieve optimum Quality verifying the quality level of a product. Quality below optimum affects reputation. Quality above optimum results in scrap or reworks, reducing profits. 8. Develops Demand for sound Materials: As size and weight decrease factor of safety is lowered. New specified quality levels on raw materials and workmanship rest on NDT for practicality. 9. Aids in product Design: The state of physical soundness as revealed by NDT shows the designer the important areas that need design changes. In a casting the design can be improved and the 'Pattern' modified to increase the Quality of Casting.
PLANNING AND SCHEDULING NDT:
Planning for NDT starts during design stage. For inspections to be meaningful consideration must be given to condition of materials, location and shape of welded joints. Design planning includes avoiding complex weld geometry accessibility for performing examination method planned. E.g. Placement of sources or film in Radiography and to facilitate movement of probes at appropriate skip-positions for scanning in ultrasonic testing, use of ring-forging for critical pressure vessels to avoid longitudinal welds etc.
Timely inspection and good construction standards result in reduction of both costs and delays due to rework. The stages of production at which the inspection is to be conducted should also be preplanned. During fabrication quality plans must be integrated with manufacturing sequence to ensure inspections are performed at proper time and to the requirements of applicable standards.
INSPECTION METHODS:
Non-destructive inspection methods are specified for materials to maintain necessary quality for their final service life. The inspection requirements of materials such as plates, forgings, pipes, valve castings and welded components employed in boiler and pressure vessel industry are stipulated by Codes which further defines sensitivity level of inspection as well as
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Learning modules material- Engineering (FB) objectionable flaw-size. The inspection methods include visual, radiographic, ultrasonic, liquid penetrant and magnetic particle. Radiographic Inspection:
Radiography is concerned with studying the homogeneity of opaque materials using penetrating radiation and is mainly used for volumetric examination of materials and welds to detect internal discontinuities present and to find-out assembly errors. It is based on differential absorption of penetrating radiation by the part being inspected. The degree of absorption depends upon the thickness as well as physical density variations in the object. The forms of penetrating radiation that are used in radiographic inspection of pressure vessels and valves are X-rays and Gamma rays. X-rays are generated in vacuum tubes when accelerated electrons are stopped by metals like tungsten. Gamma rays originate from artificially produced radioisotopes when elements such as Iridium and Cobalt are treated in a reactor with a flux of neutrons.
Radiography is the most understood and widely accepted Non-Destructive Evaluation (NDE) method. The major reason for its use is that the radiograph of a three dimensional object provides a permanent visual record as a two-dimensional interior image. In a radiograph the length and width of the part under examination is more or less truly recorded whereas the thickness plane is distorted and reduced to blacks and grays. The grays and blacks that may result from voids, changes in thickness and variations in density of materials are properly interpreted to compare them with predetermined acceptance standards. The ability to characterize indications-grays & blacks-appearing on a radiograph needs training and experience added along with it over a period of time makes one capable of distinguishing slags, cracks and lack of fusion in a weld radiograph. 113
Learning modules material- Engineering (FB) The acceptance standards are developed according to the limits of radiography and the quality level obtainable by the manufacturing practices used in making pressure vessels or valves and are well-defined in codes and standards. As a permanent visual record in the form of a radiograph is available the NDT inspectors or customers can review the radiograph at any point of time to ensure the requirements of the product have been complied with.
Application: Inspection of welds, castings & Assly.
LIMITATIONS OF RADIOGRAPHY
1. 2. 3. 4.
Impracticable to use on specimens of complex geometry. The specimen must lend itself to two side accessibility. The greatest dimension of suspected discontinuity must be ”parallel to radiation beam. Narrow discontinuities and laminar type of discontinuities are often undetected by Radiography. 5. Thick specimens require equipment of high energy potential, requires costly space utilisation and construction practices. Hence it is a relatively expensive means of Nondestructive testing. 6. Safety considerations imposed by X and Gamma rays must be considered. 7. Radiography is time consuming.
Ultrasonic inspection:
Ultrasonic Inspection is another volumetric inspection method to detect internal discontinuities similar to radiography. Both radiography and ultrasonics are not 'substitutes' to each other but rather they complement or supplement each. The basic principle is that of 'ECHO'- reflection of ultrasound by voids. High frequency electrical signals are converted as mechanical vibrations in a piezo-electric crystal; these vibrations form a wave-front and are coupled to parts under inspection with suitable medium such as oil, grease or water. These vibrations propagate through materials in longitudinal, shear or surface modes and are reflected by any metal-air interfaces that are oriented approximately normal to incident sound wave. The reflected waves from such interfaces or flaws are directed back to the same transducer and are converted as electrical signals to be presented as a pip or vertical deflection in a cathode ray tube screen (CRT) of an ultrasonic flaw detector. Straight beam ultrasonic inspection, where sound beam enters vertically from the surface of inspection is specified to detect lamination in plates and internal discontinuities in forgings and castings. Angle beam inspection ,where sound beam enters inside material at pre-determined angles to the surface, is specified for welds, ring forgings and pipes to detect cracks and inclined 114
Learning modules material- Engineering (FB) flaws. It is mandatory however the examination shall be conducted to a detailed procedure. The procedures generally refer to the amplitude of signals obtained from a calibration notch or cylindrical holes as a basis for interpretation of discontinuity signals presented on the CRT. It must be also remembered that not all slag inclusions or cracks present in production materials would produce a similar signal amplitude response and it is difficult to distinguish planar flaws (cracks) from linear-flaws. The acceptance standards of radiography and ultrasonics are almost one and the same. However ultrasonic inspection will detect cracks and side-wall lack of fusion in welds better than radiography. Another advantage of ultrasonics over radiography is its capability to size the discontinuities in the direction that reduce the cross-sectional thickness providing a base line data during pre-service inspection to make it possible to monitor growth of flaws. Ultrasonic testing is more operator dependent test method as the presentation of discontinuity indications is by indirect means and the test results depend on the interpretation skill of the operator. Advanced imaging and automatic scanning techniques reduce dependency on operators and ensure that pressure vessels remain fit for continued service by locating, characterizing and sizing the discontinuities more accurately.
NORMAL BEAM TESTING
ANGLE BEAM TESTING
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UT APPLICATIONS:
Because Ultrasonic techniques are basically mechanical phenomena, they are particularly adaptable to the determination of structural integrity of engineering materials. Their principle application consists of: 1. 2. 3. 4. 5. Flaw detection Evaluation of the influence of processing variables on materials and process. Thickness measurement Study of metallurgical structures Determination of elastic moduli
Advantages of Ultrasonic Test:
1. 2. 3. 4. 5.
High sensitivity, permitting detection of minute defects Great penetrating power, allowing examination of extremely thick sections Accuracy in the measurement of flaw position and estimation of flaw size Fast response, permitting rapid and automated inspection Need for access to only one surface of the specimen
Limitations of Ultrasonic Test
1. Unfavorable sample geometry; for example, size, colour, complexity and defect orientation. 2. Undesirable internal structure; for example, grain size, structure porosity inclusion content, or fine, dispersed precipitates. 3. Coupling and scanning problems 4. Difficult to detect point reflecors such as porosity
Magnetic Particle Inspection:
Magnetic Particle examination is widely used on ferromagnetic parts on edge preparations of welds and on the welds before and after the vessel has been subjected to hydrostatic test. Magnetisation is done by passing current through copper electrodes and sometimes by a hand116
Learning modules material- Engineering (FB) held Electro-magnetic yoke. This technique is best employed for both detecting surface discontinuities that are open and too fine to be seen by naked eye and any discontinuities that may lie below surfaces. When a part or area of a part is magnetised, magnetic lines of force or magnetic flux will be developed. If a medium such as iron-powder is dusted on the surface of the part during magnetisation, any discontinuity which interrupts the lines of force more or less in perpendicular direction will set-up flux-leakage at the surface and thereby attracting the powder to form a build-up resembling the discontinuity. All linear discontinuities that are greater than 1.5 mm are generally analyzed and taken for re-work .
CIRCULAR MAGNETISATION- PROD INSPECTION METHOD
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LONGITUDINAL MAGNETISATION- YOKE
Application
1. Widely used both in fabrication and maintenance of ferritic materials 2. Used for inspection of castings and ferritic Butt, corner &Tee joints in boiler & pressure Vessels.for defect locations and to identify lamination and cracks on gas cut edgess of plates-and exposed faces and edge prepared grooves in ferritc joints. 3. Any modification work during periodical overhaul of the boiler & Nuclear is tested by magnetic particles inspection method. Limitation 1. Metals such as austenitic stainless steels, Aluminum alloys ,titanium, and non-mtals such as ceramics and composites (non-ferro magnetic materials ) reduce scope for application of Magnetic Particle Inspection. 2. Effective for for surface and near-surface defet detection only and components need to be magnetised in more than ONE direction. Materials of complex geometry pose problems for testing. 3. Difficulty in btaining permanent recordsof test. 4. Certain components need demagnetising after test.
Liquid penetrant Inspection:
Liquid Penetrant examination: is used on all ferro and non-ferromagnetic materials that are essentially non-porous. They are also applied on edge preparation of welds, on the final welds after the vessel has been subjected to hydrostatic test. Liquid penetrant that is deep red in colour seeps or pulled into minute surface openings or cavities or cracks by capillary action. The surface is cleaned-off the excess penetrant after a period of penetration time using a cloth by wipe technique. A thin developer coating made on the dried surface reveals the presence of flaws as a red indication against white developer background, which makes the flaw indications easily noticeable. The limitation of this method is the defects should be open and
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Learning modules material- Engineering (FB) connected to surface. Hence this is considered as an aid to visual inspection as the discontinuities are too fine and are not seen by naked eye. Spherical or round indications are tolerated to a certain size but all linear discontinuities that are greater than 1.5 mm are generally probed for existence of material-separation and taken for re-works, if found to be so.
Reverse Capillary action- Blotting action
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Applications:
1. Penetrant Inspection is widely used both in fabrication and maintenance. -Fillet welds in drum-dished ends, header hand hole plates etc. are inspected by penetrant test . 2. Any modification work during periodical overhaul of the boiler as is scalloped bar welding; attachment welding in super heaters is tested by penetrant inspection method. 3. As it can be applied on any kind of material unlike magnetic particle inspection and as is free from any external energy like electricity for operation, and as it can be operated by minimum technical knowledge, 4. It is very versatile test method. But before putting the chemicals into use, it is to be ascertained that the halogen and sulfur content should not be harmful to the material being tested. 5. The testing area should be properly ventilated, so that the chemicals will not be hazardous to the operator. Exhaust fan should be provided when test is conducted inside closed vessels.
Limitations:
The major limitation of liquid penetrant inspection is that it can detect only imperfections that are open to the surface. Extremely rough or porous surfaces are likely to produce false indications. 8.0 Water Chemistry
Water treatment
General The major objectives of water chemistry programme in modern high-pressure boilers are the control of corrosion, deposition and achieving desired steam quality. Internal corrosion and deposition cost power plants crores of rupees in repairs and maintenance. Steam turbines rated for high capacities call for stringent steam quality to avoid damages and to maintain the rated output. Thus the successful operation of power plant requires a thorough understanding of all aspects of water treatment. Make-up water treatment: Raw water contains suspended solids, dissolved solids, dissolved gases and organics. The suspended solids and turbidity are generally removed by pre-treatment stage comprising of coagulation, clarification and filtration. The removal of suspended impurities is important for efficient operation of the demineralising plant. The dissolved solids and silica are required to be 121
Learning modules material- Engineering (FB) maintained at very low levels in boiler at high pressures to control deposition and to maintain steam quality. The dissolved solids and silica are removed in the demineralising plant and thereby producing acceptable make-up water quality for high-pressure boilers with specific electrical conductivity of less than 0.2 µS / cm with hardness completely removed and silica less than 0.02 ppm. Feed water treatment: The reaction of feed water and steel is spontaneous and rapid at high temperatures. The corrosion end product, magnetite (Fe3O4) forms a protective barrier on the boiler steel surfaces which minimizes further corrosion. The work of Bell and Van tack (fig.1) has been used to relate the relative corrosion of steel over a wide range of pH values. Minimum corrosion of steel is indicated at pH values of 8.5 to 11.0. Considering boiler components with mixed metallurgy, an optimum pH of 8.8 to 9.2 is recommended for feed water to minimise the pre-boiler corrosion. Ammonia is generally used to elevate the pH of feed water by the feed of not more than 0.5 ppm of ammonia to minimise copper corrosion. The exclusion of dissolved oxygen, another contributing element in corrosion of steel is essential to avoid corrosion in feed water. With main oxygen removal by deaeration, residual oxygen (0.01 to 0.02 ppm) in small quantities can be reduced further by reducing agents such as sodium sulphite for low pressure boilers and hydrazine for high pressure boilers. Boiler water treatment: It is recommended to use co-ordinated phosphate-pH treatment method for high pressure drum type boilers, by the injection of mixtures of phosphates in to the boiler drum so that sodium to phosphate molar ratio is less than 3.0. This method of treatment excludes free caustic from boiler water. Even if bulk boiler water does not contain large amount of caustic, there is a great potential for caustic to concentrate under deposits and cause corrosion . Oxygenated treatment: In oxygenated treatment [OT], Oxygen is deliberately injected in a controlled manner into the boiler feedwater to maintain a 50 to 150 ppb residual. Ammonia (20 to 70 ppb) is added to 122
Learning modules material- Engineering (FB) raise the pH to a range of 8.0 to 8.5. Compared to AVT programmes, iron oxide generation (due to corrosion of feedwater system) has been observed to be much less with OT programmes. With the controlled injection of oxygen, the base layer of magnetite becomes overlaid and interspersed with a tighter and impervious film of ferric oxide hydrate (FeOOH). This compact layer is more stable than magnetite and releases very little dissolved iron or suspended ironoxide particles to the fluid. The key to an OT programme is control. Input water must be extremely pure (cation conductivity < 0.15 µS / cm). Poor water quality would cause deposition leading to differential oxygen cells formation and subsequent under-deposit corrosion and pitting. Hence boilers operating on OT are always equipped with condensate polishers. OT cannot be used in systems that contain copper-alloy feedwater heater tubes, as the corrosion would be very high. OT has become very popular in once-through units. Chemical cleaning: In spite of high purity make up water and improved boiler water treatment program, trace levels of corrosion products in feed water and impurities from condenser leakage are carried over and accumulated on boiler heating surfaces. Water side deposits can reduce heat transfer, influence corrosion of the underlying metal and ultimately result in tube failure if they become excessive. If boiler performance is to be maintained at design values and availability is to be assured, deposits will have to be removed periodically by chemical cleaning.
The principal criterion for determining the need to clean boilers is the deposit density in milligram per cm2. BIS: 10391-1982 provides guidelines for permissible deposit weight limits and its relationship with boiler cleanliness for high pressure subcritical boilers.
Internal deposit weight (mg/cm2)
< 15.0 15 – 40 > 40
unit cleanliness
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Chemical cleaning is recommended whenever deposit weight is more than 40.0 mg/cm2. These limits have proven effective in avoiding overheating and corrosion related problems in a large number of utility boilers.
RECOMMENDED FEED WATER LIMITS
DRUM OPERATING PRESSURE kg/cm2 (g) TREATMENT TYPE 1.Hardness ppm (max) 2.pH at 25℃ Mixed metalallurgy All ferrous metallurgy 61-100 PO4 NIL 8.8-9.2 9.0-9.4 100 and above PO4 NIL 8.8-9.2 9.0-9.4 0.20 AVT NIL 8.8-9.2 9.0-9.6 0.20
3.Sp. electrical conductivity after cation 0.30 exchanger in H+ form at 25℃, microsiemens/cm (max) 4.Dissolved oxygen, ppb (max) 5.0 5.Silica as SiO2 , ppb (max) 6.Iron as Fe , ppb (max) 7.Copper as Cu , ppb (max) 8.Residual Hydrazine , ppb 9.Total organic carbon, ppb (max) 20.0 10 5 10-20 200
5.0 20/10* 5 3 10-20 200
5.0 10 5 3 10-20 200
* Should match with the corresponding values to be maintained in super heated steam.
BOILER WATER LIMITS
(FOR DRUM TYPE BOILERS- NORMAL OPERATION) DRUM OPERATING 61-90 PRESSURE kg/cm2 (g) TREATMENT TYPE PO4 1.Total Dissolved solids ppm (max) 100 91-125 PO4 100 125-165 PO4 50 165-180 PO4 15 AVT 2.0 181 & above PO4 10 AVT 1.0
2. Sp. electrical 200 conductivity microsiemens/cm (max)
200
100
30
4.0
20
2.0
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3. Silica as SiO2 ppm (max) 4.Chlorides as Cl ppm (max) 5. pH at 25℃
To be controlled as per fig. 2&3
3.0 9.0 10.0 2.0 – 9.0 – 10.0 1.4 9.1 – 9.8
0.1 0.6 9.1 9.6
0.1 0.03 – Note
0.1 0.5 9.1—9.6
0.1 0.02 Note
6.Phosphate residual 5-20 5-20 5-10 2-6 N/A 2-6 as PO4 ppm NOTE: pH should be monitored continuously; immediate shutdown if the pH goes below 8.0
N/A
GUIDELINES FOR EMERGENCY OPERATION
(DRUM TYPE – PHOSPHATE TREATEMENT)
SL.No.
01
Pressure range Hot well kg/sq.c.m (g) solids ppm 61-125 0.5-2.0 (ABNORMAL)
Operational Limitations Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3
Control Limits
Boiler water Control TDS<200 ppm NOTE 2 pH 9.1-10.1 PO4 5-40 ppm -DONOTE 4
>2.0 (EXCESSIVE) 02 126-165 0.5-2.0 (ABNORMAL) >2.0 (EXCESSIVE) 03 166-180 0.25-1.0 (ABNORMAL) >1.0 (EXCESSIVE) 04 181-203 0.1-1.0 (ABNORMAL) >1.0 (EXCESSIVE)
TDS<100 ppm NOTE 2 pH9.1-10.1 PO4 5-20 ppm -DONOTE 4
TDS<50 ppm NOTE 2 pH 9.1-10.1 PO4 5-20 ppm -DONOTE 4
TDS<50 ppm NOTE 2 pH 9.1-10.1 PO4 5-20 ppm -DONOTE 4
NOTE 1: NOTE 2:
Schedule Inspection and repair of condenser as soon possible Immediately start chemical injection to achieve higher phosphate and pH conditions,
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Do not continue operation if pH cannot be maintained above 8 total solids below Specified limits. Avoid use of desuper heating spray. NOTE3: Immediately reduce load to permit isolation of damaged condenser and prepare for orderly shutdown if hot well TDS cannot be reduced quickly below specified limits. NOTE4: NOTE5: Prepare for wet lay up of the boiler. Control silica in boiler water in accordance with graph provided.
HOT WELL CONDITIONS FOR ALL VOLATILE TREATMENT
(FOR DRUM TYPE BOILERS) PRESSURE RANGE (kg/sq.cm) 126-165 Above 166 Note: HOT WELL SOLIDS (PPM) NORMAL OPERATION < 0.05 < 0.05 EMERGENCY OPERATION < 0.1 PPM < 0.25 PPM
Switch over to phosphate treatment when hot well solids exceed emergency operation levels.
GENERAL INSTRUCTIONS 1. All Feed water measurements shall be made at high pressure heater outlet or economiser inlet 2. Oxygen can also be additionally measured at deaerator outlet to determine the quantity of N2H4 dozing. 3. The recommended pH in feed water can be obtained by dozing ammonia, morpholine or any volatile amine. The concentration of volatile chemical in the feed water should not exceed 0.5 ppm. (expressed as Ammonia) 4. The phosphate and pH are recommended in accordance with co- ordinate phosphate curves (Figs.4 to 6) to prevent presence of free hydroxide in boiler water. 5. Water levels in drum should be maintained within limits during all operational modes, start - up, load fluctuation and normal operation. 6. The alignment of drum internals should be checked and ensured to be in order at least once every year. 7. It is needles to emphasize that correct sampling accurate measurements with the use of reliable at adequate intervals and proper logging of reading go a long way in ensuring trouble free operation.
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RECOMMENDED FEED WATER AND BOILER WAETR LOW AND MEDIUM PRESSURE WATER – TUBE BOILERS
[For drum pressure below 60 kg / cm2(g)]
LIMITS
FOR
GENERAL The following recommendations hold good for water – tube boilers with drum pressure upto 60 kg/cm2 (g) where high purity feed water is available by use of demineraliser. It is recommended that demineraliser shall only used for all chemical recovery boilers irrespective of drum pressure.
RECOMMENDED FEED WATER LIMITS -- (Note 6) Drum Operating pressure [kg/cm2 (g)] Hardness Max-ppm pH at 25℃ Oxygen max – ppm Total iron max –ppm Total copper max –ppm SiO2 max –ppm Upto 20 1.0 8.8-9.2 0.02 0.05 0.01 1.0 21-40 0.5 8.8-9.2 0.02 0.02 0.01 0.3 5.0 41-60 Nil 8.8-9.2 0.01 0.01 0.01 0.1 2.0 Note 4 Note 4 Remarks Note 4 Note 1
Conductivity at 25℃ measured 10.0 after cation exchanger in H+ form and after CO2 removel max .( µs /cm) Hydrazine residual –ppm -
-
0.02-0.04
RECOMMENDED BOILER WATER LIMITS Drum Operating pressure [kg/cm2 (g)] pH at 25℃ Phosphate residual -ppm TDS – max – ppm Specific electrical conductivity at 25℃ max –(µs/cm) Silica max –ppm Sodium Sulphite as Na2SO3-ppm 10.0-10.5 20-40 500 1000 25.0 20-40 10.0-10.5 20-40 200 400 15.0 5-10 9.8-10.2 15-25 150 300 10.0 Note 2 Note 3 Note 3,5 Less than 20 21-40 41-60 Remarks
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NOTE:1. Morphline or any other volatile amine can be used toelevate pH. The concentration of volatile chemical in feed water shall not exceed 1 ppm (expressed as Ammonia ) Sodium sulphate shall be dozed in the feed water, after the tapping point for Desuperheating spray so that it does not get contaminated. The phosphate and pH shall be maintained in accordance with co-ordinated phosphate –pH curve (fig 7) to prevent pressure of free hydroxide in boiler water . If feed water is used for Desuperheating spray, (a) Hardness shall be nil (b) SiO2 shall not exceed 0.02 ppm (c) Conductivity at 25℃ measured after cation exchanger in H+ form after CO2 removal shall not exceed 2 micro mho/cm. Total Alkalinity in boiler water shall not exceed of TDS. Pressure of oil or organic matter is not allowed in feed water which will induce foaminess and cause carryover of impurities into steam.
2. 3.
4.
5. 6.
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FIG 2
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FIG 3
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FIG 4
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FIG 5
FIG 6
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FIG 7
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9.0 Boiler operation, Availability and Reliability, Boiler Tube failure mechanisms.
9.1 Boiler Operating Modes:
Increasing focus on plant efficiency, need to respond to grid fluctuations, conservation of fuel etc. warrant the boilers to be designed to meet varying operating modes like: Base Load Operation Cycling Two-shift Constant pressure Sliding pressure ( Natural & Modified) Trip to house load Response to FGMO( Free Governing Mode of Operation)
. Natural sliding pressure operation has the advantage of essentially eliminating first stage temperature changes inside the turbine, as long as the main steam and reheat steam temperatures are held constant. 134
Learning modules material- Engineering (FB) However, the response of the steam generator-turbine combination, with control valve in wide open condition, to changes in load demand is relatively slow and may not satisfy the power system control requirements. In order to overcome this, the modified sliding pressure operation is in use. In the modified sliding pressure mode, the steam pressure is kept constant down a certain load say from 100% to approx. 90% and then allowed to slide down along with the load. By this, turbine inlet valves are kept at a position lower than full open position and the admission cross section at the turbine is altered briefly when the load is varied, so that the accumulated steam in the steam generator is discharged at once. The dynamic behaviour of the steam generator is improved to take care of load fluctuation to a limited extent.
9.2
Availability & Reliability
Plant availability and reliability can be improved by adopting various measures like: Use of well proven equipment & design practices Redundancy for Critical equipment Use of early warning systems Tube leak detection system Operator alarms Sophisticated controls, Instrumentation, safety interlocks Vibration monitoring Equipment trip provisions Leveraging IT for performance analysis & optimization and for smart operation
9.3 Boiler Tube failures
9.3.1 Major Tube Failure Mechanisms : The 22 primary mechanisms responsible for boiler tube failures as per the EPRI (Electric Power Research Institute) statistics are given below. Table 1: Major Tube Failure Mechanisms
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Learning modules material- Engineering (FB) Stress Rupture
Short Term Overheating High Temperature Creep Dissimilar Metal Welds
Fatigue
Vibration Thermal Corrosion
Water-side Corrosion
Caustic Corrosion Hydrogen Damage Pitting Stress Corrosion Cracking
Erosion
Fly Ash Falling Slag Soot Blower Coal Particle
Fire-side Corrosion
Low Temperature Water wall1 Coal Ash1 Oil Ash
Lack of Quality Control
Maintenance cleaning damage Chemical Excursion damage Material Defects Welding Defects
1. Not observed in Indian power stations.
9.3.2 Aspects to be considered for boiler pressure parts Availability at various stages : Design stage: Selection of material during design stage should take care of the following aspects. Compatible for working pressure / temperature. Based on quantity of steam flow and the velocity / pressure Heat transfer characteristics / surface effectiveness / metal temperature Thermal expansion / constraints Radius of bends Attachments Weldments
Manufacturing aspects : Most of the pressure parts of the high pressure and high temperature boilers are manufactured by welding. Therefore the weld design and welding process are to be carefully chosen. Since the different material sizes and specification are used in combination for an optimal design at economic cost, the welding process to be adopted do vary widely. preheating, post heating, stress relieving are important factors included in the welding process to ensure flawless fabrication. Transportation / Handling :
136
Learning modules material- Engineering (FB) Long pipes, panels of tubes and headers do suffer due to self weight. Hence, during handling (loading, unloading) enough care to be taken so that permanent sag does not setin in the components. Cases of tube failures have been faced due to scoring / cutting cost by wire ropes used for tying up during transportation. Storage : Normally the large capacity boilers takes several months for erection and commissioning. Hence the materials received from manufacturers are to be properly stored at erection sites. Particularly pressure part tubes are to be preserved by properly closing the open ends with end covers after putting the necessary preservatives. Open yard storage for prolonged duration may need periodical repainting of tubes with rust preventive coating. Plants near to costal area needs extra care due to salubrious atmosphere prevalent.
Erection : The high capacity of boilers are manufactured in parts and pieces from the point of view of handling, transportation and site erection. For facilitating field erection it is recommended for sequential erection with different modules which has to be adopted at sites. Welding at site needs due care and carried out as per recommended weld procedures to minimize / avoid the pressure part failures later, during service of the boiler.
Commissioning : After completion of erection, normally a hydraulic testing is done at site even though various pressure parts are hydro tested at manufacturing works. Since there are many site weld joints involved at erection sites the soundness and reliability of these are ensured by field hydro testing. Moreover this is also a statutory requirement as per IBR. Followed by hydro testing alkali boil out and acid cleaning and steam blowing operations are carried out to remove the loose deposits, mill scale, weld slag, muck, loose rest etc. During these commissioning activities especially during hydro test any weak / defective weld joint will be revealed during this hydro test and the same can be corrected. Operation and Maintenance: As per the statistical evidences gathered so far, the pressure part tube failures due to design / manufacture / erection and commissioning accounts for 20 to 30% whereas the rest 70 to 80 % are traceable to operation and maintenance. By taking prudent steps in O&M, the number of failures can be minimized if not eliminated.
Tube failure reduction through operation and Maintenance Over heating – short term / long term: 137
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Overheating of tubes may be due to any or combination of the following: Partial or full choking of tubes resulting in flow starvation. Internal deposits due to poor water chemistry, carry over of solids from the steam drum. Flame impingement due to faulty or improperly aligned burners. Extracting more auxiliary steam flow than the designed level may also lead to starvation in super heater / reheater.
Excess air levels: In order to ensure complete combustion, it is usual practice to operate the boiler with 5 to 10% (oil & gas firing), 15 to 20% (coal firing) more air than the stoichometric quantity. But too much of excess air leads to cooler furnace and higher heat absorption in convective paths. Too little excess air leads higher furnace temperature result in higher radiation pickup and also promotes slagging in the furnace. More excess air also accelerates the erosion potential of the ash and external metal wastage resulting in thinning of tubes and consequent tube failure. Fuel types : Fuel oil (% of sulphur and vanadium content): Vanadium present in the fuel gives raise to high temperature corrosion. The sulphur content in the fuel results in cold end corrosion when the sulphur percentage increases.
Coal : The coal having higher ash content and higher percentage of Alpha quartz results in higher erosion rate and metal wastage in the convection pass especially in the LTSH and economizer zones. When firing coal with ash containing more than 4% sodium having conventional tubes spacing results in severe plugging in gas path leading to overheating failures. Other fuels : In Industries the process wastes, industrial gases and bio fuels when used bio mass when used as fuel in steam generators, the properties of such fuels are to be analysed in detail and steam generator are to be designed and operated cautiously and carefully according to their characteristics to avoid any unforeseen failures. Soot blowing : 138
Learning modules material- Engineering (FB) While soot blowing helps in cleaning any ash / slag deposits on the tubes, excessive pressure of steam used for blowing may cause external metal wastage due to erosion of steam with fly ash in the flue gas. Excessive usage of soot blowers, struck-up soot blower lance which fail to retract, jammed soot blower valves, condensate in blowing steam, etc., are few of the reasons causing erosion of pressure part tubes and consequent failures. Correct drum level operation: Incorrect drum level operation beyond permissible limits is unsafe. Low drum level leads to steam entrapment in the down comer, leading to sudden loss of density of circulating fluid in the down comer. This in turn, affects the circulation in the water wall tubes resulting in overheating of tubes. The damage may range from busting of water wall tubes to bowing in of the water wall panels. Snapping of the buck-stay connections to the water walls may also occur. Typical situations leading to low drum level operation are as follows: Stoppage of feed water to the boiler due to inadvertent closure of feed system valves or loss of deed pump. Lack of calibration of drum water level measurement or control instrumentation. Level protection devices either by passed or in-operative. Operation of the Unit without requisite level instrumentation
Water Chemistry: Quality of water and steam used in modern high pressure steam generator is of utmost important for trouble free performance of fossil fuel fired steam generators. When water chemistry are maintained within the prescribed limits recommended by the designers or qualified consultant, corrosion damage may occur in water wall and economizer tubes. Water wall corrosion problems can generally be avoided in boiler if the following points are taken care : Recommended water treatment controls are followed. Corrosion products formed in the feed water system are kept within the specified limits. Feed water oxygen concentration is properly controlled. Precaution are taken during chemical cleaning operations to prevent metal attack.
Tube internal deposits problems can be avoided if: Hydraulic test water, superheater fill water and de-superheater spray water are free of solids. It is preferable to use DM water for these operations. Drum internals and drum water level controls are maintained in good working order.
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Learning modules material- Engineering (FB) Silica concentration in the boiler water is held within the acceptable limits.
Low pH damage: Corrosion failures occur when acid or alkaline salts are concentrated. Hydrogen induced brittle fracture occurs beneath a relatively dense deposit and is most likely to occur when boiler water pH is too low. Some of the hydrogen produced in the corrosion reaction diffuses into the tube metal where it combines with carbon in the steel. Methane is formed and it exerts internal pressure within the steel causing grain boundary fissuring. Brittle fracture occurs along the partially separated boundaries. In many cases an entire section is blown out of a damaged tube leaving window opening. Restoration of proper boiler water treatment may not be sufficient to prevent further hydrogen attack unless the dense corrosion product deposits are removed. Even repeated chemical cleanings sometimes will not remove them. Replacement of tubes where metal attack exists becomes necessary. Generally Hydrogen damage is difficult to detect using NDT means. To some extent UT may pin point some damaged areas, but positive identification of all failure prone tubes is not possible.
High pH damage: Concentrated hydroxide salts such as sodium hydroxide in the boiler water will cause gouging type of corrosion leading to ductile failures. Ultrasonic tube wall thickness checks can detect tubes with metal loss. Proper boiler water treatment can minimize further corrosion. Minimising corrosive attack : Concentration of salts promoting corrosion generally forms at the surfaces of the tubes when acidic or alkaline producing environment prevails. This condition may happen when water treatment conditions deviate from the recommended parameters. If the recommended specifications are followed during operation of the Boilers, the corrosive attack can be minimized. Causes of high and low pH: It is a well known fact that the high or low pH do cause damage to pressure parts as explained in the previous paragraphs. Condenser leakage is the primary cause for acidic and caustic boiler water conditions. The raw cooling water that leaks into the condenser essentially ends up in boiler water. The cooling water source determines whether the inleakage is either acidic producing or caustic producing. Fresh water from lakes and rivers usually provide dissolved solids that hydrolyze in the boiler water environment to form a caustic such as sodium hydroxide. By contrast, sea water and water from re-circulating cooling water systems with cooling towers contain dissolved solids that hydrolyze to form acidic compounds.
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Learning modules material- Engineering (FB) Another potential source of acidic and caustic contaminants is the make up demineralised, where regenerant chemicals like, sulphuric acid and caustic may inadvertently enter the feed water system. Chemicals incorrectly applied during boiler water treatment also can be corrosive. For example sodium hydroxide used in conjunction with sodium phosphate compounds to treat boiler water. Corrosion can occur if the sodium hydroxide and sodium phosphate are not added to the water in proper proportion. Minimising pitting of Boiler tubes : Pitting caused by dissolved oxygen can be prevented by maintaining feed water oxygen level within the 5 ppb limit. Attack by chemical cleaning solvents can be eliminated by carefully following the cleaning procedures. During shut down period it is necessary to protect all internal surfaces. Wet lay-up together with a positive nitrogen pressure cap of about 3.5 psig will protect metal surfaces from corrosion. Avoiding steam side deposition: Internal surfaces of steam side components like superheaters and reheaters are deposited with salts carried over along with steam from drum. These deposits impairs with heat transfer because of its insulating effect and leads to overheating failures. Boiler manufacturers help in limiting the solid carry over from steam drum by proper design and fitment of drum internals. From operational point of view proper blow – down, controlled dosing and proper drum level control will help in avoiding deposits in superheaters / reheaters. Solids carry over by the steam into the turbine also can cause turbine damages. To sum up, good water chemistry with prudent boiler operations with vigil will go a long way in minimizing the tube failures to a great extent. While extensive elaboration on technique of water treatment and water chemistry is outside the purview of this paper, an attempt has been made to give a brief coverage in this paper. Hydrogen Damage: Boiler tube failures caused by hydrogen damage result from fouled heat transfer surfaces and an acidic (low pH) condition of the boiler water. Hydrogen damage is some times referred to as “ hydrogen attack‟ or hydrogen embrittlement”. The tube steel will become brittle from the combination of hydrogen and carbon, which forms gaseous methane (CH4) at the grain boundaries in the tube steel. Hydrogen damage develops from the generation of hydrogen during rapid corrosion of the internal surface of the tube. The hydrogen atmos `migrate through the tube steel where they can react with the iron carbide (Fe3C) to form the methane. The larger methane gas molecules become trapped between the grain boundaries and cause a network of discontinuous internal cracks to be produced. These cracks grow and some will link up to cause a through wall fracture. Causes of Hydrogen Damage : Hydrogen is generated under the following circumstances :
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Learning modules material- Engineering (FB) Operation of the boiler with low pH water chemistry from the ingress of acidic salts through condenser leakage, contamination from chemical cleaning, or malfunction of the chemical control subsystem. Concentration of the corrosive contaminants within the deposits on the internal tube wall, especially in crevices, shallow pits, and under weld backing rings.
Caustic corrosion: Boiler tube failures caused by caustic corrosion result from fouled heat transfer surfaces and an active corrodants in the boiler water. Caustic corrosion is some times referred to as “caustic attack”, “caustic gouging, “or “ductile gouging”. Caustic corrosion develops from deposition of feed water corrosion products in which sodium hydroxide (Na OH) can concentrate to high pH levels. At high pH levels, the tube steel‟s prot ective magnetite oxide layer is solubilized and rapid corrosion occurs. The tube surface deposits accumulate at locations where flow is disrupted such as just downstream of welds with backing rings, at bends, in horizontal tubes heated from above or below, and at high heat input locations. Most of the feed water corrosion products deposit on the heated side of the furnace wall tubes since deposition is heat flux related and will favor the tubes within the highest heat absorption zones. Causes of Caustic Corrosion: Caustic corrosion occurs through: Selective deposition of feed water system or pre-boiler corrosion products at locations of high heat flux. Concentration of sodium hydroxide from boiler water chemicals or from upsets in the water chemistry.
Stress Corrosion cracking: Boiler tube failures have been caused by stress corrosion cracking (SCC) and result from the combined effects of tensile stress, a corrosive environment, and a susceptible material. Stress corrosion cracking failures in a boiler usually occur in the austenitic stainless steel used for superheater and reheater tubing. However, SCC failures have occurred in some ferritic reheater tubing when high levels of caustic were introduced from the desuperheating or attemperator spraying station. Causes of Stress Corrosion Cracking : Conditions for stress corrosion crack initiation and propagation exist under the following circumstances: Contamination of boiler water or steam with chlorides or hydroxides. Introduction of high stresses from service conditions. Production of high residual stresses during fabrication and assembly. 142
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Tube failures indications: During the regular operation of the Steam Generator if there is any tube failure occurs it will be indicated by any one or combination of the following points: Sudden or abnormal loud noise Continuous hissing noise Furnace draft fluctuations / pressurization Flame out Falling drum level Increased quantum of make-up water Steam / Water leakage Uncontrolled Boiler Feed Regulation / Feed Pump Trip
SHORT OVERHEATING
TERM
LONG OVERHEATING
TERM
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STEAMSIDE OXIDE SCALE
OVERHEATING, CREEP INCORRECT MATERIAL
–
OVERHEATING – BULGING, SATELLITE SCALE CRACKING
OVERHEATING DEPOSITS
–
WATERSIDE
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DISSIMILAR METAL FAILURE IN SERVICE
WELD
DISSIMILAR METAL WELD MICRO AT TRANSITION
FLATTENING TEST FAILURE HYDROGEN EMBRITTLEMENT
GRAIN BOUNDARY CRACKSHYDROGEN EMBRITTLEMENT
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HYDROGEN EMBRITTLEMENT ETCHED
ROUNDED DISSOLVED PITTING CORROSION
GAS
LONG TERM OVERHEATING WATERSIDE DEPOSTS
DISSIMILAR METAL WELD FAILURE
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CAUSTIC GOUGING STRATIFICATION
CAUSTIC GOUGING
HYDROGEN EMBRITTLEMENT
HYDROGEN EMBRITTLEMENT
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DISSIMILAR METAL FAILURE IN SERVICE
WELD
DISSIMILAR METAL WELD MICRO AT TRANSITION
FLATTENING TEST FAILURE HYDROGEN EMBRITTLEMENT
GRAIN BOUNDARY CRACKSHYDROGEN EMBRITTLEMENT
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HYDROGEN EMBRITTLEMENT ETCHED
ROUNDED DISSOLVED PITTING CORROSION
GAS
STRESS CORROSION CRACK IN COLD BENT TUBE
TRANSGRANULAR STRESS CORROSION CRACKS
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INTERGRANULAR STRESS CORROSION CRACKS
CORROSION FATIGUE CRACKS
CORROSION FATIGUE CRACKS
SIDE WALL LACK OF FUSION IN A WELD
DISSOLVED OXYGEN PITTING CORROSION
EXTERNAL PITTING IMPROPER STORAGE
–
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STRESS CORROSION CRACK IN SS 304H
LOW TEMPERATURE CORROSION
LOW TEMPERATURE CORROSION
WATER WALL CORROSION
FIRE
SIDE
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COAL ASH CORROSION
OIL ASH CORROSION
FATIGUE CRACK
FATIGUE CRACK
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FATIGUE CRACK – INADEQUATE FLEXIBILITY
THERMAL FATIGUE
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FATIGUE CRACK ATTACHMENT WELD
–
CORROSION FATIGUE CRACK IN COLD BEND
CORROSION CRACKS
FATIGUE
CORROSION FATIGUE CRACKS
DAMAGE DUE TO TUBE INSIDE TUBE
PIPE MANUFACTURING DEFECT LAP
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INCORRECT MATERIAL
TUBE MANUFACTURING DEFECT - FOLD
TUBE MANUFACTURING DEFECT - SEAM
TUBE BENDING CRACK – STRIP HEATED BEND
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START STOP GAP IN PULSED MAG WELDING
BURN-THROUGH IN SMAW
TRANSPORTATION DAMAGE AT WIRE LASHING REGION
INDUCTION PRESURE WELD – PASTY JOINT
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LIGAMENT HEADER
CRACKING IN
CLOSE UP VIEW OF LIGAMENT CRACK
INTERNAL EROSION IN A TUBE BEND
CREEP FAILURE OF A LINK PIPE – IMPROPER SUPPORT
Tube failure investigation: An Investigation into a tube failure in an electric utility steam generating boiler has the potential to determine the root cause of that failure. Determination of the root cause can lead to implementation of corrective actions which could reduce or eliminate the likelihood that a similar type of failure will occur. These economic and morale benefits can only be achieved if an investigation into the tube failure is conducted which correctly identifies the 157
Learning modules material- Engineering (FB) failure mechanism and recommends the corrective actions that will control the root cause for that failure. Such an investigation can be a complex process requiring effective communications between equipment operators, maintenance mechanics, plant management, equipment manufacturers, and technical experts in materials, chemistry, and mechanical engineering. For an investigation to be successful, the following activities must be performed by plant personnel : Information and data concerning the tube failure must be gathered quickly before repair activities can begin. Failure descriptions, operating conditions at the time of failure, historical records, and tube samples must be acquired and transferred to others who will conduct the investigation while repairs are being performed. Immediate corrective actions based on the initial results of the investigation must be approved and implemented before repairs are completed. Follow-up corrective actions based on the complete results of the investigation must be planned and implemented before additional failures are experienced.
The likelihood that a failure investigation will be successful and produce the proper corrective action can be enhanced when the plant personnel have knowledge of the basic failure mechanisms that produce tube degradation, cognizance of the root causes for each failure mechanism, recognition of the ways to verify a root cause, and conviction to follow a planned approach to document the failure with pertinent data. Human errors are also an important factor in boiler tube failures. Errors can occur in the design, manufacture, shipping, storage, construction, operation, and maintenance of the boiler tubing. The wrong material can be installed at a critical location, leading to premature failure of the tube. This error can be the result of lack of quality control at the supplier ‟s factory or in the utility‟s storage and stock disbursement process. Boiler tube failures have been experienced due to lack of quality control in maintenance cleaning, welding, chemical cleaning, and tube manufacturing.
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For Engineer Trainees
Engineering /Fossil Boilers
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Index
1. Boiler - Design 1.1 Steam Generation 1.2 1.3 1.4 Types of boilers Boiler Circulation system Boiler Design – Specification & Parameter
2. Engineering processes 2.1) Boiler Performance and proposals 2.2) Product Engineering 2.2.1) Boiler layouts 2.2.2) Pressure parts, pressure parts arrangement and Stress analysis 2.2.3) Boiler Mountings 2.2.4) Ducts and Dampers 2.2.5) Fuel Systems 2.2.6) Lining and Insulation 2.2.7) SS and Buckstay 2.3) ITS &S 2.4) Controls and Instrumentation 2.5) FES and R&M
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3. Boiler Materials 1. Materials used in boiler 4. Boiler auxiliaries 1. Pulverisers 2. Fans 3. Air Pre-heater 4. Dust Collector 5. Environmental Pollution 1. Indian Pollution control board guidelines 6. Codes and Regulations: 1. Material testing codes 2. Coal Analysis Standards 3. Boiler efficiency 7. Destructive and non-destructive testing 8. Water Chemistry 9. Boiler operation, Availability and Reliability and Boiler Tube failure mechanisms.
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1.1
Steam Generation
Modern Thermal power plants operate on the Rankine cycle . In a typical thermal power plant, the heat released during combustion of the fossil fuel is transferred through the walls of the boiler ( waterwalls formed of tubes) to water that flows through the tubes, thus generating steam. In drum type boilers, normally used for subcritical pressure application, a mixture of steam and water leaves the waterwalls. The steam is separated from the water in the boiler drum, superheated in the superheaters and is sent to a steam turbine. In the case of once thru boilers, all the water is converted to steam ( in once thru mode of operation) . The rotor of the steam turbine rotates as the steam passes over the turbine blades, which in turn rotates the generator, connected to the rotor, thus producing electric power. The steam after expansion in the turbine is condensed in the condenser and is pumped back to the boiler.
Steam generation Process Boiler is thus a very vital component of the thermal power plant. Pulverized coal fired boilers today form the backbone of thermal power generation in almost all countries due to the abundant availability and low cost of coal. The increasing concerns on the atmospheric pollution warrant that power be generated with minimal pollution. It is therefore of paramount importance that coal utilisation is done in an environmental friendly manner, which can be attained by efficient processes, technologies and equipments. Increase of plant efficiency is one of the important ways of reducing the fuel consumption and consequently reducing the plant emissions and conserving energy resources. Adopting any one
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Learning modules material- Engineering (FB) or more of the following measures can increase the overall efficiency of pulverised coal fired power plants: 1.2 Increasing main steam pressure Increasing Superheat and Reheat steam temperature Adopting double reheat cycle. Increasing the vacuum of condenser Increasing final feed water temperature Reducing boiler flue gas exit temperature Reducing excess air Boiler Types:
Boilers can be broadly classified on the following basis: Use/ Application: Drum arrangement Heating surface arrangement: Circulation: Operating Pressure: 1.2.1 Boiler application Industrial boilers are mainly for use in process industries and are normally non-reheat units and have partial steam generation in boiler bank tubes. Utility boilers are large capacity boilers used for electric power generation 1.2.2 Drum Arrangement Industrial / Utility application Single drum/ bi-drum/ once thru (no drum) Tower type/ two pass type/ box type/ close coupled type Natural Circulation, Forced/ Controlled circulation Sub critical, supercritical
Drum type boilers employ either Natural circulation or Forced circulation . In drum type boilers steam generation takes place in the furnace water walls and has a fixed evaporation end point - the drum. Steam -water separation takes place in the drum. Separated water is mixed with incoming feed water and flows back to the waterwalls. Bi-drum boilers: In lower pressure ranges it is common to incorporate a boiler bank for heat transfer in the evaporator circuit. The boiler bank is arranged between two drums and hence the name bidrum boilers.
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Learning modules material- Engineering (FB) Single drum boiler: With increase in operating pressures, the drum plate thickness increases considerably and the bi-drum arrangement is not preferred because of the large thickness. At steam pressures above 100 kg/cm2, single drum boilers are hence normally employed. 1.2.3 Heating Surface Arrangement: The heating surfaces like Superheater, Reheater and economizer are dispositioned in a boiler to achieve the most optimum heat transfer. There are different arrangements that could be adopted. Tower type designs have all heat transfer surfaces arranged as horizontal sections above the furnace. Two pass designs employ a combination of pendant and horizontal sections. Box type and close coupled are compact designs normally used with oil and gas fuels. 1.3 Boiler Circulation System:
Choice of Circulating system depends on operating Pressure. The density difference between water and steam provides the driving force for the circulating fluid. Higher pressures units warrant pumps to ensure circulation or alternately the components are to be sized bigger to reduce the frictional resistance, so that natural circulation can still be employed. Natural Circulation Boiler Circulation thru water walls by thermo-siphon effect
Forced/ Controlled Circulation Boiler Thermo-siphon effect supplemented by pumps
Once-through Steam Generators: In once through steam generators, the boiler feed pump forces a once-through type flow through the complete system in the boiler (through the economiser, water walls and superheater sections) in one single continuous pass. The concept is shown in Fig.1
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Fig.1 Concept of Once Through Steam Generation Once-through steam generators can be designed for both sub-critical and super critical pressures. The important advantages of once-through boilers such as short start-up time, load ramp behaviour and flexibility are available with subcritical design parameters also. Once through steam generators are ideally suited for sliding pressure operation due to the absence of thick walled components and lesser storage requirements as compared to drum type steam generator. In sliding pressure operation, the turbine inlet valves are kept full open during normal operation. Hence, the live steam pressure is directly proportional to the steam flow. Increase in the steam parameters, i.e. temperature and pressure, is one of the most effective measures to increase the efficiency. The supercritical cycle (pressure higher than the critical pressure) offers a ‘burn less fuel for the same output’ approach. However higher steam condition is limited by the availability of the materials required to withstand the duty conditions Development of once through technology, new materials and improvement in design has led to adoption of supercritical cycles. In the 80’s, with the development of high temperature steels, number of new units adopted higher temperature of 565 0C and 5800C. In the 90’s, a few utilities adopted advanced steam parameters of 285 bar and 5800C to 6000C, which are termed as ultra supercritical parameters. The current trend in advanced countries is to go in for increase in steam pressure as well as temperature. In countries like Japan, the parameters in new projects are even higher (300 bar / 600OC / 600OC). World-wide R & D efforts are going on to develop power plants with ultra supercritical steam parameters.
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1.4.
Boiler Design parameters
The following parameters shall be specified as a minimum for designing a boiler: Feed Water & Steam parameters – flow, pressure, temperature, quality Control load – the load range in which superheat and reheat steam temperature are maintained at rated value. Fuel Firing – Fuels to be fired, fuel combinations, fuel properties, fuel parameters at terminal point. Site conditions – seismic data, wind velocity, altitude, rainfall, ambient temperature, cooling water temperature, humidity etc. Layout constraints/space availability Boiler operating modes (Base load, Cycling, Two-shifting, Constant pressure, Sliding pressure etc.) Emission limits Specification of major components/auxiliaries / systems (type, number, sizing criteria etc)
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Learning modules material- Engineering (FB) 2. Engineering Processses: 2.1 Boiler Performance and Proposals:
Boilers are designed to meet the requirements specified by the Customer. Requirements vary in terms of output, inputs, operating requirements, emission norms, space constraints, preferences on equipment type, level of technology, individual equipment specifications etc. Each boiler is unique, as it is designed to meet these varying requirements. In BPP, the thermal and system design of the boiler is developed to meet the desired performance and configuration requirements. Proposals BPP prepares the technical portion of the tenders against enquiries from Customers for Utility, Industrial and Chemical Recovery boilers. Proposal packages include details of the proposed boiler (Technical specification, Technical data sheets, Drawings etc.). Contract Performance Engineering On award of Contract, the Performance Engineering of the boilers is carried out towards ensuring the required boiler performance. Based on this, necessary engineering inputs are furnished to Product engineering sections for further engineering. Field Performance Data Analysis On commissioning of the boiler, the operating data from the unit is collected, compared with the predicted performance data and reasons for variations, if any, are analyzed. Designs are continuously updated to incorporate the feedbacks to obtain better performance.
Renovation & Modernization (R&M) of Boilers In R&M, the performance aspects of the boilers are studied and the required inputs are generated and furnished to the related agencies. Functions of BPP fall under three categories: Proposal Contract FES support for performance
Proposal Phase BPP Proposals come from the three major business sectors: Power sector /Marketing ( utility boilers)
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Industries sector International operations
Inputs Enquiry specification , addendum / clarification by customer from commercial tendering and estimation Boiler parameter from PEM /PED as applicable Scope, terminal points and exclusions from power sector/ marketing
Deliverables Filled up specification review Filled in proposal datasheets PG wise BOM with estimated weight Technical write up and proposal Details required for erection estimate List of information of spares Proposal valve schedule to valves
BPP processes Enquiry registration Receive enquiry specification from commercial Assign proposal number Detailed proposal Budgetary proposal Detailed proposal Prepare the specification review list Finalize scope, exclusion and terminal points between units consortium partners Seek clarification from customer Seek clarification /comments from PEB, QA, BAP, PC, HYD , valves and C and I
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Obtain feedback from the FES for similar running contracts Design processes Customer clarification Finalize design approach Furnace design SH , RH , ECO arrangement Performance calculation Auxiliary selection Prepare technical deviations Presentation to top management Prepare BOM Prepare support documents Performance Calculation General data Proximate analysis to ultimate analysis Slagging characteristics of coal Air and gas weight calculation s Heat duty Efficiency calculations Flue gas analysis Furnace performance Plan area Volume Effective projected radiant surface Arrangement data Heating surface area
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Flue gas area SH , RH , ECO performance IBP runs Mills AH Check for design limiting values Material selection Metal temperature program Tubing list for PP
Headers and piping Pressure drops Fuel firing Mill selection wind box selection Airheaters Tubular Bisector (a) Trisector Fans, ducts and losses Duct area calculation Fan selection Duct draft loss calculation Circulation Controlled and natural circulation Pump selection CIRGEN
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DNB check Auxiliary selection EP selection Ash collection data Chimney selection data Safety valve selection C and I parameters Boiler performance Guarantee schedule Efficiency program Auxiliary power consumption NOX, SOX Wear life of mill rolls Performance curves Prepare support documents Guarantees Technical data sheets Spares list Technical offer Erection input to regions Finalize technical offer Review by management Presentation to top management Prepare and submit technical offer Attend post bid meeting with customer/ consultant and resolve points Revise BOM, if required
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Revise technical offer, if required Finalize technical offer Submit to commercial
2.2 PRODUCT ENGINEERING(Detailed Engineering): Various Section in Product Engineering are Layouts Pressure Parts, Pressure Parts Arrangement and Stress Analysis Boiler Mountings Ducts and Dampers Fuel System Lining and Insulation Supporting Structures and Buckstays
2.2 .1 Layouts
Various types of Layouts used in the power plants are as follows:
(a) Conventional / Front mill layout
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(b) Vijayawada Layout
(c) Rear mill / Panipat Layout
(d) Side mill layout
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. In the Conventional Layout, mill bay is located adjacent to the TG set. The mill reject started accumulating in TG set causing inconvenience to the people working with it. This led to the change in the location of mills in some of the subsequent boilers. Due to this, dust problem to TG set was rectified and sufficient ventilation was also available as compared to the conventional layout. As this was implemented first in Vijayawada, the layout was named as Vijayawada Layout. In Vijayawada Layout, duct carrying the primary air to the mill bay and the pulverized coal to the furnace, covers the entire area around the furnace. This problem was rectified using the Rear Mill layout. As it is first used for Panipat it is also called as Panipat Layout. As the number of mills operating in the 500MW unit is around 10 and occupies more space in the conventional layouts. Mills are arranged along both sides of the furnace in this layout, named as Side Mill Layout.
2.2. 2 Pressure parts,Pressure Parts Arrangement and stress analysis: DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 1 For a 500 MW, To prepare Production & Erection documents for superheater/reheater assembly and header is being done for each project as per the boiler specification based on the design guideline. This project involves the detailed study of each components in the superheater/reheater assembly and header, classification and analysis of their variant drawings, need for these variants and scope for standardization. It also involves collection of all standards required for the design into a single document for easy reference. The superheater/reheater assembly of various Projects have been analyzed to find the variants involved in each component. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 2
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DESIGN OF SUPERHEATERS & REHEATERS INTRODUCTION: The Superheaters & Reheaters are part of the pressure parts systems of a boiler. The other pressure parts which act as heat absorbing surfaces are waterwalls or furnace walls. The headers, Drum, Downcomer and other connecting lines and links will also be called as pressure parts in a boiler. Superheaters are used to raise the steam temperature above the saturation temperature by absorbing heat from flue gas to increase the cycle efficiency. Due to superheating the useful energy that can be recovered increases, thus the cycle efficiency also increases. For utility boilers which are meant for power generation, the superheater outlet temperature is limited to 540 + 5 C because the maximum temperature is dictated by the metallurgy and economy in initial cost and maintenance cost. Superheating also eliminates the formation of condensate during transporting of steam in pipe lines and inside the early stages of turbines which is harmful to the turbine blades and pipe lines. Reheaters are used to raise the steam temperature to the same superheat temperature but at a lower pressure since the steam flow through reheater takes place after H.P turbine. The reheating of steam improves the cycle efficiency and reduces the damage to the turbine blades due to condensation of steam at turbine ends. In the case of reheaters this method is used only as an emergency purpose and not as a regular means as direct admission of water in reheater adversely affect the system efficiency. Titling the burners up and down is the major method of control used generally for Reheater to control the temperature. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 3 TYPES OF SUPERHEATRS AND REHEATERS: These heating surfaces are in the form of coils which are made by bending the tubes in cold or hot forming. The hot bends are called squeezed bends which are done in a special hydraulic pressing machine called squeezing press. Squeezed bends are used in the coils where the ratio between Radius of bend and diameter of tube is less than or equal to1.5 (R/D 1.5). The inlet and outlet end of circuit or coil is connected to inlet and outlet heaters correspondingly which will act as supplier and receiver for each stage. The Assemblies may have one or more than one circuits depending on the performance requirements of the boiler. Depending on mode of heat transfer the superheaters and reheaters coils are generally classified as Radiation or convection type and depending on location or Arrangement in the boiler this is further classified as Horizontal and Vertical (pendant) types. In both the Radiation and Convective types, the Horizontal or Vertical arrangement is used based on type of boiler. In a conventional type of boiler vertical and Horizontal superheaters are used with vertical or pendant Reheater. But in Box type units all superheaters and Reheaters are arranged Horizontally which will act as Drainable type.
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The vertical arrangement is simpler in supporting and allowing for expansion and this arrangement is called pendant type. Horizontal superheaters or Reheaters needs supporting of the tubes at multipoints to avoid segging and expansion movement should also be permitted with the advantage of draining. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 4 RADIANT SUPERHEATER/REHEATER: The reheaters or superheaters which can view the flame is called radiant type. The Radiant surfaces are kept at the top of combustion chamber and in our conventional units, only one Radiant superheater or Platen superheater are used. Since the projected area only is the design criteria for radiation heat transfer the radiant superheaters have tubes closely pitched along the flue gas flow which is called longitudinal pitching or spacing between tubes SL. The pitches across flue gas flow between coil assemblies is called as Transverse pitch ST which is wider in the case of Radiant superheater to reduce the velocity of gas and the bridging the surfaces by the ash and they are arranged in line fashion. Because the heat absorption of furnace surfaces does not increase in direct proportion to boiler output but at a considerably lesser rate curve of radiant superheat as a junction of load slopes down ward with increase in boiler output. In the case of typical 210 MW boiler the Radiant superheater is composed of 29 assemblies of dia 51 tubes spaced at 457.2 mm centers along the width of the furnance. Since the adjacent superheaters are closely arranged generally squeezed bends are used to form the coils CONVECTION SUPERHEATER/REHEATER: Convection surfaces are located at moderate flue gas temperatures and also kept in the rear pass. Since the total circumferential area (surface area) is the criteria for heat transfer the pitch between tubes SL along the flue gas flow will be wider to allow flue gas will flow around the tubes. The transverse pitch ST (pitch between assemblies) will be closer when compared to radiant superheater. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 5 In a conventional type unit the reheaters will be of pendant convection arrangement and also the final superheater. The superheater which is placed at lower flue gas temperature region in the second pass of the boiler will be of Horizontal convective type and it is generally called as low temperature superheater (LTSH). Since the tubes are very widely pitched along the flue gas flow the convective SH/RH. Since convection heat transfer rates are almost a direct function of output, the total absorption in the superheater increases with increase in boiler output. The control of combination radiant convection superheaters are relatively simple because of their compensating characteristics. The combination of these two superheaters is generally used in all utility units to give flat superheat curves (to maintain constant temperature) over wide ranges in load. The fundamental considerations governing Superheater design also apply to Reheater design. However the pressure drop in reheaters is critical because the gain in heat rate with the reheat cycle can be
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completely nullified by too much pressure drop through the reheater system. Hence steam mass flows are generally somewhat lower in the reheater. In conventional type units the reheater is composed of two stages or sections, the front pendant vertical spaced section and the rear pendant vertical spaced section. The rear pendant vertical spaced section is located above the furnace arch between the water cooled screen wall tubes and rear water wall hanger tubes. The front pendant vertical spaced section is located between the rear water wall hanger tubes and the superheater platen section. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 6 RELATIONSHIP IN SUPERHEATER/REHEATER DESIGN : Effective superheater/reheater design calls for the resolution of several factors. The outstanding considerations are: 1. The steam temperature desired. 2. The superheater surface required togive this steam temperature. 3. The gas temperature zone in which the surface is to be located. 4. The type of steel, alloy, or other material best suited to make up the surface and the supports. 5. The rate of steam flow through the tubes which is limited by the permissible steam pressure drop but which in turn, exerts a dominant control over tube metal temperature. 6. The arrangement of surface to meet the characteristics of the fuels anticipated, with particular reference to the spacing of the tubes to prevent accumulation of ash and slag. 7. The physical design and type of superheater as a structure. A change in any one of the first six items will call for a counter balancing change in all other items. Steam Mass velocity, steam pressure drop, and superheater tube metal temperatures are calculated after the amount of surface is established. The proper type of material is then selected for the component tubes, headers and other parts. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 7 The same general similarity exists between superheater and reheater considerations, but the reheater is limited in ruggedness of design by the permissible steam pressure drop. The outside diameter of reheater tubes will be bigger than that of superheater tubes as more volume is to flow through reheaters, operating at low pressures. Since the superheaters are at high pressure, their thickness will be higher than that of reheaters, superheater tubes normally vary from 44.5 OD to 54 mm OD whereas reheater tubes vary from 47.63 mm OD to as high as 63.5 mm OD. The thickness for superheaters goes as high as 10 mm whereas reheater thickness do not exceed about 5 mm. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 8
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MATERIAL CONSIDERATION: Oxidation resistance, maximum allowable stress and economics determine the choice of materials of materials for superheater and reheater tubes. The use of carbon steel is extended as far as these considerations permit. Beyond this point carefully selected alloy steels are used. The majority of superheaters and reheaters are made of low & high alloy steels. The steels commonly used for this application are shown in Table. Higher chromium content increase the resistance to scaling or oxidation. Stainless steels are also used to a limited extent wherever the skin temperature of superheaters or of wrapper tube which is exposed to furnace is made of stainless steel. Stabilised stainless steel has Niobium, Titanium, Cadmium, Tungsten etc. added in traces to the steel prevents carbide precipitation in the grain boundaries. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 9 1. Carbon SA192 SA106 Gr.B SA515 Gr.70 SA105 SA216 427 C Waterwalls. Fin welded Steel panels, Economiser. LTSH Lower bank (some portion) Waterwalls headers. SH Headers upto SHH9. Suspensions for plates. 2. Carbon SA209 T1 ----- SA204 SA182 F1 SA217 WC1 482 C Tubes are used for SH Radiant ½ % Mo roof, LTSH lower bank, LTSH upper bank, (some Portion), RH inlet. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 10 3. 1 % Cr SA213 T12 SA335 P12 SA387 Gr.12 SA182 F12 SA217 WC6 535 C Pipes are used for SH hdrs. ½ % Mo (Rod) SHH13. Plates for suspensions. Forged rod fot tie rod Suspensions & nozzles. 4. 1 ¼%Cr SA213 T11 ----- ----- SA182 F11 ----- 552 C tubes are used for LTSH ½%Mo upper bank, terminal tubes. 5. 2¼%Cr SA213 T12 SA335 P22 SA387 Gr.22 SA182 F22 SA217 WC9 577 C Used in platen SH, RH front
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1 % Mo & rear, Final SH pipes are used for final SH & RH outlet headers Plates are used For supporting purposes. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 11 6. 9%Cr. SA213 T91 SA 335P91 635 C Used in SH Platen,SH Final 0.25%Mo RH final coils. 7. 18%Cr SA213TP ----- SA240 SA182 ----- 704 C Tubes are used for wrapper 10%Ni &cb,Ta 347H Gr.347 F347 tube portion of platen SH. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 12 SPACERS FOR SUPERHEATER/REHEATER: Spacers are provided to maintain pitches along and across coil assemblies. The spacers must be able to do the function both in cold and hot conditions. The type of spacers generally used are transverse spacers and alignment ties. Transverse spacers are used to maintain pitch between assemblies ST. Fluid cooled spacers are mechanical spacer bar are used as alignment ties. Alignment ties are used to maintain pitch between tubes in the same assembly i.e., SL. Flexible connector and alignment band are used as transverse spacers. Flexible connectors in combination with fluid cooled spacers are used when the maximum average gas temperature exceeds 900 C in coal and gas fired units and 593C in oil fired units. Mechanical spacer bars in combination with alignment band are used at temperatures below these. The pendant spaced sections have lot of offset bends only to accommodate these small size spacers. Unless the spacers are small or cooled by medium they will be burnt at these higher temperature. Flexible connectors have one male and two female connectors welded to second and first tube as shown in Fig.3. These connectors will allow the tube to expand downwards during operation but at the same time they will maintain SL pitch between tubes. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 13 In the case of fluid cooled spacers, steam cooled spacer is generally used in all our natural circulation units. A tube from low temperature SH header (I utility boilers this tube is taken form Front SCW inlet header) will be taken and passed across the coil assemblies along the furnace width. Spacer plates are welded on either side of this tube in between assemblies to maintain ST which is cooled by steam
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flowing inside the tube. The entire tube will be resting on support lugs which are welded to tubes of coil assemblies as shown in Fig. After the spacer passes through all assemblies the same will be routed thro the gap available between assembly and side wall and it will be connected with low temperature SH header. Alignment band is a strip of 5 mm and height of Maximum 100 mm is wound all round the assembly as shown in figure and the ends are welded. Bar strips will be welded over the band in between tubes to maintain SL and the entire band will be resting on support lugs which will be welded with the tubes of coil assembly. Mechanical spacer is a scalloped bar which will be running across the assemblies over the alignment band and the first tube of alternate assemblies will be connected with this bar by U-Rod .The spacers are all made of stainless steel. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 14 SUPPORTS & SUSPENSION OF SUPERHEATER/REHEATER: The vertical or pendant superheaters or reheaters are suspended from the ceiling and horizontal coils are either self supported or supported by hanger tubes. In pendant SH or RH Assemblies, the tangent ties are welded in between tubes at the top row to transfer the load from center to end terminals on either side. The Horizontal superheaters are supported by Economiser hanger tubes thro tube saddles as shown in Figure 1. On either side of Eco Hgr. tube these saddles are welded along the length and the other side of saddles is welded to horizontal tubes of LTSH. These saddles provide very good spacer cooling from close tube contact and permits each horizontal tube to be picked up individually from the hanger tubes. The pendant superheaters and reheaters are separately suspended by high crown supports. High crown plates are welded on either side of the seal band which is already welded on wither side of the seal band which is already welded to the terminals tubes of coil assemblies as shown in Figure.4. The ends of high crown plates will be welded to end plates. So the load available at terminal tubes will be transferred to end plates by high crown plates. The end plates will be suspended from the ceiling by means of tie rod assemblies. The headers which are required for supports will be independently suspended from the ceiling thro tie rod assemblies. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 15 SUPERHEATER WALLS: In power boiler, the horizontal and rear passes are covered by superheater walls just like furnace is covered by superheater walls just like furnace is covered by waterwalls. But the superheater walls are fin welded whereas water walls are fusion welded. Fin welded walls have flats welded in between tubes rigidly and the walls are gas tight. The fin welded walls are an improvement over skin cased enclosures where the walls are peg finned. The radiant roof will be of peg fin welded construction where these small pieces of flats are welded along the length of each tube with gap and there will be gap in between
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adjacent tubes also these gaps are covered by castable refractory. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 16 COMPONENTS OF SUPERHEATER/REHEATER 1) Super Heater / Reheater circuits 2) Roof seal band 3) Heat shields 4) Connectors (i) Flexible Male connectors (ii) Flexible Female connectors 5) Lugs a. Spacer lug b. Support lug c. Lifting lug 6) Flats 7) Welding bifurcate DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 17 INPUT REQUIRED: (A) BPP transmittals: CPT 1301 - SH, RH & Economiser Arrangement Data (Ref :Annex:A) CPT 1501 - Tubing list of RH,SH, Economiser(Ref:Annex:C) (B) PPA drawing: General Arrangement Drg Sectional Side Elevation – Upper/First pass (C) Applicable standards (if any) of Roof seal band Flexible male/female connectors
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Lifting Lug Spacer Lug Support Lug DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 18 VARIANT OF REHEATER: (i) No of stages ( single or Double) (ii) No of elements (iii) Usage of connectors or Alignment bands VARIANT OF SUPERHEATER: In a 500 MW Boiler, Superheating of steam takes place in 3 stages: i) Low Temp Superheater (LTSH) ii) Divisional panellete iii) Final Vertical platen Superheater In a 250 MW Boiler, Superheating of steam takes place in 3 stages: i. Low temp Superheater (LTSH) ii. Pendant platen Superheater iii. Final Vertical Spaced Superheater DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 19 1) SUPERHEATER COILS: The material specification, no of circuits, design temperature and pressure are obtained from the BPP transmittal – CPT 1501 and CPT 1701 gives the rough sketch with welding bifurcation and location with respect to roof. The position and location constraints are obtained from the PPA – GA drg. The design of the individual reheater circuits are done based on the BPP transmittal which gives the tubing list along with the different material specification that should be used, along with the approximate length of each specified tube. The designer shall ensure that material transistion doesn’t occurs at the bend of the tubes. The minimum Bend Radius that is feasible in shop based on the tube diameter. For Choosing Coil Bend Radius use ‘List of Rotary Bending Tools’ (Refer Annex :F).
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2) ROOF SEAL BAND: Roof seal band are provided with the coil assembly for the purpose of transferring the load of the coils to the roof girders. It transfers the entire load of the coil through the Hi-crown plate, which is welded with the End bar. The end bar transfers the load to the ceiling. Dimension of the roof seal band and hi crown support is determined by the load carried by the reheater coil, which is given by the Stress Analysis dept. The distance between the end bar are governed by the overall size of the reheater coils and this is given in the PPA drawings. The roof seal band varies with the diameter of the coil tube, pitch distance between the two circuits. The plate width is governed by the entire assembly load. DESIGN STANDARDS OF SH/RH COILS & HEADERS FOR 500 MW PRESSURE PARTS/PRODUCT ENGINEERING 20 Inputs from BPP to LAYOUTS: Drawings Proposal drawing Scheme of water and steam Scheme of air and gas path Scheme of pulverizer system Scheme of oil system Basic scheme of spout cooling water Secondary air fan and tertiary air fan selection Piping for GL/BL system
Transmittals Boiler parameters Special operating conditions Soot blower selection data Furnace and back pass sketch Wind box selection data Mill selection data Feeder selection data
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Fuel pipe data SH, RH and Economizer arrangement data Air heater selection data SH, RH and Economizer arrangement sketch SCAPH selection data Duct design data FD fan specification ID fan specification PA fan specification GR fan specification Header and piping list for SH, RH and Economizer Header and piping list for circulating system Safety valve selection data Pent house cooling fan selection data Flow measurement device characteristics and data Contact data sheet ESP ash tank data Pressure part arrangement group prepares the detailed drawings of the Pressure Parts of boiler. Pressure parts include steam and water circuits starting from the Economiser inlet header to the Main Steam supply line and Reheater Assemblies. These drawings will be useful for manufacturing and erection purposes.
Inputs: BPP Transmittals PPA sketches Layout drawings Contract specification and scope
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Proposal BOM from BPP
Activity Sequence Preparation of applicable PGMA Preparation of PP specification review list Preliminary strength calculation Preparation of material forecast IBR fees payment based on heating surface area of boiler For NTPC contracts, a pressure parts schedule is prepared DDR as per CCST in theory Inputs : BPP transmittal PPA drawings For new designs Coil arrangement drawings are drawn to scale for verifying the inner tube allowances, interferences, and geometries. For industrial boilers, type of drum internals, screen dryer or turbo separator etc may vary During DDR, design of vertical coils is interlinked with design of suspensions for the same Pressure parts and their corresponding supports are released together Sequence is usually Drum Top header Upper belt of panels Roof panels Middle portion Lower panels including re heater Bottom headers IBR submission Erection welding schedule preparation
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Unit material diagram preparation
Various Drawings prepared by this group: General arrangement of pressure parts Expansion movement diagram Plan over headers and links of furnace Ceiling loading plan
Various Schemes supplied for the execution of above Scheme of boiler water and steam with valves and fittings and instrumentation Scheme of boiler water circulating pumps with valves fittings and instrumentation Scheme of SH and RH system with valves fittings and Instrumentation Scheme of soot blowing system with valves fittings and Instrumentation
Stress Analysis Stress analysis group carryout the stress analysis for following components Drums and hanger rods Hanger tubes Ceiling loading Pressure parts hanger rods Header lugs Furnace guide loads Horizontal tube support spacing Header nipple flexibility VLH and CLH Roof tubes supports Flexible connections Steam cooled spacer
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Height of header above roof Strap type support PANTO support Bent tube lugs U-rod supports Collector channel Washer plates Thermal expansion movements MS support lines and links Drum rocker settings Allowable deviations in tube bends Vibration sobers Fin width Fillet weld designations Eco inlet header support
2.2.3 Boiler Mountings Inputs: Contract scheme of water and steam circuit from BPP BPP Transmittals GA drawings Instrumentation required from C and I Contract and tender spec PPA drawings Dished end tappings for Drum
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This group prepares the following layouts. instrumentation riser tubes. After preparation of the schemes, valve schedule will be provided. Scheme of boiler water circulating pump with valves, fittings and instrumentation Scheme of soot blower piping with valves, fittings and instrumentation. Collection of water and steam mixture raised through water walls and Scheme of water and steam circuit with valves, fittings and instrumentation
Scheme of super heater and re-heater steam circuit with valves, fittings and
Apart from the schemes mentioned above, this group involves in designing, quoting and procurement of sub delivery items like Control valves Circulating pumps
NTPC approved vendors for procurement of circulating pumps are Hayward Tyler, U.K. and KSB, Germany. Apart from the above two BHEL approved Torishima, Japan for circulating pumps. Transmittals will be sent from BPP about pump characteristics, control valves and block valves. Based on the transmittals received this group orders for the pumps and control valves 2.2.4 Ducts and Damper This group deals with Ducts Dampers Guillotine gates Expansion joints Duct support Flow measuring device Guide vanes
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The ducts are designed for flue gas, secondary air, and primary air circuits. Dampers are used when 100% leak proof is required. It is only a on/off system. Guillotine gates are big in structure so cannot be used inside the furnace. Ducts are made of IS2062 carbon steel. Its thickness will be 4 mm to 8 mm and the expansion rate is 1mm/100°c/1m.To absorb thermal movements, expansion joints are provided. Three types of duct support are there bottom support, top support, and restraints to restrict the expansion movement in the desired direction. Flow measuring devices like Airfoil in secondary air and venturimeter in primary air. Guide vanes are provided to distribute the flow The following loads are considered in the Design the Ducts. Dead load Live load Pressure load Ash load Wind load
Dampers are used for isolation. It gives an efficiency of 98.5%, but by providing Guillotine gate one can achieve 99.98% efficiency. Generally in ID Fan, guillotine gates will be provided. In case of space restriction, dampers will be used. Location of guillotine gate: PA Fan outlet ESP inlet ESP outlet ID Fan inlet ID Fan outlet Hot air mill inlet Cold air mill outlet
Types of dampers Biplane damper Louver damper
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Location of dampers: FD Fan outlet SAH air inlet SAH air outlet PAH air inlet PAH air outlet AH Gas inlet
Types of expansion joints: Metallic joint Non-metallic joint
2.2.5 Fuel Systems Firing Systems
Coal Firing Systems
Tangential Firing system Wall firing (turbulent/vortex burners) Direct firing/Indirect firing Stoker firing Fluidised Bed combustion
Tangential Firing System:
In tangentially fired boilers, four tall windboxes (combustion air boxes) are arranged, one at each corner of the furnace. The coal burners or coal nozzles are located at different levels or elevations of the windboxes. The number of coal nozzle elevations are equivalent to the number of coal mills. The same elevation of coal nozzles at 4 corners are fed from a single coal mill. In some designs one mill feed two elevations of burners. The coal nozzles are sandwiched between air nozzles or compartments. That is, air nozzles are arranged between coal nozzles, one below the bottom coal nozzle and one
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above the top coal nozzle. If there are 'n' number of coal nozzles per corner there will be (n+1) number of air nozzles per corner. The coal fuel and combustion air streams from these nozzles or compartments are directed tangential to an imaginary circle at the centre of the furnace. This creates a turbulent vortex motion of the fuel, air and hot gases which promotes mixing, ignition energy availability and thus combustion efficiency. The air nozzles in between coal nozzles are termed as „Auxiliary Air nozzles‟, and the top most and bottom most air nozzles as „End Air Nozzles‟. For a boiler equipped with 10 mills, (or 5 double ended tube mills), the coal nozzle elevations are generally designated as A, B, C, D, E, F,G,H,J and K from bottom to top, the bottom end air nozzles as AA and the top end air nozzle as KK. The auxiliary air nozzles are designated by the adjacent coal nozzles, like AB, BC, CD, DE , EF, FG, GH, HJ and JK from bottom to top. The four furnace corners are designated as 1, 2, 3 and 4 in clockwise direction looking from top, and counting front water wall left corner as '1'. Each pair of coal nozzle elevations is served by one elevation of oil burners located in between the auxiliary air nozzles.ach oil gun is associated with an ignitor arranged at the side. Combustion Air Distribution: Of the total combustion air, a portion is supplied by primary air fans that goes to coal mill for drying and carrying the pulverised coal to the coal nozzles. This primary air flow quantity is decided by the coal mill load and the number of coal mills in service. The primary air flow rate is controlled at the air inlet to the individual mills by dampers.
The balance of the combustion air, referred as Secondary Air, is provided from FD Fans. A portion of secondary air (normally 30% to 40%) called 'Fuel Air', is admitted immediately around the coal fuel nozzles (annular space around the casting insert) into the furnace. The rest of the secondary air called 'Auxiliary Air' is admitted through the auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD Fan inlet guide vane regulation. The proportioning of air flow between the various coal fuel nozzles and auxiliary air nozzles is done based on boiler load, individual burner load, and the coal oil burners in service, by a series of air dampers. Each of the coal fuel nozzles and auxiliary and end air nozzles is provided with a louver type regulating damper at the air entry to individual nozzle or compartment.
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Wall firing The desired intensity and completeness of pulverized fuel combustion in the furnace space can be achieved through the proper supply and intermixing of pulverized fuel with
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secondary air in a burner assembly. The intermixing in the furnace space is ensured mainly by an appropriate arrangement of burners on the furnace walls and by providing a particular aerodynamic pattern of jets in the furnace space. There are two main types of burners, Straight flow burners: Burners of this type turbulise the air flow less substantially and produce a long ranging jet with a low expansion angle and weak intermixing of the primary and secondary flow. It may be fixed or tiltable burners which facilitate combustion control. They are mainly employed with high reactive fuels.- is this necessary? Turbulent Burners: In this burner, dust air mixture and secondary air are fed as whirled (turbulized) jets which form a cone shaped expanding flame in the furnace space. It has circular cross section. It can be used with any kind of solid fuel but are used mostly widely for low volatile grades.
Stoker Firing: Among renewable energy sources bio - mass fuel occupy an important place. Bagasse, the bio-mass fuels from Sugar mills, is being utilised for generating steam and power. As sugar cane cultivation is seasonal, availability of bagasse is not ensured throughout the year. To overcome this , the generating units are operated using bagasse whenever available and for the rest of the periods the unit can be operated with other fossil fuels like coal or with other bio-mass fuels like bark, peanut husk, coconut shell, wood chips etc These bio-mass fuels are efficiently burnt in stoker grates, which release heat for steam generation. There are a variety of stoker grates viz., inclined grates, vibrating grates, roller grates, dump grates and travelling grates . The choice is based on their capacity and application. Travelling grate stoker is unique in its design and is capable of burning variety of bio-mass fuels as well as burning coal . The travelling grate stoker has a firing bed which moves continuously between two sprockets . Fuel starts burning at one end, and is discharged at the other end as ash. Hence the travelling grate stoker is otherwise called as continuous ash discharge stoker (CAD) There are two types of travelling grate stokers viz.,
Non -catenary type Catenary type
Oil Firing Fuel Oil Atomisation 35
Learning modules material- Engineering (FB) Atomisation is the process of spraying the fuel oil into fine mist, for better mixing of the fuel oil with the combustion air for efficient combustion. While passing through the spray nozzles of the oil gun, the pressure energy of the fuel oil is converted into velocity energy, which breaks up the oil stream into fine particles. Poorly atomised fuel oil would mean bigger spray particles which takes longer burning time resulting in carry over and makes the flame unstable due to low rate of heat liberation and incomplete combustion. Other than pressure, viscosity of the oil is the major parameter which decides upon the atomisation level. For satisfactory atomisation the viscosity shall be within 15 to 20 cst. Fuel Oil Pump & Heaters
Pumping the oil is a major preparatory work on fuel oils for atomization and burning. The fuel pumps used in the fuel pumping house are of positive displacement type. A positive displacement pump causes a liquid to move by trapping a fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge pipe. Normally screw pump is used for pumping the fuel oil up to required pressure. The screw pump is compact in design and more silent in operation compared to the gear pump and vane pump. The screw pump can be operated at high speeds (around 4500 RPM) compared to the gear pump and vane pump. So, it can be directly coupled to the driving motor. Also it can develop very high pressure i.e. 150000 kpa, whereas a gear pump can develop pressure upto 1000 kpa and the vane pump can develop pressure up to 70000 kpa. The main advantage of screw pump is that it is vibration free and hence a smooth pulsation free delivery of oil is possible. Heaters: The viscosity of fuel oil at atmospheric temperature is very high. In other words the pouring temperature of fuel oil is higher than the atmospheric temperature. So to maintain the flow ability fuel oil heaters are used. There are two types of heaters namely steam coil heater and electrical heater. In steam coil heater steam is allowed to pass through the coil to heat the fuel oil up to the required temperature. In electrical heaters the heat energy induced by the electrical supply to the coils is used for heating the fuel oil.
Fuel oil strainers Fuel oil strainers or filters are essential to prevent the mechanical impurities reaching the small clearance and intricate passages in the screw pump. HFO suction strainers are provided with SS mesh of 500 microns filtration and LFO suction and HFO discharge strainers are provided with SS mesh of 250 microns filtration.
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Learning modules material- Engineering (FB) When one of the strainers is in line, the other is serviceable; when the pressure drop across the operating element exceeds the set value, the changeover may be effected by opening and closing the isolation valves. Suitable alarms are provided to indicate such clogged filters. Before such changeovers it is necessary that the standby strainer is filled with oil to avoid air locking and for smooth continued running of the pump. The clogged strainer should be immediately cleaned in solvent, wiped and air blown and replaced in position for ready availability. Strainer and strainer baskets must always be clean. Air Cooled Oil Gun: The atomiser assembly of an operating oil gun is protected from the hot furnace radiation by the flowing fuel oil and steam, which keeps it relatively cool. Once the burner is stopped, there is no further flow of oil or steam. Under such situation, it is required to withdraw the oil gun from firing position or provide some other means provided, in order to protect the atomizers from damages due to overheating. Certain designs employ stationary oil guns and utilize cooling air to keep the guns protected from heat. Before stopping the oil burner, the oil gun is scavenged with steam to keep the small intricate passages of the atomiser parts clean. Oil gun is always scavenged with associated igniter in service, to burn out the oil.
This group involves in Material Handling Coal firing Oil system Feeders and Handling equipment Air system
Inputs required: Scheme of pulveriser system and instrumentation Scheme of oil system GA of boiler Fuel pipe diameter Mill selection data from BPP
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Windbox selection Furnace sketch Key plan of boiler GA of mill Vertical bracing arrangement
There are two types of fuel systems used for Fossil boilers Gravimetric Feeder Volumetric Feeder
In the case of gravimetric feeders weighed coal will be sent to the boiler. In the case of volumetric feeder, rather than belt conveyer and weight measuring system, volume-measuring system will be adopted. The schematic diagram of the gravimetric feeder is as shown in figure followed by its working. 25mm crushed coal will be supplied to raw coal bunker Raw coal bunker outlet will be connected to feeder by means of coal valves and downspout Two no. of coal valves will be used, one for isolation and other for control Isolation valve is controlled manually, whereas flow regulating valve is motor controlled. Height of the bunker above the feeder will be such that seal air sent to feeder must not escape through the downspout. Coal reaching the feeder will be conveyed using belt drive Coal weight measurement system will be provided in between to weigh the coal. Weighed coal will be sent to the mill through the feed pipe.
The schematic diagram of the gravimetric feeder is as shown in figure.
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The schematic diagram of the Volumetric Feeder is as shown in figure.
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Disadvantage associated with volumetric feeder against the cost advantage is, the heat-input measurement in terms of volume for solid fuels is inaccurate, as there will be improper filling in specific volume. For handling chemical ash in the case of chemical recovery boilers, drag link chain feeder will be used, as the chemical ash is highly corrosive. In case of industrial boilers, drum type feeders will be used. This is because of the high bulk density, high moisture content and fibrous nature of biomass-based fuel.
Oil Firing scheme For the start-up of the boiler LDO and HFO will be used. Oil support to the boiler will be continued till the load reaches 30% and above this oil support will be withdrawn. Typical arrangement diagram for oil system is as shown in the following diagram.
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As the heavy fuel oil is highly viscous, there is need for heating. This results in fewer chances of choking of fuel line. Apart from above, this group also involves in handling equipments like Fan Circulating pumps Air heater and SCAPH Mills Pressure parts (LTSH and ECO) Furnace maintenance platform
2.2.6 Lining and Insulation Wherever the temperature of components is more than 60oC, insulation needs to be provided in all those areas in order to prevent the accidental damage to men and material working in the area. Various components in which insulation is provided are as follows Insulation: Insulation is the material which resists the heat flow from one medium to other. Types of Insulation Materials: Furnace bottom Rear arch Roof tubes Second pass header Furnace first pass & second pass Tubes and pipes >60°c Fuel lines Down comers, economiser coils, oil lines Main steam line Enclosure area
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Lightly Bonded Wool Mattress Rock wool Slag Wool Glass Wool
This material is used on water walls, enclosures, ducts, pipes, oil lines and EP. Service Temperature: 550 C & Density: 100 Kg / m3. Pourable Insulation This material is used in pent house roof deck and water wall Buckstays. Service temperature 650oC & density 650 Kg /m3. Calcium Silicate Slab
This material is used on water walls, Ducts, and Pipe lines. Service temperature 600oC & density 350 Kg /m3. The other insulation Materials are Lining : Lining is the material which can withstand high temperature. Different types of lining, their usage, temperature upto which they may withstand the heat and their densities are as follows. Insulating Bricks. Asbestos Mill Board Asbestos Rope.
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.
2.2.7 Boiler Supporting Structure Introduction A boiler is made up of complex masses either hanging or ground / floor supported. The hanging mass is primarily pressure part system consisting of components like Drum, Water wall, Super heaters, Repeaters, Economizers and these masses will be supported at the top most ceiling level of boiler through a ceiling grid consisting of main girders, cross girders and intermediate beams. The boiler structure also supports other equipments like Air heaters, Hot and Cold air ducting, Fuel pipes, Soot blowers, Critical pipes and also takes care of loads due to platforms provided for access and maintenance purpose. Both pressure part and non pressure part system contribute vertical load due to mass and horizontal load due to pressure, expansion etc. Further a boiler structure is subjected to dynamic loads like wind/seismic forces. A boiler supporting structure primarily does the function of supporting all the above masses for vertical as well as horizontal forces and safely transfers the load to foundation. The boiler structure is analyzed for all the above loads and sized using advanced software packages.
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Structural Arrangement and Load Transfer Mechanism: A boiler supporting structure is a steel structure of complicated arrangement consisting of girders, cross girders, intermediate beams, columns, beams, vertical bracing in both longitudinal and transverse directions along with horizontal floors and bracing system at different levels. The boiler structure primarily does the function of supporting the loads due to all the boiler equipments and platforms /floors (vertical) as well as wind and seismic loads (horizontal) and transfers the loads to the foundation. It acts in a composite form/ integrated form to resist all the induced forces on the structure. Since the basic pressure part system is hanging and subjected to downward thermal expansion the horizontal load transfer from pressure part to main boiler structure is affected by means of boiler guides. These guide forces either due to stability or due to wind/seismic will be transferred to boiler main braced levels and horizontal bracings at these levels will transfer these guide forces further to the vertical bracing planes, which in turn will transfer the force to ground. For seismic analysis of boiler structure, dynamic analysis using response spectrum method as per code IS 1893:2002 is performed. In short, all vertical loads will be transferred to columns through beams/ girders or directly like Airheater loads and the vertical load from columns are transferred to foundations through suitable base plates. The horizontal load acting on the boiler at various locations will be picked up locally by horizontal bracing of main braced levels, which will be transferred to vertical bracing nodal points at those elevations. These vertical bracings will transfer the horizontal load in X-X (along the axis of boiler/longitudinal) and Y-Y (transverse/perpendicular to boiler axis) directions to the column base. Counter balancing couples of reactions at column bases will resist the external moment created by the horizontal forces at the column base. The base shear will be transferred to the foundation through suitable shear lug provision at the bottom of base plate. All the mainframe analysis, ceiling analysis, Main Brace Level (MBL) / Horizontal Floor analysis etc., are accomplished with the help of computer programs “like in house program Anchoring System, MAIN, FLAT and bought out program like STAAD PRO”. Analysis of main boiler structure can be done in two ways namely 2-D or Plane Frame Analysis, 3D or Space Frame Analysis. The complete boiler structure is analyzed in 3 to 4 separate 3D structural models i.e., Main Boiler structural Frame, Boiler ID system frames like Before ESP, After ESP and Near Chimney. The boiler structural frame will consist of columns, beam, vertical bracing and sometimes horizontal bracing are also included for the analysis.
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Methods of Design: The column / bracing sizes taken from the STAAD output file “Member Selection” are altered to suit the available raw material inventory using program like “ALDESIGN” and “Opt-Plus”. The beam sizes will be designed by taking the forces from the STAAD output file “Member forces “and also the force actually coming in the beam like Floor & Duct load etc,. For Ceiling Structures separate IN HOUSE DEVELOPED PROGRAM will be used to prepare the input file and analyzed in the STAAD.
Engineering transmittals and material forecast: The material forecast will be prepared based on the above and send to Material Planning for the purpose material procurements in advance. The Engg Transmittals are prepared incorporating the final sizes of the members. Based on the transmittal the fabrication, the erection drawings and documents are prepared using the IN HOUSE DEVELOPED DETAILING PROGRAM (SCAD) & AUTOCAD. All foundation analysis, sizing of members for boiler supporting structure, ID structure and lift structure is done through computer programs which enables an accurate and optimum design in the least possible time with error free calculation. Timely release of foundation drawings and design documents are possible due to implement of software in each stage of analysis and design.
Sequence of Design Arrangement of vertical bracings and MBL Preparation of foundation loading table and anchoring detail for main boiler Design of Buckstays and preparation of transmittal DDR for Buckstays through program DDR for galleries and stairs Modeling in STAAD, PRO and analysis Submission of foundation drawings to customer Collection of boiler GA drawings, key plan, floor extension sketches, and vertical bracing arrangement drawings from layout.
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Collection of vertical chunk load data from sections of PE Calculation of live and dead load Calculation of the wind load Transfer of PP furnace guide loading to MBL Modeling of column , beams, vertical bracings, member releases, member properties and support Addition of vertical load to the model Stabilization of model with wind loading Analysis of model of various load combinations Perform dynamic analysis Design of interconnecting platforms Lift structures Economizer handling structures Mill platform Mill handling structures Machine room structure
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Flow chart representing the input and output of Structural Design
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Weight Breakup of Boiler Structural 42% Pressure Parts 26% Ducts 14% L&I 10% Fuel Firing 6% C&I 1% Handling system 1%
Comparison of Boilers (Typical) Capacity (MW) Flow (TPH) Weight (MT) PGMA (No.) No of DU 500 1675 31000 820 25000 250 810 14700 640 18000 210 700 12000 620 16000 120 390 7800 550 10000 60 160 4100 300 5000
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Learning modules material- Engineering (FB) 2.3 Information Technology Systems And Services
The main functions of the department are Develop programs for Engineering cycle time reduction Engineering process automation BCR monitoring system Erection drawing viewing system Collaborator documents transfer system Development of Dolphin drawing management system Re- Engineering technical engineering design fortran programs Operations and Maintenance manual management system Plant modeling
The works dealt till now are Developed a comprehensive O&M Generation and Management System covering aspects from preparation to despatch featuring online viewing of O&M Manuals and easy status tracking Automated PDF document generation and successful deployment in CPT and BCR systems Enhanced security features in online viewing of Dolphin drawings Implemented BCR monitoring system Developed CPT workflow system with extensions to general engineering transmittals Deployed Design Directives, Technical Information and CIP‐Contract Information package in quest Automated generation of 25 CPTs Enhanced Windows‐based Integrated IBP/AH/Mill programs and link programs for selection of Auxiliaries Completed IBP Input & Output storage to database for integration with CPT generation systems Developed programs for ESP Selection, Chimney Selection, Duct Sizing, Drum Length calculation, OCF calculation Heating Surface Area calculation for 500MW boilers and Fan Selection
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Implemented Single‐stroke multiple load IBP Input Generation system Developed C&I proposal document generation system that generates all the 8 documents Implemented AIX‐based systems, NOWA/WANSYS, RHBP, Metals, Filmx and Supernova for super‐critical boilers on temporary server Quest, an enhanced Technical Information Gateway to access all online engineering applications, including technical reference information with built‐in access control Provided facility for viewing and downloading of Erection Drawings Tender Documents online reference facility was developed & Implemented Developed an online PIR management system to manage developmental projects Implemented CIB System for preserving and accessing CIB documents online. ASTM, BIS, BS, IEC standards made online Facilitated Tour Management System for FES General Arrangement drawing extracted from PDMS model of Budge Budge project: Boiler sectional side elevation Pressure parts arrangement Layout of cold air ducting plan and elevation Marks a major milestone in plant modeling
2.4 Controls and Instrumentation Department of Controls and Instrumentation (C&I) has 5 groups as follows Project management (PM/C&I) Product engineering (PE/C&I) Field engineering services and renovation and modernisation (FES, R&M) C&I Centre Quality assurance and control (QAC/C&I)
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Department of C&I mainly look after all the controls required for the boiler. A few of the controls are designed and fabricated in BHEL, Tiruchirapalli and the rest are outsourced.
C & I Centre Various Instrumentation getting assembled and tested at C&I centre is Electronic water level indicator (BHEL VISION 20M) BHELMHO level switch Flame scanner head assembly Acoustic steam leak detector Gravimetric Control
Electronic water level indicator is the indigenously developed technology to know the drum level. It works on the principle of difference in the electrical resistance of the steam and water. Steam will have the electrical Resistance in Mega Ohms whereas the same for the water in Kilo Ohms. By measuring the resistance signal will be passed to activate either green or red bulb to indicate the water or steam. Logical checks for the steam below water and water above steam also provided to get the correct signal. System which is operating with 24 V DC supply got certified by the Canadian Standards Association and 230 V AC supply system is under the process of certification. BHELMHO Level Switch will be used in HP Heaters and condenser drying parts. Acoustic steam leak detector works on the principle of steam leaking will have some sound with some frequency. Based on this sound level it will be found whether steam leak is occurring or not and if it is, at what location, such that it can be rectified during the maintenance or annual shutdown. This reduces the damage by preventive action Flame scanner head assembly will be used to detect the flame inside the Combustion chamber and to transfer the signal to FSSS. It works on the principle of intensity discrimination and flicker frequency discrimination. Intensity discrimination decides based on the quantity of light signal received and flicker frequency discrimination is based on that there will be flicker if coal burns in the boiler. Flexible fibre optic cable will be used to tilt the head as coal nozzle tilts and to transfer the light signal.
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C&I -Product Engineering (PE /C&I): Product engineering activities consist of detailed engineering of activities pertaining to C&I area. This group designs the furnace safeguard supervisory system. Furnace safeguard supervisory system is the systems, which consist of control systems for all electrical components such that plant works healthy. It contains the facilities to trip the boiler or particular equipment at any abnormal operation. It also contains the soot blower control system, secondary air damper control system and burner management system. FSSS manufactured by EDN, Bangalore. Apart from the design of FSSS, this group carries out the procurement of Instruments required.
C&I -Field Engineering Services FES (C&I) activities start with the activities of FES (FB). Site action request and commission action request is the ways of giving complaints. The FES (C&I) group will deal complaints pertaining to commission action request. PG Test and pending work, which will be raised by site or customer, will be taken care off. Responsibility of training the customer for operating the plant will go to FES (C&I).
C&I- Quality Control and Quality Assurance Quality assurance prepares the plan for various items prepared at BHEL. Checking or inspection carried out at various stages like raw material inspection, in-process inspection and final product inspection. There are 4 types of quality plans viz., standard quality plan, vendor quality plan, contract quality plan and reference quality plan. Quality control is the post quality activities of the purchase order. Activities performed by C & I group Proposal Activities : Technical offer preparation, bill of material preparation, marking of instruments in p & id’s, filling up of data sheets, engineering support for obtaining budgetary offer for new items, attending proposal meetings with clients Digital controls boiler interlocks Burner management system Soot blower control Analog controls drum level control Main steam pressure control Main steam temperature control Furnace pressure control
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Cbd tank level control Local instruments Level measurements Direct level gauge ,electronic water level indicator, transmitters Preparation of specifications and Indenting. Pre bid discussions. Evaluation of offers. Technical recommendation. Scrutiny of vendor documents
C&I PRODUCTS & THEIR INTERFACES WITH BOILER CONTROLS” 1) INTRODUCTION 1 2) C&I PRODUCTS 2 GRAVIMETRIC FEEDER 3 BHEL SCAN 12 BHEL VISION 16 BHELSONIC 18 ELECTROMATIC RELIEF VALVE 21 FURNACE TEMPERATURE PROBE 23 INTRODUCTION Every process available with and used by Us were created with an objective and to serve purpose. While doing the same the process should be in our control. Uncontrolled process often end up in creating harms. The process when controlled manually deprive lot of man power and are less accurate in measurements. The solution for providing a efficient control system is to automate the same and having Soft controls to control the same. Every process created by us need to be monitored for the datas provided by the process. When the datas are plenty it becomes less possible to track its
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Learning modules material- Engineering (FB) performance. Automation provides features to have these datas stored and retrived when necessary. When a process is at risk or at saturated level the conditions need to be communicated for further proceedings to be made with respect to the behavior of the process. The speed of communication is a highly necessary factor when the process is most critical. Automation has also provided Indication and Alarm features which reach the process owners in the same instant of risk formation. C&I PRODUCTS The Instruments or systems supplied in the Boiler serve any of the following purposes like Indication, Control, Quantisation, Alarm generation or Interlock. The In-house products developed by C&I like BHELFEED, BHELSONIC, BHELVISION and BHELSCAN also serve these purposes. BHELFEED controls the coal flow, Quantize the amount fed into the boiler, creates alarm and trip interlocks for malfunctioning observed. BHELVISION indicates the drum water level and also used for Drum level control. The same system also creates Alarm and trip conditions for the starvation and over flow of the drum level. BHELSONIC indicates the Boilertube leaks and creates alarm that are useful in preventing damages to adjacent tubes. BHELSCAN indicates the flame availability and the distinguishes the fuel causing the flame. It also creates Boiler trip when the fuel injection is available but the flame is not sensed. ERV, Electromatic relief valve is a supplementary valve to relieve Boiler pressure in addition to Safety valve. It avoides frequent opening of Safety valve. FTP, Furnace temperature probe is used to indicate the boiler furnace tem perature during Start up. GRAVIMETRIC FEEDER GENERAL ARRANGEMENT OF A GRAVIMETRIC FEEDER The path of the coal flow is from the Bunker Outlet to the Mill and the Gravimetric feeder regulates the flow of the coal in the middle. The purpose of Feeder is to control the speed of the belt according to the fuel demand and hence controlling the Coal flow into the Mill. The
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Learning modules material- Engineering (FB) general arrangement of a feeder has various parts both Mechanical and Electrical for the efficient functioning and data transmission purpose.
FEEDER GENERAL ARRANGEMENT The Gravimetric Feeder weighing system consists of two load cells to measure the mass on belt. Load cells measure the change in resistance in strain gauges to measure force. The control system of gravimetric feeder consists of two electronic assemblies. One mounted along with the feeder and the other a remote power panel at the control room. The electronic assembly mounted to the feeder will have signal conditioning modules for load cell and tachometers, panel mounted rotary and push button switches, lamp indicators for feeder status. The remote power panel will have all necessary power circuits, controller etc.. The controller is a 32 bit embedded processor of Pentium architecture. A 32 switch membrane key panel and two row vacuum fluorescent display of 40 characters each, are provided on the panel door for operator interaction. All the I/O modules are intelligent, EURO standard rack mounted. The controller works on a real time operating system. The system contains the 32- bit microprocessor, memory, digital circuits and a keyboard display etc. All I/Os are optically isolated with respect to processor and its associated circuits. Analog 55
Learning modules material- Engineering (FB) circuits used to amplify and convert the load cell outputs are located separately in the feeder local panel. The display / keyboard located on the Remote panel door provides the means to communicate and receive information from the processor, for parameter setting, mode selection etc. The display / keyboard assembly consists of a vacuum fluorescent display for written communication, in addition to the display of process numerical data. The vacuum fluorescent display consists of two lines with 40 characters each, which indicates totalized weight, feed rate, motor RPM, material density etc. In addition, 9 LEDs serve as system status indicators. The keyboard consists of 32 keys with various functions / operations. Since the feeder control system employs advanced digital processing with 32-Bit microprocessor, the control gives better weighing accuracy and better repeatability as compared to analog electronics based controller.
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OPERATIONAL DETAILS: The MPC based feeder control receives signals from the two load cells, which represent the weight of coal acting on the weigh span. Also the system receive two speed signals from two independent speed sensors (tacho-generators) which represent the speed of the feeder belt. The system is designed to automatically switch to the secondary tacho-generator if the primary tachogenerator fails. The application software computes the feeder delivery as follows: a measurement is taken of the output of one load cell. This signal is converted in to a digital signal with high resolution. This value is compared against parameters stored in temporary memory. The same operation is then performed on the other load cell. The signals are validated and if it is invalid, the feeder is switched to volumetric operation and the controls use a simulated load cell output generated from historical average value stored in the memory. If the signals are valid, the two load cell output signals are summed and the tare is subtracted. The result is multiplied by a calibration factor determined during calibration to arrive at weight of material per unit of belt length. The speed signal from the tacho-generator is multiplied by another calibration factor to arrive at a number representing belt travel speed per second. Subsequently the belt speed and weight signals are multiplied together to arrive at the feed rate. The result is then compared to the demand feed rate to determine the error and to vary the feeder belt speed through the variable frequency drive. The key board with display unit provided on the front door of the remote control panel facilitate user friendly communication which clearly communicate text messages to the operator during parameter setting which are project specific (during initial start-up), calibration, self check and self diagnostics. The display unit has 2 lines of 40 characters each with Vacuum Fluorescent Display. The embedded software is suitable for controlling the gravimetric feeder in real time. The mode of operation and parameters are entered through keyboard and the Vacuum Fluorescent Display unit show the status of operation and indicate the values like Feed rate, total coal delivered, motor speed, coal bulk density etc. The system provides four numbers of 4 - 20 mA DC analog feed rate feedback signals to DDCMIS. In addition, one pulsar unit is provided with two pulse outputs – one for customer use 57
Learning modules material- Engineering (FB) and the other for remote integrator. (For every 100 Kg. of coal delivered, the pulsar unit will give one pulse output) Each feeder controls shall receive federate demand signal from DDCMIS for feeder speed controls. The demand signal provided by DDCMIS shall be galvanically isolated 4-20 mA DC. SALIENT FEATURES : The salient features of the micro Processor based gravimetric feeders are: Automatic digital calibration of feeder to measure and record Tare and Span factors independently requiring no potentiometer adjustments. Providing user interface through Keyboard and display unit by which the feeder controls can be programmed and operated. This includes setting the mode of operation, programming of parameters and display of various status. Automatic changeover from Gravimetric mode to volumetric mode in case of errors in load cell signals. Data logging and storage of process parameter like total coal consumption, historic density and trip details and recovery of data in the event of power interruption
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Automatic change over from main tacho to redundant tacho when main tacho fails. In built diagnostic features, which assist the operator in system trouble shooting, thus resulting in less down time of the feeder. INTERLOCKS RELATED TO FEEDER : TO START A FEEDER : The following conditions are required to start a Gravimetric feeder in the remote mode. i) Adjacent Oil Elevation should be in service to give start permit to feeder. ii) The Primary air capacity also determines the number of feeders that can be put into service. 59
Learning modules material- Engineering (FB) When no PA fans are in service no feeder can be started. When only one PA fan is in service only lower half elevations can be put into service. The Feeders corresponding to the lower half elevations should have been provided with other start permissive. When both the PA fans are in service all the feeders can be put into service provided the start permissive from FSSS is available. During operation while the PA fan trips the boiler load has to be reduced and hence the feeders corresponding to the upper half elevations are taken out of service. Similarly the remaining feeders also stopped while the other fan trips. SIGNALS TO OTHER SYSTEMS : a) The Feeder signal are interfaced with the SADC system. The 4-20 mA output given from the Feeder to the SADC system regulates the Secondary Air Damper. b) Feeder proven signal is given by the feeder when the federate crosses 50%. When the Feeder is proven the Fuel air damper is in regulating mode according to the boiler load. c) When the feeder is OFF the status indicates that coal fuel is nopt available in that elevation and Oil fuel has to be introduced to support the Elevation flame. SIGNALS GIVEN BY FEEDER : i) Feeder Trip : The Feeder Trip is caused by the following conditions. a) Loss of Both tachos. b) Feeder Discharge plugged c) No coal on Belt in Remote d) Material on belt in local/calibration e) Belt motion monitor timed out f) Motor starter fault g) RPM Deviation h) Coal flow monitor fault 60
Learning modules material- Engineering (FB) i) Loss of flow j) AIM/AOM/DIM/DOM/PIM module BAD When the feeder trips due to any of the trip condition the Feeder Off signal is given to the FSSS. If the same happens for all the feeders then All feeders Off signal is generated which is a condition for Loss of all fuel trip. ii) Feeder in Remote : When the feeder is running in remote mode the Feeder remote signal is available and the feeder cannot be operated locally through Local control panel. The operations done locally will be null and void. iii) Feeding Volumetric : When the loadcell difference goes above 12.5% of their count feeder switches to Volumetric mode.The signal given to DCS indicates whether the feeder is currently running in Volumetric or gravimetric mode. iv) Feeder Alarm : The Gravimetric Feeder gives Alarm output for the following Conditions. a) Load cell signals Out of range b) Demand signal out of range c) Loss of any one tacho d) Remote TCI increments too small e) Feed rate Error For all alarm conditions,Feeder Alarm status is communicated to the control system through the Feeder alarm Signal to initiate necessary actions. v) Feeder Running Reverse : The Feeder when taken for maintenance to clean the coal deposited on the belt the feeder will be run in the reverse direction. During this condition feeder cannot be operated from Remote condition. BHELSCAN
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Learning modules material- Engineering (FB) Flame scanner is a digital and reliable way of flame detection. Advancements in flame detection has moved from just identification to discrimination of flame and processing the multiple flames. The advantage of the same is the elimination of individual scanners for different type of fuel to discriminating operation for multiple fuels. PRINCIPLE OF OPERATION : The basic physics of Flame scanner is Light detection. The further processing of the signal is to derive further observations as inputs for other systems. It consists of a Head electronics and a control unit. The Head Assembly of the scanner consists of a Quartz lens for collection of light signal and a fibre optic cable for transmission of light signal to the Head electronics. In the Head electronics a Photodiode is used to convert the light signal to electrical signal.
BHELSCAN SYSTEM CONFIGURATION The electrical output of photo diode is amplified by log amplifier. The amplified signal is then compensated for back ground radiation of the furnace. The flame intensity component and flicker component of the signal is separated and processed for characteristic frequencies of coal and oil flame by the control unit which consists of four signal processing card and 2/4 logic card.
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SIGNAL PROCESSING IN THE BHELSCAN The software ensures discrimination of different flames through proper detection of characteristic flicker frequency range of the flame. For an elevation, all the four corners are centrally monitored by a fifth microcontroller, to give flame out signal by employing 2/4 logic and establishes the connectivity to DCS through RS232C.
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Learning modules material- Engineering (FB) FLAME PROCESSOR MODULE : The individual flame processor module has the following architecture. The Input signal is conditioned and compared with the previous set values done through Dip switch. The display mode can be changed with the Mode Pushbutton. The final output of the availability of flame is given through LED outputs.
FLAME PROCESSOR MODULE INTERFACE FROM BHELSCAN FOR BOILER CONTROLS : 2/4 Flame output from the flame scanner is taken for voting the elevation flame or No flame. The Flame signal along with the open condition of HFO, LDO Trip and Nozzle valves shows that the flame is due to Oil firing.
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Learning modules material- Engineering (FB) The Flame signal along with the Feeder proven(>50%) feed rate shows that the flame is due to Coal firing. These two signals are supporting the discriminating mode of operation of the BHEL SCAN. The No Flame signal is an input toElevation No flame which in turn goes to the Unit Flame failure trip.
The Intensity of the Flame sensed by the Scanner is given as 4– 20 mA signal to DDCMIS for monitoring purpose.
BHELSCAN HEAD PORTION INSTALLATION AT SITE BHEL VISION The BHEL VISION is a Water level Indicator. It works on the Principle of Conductivity. The Conductivity difference between Steam and water is used to distinguish the fluid in the Drum.
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Learning modules material- Engineering (FB) The Drum tappings are connected with a Pressure vessel in which the fluid are almost at the same operating conditions of the Drum. The level in the pressure vessel does not reflect the Drum level directly due to the density difference of the fluid caused due to the pressure difference between the Drum and the pressure vessel. The level difference is corrected in the software. The Pressure vessel is divided in equal parts. The Electrodes are fit one per region. The Steam or the Water in the region induces conductivity as the Electrode is a conductor. The Conductivity for the Water is greater than Steam. The Resistance in the electrode is sensed and measured. Steam region will have the higher resistance due to lower conductivity of the range of 200 K ohm. Water being a good conductor offers less resistance of the order of 100 K ohm. The Resistance observed are compared with the set value for that region. If the Resistance is high the region has been voted for Steam and for lower resistances water is voted. The output has been indicated through Red and Green LED at the display units fixed at the panel door and in UCB. BHEL VISION AT SITE INTERFACES FOR BHELVISION WITH BOILER CONTROLS: The Drum level has been transmitted to DDCMIS as 4 – 20 mA output. Drum Level control is used in Auto control for 2/3 redundancy in evaluating the drum level along with Direct water level gauge and Level transmitters. The Drum level Low alarm and H igh alarm are used as alarm indications to the Drum water level Low or High status.
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Learning modules material- Engineering (FB) The Drum level very low with a 5 seconds time delay is taken as a Low trip signal for Master fuel trip. Similarly, Drum level very high with a 10 seconds delay is ta ken as High trip signal for Master fuel Trip. The probe fault and the System fault along with the Alarm signals for Drum level are taken to the marshalling panel in DDCMIS. There are provisions for disabling the Trip signal also for User’s convenienc e. BHEL SONIC The BHELSONIC is an advancement of the early days manual tube leak detection. This system provides early tube leak detection through acoustic emission.
BHELSONIC SYSTEM ARCHITECTURE BHELSONIC consists of the following operating parts. Sonic tube assembly makes contact with the boiler tube wall and the acoustic waves passes through it to reach the acoustic sensor. 67
Learning modules material- Engineering (FB) Acoustic sensor converts the acoustic signal to electrical signal. The Field amplifier box eliminates the unwanted ba ckground noise through a filter and amplify the signal. The Voltage to Current convertor produces an output of 02.2– 20 mA and it is taken to the panel. The Ascertor modules in the panel convert the signal back to voltage and process them and give to the dB scanner. SIGNAL PROCESSING IN BHELSONIC The scanner scans all the channels and displays the amount of leak in the form of sound decibels. The Scanner displays 0 – 40 dB corresponding to 74 – 114dB in actual sense.
The Alarm preset and tim e delay for alarm are set through manual selection. The PC attached in the panel gives pictorial representation of the Boiler arrangement through mimic and gives the leak pattern of regions as bargraph, Trend and history. 68
Learning modules material- Engineering (FB) INTERFACE FOR BHELSONIC WITH BOILER CONTROL: Sonic tube leak detection has no Trip Interlock attached with it. When the leak is detected after the preset time delay Alarm is generated through the Bhelsonic panel. The Bhelsonic performs the discrimination of Steam leak and Sootblower operation as both the operations are steam flow. The Alarm feature identifies the possible location of steam leak and operator can perform remedial measures to avoid the steam leak causing damage to nearby tubes. ELECTROMATIC RELIEF VALVE The Electromatic relief valve is an automatic, electrically actuated, pressure relief valve which can be set for one percent or less differential between opening and closing by means of a pressure sensitive element that precisely and automatically relieves pressures within very close limits. The application of this valve places at the command of the plant operator a means of instantaneous opening and closing a relief valve on remote header. The electromatic relief valve does not replace the spring-loaded safety valve. Rather, it has been designed as a supplementary operating valve that will conserve power and increase efficiency of a steam generating plant. Regular spring-loaded safety valves rarely pop in service when the electromatic relief valve reduces safety valve maintenance substantially. The relief valve assures more accurate, balanced boiler operation at peak loads, plus a more uniform line pressure. ERV SYSTEM ARCHITECTURE
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Learning modules material- Engineering (FB) ERV PRESSURE SWITCH : The electromatic safety valve is provided with local controller. The controller is provided with Barksdale make pressure switch (B2S-H32SS), having micro switches with dual set points. One micro switch with change over on increasing parameter (PSH) and the other on change over on decreasing parameter (PSL) OPERATION: The UCB mounted control station has a 3-position switch. When the switch is in "OFF" position, the DC supply to the contactor is cut off and the solenoid valve is not energised and hence in closed position. When the switch is in "MANUAL" position, the relay coil of the interposing relay mounted in the relay panel gets energised; and through the contacts of interposing relay, DC power contactor housed in local (in ERV controller) gets 220VDC power supply and gets energised. The DC supply gets connected to the ERV solenoid valve through contacts of this DC contactor and the ERV opens, relieving the steam pressure. The ERV remains open so long the switch is in "manual" position and closes when the switch is turned to the "OFF" position. When the switch is in "AUTO" position, the pressure switch also gets connected in series with the contactor relay coil. When the steam pressure is in normal operating range, the DC supply is not extended to the contactor coil. When the steam pressure reaches a preset relief high level, the PSH is activated, contactor is energised and power supply reaches the solenoid valve energising and opening it. ERV opens and the steam is let off and the pressure is relieved. Subsequently, when the steam pressure reduced to the preset low level, the PSL resets and the control supply to the contactor is cut - off, de-energising the solenoid valve. Solenoid valve closes and ERV closes closing steam line. Hence, normal operation continues without affecting the process even if process pressure overshoots using ERV control INTERFACE WITH BOILER CONTROL : The ERV valve opening and close status are available at the DDCMIS to ensure the action taken against the Boiler pressure relief. FURNACE TEMPERATURE PROBE The furnace temperature probe is intended to measure the temperature in the furnace during the initial start-up of boiler. The probe consisting of: 70
Learning modules material- Engineering (FB) i. A duplex chromel - Alumel thermocouple mounted at the tip of the probe. ii. Limit switches for ‘Probe extended’ and ‘Probe retracted’ positions. iii. Local push-button integral with the probe for local operations. iv. An electrical drive to extend or retract the probe as necessary. v. A helipot potentiometer for remote position indicator. OPERATION : Switch on the power supply switch (PSS). Press the 'EXT' push-button from field or ‘extend’ command from DDCMIS to advance the probe. Power contactor 'EXT' energises and hence the electrical drive is switched ‘ON’ and the probe advances to furnace. Once the probe reaches the fully extended position, the limit switch LS-R changes over to thus cutting off power supply to the 'EXT' power contactor. FTP stops at the forward end. The Cr.Al thermocouple measures the furnace temperature and fed to DDCMIS for remote indication. The probe can be retracted from the forward end position by pressing ‘RET’ pushbutton from the field or by ‘Retract’ command from DDCMIS. The limit switch LS -S is provided to monitor the home position of the probe. Once the probe reaches the home position, LS-S resets tripping ‘RET’ power contactor and tripping the power to probe motor. If intentionally the probe is to be retracted to its home position during the advance / retract cycle of operation, press the retract push button / give retract command from DDCMIS. The contactor RET energises and trips ‘EXT’ contactor, if it is energised already and probe withdraws to home position as described above. Once the probe reaches the ‘home position’ the limit switch ‘LS-S’ stops probe operation. Temperature probe can be stopped inside the furnace at any location within its operating range by pressing ‘STOP’ pushbutton or by giving ‘STOP’ command from DDCMIS. This can be used for measuring the temperature of furnace at any location. From that particular location FTP can be advanced/ retracted using extend/retract commands. FTP INTERLOCKS : i. When the temperature exceeds 540°C, the interposing relay ‘R4’ is energised from DDCMIS, ‘NO’ contact of the same energises the RA contactor, thus bringing the probe to its ‘home position’. ii. When probe motor gets over-loaded FTP will be retracted to home position.
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Learning modules material- Engineering (FB) iii. At any position temperature probe can be stopped by pressing ‘Stop PB’ from local or from DDCMIS, to register steady temperature. iv. Spare potential free contacts for X1, X2, X3, 74, EXT & RET are provided in the starter box for annunciation. v. For remote position indication of temperature probe position, 4-20mA signal will be provided from the local starter box to DDCMIS. INTERFACE WITH BOILER : FTP has no part to play with the Boiler control and it is used only for the Indication purpose. The Operator should ensure fluid flow in the Reheater when the set temperature is acquired. FTP is meant for Reheater Protection. Probe in operation and probe motor overloaded indications are provided in the starter box. 2.5 FES AND R&M:
Field Engineering Services Activities of field engineering services start after the erection of boiler. Key Activities are as follows Project Follow – Up Project Follow–up consist of Advice to customer/site on pre-commissioning checks Support to customer/site in commissioning and trial operation Feed back to the concerned Engineering sections regarding technical problems /equipment performance Recommendations with required documents for repair / modification in case of failure Disposition of CARs and customer complaints Technical directives to site/Region Feed back to similar units for corrective and preventive action Coordination of interface activities between Engineering sections and site during repair/modification
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Performance Testing Performance Testing Includes Functions: Clarification (CL) : Performance Guarantee test Proposal Performance Evaluation test Proposal Performance Guarantee test reports. Performance Evaluation test report Performance test data.
Complaints requiring only Clarifications / advice without involving any material supply / man power are classified under this category. FES activity is complete when the clarification is furnished.
Material supply (MS):
Problems for which materials are to be arranged / supplied are categorized under Material Supply. Phase -I activity of FES is complete when the Work Order is obtained and document is released (if required) by FES and communicated to customer /site.
Phase I is completed with the following activities: Sending of WO Request to Contracts as per format 353-012 Receipt of WO from Contracts Group DDR as applicable (No DDR required for raw material supply or repeat manufacturing of NS-I document) Communicate the proposed action to Contracts / Site / Customer as applicable.
Phase II is completed with the following activities: Contracts will arrange for material supply to BHEL site / Customer and inform FES on despatch of materials.
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On receipt of material at site, FES will further follow-up for implementation and for obtaining feedback. If no complaints are reported within three months of the implementation, it will be construed as the issue is resolved satisfactorily with the customer and treated as closed.
Site work (SW) :
Based on the advice / solution recommended by FES, customer / site has to carry out work / modification. No material supply is involved from the unit. These are categorized under Site Work.
Phase I is completed with the following activities. FES will coordinate with the BHEL site / customer enabling them to arrange material, man-power and work schedule.
Phase II is completed with the following activities: Material supply will be organized by BHEL site / customer as applicable. On receipt of material at site, FES will further follow-up for implementation and for obtaining feedback. If no complaints are reported within three months of the implementation, it will be construed as the issue is resolved satisfactorily with the customer and treated as closed.
Engineering Analysis (EA) :
Complaints received from customer / BHEL site requiring engineering analysis for deciding the solution are classified under this Category. These are referred to concerned Engineering sections whenever required and FES coordinates for solution. Phase I is completed when suitable advice to customer / BHEL site is communicated. However, an interim reply informing the action being taken will be sent to customer / BHEL site. Based on the recommendation, execution will be done by Customer / BHEL site. Follow up of implementation and feedback on performance will be done by FES as Phase II activity. Wherever material supply is needed, WO will be obtained by FES and Document released, if required. Generic problem (GP) : Generic Problems are those which are repetitive in nature for more than three times either in the same boiler or in different boilers.
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The Generic Problems which are reported either by the Project Coordinator or Group Head or HOS are referred to a Select Committee which will review and advise appropriate actions to avoid recurrence / occurrence.
Renovation and Modernisation
Proposals stage: This checklist is designed by incorporating all the details related to the R&M proposal preparation starting from dates, observation and values. The checklist is divided into 6 sections, for easy application and maintenance of data. They are General information about the project R&M requirement in component wise
Proposal Finalisation and Date of Commitment.
Enquiry.
Schemes and BOM Preparation
Proposal document Checklist while forwarding proposal
SECTION 1: GENERAL INFORMATION ABOUT THE PROJECT This section contains general information about the R&M proposal. Some of the main details available here are, Place - from where we received the offer, Type of R&M - which is to be placed and Factors - based on which the R&M is proposed. The enquiry/ tender may be placed by the Spares and service Business Group head quarters or by the corresponding SSBG region. Sometimes the enquiry will come from customer directly to R&M commercial department or R&M engineering department. These are the four ways of getting the R&M proposal from customer. Type of R&M will be decided by following factors,
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Learning modules material- Engineering (FB) Major R&M – Nameplate Rating : To meet the name plate rating of the boiler, which is currently running below the specified rating. Major R&M – with Up rating : Here the R&M is based on increasing the capacity of existing boilers by 5 to 15 MW. C&I R&M : Here the R&M is based on Control and instrumentation requirements and state of art improvements in electrical and electronics field Part R&M – due to requirement from customer for specific replacement of components due to failures and damage.
Requirement of R&M is based on following factors: They are various reasons why R&M of old units is required. To meet the growing demand of power, the R&M of existing old units is a viable option for our government. Sometimes due to some peculiar problem also the customer want the renovation and modernization the unit.
Decrease in Plant Load Factor of the plant Increase in Heat Rate Generic Problem in the thermal station Lost capacity of the power plant Pollution Control regulations Availability / Reliability of the power plant Section 2: R&M requirement in component wise :
The component vise R&M requirement can be broadly classified into Pressure parts, Fuel system, Non- Pressure parts, C&I requirements, Mills, Gates and Dampers and Auxiliaries like Fans, Air pre-heater and Electrostatic Precipitator.
Pressure parts: The R&M of pressure parts may be required due to erosion, clinkering, High gas temperature at AH outlet, second pass modification due to overheating. It may also due to coal property change or modification due to uprating. Fuel system: In fuel system the R&M activity may be carried out due to state of art improvements or coal property change.
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Non Pressure Parts: In Non pressure parts the R&M activity may be based on conditional assessment carried out in the boiler or due to customer requirements. C&I Requirements: The state of art improvements of C&I equipments will create interest in customer minds for R&M of C&I equipments. Fans: The changes in ESP modification and ID system layout change will be sorted out by the corresponding change in fans capacity through R&M activity. ESP : The R&M of Electrostatic Precipitator may be due to state of art improvements in the ESP field or may be due to customer requirements through communication meetings. Gates and Dampers: In gates and dampers, the R&M activity may be based on state of art improvement in the same field or may be due to customer requirements. Mills: Change in coal property change and state of art improvements in mills field will induce the R&M of Mills. Section 3: Proposal Finalisation and Dates of commitments: The important aspect of the checklist is to follow up with dates of commitments, on which certain actions are planned. Because this commitment dates will decide the time taken for the proposal preparation. The first important document, what we get for the proposal preparation is the tender document from the SSBG head quarters. This date is the starting date in proposal preparation schedule. On receipt of this, we will duplicate and distribute the copies to various related departments. Based upon the documents, the next step will be compilation of missing data. After this both the customer and supplier will negotiate between them for the deviation observed in the tender. Then the negotiated deviations have to be discussed with the relevant departments for their concerns and it has to be reported back to SSBG and Customer. Then list of proposal drawings to be submitted will be finalized. After this scope, exclusions, terminal points, codes, standards and division of work among departments will also be finalized. Then list of major sub deliveries will be intimated to corresponding sources. Floating enquires for sub deliveries will issued to the dealers.
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Section 4 : Enquiry This section deals with proposal calculation. The first step is the Verification of Boiler thermal calculation & performance variation from original design condition. If there is no deviation observed then design of fuel analysis will be done and in case if it is found that mill cannot support full load it will be intimated to customer. The boiler parameter will be checked against the margins specified by customer. The Air temperature entering Air- pre Heater is designed accurately based on the calculation. The gas temperature leaving air- heater is another important part of proposal calculation. It mainly depends on the acid dew point and draft loss level at various loads. The excess air required will be fixed, based upon the calculation. After this the air and gas weight calculation will be done. After this, The efficiency calculation and furnace leakage percentage will be calculated. The selection of pulverizer will be based on the spare mill required, mill up rating in future, motor rating to IS, mill reject handling system input and power consumption guarantee given to the pulverizer. Similarly the wind box selection is decided by the Number of oil elevation, maximum oil load, type of scanner, type of ignitor, scanner used, gun cooling air fans, ignitor air fans. In this stage, few data has to be furnished to oil system. They are oil dew point, line tracing, common/ individual system and simple and duplex strainer. Like this some data has to be furnished to C&I for furnace supervisory safe guard system (FSSS). Some of the details including pump house, boiler overall new design, latest design requirement will be given to C&I. The Air Pre- Heater selection will be based on TG min, Sector angle required, modn check op air/ gas parameters. Then Steam coil air preheater (SCAPH) will be designed . The fan selection for Forced draft, Induced Draft, Secondary air and Primary air sectors is dependent on parameter like margin available with existing fans, special drives, cold/ hot PA, checking healthiness of motor by IS, recirculation required by FD and stalling point check.
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Then the parameter for dust collecting plant will be discussed with BHEL Electronics division, Bangalore and BHEL PDX, Bhopal. The Furnace outlet temperature is calculated based on site performance data analysis. Surface effective factor is also calculated based on site performance data analysis. The next step will be performance calculation for upper furnace. The pressure drop calculation is based on margin of drum design pressure and gap between D. O. P and SV closing pressure. Details will be given to stress analysis on pressure parts like check on minimum pressure drop required of coils and SCW. The soot blower selection for repair, replacement, addition of long retractable soot blower is based on the ratio of 100% bypass to the 30% adequacy of steam parameter, additional LRSB layout or interference location of heat flux probe, Location of ASLD, location details of smart soot blowing system, replacement requirement of soot blower central structures, thermal drain system. The data for structural design is based on beam bend second pass modification or change of mill layout. Furnace details will be provided for insulation purpose and the main components will be taken into account are skin temperature required, wind velocity. Similarly furnace details will be provided to stress analysis group for designing buck stays, beam support, eco modification and for column strengthening when change of load transfer. The data sheets for gates and dampers will be issued from BAP, Ranipet. Specific information about Line & insulation and painting can be given to corresponding departments. Section 5 : Schemes and Bill of Material The collection of data for the preparation of schemes and Bill of material starts with oil system, boiler mountings, C&I , structures, L&I, Ducts, Pipe lines, Air Heater and Fans sections. Then the component code wise, bill of material preparation will start. The group wise weights and erection details for PPA will continue after BOM preparation. The suppliers for sub-deliveries will be finalized. Then the BOM and sub deliveries list will be sent to commercial for estimation. After this, list of spares will be finalized. Inputs to erection and commission agency will be furnished along with list of drawings and documents. Section 6: Proposal documents checklist while forwarding the proposal.
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This section deals with options, which have to be checked before forwarding the proposal. Some of the items to be checked are List of tender deviations & clarifications, filling up data sheets, preparation of technical specification and preparation of design summary letter. Then design summary letter will be prepared. After this technical specification and drawings will be sent to SSBG. The list of proposal input to other units will be given. Then erection inputs will be furnished to erection department. Then inter unit scope matrix will be prepared. The sister units inputs will be forwarded to corresponding units. General arrangement drawings including plan and elevation drawings will be issued. Proposal Schemes of steam and water will be prepared followed by proposal schemes for valves and fitting. This will be followed by schemes for air and gas path along with schemes of oil system. Drawing inputs for pressure part arrangement and ID system inputs will be given to respective department. Sometimes specific component designs will be given if applicable. Then the plot plan will be drawn by adding all the changes in the layout details.
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Remaining Life Assessment:
Life Assessment and Life Extension program helps to identify and implement strategies so as to ensure continued running of the unit in a cost - economical way. A team effort is needed between the utility and designer to ensure safe continued operation of steam generators with minimum unscheduled outages. A complete analysis combining the unit‟s operational data with design expertise and problem solving knowledge is basic requirement for success of the exercise. Boiler pressure parts like super heater tubes, Steam pipes and headers operating in the creep range are designed for certain minimum lifetime. As units age, critical Components may distress through mechanism such as oxidation, corrosion, creep, fatigue and interaction of above mechanisms. These components deteriorate continuously during service as a result of the above time dependent material degradation process. In actual practice material damage results from interaction of two or more of these mechanisms causing unanticipated failures. Sometimes such failures may be catastrophic resulting in huge loss. Condition of critical components and the remaining life / service time available before replacement or major repair are the main consideration in life assessment study. Units originally designed for base load require improvements in equipment and controls. Additionally, modification in operating philosophy is also considered during life assessment program. The useful life of components in service may well exceed or fall significantly short of the design life. The reasons for such behaviors are related to design, operational and metallurgical conditions, considerable reserve strength and longevity even at the end of the predicated design life are not uncommon. This happens because of the conservatism built into the original design, over estimation of oxidation effects especially in case of thick walled components and use of large factor of safety and lower bound properties. On the other hand the components may fail prematurely due to unforeseen system, stress concentrations and operating conditions envisaged during design stage. Local conditions and operational factors associated with the particular unit dictate the type and
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extent of damage in a component. Hence even though a station has units of similar type, each unit needs independent study so that exact status of each unit can be established.
TESTS AT SITE:
1. VISUAL EXAMINATION: Visual examination is carried out to assess material wastage due to oxidation, erosion /corrosion problems, fouling conditions of heat transfer surfaces, integrity of attachments in coils and hanger supports in piping. This includes inspection of drum internals to ensure proper steam/water separation. During visual inspection the observations made with reference to discoloration of coils, misalignment is considered in deciding sample tubes removal for metallurgical examination.
Prior evaluation of pressure part condition, based on experience and design knowledge from similar plants makes sample selection more rational Samples from the regions thus determined to be most susceptible to failures and samples depicting the general condition of each component, are selected for an evaluation of the metallurgical condition. 2. DIMENSIONAL MEASUREMENTS: Essentially, thickness and outside diameter measurements form the dimensional
measurements. Thickness measurement at critical areas gives a value of thickness loss over the years, due to erosion and corrosion. Outside diameter measurements are generally employed to determine the swelling (bulging) due to creep. The thickness measurements are made using ultrasonic thickness meters supplied by M/S Kraut Krammer GMBH, West Germany and for diameter measurements, digital vernier calipers are used. 3. NON-DESTRUCTIVE EXAMINATION: The following NDE will be carried out prior to examination by replica technique. A) Liquid Penetrant Inspection:
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This technique is adopted primarily for detection of cracks or crack like discontinuities that are open to the surface of a part, like surface porosity, pitting, pinholes and other weld defects.
In principle, the liquid penetrant is applied to the surface to be examined and allowed to enter into the discontinuities. All excess penetrant is then removed surface dried and the developer applied. The developer serves both as a blotter to absorb the penetrant coming out by capillary action and also as a contrasting background to enhance the visibility of the indication. The testing is as per ASTM-E165-80. B) Magnetic Particle Examination: This technique is adopted for locating surface and sub-surface discontinuities like seams, laps, quenching and grinding cracks and surface rupture occurring on welds. This method is also used for detecting surface fatigue cracks developed during service. Magnetic particle inspection helps to detect cracks and discontinuities on or near the surface in ferromagnetic materials using dry magnetic particle testing equipment. The testing is done by magnetising at least two mutually perpendicular directions to ensure detection of defects in all possible orientations. The magnetic particle testing is carried out as per ASTM-E-709-80 and the adequacy of the magnetic field strength is verified by using octagonal field indicator (ASTM 275). C) ULTRASONIC TESTING: By using high frequency sound waves, the surface and sub-surface flaws can be detected. Cracks, laminations, shrinkages, cavities, flakes, pore and binding faults that act as discontinuities in metal gas interfaces can also be easily detected. 4. SAMPLING FOR LABORATORY ANALYSIS AND STRESS RUPTURE TESTING: Tube samples carefully selected after the visual inspection from super heater / reheaters are analyzed in laboratory for material degradation, extent of oxide scaling and corrosion/erosion. Tube deposit analysis, internal and external surface condition assessments aid in identifying the root cause of failures/degradation noticed.
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4.1. WATERWALLS AND ECONOMISER: Water wall tube samples will be removed from high heat flux zone for evaluating the deposit content and constituents of the deposit. The need for chemical cleaning will be decided on the deposit content and the constituents of the deposit. The metallic constituents of the deposit are determined using atomic absorption
spectrophotometer and the morphology is found using X-ray diffraction analysis. The analysis report may include the need or otherwise for chemical cleaning. Recommendation on solvent for ensuring the effective removal of the deposit will also be included as per requirement.
4.2. METALLURGICAL EXAMINATION OF HIGH TEMPERATURE TUBES: The tube samples removed from super heater will be analyzed for any metallurgical degradation in service. Transverse ring segments from the tubes will be metallographically prepared and examined using light optical microscope up to a magnification of 500X. Carbide morphology and distribution, presence of creep bulging, and tube wall thinning will be evaluated. The oxide scale thickness on steam side surface will be measured and used in estimating the extent of damage as also the general operating temperature for the running hours. The analysis is done according to internationally accepted micro structural criteria as noted below: 5. IN-SITU METALLOGRAPHY BY REPLICA TECHNIQUE: The high temperature components in utilities when subjected to high stress for a long time undergo steady changes in transformation of strengthening carbide phases followed by creep cavitations. This is the beginning of creep or slow plastic deformation leading to gradual bulging of pressure parts. Three distinct stages of creep occur in several alloys. The first stage of creep occurs in a short period which is transient. The second stage or steady state creep occurs over a very long duration of several years. The metallurgical changes like carbide transformation and dispersion occurs. Formation of minute creep voids along the grain boundary surfaces also accompanies creep deformation. In the third stage of creep, the creep voids increase in number and size and get oriented and connected They generate micro cracks, and the micro cracks connect
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themselves resulting in the initiation and growth of macro crack with sudden fracture in some zones depending on the operating stress at that zone. The replication is the technique adopted to obtain the micro structure "in-situ' by non-destructive metallography. The technique is used in areas where sample removal is difficult and not viable on cost economic aspects. 6. REPLICATION TECHNIQUE: The process involves preliminary preparation of the metal surface using polishing equipment. When the spot is ensured free from rust and scale polishing will be done using abrasive paper of varying grits from, 120, 200, 400, and 600 in sequence. Subsequently diamond paste lapping is done followed by etching with 3% Nital to reveal the structure. The surface preparation can also be done by adopting electro polishing. After the preparation of the surface, the micro structure of component is truly transferred to a film. Transparent film with green reflecting foil can be used which can be examined in laboratory with magnification up to 500X to assess the metallurgical damages like creep cavitations. For examination at higher magnification, the microstructure of the components can be transferred to cellulose acetate replicating tape. A cellulose acetate film of 0.1 to 0.15 mm thickness and 20 x 40 mm size is cut from roll or sheet. A few drops of acetone will be applied on one surface for about 5 seconds and this makes the acetate film soft on one side and retains hardness on the reverse side. The soft side is pressed uniformly over the etched surface using clean and plain rubber and exerting the force of the thumb for about 10 seconds. It will be protected against dust and left for some time for drying. The dried film will be lifted up using fine knife and will be kept between parallel glass slides. Silver shadowing or gold sputtering in vacuum can be done on the impression side to improve reflection. This helps in micro structural examination using light optical microscope or scanning electron microscope at higher magnification.
R.L.A. DETERMINATION BASED ON ACCELERATED CREEP RUPTURES TESTING.
Super heater tubes operating at higher temperatures (more than 450 °C) are subjected to a time dependent phenomena known as creep. The sample tubes removed from boiler will be subjected to creep rupture tests at accelerated temperature and at service pressure in the test
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facility available at laboratory. The test parameters can be controlled within close limits. To predict remaining life from the results of rupture tests, the following method is adopted. Specimen from each sample tube will be subjected to a specified stress and temperature. Time to rupture versus temperature will be plotted and the extrapolation will be done for the operating temperature to decide the remaining life. The assumptions made in the above method are; a) Thickness variation is not considered and hence the operating stress is assumed as uniform. b) Metal temperature considered for extrapolation is assumed as constant and metal temperature increase due to building of oxide scale over a period is not accounted.
R.L.A. CALCULATION BASED ON SERVICE TEMPERATURE:
The tube samples removed from boiler will be evaluated for micro structure classification based on which the service temperature can be evaluated taking into consideration the operating hours collected from the plant record. Another method of estimating operating temperature is based on oxide scale measurement. As steam passes through the tubes at high temperature, the metal is oxidized. Knowing the operating hours and oxide thickness measured in mills, the average temperature‟t‟ can be calculated.
CALCULATION OF REMAINING LIFE:
Assuming oxidation rates for a specific period, the average stress can be calculated for the aging duration considered. With average stress value Larsen - Miller parameter can be calculated for the particular material. With the Larsen - Miller parameter rupture life can be calculated using metal temperature values. Fraction of life consumed is the ratio of operating period divided by Rupture life is consumed. The following assumptions are made in this method. 1. Uniform oxidation rate is assumed. 2. Metal wastage is assumed as linear for computing operating stress. 3. Larsen Miller parameter is computed based on lower bound stress.
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3.Boiler Materials
The boilers being built today are demanding in terms of unit sizes and operating requirements ; hence the choice of materials is of prime importance for ensuring satisfactory performance of the boilers.
3.1 VARIOUS STEEL GRADES OF TUBES / PIPES IN BOILERS
Sl. No.
Nominal Composition
MATERIAL SPECIFICATION ASME Section-I DIN 300 – TRD BS 1113
01.
Carbon Steel
SA178 SA192, Gr.A1 & Gr.C
Gr.C, St 35.8 SA210 St 45.8
BS3059 P2 S2 360, 440 BS3602 P1 360, 430, 500 Nb
SA106 Gr.B, Gr.C
02.
½ Mo
SA209 T1
15 Mo3
----
03.
1 Cr ½ Mo
SA335 P12 SA213 T12
13 Cr Mo 44
BS3059 P2 S2 620 BS3604 P1 620 – 440
04.
1¼ Cr ½ Mo
SA213 T11 SA335 P11
----
BS3604 P1, 621
05.
2¼ Cr 1 Mo
SA213 T22 SA335 P22
10 Cr Mo 910
BS3059 P2 S2 622490 BS3604 P1, 622
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06.
9 Cr 1 Mo ¼ V
SA213 T91 SA335 P91
X 10 Cr Mo V ----Nb91
Area Application
of
Material type
Typical spec. for Plates, Tubes, Pipes
Guiding Reason for Upper Limit
07.
12 Cr 1 Mo ¼ V
-----
X 20 Cr Mo V BS3059 P2 S2 762 121 BS3604 P1 762
08.
18 Cr 8 Ni
SA213 TP304 H
-----
BS3059 P2 304 S51 BS3605 – 304 S59 E
09.
18 Cr 10 Ni Cb
SA213 TP347 H
-----
BS3059 P2 347 S51 BS3605 347 S59 E
3.2 Conventional Boiler Materials
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Drum Water walls, Economiser Superheater and Reheater C Steel/ Low Alloy Steel C Steel C ½ Mo steel 1Cr ½ Mo 2 ¼ Cr 1Mo 18 Cr 8 Ni 18 Cr 10 Ni Cb Modified 9Cr 12%Cr SA299 SA192, SA106 A209 T1 SA213T11, SA335P11 SA213T22, SA335P22 SA213 TP304 H SA213 TP347 H SA213T91, SA335P91 X20CrMoV12 1 ASME code German Code SA210, Graphitisation Graphitisation Oxidation/ corrosion, Flue gas Oxidation/ corrosion, Flue gas
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MATERIALS FOR ADVANCED SUPERCRITICAL PLANTS
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3.3
NEW GENERATION STEELS FOR SUPER CRITICAL AND ULTRASUPER CRITICAL BOILERS The changes in boiler design in super critical and ultra super critical boiler calls for advanced
creep resistant steels for the enhanced steam parameters of temperature and pressure. Some of the steels that have been developed and are in service in European and Japan utilities are given below. Ferritic steels of 9Cr-2W-0.5Mo and 2.25 Cr-1Mo-2W steel Austentic stainless steels 17Cr-8Ni-3Cu 9Cr-2W-0.5Mo Steel(T92/P92) The 9 Cr- 0.5 Mo- 2W steel for tube and pipe has been approved for boiler and pressure vessel manufacture and has been included in ASME specification. The tube material (T92) has been listed in SA 213 and the pipe material (P92) in SA 335 of ASME sec IIA. P92 is a modification of P91 steel which is now well established steel in power plants. The P91 steel is modified by reducing the molybdenum content to about 0.5% and adding about 1.7% tungsten plus a few parts per million of boron. Controlled micro alloying in the form of niobium vanadium and nitrogen is retained. This composition modification gives rise to very stable carbides and carbon-nitrides, which improve long term creep strength. The carbon content has been kept low to ensure welding processing characteristics. This steel is designed to operate at temperatures up to 625°C. It is claimed that high temperature rupture strength is 30% more than for P91. For example at 600°C the 100,000hour creep rupture strength of P91 base material is about 95MPa whereas that of P92 is about 123MPa. The steel composition and tensile properties of P92 grades are shown in Table I and II. The steel grade has higher yield and tensile strength compared to P91 grade. Table I Chemical composition of P92 steel C 0.070.13 Mn 0.300.60 S 0.010 P 0.020 Si 0.50 Cr 8.509.50 Mo 0.300.60 Others V 0.15-0.25 ; Cb 0.04-0.09 B 0.001-0.006; N 0.03-0.07 Ni 0.40; Al-0.04 W-1.5-2.0
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Table II Tensile properties of steel for P92 and P91 Steel SA 335 P92 SA 335 P91 *mimnimum 2.25 Cr- 1.7 W steel (T/P23) The steel grade T/P23 was validated and included in the ASTM A 335 in 2001 and incorporated in ASME section II in 2004 .The steel has also been approved for other product forms like fittings, flanges under ASTM A 182 F22. The steel is the modified form of well-known creep resistant steel SA 335 P22 grade. It is a bainitic steel with addition of tungsten upto 1.7% as well as micro alloying with vanadium, columbium (niobium), nitrogen and boron. The carbon content is intentionally lowered to improve the steel welding and processing characteristics. Due to its chemical composition T/P23 developed a bainitic martensite structure and the maximum hardness is only about 350 HV due to low carbon content. It is used in water wall panels in advanced new power boilers such as super critical boilers and in superheater, reheater tubes and water wall panels in conventional power plants and HRSG heat recovery steam generators. The steel has higher yield and tensile strength compared to P22 steel. In addition the long term creep properties are much higher for P23 and they are close to P91 grade. The steel composition and tensile properties are shown in Table III & IV. Table III Comparison of chemical composition of rP22 and P23 grades Grade C Mn 0.300.60 S 0.025 P 0.025 Si 0.50 Cr 1.90-. 2.60 Mo 0.871.83 Others NS *UTS(MPa) 620 585 *YS(MPa) 440 415 *Elong.(%) 20 20 Hardness max.(Hv) 265 265
SA 335 0.05P22 0.15
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SA 335 0.04P23 0.10 0.100.60 0.010 0.030 0.50 1.902.60 0.050.30 V-0.20-0.30; Cb-0.02-0.08 B-0.0010-0.006; N-0.0015 W-1.45-1.75; Ti 0.0050.06
Table - IV Comparison of mechanical properties for P22 and P23 grades Steel *UTS (MPa) SA 335 P22 SA 335 P23 415 510 *YS (MPa) 205 400 *Elong (%) 30 20 *Hardness ( Hv) Not specified 220 HB Allowable stress at 600 :C (MPa)
23.5
57.0
*Maximum limit
Super 304H austentic stainless steels Austenitic stainless steel pipes are primarily used for boilers in thermal power plants and the energy sector. High temperature strength has become one of the most important aspects for the application of boiler tubes. The 18Cr-8Ni steel grade 304 is a conventional austenitic stainless steel. Though the addition of 3wt% of copper to the standard grade, an increased carbon content and certain amounts of niobium and nitrogen, the elevated temperature strength and especially the creep properties are improved in the grade Super 304H. The addition of nitrogen leads to a solid solution strengthening of the material. The steel composition and tensile properties are shown in Table V & VI. This increases the allowable tensile stresses. The allowable stress of this stainless steel is more than 20% higher compared with that of SA-213 grade TP347H. This excellent creep rupture strength is based on the precipitation strengthening effect of fine Cu-rich phase which precipitates coherently in the austenitic matrix during service-exposure. The corrosion resistance of this stainless steel is almost same as that of fine-grained TP 347H. The super 304H tubes have been service exposed as Superheater tubes and reheater tubes, and the 6.5 years service-exposed tubes confirmed that this new stainless steel applicable to the boiler material. Table V CHEMICAL REQUIREMENTS of super 304H C 0.07 – 0.13 Mn 1.00 S 0.010 P 0.040 Si 0.3 Cr 17.00 19.00 Ni Cu Mo 0.871.83 Cb N B 0.001 –0.01 Al 0.0030.03
7.5 – 2.510.50 3.5
0.3- 0.050.6 0.12
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Table VI MECHANICAL PROPERTY REQUIREMENTS of super 304H Tensile strength, min, ksi ( M Pa) Yield Strength , min.,ksi (M Pa) Elongation in 2 in., min, % 85 (590) 34 (235 ) 35
4.0 4.1
Boiler auxiliaries Pulverisers
The pulverizing process is composed of several stages. The first is the feeding system, which must automatically control the fuel-feed rate according to the boiler demand and the air rates required for drying and transporting pulverized fuel to the burner. Because coals have varying quantities of moisture, drying is an integral part of pulverizing process. Part of the air from the steam-generator air preheater, the primary air, is delivered to the pulverizer by the primary air fan. There it is mixed with the coal as it is being circulated and ground.
Grinding is accomplished by impact, attrition, crushing, or combinations of these. There are several commonly used pulverizers, classified by speed:
Types of Pulverisers Low speed (Ball mill/ Tube mill) - Usually rotating between 15 to 25 rpm Medium speed (Vertical Spindle – Bowl / Ball & Race / Roller Mills) – usually rotates between 50 to 100 rpm. High speed (Beater Wheel) - runs at high speed, normally 750-1000 rpm.
The selection of number of mills and capacity of mill shall meet pulverizing requirement for the range of coals specified; Pulveriser selection shall also ensure one spare mill to account for outage for maintenance.
Low speed mills The low-speed ball-tube mill is basically a hollow cylinder with heavy-cast wear-resistant liners, less than half-filled with forged steel balls of mixed size. Pulverization is accomplished by attrition and impact as the balls and coal ascends and falls with cylinder rotation. Primary air is circulated over the charge to carry the pulverized coal to classifiers. The ball tube mill requires low maintenance, but it is larger and heavier in construction and consumes more power than others.
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Medium speed mills The medium-speed ball-and-race and roll-and race pulverizers are the types mostly in use nowadays. They operate on the principles of crushing and attrition. Pulverzation takes place between two surfaces, one rolling on top of the other. The rolling elements may be balls or ring-shaped rolls that roll between two races, in the manner of a ball bearing. The balls are between a top stationary race or ring and a rotating bottom ring, which is driven by the vertical shaft of the pulverizer. Primary air causes coal feed to circulate between the grinding elements, and when it becomes fine enough, it becomes suspended in the air and is carried to the classifier. Grinding pressure is varied for the most efficient grinding of various coals by externally adjustable springs on top of the stationary ring.
Bowl Mill: It is one of the most advanced designs of coal pulveriser. The bowl mill essentially consists of a revolving bowl which is driven by a reduction gear mechanism coupled to an electric motor. It is provided with a set of 3 grinding rollers. The coal fed to the centre of the revolving bowl is forced between the grinding roll and the bowl for getting pulverized. The required grinding pressure is given by means of a set of heavy duty springs. Hot air is sent through an air chamber provided beneath the bowl to dry and transport the pulverized coal. The advantages of Bowl mill are Low power consumption, Reliability, Minimum resistance, Wide capacity, Quiet and vibration less operation and ability to handle wide range of coals. High speed mills High-speed pulverizers use hammer beaters that revolve in a chamber equipped with high-wear resistant liners. They are mostly used with low-rank coals with high-moisture content and use flue gas for drying. Classifier The classifier is located at the pulverizer exit. It is usually a cyclone with adjustable inlet vanes. The classifier separates oversized coal and returns it to the grinders to maintain the proper fineness for the particular application and coal used. Adjustment is obtained by varying the gas-suspension velocity in the classifier by adjusting the inlet vanes.
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4.2 Fans Introduction: Fan is a rotating machine with a bladed impeller which maintains a continuous flow of air or gas. It is continuous because the flow at entry and exit and also through the impeller is steady. Fans may be classified into two major types : Axial flow and radial flow.
Selection of fans for a given application depends on the following parameters. Capacity in m3/sec. Pressure at fan inlet. Total pressure rise required in mbar/mmwc. Specific weight of medium in m3/kg Operating temperature in deg.C.
-
The mechanical design of the fan is governed by the tip speed and the maximum operating temperature. Axial fans may be classified further into Impulse Type and Reaction Type fans. In the Impulse type fans, most of the energy coming out the impeller is Kinetic Energy. It is converted into Pressure Energy in the Outlet Blades and the diffuser. Hence these fans are called Impulse Fans. In the reaction type of Axial Fans, most of the energy coming out of the impeller is in the form of Pressure Energy. Axial fans are best suited for handling large capacities compared to pressure rise with good efficiencies, less floor space and less weight. AN fan has blades particularly suitable for operation with air and dust laden gases. Even with advanced wear due to erosion the performance hardly changes. `AN’ fan is of mechanically simple design. All major components are easily accessible. Where load changes are frequent, Axial reaction fan (`AP’ type) has distinct advantages – because of the highly efficient profiled blades. `AP’ fan is generally offered for clean air application than dust laden gases because the profiles will be affected by erosion.
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Radial fans: Based on the configuration of the blade with respect to the direction of rotation of the impeller, it is called Backward Curved, Forward Curved and Radial Bladed Impeller. Application: Based on the selection parameters, single inlet or double inlet radial fans can be offered. as forced draft, induced draft, primary air and gas recirculation fan in power stations. for refineries, steel, cement, fertilizer, petrochemical, palletizing, sinter plants. for ventilation application including mine ventilation.
Forced Draft fans: Forced draft fans supply air necessary for fuel combustion and this shall deliver stochiometric air plus the excess air needed for proper burning of the specific fuel for which they are designed. These fans supply the total air flow taking in to account the air preheater leakage and some sealing air requirements. Primary Air Fans: These fans supply the air needed to dry and transport the coal either directly from pulverising equipment to a furnace or to an intermediate storage bunker. Induced draft Fans: These fans exhaust combustion products from a boiler to chimney by creating sufficient negative pressure to establish a slight suction in the furnace. Gas Recirculation Fans: These fans draw gas from a point in the flue gas flow path ( normally between economiser outlet and air preheater inlet) and discharge it (for steam temperature control) in to the bottom of the furnace. These fans are used mostly in the oil & gas fired boilers. Seal Air Fans: Seal air fans take suction from FD/ PA fans fans and boost its pressure to deliver the downstream equipments like pulveriser and feeders to maintain the sealing pressure required for them to prevent leakage. Scanner Air Fans: These fans provide air to the flame scanners which are operated at higher temperature zone for cooling of the scanner heads. Normally 1 AC and 1 DC fans are provided to ensure the availability even during power failures. Igniter Air Fans: These fans are provided to supply air to take eddy plate oil/gas igniters and normally suction is taken from FD fans. They ensure the combustion air requirements of the igniters.
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Learning modules material- Engineering (FB) 4.3 Airpreheaters
Schematic diagram of typical coal-fired power plant steam generator highlighting the air preheater (APH) location. An air preheater (APH) is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil. The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).
Types
There are two types of air preheaters for use in steam generators in thermal power stations: One is a tubular type built into the boiler flue gas ducting, and the other is a regenerative air preheater. These may be arranged so the gas flows horizontally or vertically across the axis of rotation.
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Tubular type
Construction features
Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the preheater tubes, transferring heat from the exhaust gas to the air inside the preheater. Ambient air is forced by a fan through ducting at one end of the preheater tubes and at other end the heated air from inside of the tubes emerges into another set of ducting, which carries it to the boiler furnace for combustion.
Regenerative air preheaters
There are two types of regenerative air preheaters: the rotating-plate regenerative air preheaters (RAPH) and the stationary-plate regenerative air preheaters (Rothemuhle).
Rotating-plate regenerative air preheater
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Typical Rotating-plate Regenerative Air Preheater (Bi-sector type)
Principle function for the regenerative preheater. The rotating-plate design (RAPH) consists of a central rotating-plate element installed within a casing that is divided into two (bi-sector type), three (tri-sector type) or four (quad-sector type) sectors containing seals around the element. The seals allow the element to rotate through all the sectors, but keep gas leakage between sectors to a minimum while providing separate gas air and flue gas paths through each sector. Tri-sector types are the most common in modern power generation facilities. In the tri-sector design, the largest sector (usually spanning about half the cross-section of the casing) is connected to the boiler hot gas outlet. The hot exhaust gas flows over the central element, transferring some of its heat to the element, and is then ducted away for further treatment in dust collectors and other equipment before being expelled from the flue gas stack. The second, smaller sector, is fed with ambient air by a fan, which passes over the heated element as it rotates into the sector, and is heated before being carried to the boiler furnace for combustion. The third sector is the smallest one and it heats air which is routed into the pulverizers and used to carry the coal-air mixture to coal boiler burners. Thus, the total air heated in the RAPH provides: heating air to remove the moisture from the pulverised coal dust, carrier air for transporting the pulverised coal to the boiler burners and the primary air for combustion. The rotor itself is the medium of heat transfer in this system, and is usually composed of some form of steel and/or ceramic structure. It rotates quite slowly (around 3-5 RPM) to allow optimum heat transfer first from the hot exhaust gases to the element, then as it rotates, from the element to the cooler air in the other sectors.
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Dust collectors
Types of dust collectors
Five principal types of industrial dust collectors are:
Inertial separators Fabric filters Wet scrubbers Electrostatic precipitators Unit collectors
Inertial separators
Inertial separators separate dust from gas streams using a combination of forces, such as centrifugal, gravitational, and inertial. These forces move the dust to an area where the forces exerted by the gas stream are minimal. The separated dust is moved by gravity into a hopper, where it is temporarily stored. The three primary types of inertial separators are:
Settling chambers Baffle chambers Centrifugal collectors
Neither settling chambers nor baffle chambers are commonly used in the minerals processing industry. However, their principles of operation are often incorporated into the design of more efficient dust collectors.
Fabric filters
Commonly known as baghouses, fabric collectors use filtration to separate dust particulates from dusty gases. They are one of the most efficient and cost effective types of dust collectors available and can achieve a collection efficiency of more than 99% for very fine particulates. Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape. The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags. The fabric primarily provides a surface on which dust particulates collect through the following four mechanisms:
Inertial collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
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Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size. Brownian movement - Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces. Electrostatic forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture.
A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned periodically.
Wet scrubbers
Dust collectors that use liquid are commonly known as wet scrubbers. In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles. The greater the contact of the gas and liquid streams, the higher the dust removal efficiency. There are a large variety of wet scrubbers; however, all have one of three basic operations:
Gas-humidification - The gas-humidification process conditions fine particles to increase their size so they can be collected more easily. Gas-liquid contact - This is one of the most important factors affecting collection efficiency. The particle and droplet come into contact by four primary mechanisms: o Inertial impaction - When water droplets placed in the path of a dust-laden gas stream, the stream separates and flows around them. Due to inertia, the larger dust particles will continue on in a straight path, hit the droplets, and become encapsulated. o Interception - Finer particles moving within a gas stream do not hit droplets directly but brush against them and adhere to them. o Diffusion - When liquid droplets are scattered among dust particles, the particles are deposited on the droplet surfaces by Brownian movement, or diffusion. This is the principal mechanism in the collection of submicrometre dust particles. o Condensation nucleation - If a gas passing through a scrubber is cooled below the dewpoint, condensation of moisture occurs on the dust particles. This increase in particle size makes collection easier. Gas-liquid separation - Regardless of the contact mechanism used, as much liquid and dust as possible must be removed. Once contact is made, dust particulates and water droplets combine to form agglomerates. As the agglomerates grow larger, they settle into a collector.
The "cleaned" gases are normally passed through a mist eliminator (demister pads) to remove water droplets from the gas stream. The dirty water from the scrubber system is either cleaned and discharged or recycled to the scrubber. Dust is removed from the scrubber in a clarification unit or a drag chain tank. In both systems solid material settles on the bottom of the tank. A drag chain system removes the sludge and deposits in into a dumpster or stockpile.
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Learning modules material- Engineering (FB) Electrostatic Precipitator
An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. In contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity).
The plate precipitator
The most basic precipitator contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented vertically, with the plates typically spaced about 1 cm to 18 cm apart, depending on the application. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the stack of plates. A negative voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough an electric (corona) discharge ionizes the gas around the electrodes. Negative ions flow to the plates and charge the gas-flow particles. The ionized particles, following the negative electric field created by the power supply, move to the grounded plates. Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure (given from layer resistivity, electric field, and current flowing in the collected layer).
Unit collectors
Unlike central collectors, unit collectors control contamination at its source. They are small and selfcontained, consisting of a fan and some form of dust collector. They are suitable for isolated, portable, or frequently moved dust-producing operations, such as bins and silos or remote belt-conveyor transfer points. Advantages of unit collectors include small space requirements, the return of collected dust to main material flow, and low initial cost. However, their dust-holding and storage capacities, servicing facilities, and maintenance periods have been sacrificed.
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5.0
Environmental Pollution Control Measures
Major pollutants from a coal fired boiler are particulates, CO2, NOx and Sox. Minimising the emissions calls for provision of dust collectors, Low NOx firing system, and DeNOx system in addition to the inherent reduction achievable by adoption of suitable firing system and by improving boiler efficiencies. Nox emission from a power plant is dictated by the fuel nitrogen (fuel Nox) and the temperature of the flame, which are determined by the operating condition and the load on the unit. Firing system adopted for a particular power plant dictates the Nox emission from the plant. Over fire air system reduces the Nox emission. In coal and oil fired units the SOx emission depends upon the % of sulphur in the fuel and the calorific value of the fuel. Since most of the Indian coals have sulphur content around 0.5 to 0.6 % only, the SOx emission from the power plant is not a problem with the current environmental regulations. However with the use of Imported coals containing high sulfur, Sox emissions need to be focused on. Particulate emissions can be reduced to very low levels by high efficiency Electrostatic precipitators and Bag filters.
5.1 Indian Pollution Control Board Guidelines: EMISSION REGULATION – JULY 1984 - GUIDELINES BOILER SIZE PROTECTED AREA OTHER AREA OLD (BEFORE 1979) < 210 MW > 210MW 150 mg/NM3 150 mg/NM3 600 mg/NM3 ---NEW (AFTER 1979) 350 mg/NM3 150 mg/NM3
MINISTRY OF ENVIRONMENT AND FOREST NOTIFICATION- GUIDELINES. MAY 1993 - Emission 150 mg/NM3 .
MARCH 2003 - to examine feasibility to limit 100 mg/NM3 for existing power plants. 100 mg/Nm3 for new plants. 20-50 mg/NM3 are being followed for most new projects.
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o o
For New Plants PLANT SIZE < 50 MWe < 500 MWe >500 MWe 100 50 50 EMISSION mg/m3
b) Plants in Areas with Degraded or Poor Air Quality ALL 50
6.0
Codes & Regulations
6.1 Material testing codes
List of IS codes applicable for structural design
1. IS:800-2007- Code of practice for General Construction in Steel 2. IS:808-1989- Dimensions for hot rolled steel beams 3. IS:875-Part-1-1987- Dead Loads 4. IS:875-part-2-1987- Live Loads 5. IS:875-part-3-1987- Wind Loads 6. IS:875-part-4-1987- Snow Loads 7. IS:875-part-5-1987- Special Loads and Load combinations 8. IS:1893- Criteria for Earthquake Resistance Design of Structures 9. IS:2062-2006 Hot Rolled Low, Medium and High Tensile Structural Steel 10. IS:456- 2000 Code of Practice for Plain and Reinforced concrete
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6.2 Coal Analysis Standards
Sl No 1 Subject / title BIS number ASTM number D 2234 - 89 and D 2013 - 86
Methods for sampling of coal and IS 436 (Part1/Sec1) coke (manual sampling) Reaffirmed 1996 Methods for sampling of coal and IS 436 (Part1/Sec 2) coke (mechanical sampling) Reaffirmed 1994 Methods of test for coal and coke : Proximate analysis IS 1350 ( part 1 ) Reaffirmed 2001
2
3
D 3302 – 91 D 1412 – 89 D 3172 – 89 D 3173 – 89 D 3174 – 89 D 3175 - 89
4
Determination of calorific value
IS 1350 ( Part II ) Reaffirmed 1994
D 2015 – 91 D 3286 - 91 D 3176 – 89 D 3178 - 89 D 3179 - 89
5
Determination of carbon and Hydrogen
IS 1350 ( Part IV/ Sec 1 ) Reaffirmed 1994 IS 1350 ( Part IV/ Sec2 ) Reaffirmed 2000
6
Determination of nitrogen
7
Determination of sulphur
IS 1350 ( Part III ) Reaffirmed 2000
D 3177 - 89
8
Determination of forms of sulphur
IS 15438 :2004
D 2492 – 90
9
Instrumental determination of carbon, hydrogen & nitrogen in
D 5373 - 93
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coal & coke 10 Grindability IS 4433: Reaffirmed 1994 11 Elemental analysis of ash of coal IS 1355 : 1984 and coke Reaffirmed 2001 D 2795 – 1991 D 3682 – 1991 D 4326 - 92 12 Fusibility of ash of coal, coke, & IS 12891: 1990 lignite Reaffirmed 1995 D 1857 - 87 D 409 - 92
6.3 Boiler efficiency Determining and adjusting the efficiency of a boiler in a power plant or a process industry is essential for energy savings. The main requirements for determining the boiler efficiency is detailed below. Modern boilers of large capacity used in power plants have an efficiency ranging from 80 to 90 %. Elaborate calculation method is given in ASME PTC 4 or BSEN 12952-15:2003 which are the performance test codes for boilers. Input-Output Method: The simple method is to measure quantity of fuel input and the steam energy output. This method is the input output method. Efficiency % = Output / Input X100 = [Steam Flow kg/s x Steam Enthalpy kj/kg] - [Water Flow kg/s×Water Enthalpy kj/kg] / [Coal Flow kg/s x HHV of Coal kj/kg] ×100 In case of reheat units the reheater inlet and outlet enthalpy also has to be considered. One can determine the higher heating value by taking a sample of coal as it enters the boiler and analyzing it in the laboratory. These are normally done on a daily basis in most power plants.
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Fuel flow is more complicated. Gravimetric feeders used in modern power plants can give the coal flows to a certain degree of accuracy. Otherwise this will have to be computed from volumetric flows and bulk density of the fuels. This method, although it looks simple on paper, is not the industry preferred method because •Flow measurements are not accurate nor steady •Good quality flow instruments are costly. •Flow measurements always involve a co-efficient, which can very much alter the results. •Trouble shooting problems for determining the reasons for a lower efficiency is difficult Coal calorific value may change during the course of the day and hence calculation based on one analysis in a day need not be correct. However, this method finds use for quick calculation if the flow measurements are reliable and steady. Losses Method: Another method and a more practical approach is to measure the losses and then calculate the efficiency. Efficiency % = 100 – Losses %. The big advantage is that the calculation is on unit basis i.e.: for 1 kg of coal. This eliminates any inaccuracies in flow measurements. Air and gas quantities are determined on theoretical basis and from laboratory analysis of the fuel. This is more accurate than the field flowmeters. Since each loss is separately calculated it is easy to identify problem areas.
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Learning modules material- Engineering (FB) 7.0 Destructive & Non-destructive Testing NDT is an integral part of manufacturing system for quality control of engineering materials during all stages –raw material, in process and finished condition. The main objective of defect prevention and not just detection. In general most NDT methods in use today indirectly measure the overall quality, strength or serviceability characteristics of the items under test be it a material, component or assembly. There are a number of NDT methods available today. However, it is essential to know the material, process, dimension etc to choose the most appropriate NDE methods. The most widely used NDE techniques in the welding industry are : Surface NDT methods Visual Liquid Penetrate Testing (PT) Magnetic Particle Testing (MT)
Voluminar NDT methods Radiographic Testing (RT) Ultrasonic Testing (UT)
VISUAL INSPECTION: Widely used to detect surface discontinuities, visual inspection is simple, quick and relatively inexpensive. The only aid that might be used to determine the conformity of a weld are a low power magnifier, a boroscope, a dental mirror, or a gage. Visual inspection can and should be done before, during and after welding. Visual inspection is useful for checking the Dimensional accuracy of weldments, Conformity of welds to size and contour requirements, Acceptability of weld appearance with regard to surface rough ness, weld spatter and cleanness, Presence of surface flaws such as unfilled craters, pockmarks, under cuts, overlaps, and cracks etc.
MAGNETIC PARTICLE INSPECTION(MPI):This is used for detecting surface and near surface flaws in ferromagnetic materials. It consists of four basic operations ,viz, Establishing a suitable magnetic field in the material being inspected, Applying magnetic particles to the surface of the material ,Examining the surface of the material for accumulations of the particulars (indications ) and finally evaluating the serviceability of the material.MPI is a particularly suitable for the detection of surface flaws in highly ferromagnetic metals. The types of weld discontinuities normally detected magnetic particle inspection include cracks, LOP, LOF, and
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Learning modules material- Engineering (FB) porosity open to the surface. Linear porosity, slag inclusions, and gas pockets can be detected if large or extensive or if smaller and near the surface.
LIQUID PENETRANT INSPECTION(LPI):It is capable of detecting discontinuities open to the surface in weldments made of either ferromagnetic or non-ferromagnetic alloys, even when the flaws are generally not visible in the unaided eye. Liquid penetrant is applied to the surface of the part, where it remains for a period of time and penetrates into the flaws. After penetrating period, the excess penetrant, remaining on the surface is removed. An absorbent, light – coloured developer is then applied to the surface. This developer acts as a blotter, drawing out a portion of the penetrant that had previously seeped into the surface openings. As the penetrant is drawn out, it diffuses into the developer, forming indications, that are wider than the surface openings. The inspector looks for these coloured or fluorescent indications against the background of the developer.
RADIOGRAPHIC INSPECTION (RT): Radiography is most popular NDT method .In industrial Radiography, the usual procedure for producing a radiograph is to have a source of penetrating radiation (X-rays or gamma rays) on one side of the other side. The energy of the radiation must be chosen so that sufficient radiation is transmitted through to the detector. The detector is usually a sheet of photographic film, held in a light tight envelope or cassette having a very thin front surface which allows X-rays to pass through. Discontinuities detectable by radiography include gas porosity, slag inclusions, cracks, lack of penetration, lack of fusion, Tungsten inclusions etc. However enough care should be given to follow the safety procedures to avoid Radiation hazards.
ULTRASONIC INSPECTION(UT): Ultrasonic inspection is a nondestructive method in which beams of high frequency (0.1 & 25 MHz) sound waves are introduced into materials for the detection of surface and subsurface flaws in the materials. The sound wave travel through the material with loss of energy (attenuation) and are reflected to define the presence and location of flaws or discontinuities.UT equipment generates electric signals which are converted to Ultrasound by piezo electric tranducers and the beams reflected from defects are converted back to electric signals and presented in the Cathods Ray Tubes.UT cab detect almost all the internal defects including Cracks, laminations, shrinkage cavities, bursts, pores, lack of bonds, Inclusions etc.
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Learning modules material- Engineering (FB) EDDY CURRENT TESTING(ECT): Eddy current inspection is one of several NDT methods that use the principal of “electromagnetism” as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle. Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right. ECT can be used to detect Surface Breaking Cracks,SBC using Sliding Probes, Metal thinning(Corrosion),Tube Inspection, Verification of Thickness of Thin wall materials, Thickness of Coatings etc. ACCOUSTIC EMISSION TESTING: Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 -12 m) can be identified. Sources of AE vary from natural events like earthquakes and rockbursts to the initiation and growth of cracks, slip and dislocation movements, melting, twinning, and phase transformations in metals. In composites, matrix cracking and fiber breakage and debonding contribute to acoustic emissions. AE’s have also been measured and recorded in polymers, wood, and concrete, among other materials.
BASIC PRINCIPLES OF NON-DESTRUCTIVE TESTING
Non-destructive inspection is a testing technology based on applied physics and is exactly what the name implies-method of testing materials for cracks or flaws without damaging or altering their physical structure. The complex products and new materials that are developed today and introduced in the market need freedom from defects and assurance for trouble-free performance. Appropriate Non-destructive inspection provides such vital assurance by determining the existing state or quality of materials with a view to find out its acceptance for intended end-use and supplements percentage 'destructive-testing' assuring that all materials employed meet the required quality standards and are reliable. Reason for using NDT:
1. Saves lives & prevents accidents: Reliability to protect human life is essential. Proper NDT assures the axle in a supper fast train does not fail at high speed and prevents collapse of landing gears in an aircraft on touch down. The demand for personal safety is a strong force in the development of non- destructive tests.
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2. Ensures Product Reliability: The public expects a fabricated part to give a trouble free performance for a reasonable period of usefulness, better service year by year and longer life for a given product. 3. Makes profit for user which are tangible and intangible 4. Ensures customer satisfaction: Actual Quality and reputation for quality stands high in customer's mind when choosing among products of competing manufactures. 5. Controls Manufacturing process: The operators need be trained and supervised, the process of fabrication then will be controlled. When any element of manufacturing operation goes out of control quality drops & waste increases. 6. Saves Manufacturing costs: NDT locates undesirable defects of a material or component at an early stage. This saves money that would be spent in further processing or assembly. Using a defective material results in waste of labour and time. 7. Maintains uniform quality level: NDT helps to achieve optimum Quality verifying the quality level of a product. Quality below optimum affects reputation. Quality above optimum results in scrap or reworks, reducing profits. 8. Develops Demand for sound Materials: As size and weight decrease factor of safety is lowered. New specified quality levels on raw materials and workmanship rest on NDT for practicality. 9. Aids in product Design: The state of physical soundness as revealed by NDT shows the designer the important areas that need design changes. In a casting the design can be improved and the 'Pattern' modified to increase the Quality of Casting.
PLANNING AND SCHEDULING NDT:
Planning for NDT starts during design stage. For inspections to be meaningful consideration must be given to condition of materials, location and shape of welded joints. Design planning includes avoiding complex weld geometry accessibility for performing examination method planned. E.g. Placement of sources or film in Radiography and to facilitate movement of probes at appropriate skip-positions for scanning in ultrasonic testing, use of ring-forging for critical pressure vessels to avoid longitudinal welds etc.
Timely inspection and good construction standards result in reduction of both costs and delays due to rework. The stages of production at which the inspection is to be conducted should also be preplanned. During fabrication quality plans must be integrated with manufacturing sequence to ensure inspections are performed at proper time and to the requirements of applicable standards.
INSPECTION METHODS:
Non-destructive inspection methods are specified for materials to maintain necessary quality for their final service life. The inspection requirements of materials such as plates, forgings, pipes, valve castings and welded components employed in boiler and pressure vessel industry are stipulated by Codes which further defines sensitivity level of inspection as well as
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Learning modules material- Engineering (FB) objectionable flaw-size. The inspection methods include visual, radiographic, ultrasonic, liquid penetrant and magnetic particle. Radiographic Inspection:
Radiography is concerned with studying the homogeneity of opaque materials using penetrating radiation and is mainly used for volumetric examination of materials and welds to detect internal discontinuities present and to find-out assembly errors. It is based on differential absorption of penetrating radiation by the part being inspected. The degree of absorption depends upon the thickness as well as physical density variations in the object. The forms of penetrating radiation that are used in radiographic inspection of pressure vessels and valves are X-rays and Gamma rays. X-rays are generated in vacuum tubes when accelerated electrons are stopped by metals like tungsten. Gamma rays originate from artificially produced radioisotopes when elements such as Iridium and Cobalt are treated in a reactor with a flux of neutrons.
Radiography is the most understood and widely accepted Non-Destructive Evaluation (NDE) method. The major reason for its use is that the radiograph of a three dimensional object provides a permanent visual record as a two-dimensional interior image. In a radiograph the length and width of the part under examination is more or less truly recorded whereas the thickness plane is distorted and reduced to blacks and grays. The grays and blacks that may result from voids, changes in thickness and variations in density of materials are properly interpreted to compare them with predetermined acceptance standards. The ability to characterize indications-grays & blacks-appearing on a radiograph needs training and experience added along with it over a period of time makes one capable of distinguishing slags, cracks and lack of fusion in a weld radiograph. 113
Learning modules material- Engineering (FB) The acceptance standards are developed according to the limits of radiography and the quality level obtainable by the manufacturing practices used in making pressure vessels or valves and are well-defined in codes and standards. As a permanent visual record in the form of a radiograph is available the NDT inspectors or customers can review the radiograph at any point of time to ensure the requirements of the product have been complied with.
Application: Inspection of welds, castings & Assly.
LIMITATIONS OF RADIOGRAPHY
1. 2. 3. 4.
Impracticable to use on specimens of complex geometry. The specimen must lend itself to two side accessibility. The greatest dimension of suspected discontinuity must be ”parallel to radiation beam. Narrow discontinuities and laminar type of discontinuities are often undetected by Radiography. 5. Thick specimens require equipment of high energy potential, requires costly space utilisation and construction practices. Hence it is a relatively expensive means of Nondestructive testing. 6. Safety considerations imposed by X and Gamma rays must be considered. 7. Radiography is time consuming.
Ultrasonic inspection:
Ultrasonic Inspection is another volumetric inspection method to detect internal discontinuities similar to radiography. Both radiography and ultrasonics are not 'substitutes' to each other but rather they complement or supplement each. The basic principle is that of 'ECHO'- reflection of ultrasound by voids. High frequency electrical signals are converted as mechanical vibrations in a piezo-electric crystal; these vibrations form a wave-front and are coupled to parts under inspection with suitable medium such as oil, grease or water. These vibrations propagate through materials in longitudinal, shear or surface modes and are reflected by any metal-air interfaces that are oriented approximately normal to incident sound wave. The reflected waves from such interfaces or flaws are directed back to the same transducer and are converted as electrical signals to be presented as a pip or vertical deflection in a cathode ray tube screen (CRT) of an ultrasonic flaw detector. Straight beam ultrasonic inspection, where sound beam enters vertically from the surface of inspection is specified to detect lamination in plates and internal discontinuities in forgings and castings. Angle beam inspection ,where sound beam enters inside material at pre-determined angles to the surface, is specified for welds, ring forgings and pipes to detect cracks and inclined 114
Learning modules material- Engineering (FB) flaws. It is mandatory however the examination shall be conducted to a detailed procedure. The procedures generally refer to the amplitude of signals obtained from a calibration notch or cylindrical holes as a basis for interpretation of discontinuity signals presented on the CRT. It must be also remembered that not all slag inclusions or cracks present in production materials would produce a similar signal amplitude response and it is difficult to distinguish planar flaws (cracks) from linear-flaws. The acceptance standards of radiography and ultrasonics are almost one and the same. However ultrasonic inspection will detect cracks and side-wall lack of fusion in welds better than radiography. Another advantage of ultrasonics over radiography is its capability to size the discontinuities in the direction that reduce the cross-sectional thickness providing a base line data during pre-service inspection to make it possible to monitor growth of flaws. Ultrasonic testing is more operator dependent test method as the presentation of discontinuity indications is by indirect means and the test results depend on the interpretation skill of the operator. Advanced imaging and automatic scanning techniques reduce dependency on operators and ensure that pressure vessels remain fit for continued service by locating, characterizing and sizing the discontinuities more accurately.
NORMAL BEAM TESTING
ANGLE BEAM TESTING
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UT APPLICATIONS:
Because Ultrasonic techniques are basically mechanical phenomena, they are particularly adaptable to the determination of structural integrity of engineering materials. Their principle application consists of: 1. 2. 3. 4. 5. Flaw detection Evaluation of the influence of processing variables on materials and process. Thickness measurement Study of metallurgical structures Determination of elastic moduli
Advantages of Ultrasonic Test:
1. 2. 3. 4. 5.
High sensitivity, permitting detection of minute defects Great penetrating power, allowing examination of extremely thick sections Accuracy in the measurement of flaw position and estimation of flaw size Fast response, permitting rapid and automated inspection Need for access to only one surface of the specimen
Limitations of Ultrasonic Test
1. Unfavorable sample geometry; for example, size, colour, complexity and defect orientation. 2. Undesirable internal structure; for example, grain size, structure porosity inclusion content, or fine, dispersed precipitates. 3. Coupling and scanning problems 4. Difficult to detect point reflecors such as porosity
Magnetic Particle Inspection:
Magnetic Particle examination is widely used on ferromagnetic parts on edge preparations of welds and on the welds before and after the vessel has been subjected to hydrostatic test. Magnetisation is done by passing current through copper electrodes and sometimes by a hand116
Learning modules material- Engineering (FB) held Electro-magnetic yoke. This technique is best employed for both detecting surface discontinuities that are open and too fine to be seen by naked eye and any discontinuities that may lie below surfaces. When a part or area of a part is magnetised, magnetic lines of force or magnetic flux will be developed. If a medium such as iron-powder is dusted on the surface of the part during magnetisation, any discontinuity which interrupts the lines of force more or less in perpendicular direction will set-up flux-leakage at the surface and thereby attracting the powder to form a build-up resembling the discontinuity. All linear discontinuities that are greater than 1.5 mm are generally analyzed and taken for re-work .
CIRCULAR MAGNETISATION- PROD INSPECTION METHOD
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LONGITUDINAL MAGNETISATION- YOKE
Application
1. Widely used both in fabrication and maintenance of ferritic materials 2. Used for inspection of castings and ferritic Butt, corner &Tee joints in boiler & pressure Vessels.for defect locations and to identify lamination and cracks on gas cut edgess of plates-and exposed faces and edge prepared grooves in ferritc joints. 3. Any modification work during periodical overhaul of the boiler & Nuclear is tested by magnetic particles inspection method. Limitation 1. Metals such as austenitic stainless steels, Aluminum alloys ,titanium, and non-mtals such as ceramics and composites (non-ferro magnetic materials ) reduce scope for application of Magnetic Particle Inspection. 2. Effective for for surface and near-surface defet detection only and components need to be magnetised in more than ONE direction. Materials of complex geometry pose problems for testing. 3. Difficulty in btaining permanent recordsof test. 4. Certain components need demagnetising after test.
Liquid penetrant Inspection:
Liquid Penetrant examination: is used on all ferro and non-ferromagnetic materials that are essentially non-porous. They are also applied on edge preparation of welds, on the final welds after the vessel has been subjected to hydrostatic test. Liquid penetrant that is deep red in colour seeps or pulled into minute surface openings or cavities or cracks by capillary action. The surface is cleaned-off the excess penetrant after a period of penetration time using a cloth by wipe technique. A thin developer coating made on the dried surface reveals the presence of flaws as a red indication against white developer background, which makes the flaw indications easily noticeable. The limitation of this method is the defects should be open and
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Learning modules material- Engineering (FB) connected to surface. Hence this is considered as an aid to visual inspection as the discontinuities are too fine and are not seen by naked eye. Spherical or round indications are tolerated to a certain size but all linear discontinuities that are greater than 1.5 mm are generally probed for existence of material-separation and taken for re-works, if found to be so.
Reverse Capillary action- Blotting action
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Applications:
1. Penetrant Inspection is widely used both in fabrication and maintenance. -Fillet welds in drum-dished ends, header hand hole plates etc. are inspected by penetrant test . 2. Any modification work during periodical overhaul of the boiler as is scalloped bar welding; attachment welding in super heaters is tested by penetrant inspection method. 3. As it can be applied on any kind of material unlike magnetic particle inspection and as is free from any external energy like electricity for operation, and as it can be operated by minimum technical knowledge, 4. It is very versatile test method. But before putting the chemicals into use, it is to be ascertained that the halogen and sulfur content should not be harmful to the material being tested. 5. The testing area should be properly ventilated, so that the chemicals will not be hazardous to the operator. Exhaust fan should be provided when test is conducted inside closed vessels.
Limitations:
The major limitation of liquid penetrant inspection is that it can detect only imperfections that are open to the surface. Extremely rough or porous surfaces are likely to produce false indications. 8.0 Water Chemistry
Water treatment
General The major objectives of water chemistry programme in modern high-pressure boilers are the control of corrosion, deposition and achieving desired steam quality. Internal corrosion and deposition cost power plants crores of rupees in repairs and maintenance. Steam turbines rated for high capacities call for stringent steam quality to avoid damages and to maintain the rated output. Thus the successful operation of power plant requires a thorough understanding of all aspects of water treatment. Make-up water treatment: Raw water contains suspended solids, dissolved solids, dissolved gases and organics. The suspended solids and turbidity are generally removed by pre-treatment stage comprising of coagulation, clarification and filtration. The removal of suspended impurities is important for efficient operation of the demineralising plant. The dissolved solids and silica are required to be 121
Learning modules material- Engineering (FB) maintained at very low levels in boiler at high pressures to control deposition and to maintain steam quality. The dissolved solids and silica are removed in the demineralising plant and thereby producing acceptable make-up water quality for high-pressure boilers with specific electrical conductivity of less than 0.2 µS / cm with hardness completely removed and silica less than 0.02 ppm. Feed water treatment: The reaction of feed water and steel is spontaneous and rapid at high temperatures. The corrosion end product, magnetite (Fe3O4) forms a protective barrier on the boiler steel surfaces which minimizes further corrosion. The work of Bell and Van tack (fig.1) has been used to relate the relative corrosion of steel over a wide range of pH values. Minimum corrosion of steel is indicated at pH values of 8.5 to 11.0. Considering boiler components with mixed metallurgy, an optimum pH of 8.8 to 9.2 is recommended for feed water to minimise the pre-boiler corrosion. Ammonia is generally used to elevate the pH of feed water by the feed of not more than 0.5 ppm of ammonia to minimise copper corrosion. The exclusion of dissolved oxygen, another contributing element in corrosion of steel is essential to avoid corrosion in feed water. With main oxygen removal by deaeration, residual oxygen (0.01 to 0.02 ppm) in small quantities can be reduced further by reducing agents such as sodium sulphite for low pressure boilers and hydrazine for high pressure boilers. Boiler water treatment: It is recommended to use co-ordinated phosphate-pH treatment method for high pressure drum type boilers, by the injection of mixtures of phosphates in to the boiler drum so that sodium to phosphate molar ratio is less than 3.0. This method of treatment excludes free caustic from boiler water. Even if bulk boiler water does not contain large amount of caustic, there is a great potential for caustic to concentrate under deposits and cause corrosion . Oxygenated treatment: In oxygenated treatment [OT], Oxygen is deliberately injected in a controlled manner into the boiler feedwater to maintain a 50 to 150 ppb residual. Ammonia (20 to 70 ppb) is added to 122
Learning modules material- Engineering (FB) raise the pH to a range of 8.0 to 8.5. Compared to AVT programmes, iron oxide generation (due to corrosion of feedwater system) has been observed to be much less with OT programmes. With the controlled injection of oxygen, the base layer of magnetite becomes overlaid and interspersed with a tighter and impervious film of ferric oxide hydrate (FeOOH). This compact layer is more stable than magnetite and releases very little dissolved iron or suspended ironoxide particles to the fluid. The key to an OT programme is control. Input water must be extremely pure (cation conductivity < 0.15 µS / cm). Poor water quality would cause deposition leading to differential oxygen cells formation and subsequent under-deposit corrosion and pitting. Hence boilers operating on OT are always equipped with condensate polishers. OT cannot be used in systems that contain copper-alloy feedwater heater tubes, as the corrosion would be very high. OT has become very popular in once-through units. Chemical cleaning: In spite of high purity make up water and improved boiler water treatment program, trace levels of corrosion products in feed water and impurities from condenser leakage are carried over and accumulated on boiler heating surfaces. Water side deposits can reduce heat transfer, influence corrosion of the underlying metal and ultimately result in tube failure if they become excessive. If boiler performance is to be maintained at design values and availability is to be assured, deposits will have to be removed periodically by chemical cleaning.
The principal criterion for determining the need to clean boilers is the deposit density in milligram per cm2. BIS: 10391-1982 provides guidelines for permissible deposit weight limits and its relationship with boiler cleanliness for high pressure subcritical boilers.
Internal deposit weight (mg/cm2)
< 15.0 15 – 40 > 40
unit cleanliness
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Chemical cleaning is recommended whenever deposit weight is more than 40.0 mg/cm2. These limits have proven effective in avoiding overheating and corrosion related problems in a large number of utility boilers.
RECOMMENDED FEED WATER LIMITS
DRUM OPERATING PRESSURE kg/cm2 (g) TREATMENT TYPE 1.Hardness ppm (max) 2.pH at 25℃ Mixed metalallurgy All ferrous metallurgy 61-100 PO4 NIL 8.8-9.2 9.0-9.4 100 and above PO4 NIL 8.8-9.2 9.0-9.4 0.20 AVT NIL 8.8-9.2 9.0-9.6 0.20
3.Sp. electrical conductivity after cation 0.30 exchanger in H+ form at 25℃, microsiemens/cm (max) 4.Dissolved oxygen, ppb (max) 5.0 5.Silica as SiO2 , ppb (max) 6.Iron as Fe , ppb (max) 7.Copper as Cu , ppb (max) 8.Residual Hydrazine , ppb 9.Total organic carbon, ppb (max) 20.0 10 5 10-20 200
5.0 20/10* 5 3 10-20 200
5.0 10 5 3 10-20 200
* Should match with the corresponding values to be maintained in super heated steam.
BOILER WATER LIMITS
(FOR DRUM TYPE BOILERS- NORMAL OPERATION) DRUM OPERATING 61-90 PRESSURE kg/cm2 (g) TREATMENT TYPE PO4 1.Total Dissolved solids ppm (max) 100 91-125 PO4 100 125-165 PO4 50 165-180 PO4 15 AVT 2.0 181 & above PO4 10 AVT 1.0
2. Sp. electrical 200 conductivity microsiemens/cm (max)
200
100
30
4.0
20
2.0
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3. Silica as SiO2 ppm (max) 4.Chlorides as Cl ppm (max) 5. pH at 25℃
To be controlled as per fig. 2&3
3.0 9.0 10.0 2.0 – 9.0 – 10.0 1.4 9.1 – 9.8
0.1 0.6 9.1 9.6
0.1 0.03 – Note
0.1 0.5 9.1—9.6
0.1 0.02 Note
6.Phosphate residual 5-20 5-20 5-10 2-6 N/A 2-6 as PO4 ppm NOTE: pH should be monitored continuously; immediate shutdown if the pH goes below 8.0
N/A
GUIDELINES FOR EMERGENCY OPERATION
(DRUM TYPE – PHOSPHATE TREATEMENT)
SL.No.
01
Pressure range Hot well kg/sq.c.m (g) solids ppm 61-125 0.5-2.0 (ABNORMAL)
Operational Limitations Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3 Limited operation Note.1 Emergency OperationNote.3
Control Limits
Boiler water Control TDS<200 ppm NOTE 2 pH 9.1-10.1 PO4 5-40 ppm -DONOTE 4
>2.0 (EXCESSIVE) 02 126-165 0.5-2.0 (ABNORMAL) >2.0 (EXCESSIVE) 03 166-180 0.25-1.0 (ABNORMAL) >1.0 (EXCESSIVE) 04 181-203 0.1-1.0 (ABNORMAL) >1.0 (EXCESSIVE)
TDS<100 ppm NOTE 2 pH9.1-10.1 PO4 5-20 ppm -DONOTE 4
TDS<50 ppm NOTE 2 pH 9.1-10.1 PO4 5-20 ppm -DONOTE 4
TDS<50 ppm NOTE 2 pH 9.1-10.1 PO4 5-20 ppm -DONOTE 4
NOTE 1: NOTE 2:
Schedule Inspection and repair of condenser as soon possible Immediately start chemical injection to achieve higher phosphate and pH conditions,
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Do not continue operation if pH cannot be maintained above 8 total solids below Specified limits. Avoid use of desuper heating spray. NOTE3: Immediately reduce load to permit isolation of damaged condenser and prepare for orderly shutdown if hot well TDS cannot be reduced quickly below specified limits. NOTE4: NOTE5: Prepare for wet lay up of the boiler. Control silica in boiler water in accordance with graph provided.
HOT WELL CONDITIONS FOR ALL VOLATILE TREATMENT
(FOR DRUM TYPE BOILERS) PRESSURE RANGE (kg/sq.cm) 126-165 Above 166 Note: HOT WELL SOLIDS (PPM) NORMAL OPERATION < 0.05 < 0.05 EMERGENCY OPERATION < 0.1 PPM < 0.25 PPM
Switch over to phosphate treatment when hot well solids exceed emergency operation levels.
GENERAL INSTRUCTIONS 1. All Feed water measurements shall be made at high pressure heater outlet or economiser inlet 2. Oxygen can also be additionally measured at deaerator outlet to determine the quantity of N2H4 dozing. 3. The recommended pH in feed water can be obtained by dozing ammonia, morpholine or any volatile amine. The concentration of volatile chemical in the feed water should not exceed 0.5 ppm. (expressed as Ammonia) 4. The phosphate and pH are recommended in accordance with co- ordinate phosphate curves (Figs.4 to 6) to prevent presence of free hydroxide in boiler water. 5. Water levels in drum should be maintained within limits during all operational modes, start - up, load fluctuation and normal operation. 6. The alignment of drum internals should be checked and ensured to be in order at least once every year. 7. It is needles to emphasize that correct sampling accurate measurements with the use of reliable at adequate intervals and proper logging of reading go a long way in ensuring trouble free operation.
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RECOMMENDED FEED WATER AND BOILER WAETR LOW AND MEDIUM PRESSURE WATER – TUBE BOILERS
[For drum pressure below 60 kg / cm2(g)]
LIMITS
FOR
GENERAL The following recommendations hold good for water – tube boilers with drum pressure upto 60 kg/cm2 (g) where high purity feed water is available by use of demineraliser. It is recommended that demineraliser shall only used for all chemical recovery boilers irrespective of drum pressure.
RECOMMENDED FEED WATER LIMITS -- (Note 6) Drum Operating pressure [kg/cm2 (g)] Hardness Max-ppm pH at 25℃ Oxygen max – ppm Total iron max –ppm Total copper max –ppm SiO2 max –ppm Upto 20 1.0 8.8-9.2 0.02 0.05 0.01 1.0 21-40 0.5 8.8-9.2 0.02 0.02 0.01 0.3 5.0 41-60 Nil 8.8-9.2 0.01 0.01 0.01 0.1 2.0 Note 4 Note 4 Remarks Note 4 Note 1
Conductivity at 25℃ measured 10.0 after cation exchanger in H+ form and after CO2 removel max .( µs /cm) Hydrazine residual –ppm -
-
0.02-0.04
RECOMMENDED BOILER WATER LIMITS Drum Operating pressure [kg/cm2 (g)] pH at 25℃ Phosphate residual -ppm TDS – max – ppm Specific electrical conductivity at 25℃ max –(µs/cm) Silica max –ppm Sodium Sulphite as Na2SO3-ppm 10.0-10.5 20-40 500 1000 25.0 20-40 10.0-10.5 20-40 200 400 15.0 5-10 9.8-10.2 15-25 150 300 10.0 Note 2 Note 3 Note 3,5 Less than 20 21-40 41-60 Remarks
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NOTE:1. Morphline or any other volatile amine can be used toelevate pH. The concentration of volatile chemical in feed water shall not exceed 1 ppm (expressed as Ammonia ) Sodium sulphate shall be dozed in the feed water, after the tapping point for Desuperheating spray so that it does not get contaminated. The phosphate and pH shall be maintained in accordance with co-ordinated phosphate –pH curve (fig 7) to prevent pressure of free hydroxide in boiler water . If feed water is used for Desuperheating spray, (a) Hardness shall be nil (b) SiO2 shall not exceed 0.02 ppm (c) Conductivity at 25℃ measured after cation exchanger in H+ form after CO2 removal shall not exceed 2 micro mho/cm. Total Alkalinity in boiler water shall not exceed of TDS. Pressure of oil or organic matter is not allowed in feed water which will induce foaminess and cause carryover of impurities into steam.
2. 3.
4.
5. 6.
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FIG 2
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FIG 3
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FIG 4
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FIG 5
FIG 6
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FIG 7
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9.0 Boiler operation, Availability and Reliability, Boiler Tube failure mechanisms.
9.1 Boiler Operating Modes:
Increasing focus on plant efficiency, need to respond to grid fluctuations, conservation of fuel etc. warrant the boilers to be designed to meet varying operating modes like: Base Load Operation Cycling Two-shift Constant pressure Sliding pressure ( Natural & Modified) Trip to house load Response to FGMO( Free Governing Mode of Operation)
. Natural sliding pressure operation has the advantage of essentially eliminating first stage temperature changes inside the turbine, as long as the main steam and reheat steam temperatures are held constant. 134
Learning modules material- Engineering (FB) However, the response of the steam generator-turbine combination, with control valve in wide open condition, to changes in load demand is relatively slow and may not satisfy the power system control requirements. In order to overcome this, the modified sliding pressure operation is in use. In the modified sliding pressure mode, the steam pressure is kept constant down a certain load say from 100% to approx. 90% and then allowed to slide down along with the load. By this, turbine inlet valves are kept at a position lower than full open position and the admission cross section at the turbine is altered briefly when the load is varied, so that the accumulated steam in the steam generator is discharged at once. The dynamic behaviour of the steam generator is improved to take care of load fluctuation to a limited extent.
9.2
Availability & Reliability
Plant availability and reliability can be improved by adopting various measures like: Use of well proven equipment & design practices Redundancy for Critical equipment Use of early warning systems Tube leak detection system Operator alarms Sophisticated controls, Instrumentation, safety interlocks Vibration monitoring Equipment trip provisions Leveraging IT for performance analysis & optimization and for smart operation
9.3 Boiler Tube failures
9.3.1 Major Tube Failure Mechanisms : The 22 primary mechanisms responsible for boiler tube failures as per the EPRI (Electric Power Research Institute) statistics are given below. Table 1: Major Tube Failure Mechanisms
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Learning modules material- Engineering (FB) Stress Rupture
Short Term Overheating High Temperature Creep Dissimilar Metal Welds
Fatigue
Vibration Thermal Corrosion
Water-side Corrosion
Caustic Corrosion Hydrogen Damage Pitting Stress Corrosion Cracking
Erosion
Fly Ash Falling Slag Soot Blower Coal Particle
Fire-side Corrosion
Low Temperature Water wall1 Coal Ash1 Oil Ash
Lack of Quality Control
Maintenance cleaning damage Chemical Excursion damage Material Defects Welding Defects
1. Not observed in Indian power stations.
9.3.2 Aspects to be considered for boiler pressure parts Availability at various stages : Design stage: Selection of material during design stage should take care of the following aspects. Compatible for working pressure / temperature. Based on quantity of steam flow and the velocity / pressure Heat transfer characteristics / surface effectiveness / metal temperature Thermal expansion / constraints Radius of bends Attachments Weldments
Manufacturing aspects : Most of the pressure parts of the high pressure and high temperature boilers are manufactured by welding. Therefore the weld design and welding process are to be carefully chosen. Since the different material sizes and specification are used in combination for an optimal design at economic cost, the welding process to be adopted do vary widely. preheating, post heating, stress relieving are important factors included in the welding process to ensure flawless fabrication. Transportation / Handling :
136
Learning modules material- Engineering (FB) Long pipes, panels of tubes and headers do suffer due to self weight. Hence, during handling (loading, unloading) enough care to be taken so that permanent sag does not setin in the components. Cases of tube failures have been faced due to scoring / cutting cost by wire ropes used for tying up during transportation. Storage : Normally the large capacity boilers takes several months for erection and commissioning. Hence the materials received from manufacturers are to be properly stored at erection sites. Particularly pressure part tubes are to be preserved by properly closing the open ends with end covers after putting the necessary preservatives. Open yard storage for prolonged duration may need periodical repainting of tubes with rust preventive coating. Plants near to costal area needs extra care due to salubrious atmosphere prevalent.
Erection : The high capacity of boilers are manufactured in parts and pieces from the point of view of handling, transportation and site erection. For facilitating field erection it is recommended for sequential erection with different modules which has to be adopted at sites. Welding at site needs due care and carried out as per recommended weld procedures to minimize / avoid the pressure part failures later, during service of the boiler.
Commissioning : After completion of erection, normally a hydraulic testing is done at site even though various pressure parts are hydro tested at manufacturing works. Since there are many site weld joints involved at erection sites the soundness and reliability of these are ensured by field hydro testing. Moreover this is also a statutory requirement as per IBR. Followed by hydro testing alkali boil out and acid cleaning and steam blowing operations are carried out to remove the loose deposits, mill scale, weld slag, muck, loose rest etc. During these commissioning activities especially during hydro test any weak / defective weld joint will be revealed during this hydro test and the same can be corrected. Operation and Maintenance: As per the statistical evidences gathered so far, the pressure part tube failures due to design / manufacture / erection and commissioning accounts for 20 to 30% whereas the rest 70 to 80 % are traceable to operation and maintenance. By taking prudent steps in O&M, the number of failures can be minimized if not eliminated.
Tube failure reduction through operation and Maintenance Over heating – short term / long term: 137
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Overheating of tubes may be due to any or combination of the following: Partial or full choking of tubes resulting in flow starvation. Internal deposits due to poor water chemistry, carry over of solids from the steam drum. Flame impingement due to faulty or improperly aligned burners. Extracting more auxiliary steam flow than the designed level may also lead to starvation in super heater / reheater.
Excess air levels: In order to ensure complete combustion, it is usual practice to operate the boiler with 5 to 10% (oil & gas firing), 15 to 20% (coal firing) more air than the stoichometric quantity. But too much of excess air leads to cooler furnace and higher heat absorption in convective paths. Too little excess air leads higher furnace temperature result in higher radiation pickup and also promotes slagging in the furnace. More excess air also accelerates the erosion potential of the ash and external metal wastage resulting in thinning of tubes and consequent tube failure. Fuel types : Fuel oil (% of sulphur and vanadium content): Vanadium present in the fuel gives raise to high temperature corrosion. The sulphur content in the fuel results in cold end corrosion when the sulphur percentage increases.
Coal : The coal having higher ash content and higher percentage of Alpha quartz results in higher erosion rate and metal wastage in the convection pass especially in the LTSH and economizer zones. When firing coal with ash containing more than 4% sodium having conventional tubes spacing results in severe plugging in gas path leading to overheating failures. Other fuels : In Industries the process wastes, industrial gases and bio fuels when used bio mass when used as fuel in steam generators, the properties of such fuels are to be analysed in detail and steam generator are to be designed and operated cautiously and carefully according to their characteristics to avoid any unforeseen failures. Soot blowing : 138
Learning modules material- Engineering (FB) While soot blowing helps in cleaning any ash / slag deposits on the tubes, excessive pressure of steam used for blowing may cause external metal wastage due to erosion of steam with fly ash in the flue gas. Excessive usage of soot blowers, struck-up soot blower lance which fail to retract, jammed soot blower valves, condensate in blowing steam, etc., are few of the reasons causing erosion of pressure part tubes and consequent failures. Correct drum level operation: Incorrect drum level operation beyond permissible limits is unsafe. Low drum level leads to steam entrapment in the down comer, leading to sudden loss of density of circulating fluid in the down comer. This in turn, affects the circulation in the water wall tubes resulting in overheating of tubes. The damage may range from busting of water wall tubes to bowing in of the water wall panels. Snapping of the buck-stay connections to the water walls may also occur. Typical situations leading to low drum level operation are as follows: Stoppage of feed water to the boiler due to inadvertent closure of feed system valves or loss of deed pump. Lack of calibration of drum water level measurement or control instrumentation. Level protection devices either by passed or in-operative. Operation of the Unit without requisite level instrumentation
Water Chemistry: Quality of water and steam used in modern high pressure steam generator is of utmost important for trouble free performance of fossil fuel fired steam generators. When water chemistry are maintained within the prescribed limits recommended by the designers or qualified consultant, corrosion damage may occur in water wall and economizer tubes. Water wall corrosion problems can generally be avoided in boiler if the following points are taken care : Recommended water treatment controls are followed. Corrosion products formed in the feed water system are kept within the specified limits. Feed water oxygen concentration is properly controlled. Precaution are taken during chemical cleaning operations to prevent metal attack.
Tube internal deposits problems can be avoided if: Hydraulic test water, superheater fill water and de-superheater spray water are free of solids. It is preferable to use DM water for these operations. Drum internals and drum water level controls are maintained in good working order.
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Learning modules material- Engineering (FB) Silica concentration in the boiler water is held within the acceptable limits.
Low pH damage: Corrosion failures occur when acid or alkaline salts are concentrated. Hydrogen induced brittle fracture occurs beneath a relatively dense deposit and is most likely to occur when boiler water pH is too low. Some of the hydrogen produced in the corrosion reaction diffuses into the tube metal where it combines with carbon in the steel. Methane is formed and it exerts internal pressure within the steel causing grain boundary fissuring. Brittle fracture occurs along the partially separated boundaries. In many cases an entire section is blown out of a damaged tube leaving window opening. Restoration of proper boiler water treatment may not be sufficient to prevent further hydrogen attack unless the dense corrosion product deposits are removed. Even repeated chemical cleanings sometimes will not remove them. Replacement of tubes where metal attack exists becomes necessary. Generally Hydrogen damage is difficult to detect using NDT means. To some extent UT may pin point some damaged areas, but positive identification of all failure prone tubes is not possible.
High pH damage: Concentrated hydroxide salts such as sodium hydroxide in the boiler water will cause gouging type of corrosion leading to ductile failures. Ultrasonic tube wall thickness checks can detect tubes with metal loss. Proper boiler water treatment can minimize further corrosion. Minimising corrosive attack : Concentration of salts promoting corrosion generally forms at the surfaces of the tubes when acidic or alkaline producing environment prevails. This condition may happen when water treatment conditions deviate from the recommended parameters. If the recommended specifications are followed during operation of the Boilers, the corrosive attack can be minimized. Causes of high and low pH: It is a well known fact that the high or low pH do cause damage to pressure parts as explained in the previous paragraphs. Condenser leakage is the primary cause for acidic and caustic boiler water conditions. The raw cooling water that leaks into the condenser essentially ends up in boiler water. The cooling water source determines whether the inleakage is either acidic producing or caustic producing. Fresh water from lakes and rivers usually provide dissolved solids that hydrolyze in the boiler water environment to form a caustic such as sodium hydroxide. By contrast, sea water and water from re-circulating cooling water systems with cooling towers contain dissolved solids that hydrolyze to form acidic compounds.
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Learning modules material- Engineering (FB) Another potential source of acidic and caustic contaminants is the make up demineralised, where regenerant chemicals like, sulphuric acid and caustic may inadvertently enter the feed water system. Chemicals incorrectly applied during boiler water treatment also can be corrosive. For example sodium hydroxide used in conjunction with sodium phosphate compounds to treat boiler water. Corrosion can occur if the sodium hydroxide and sodium phosphate are not added to the water in proper proportion. Minimising pitting of Boiler tubes : Pitting caused by dissolved oxygen can be prevented by maintaining feed water oxygen level within the 5 ppb limit. Attack by chemical cleaning solvents can be eliminated by carefully following the cleaning procedures. During shut down period it is necessary to protect all internal surfaces. Wet lay-up together with a positive nitrogen pressure cap of about 3.5 psig will protect metal surfaces from corrosion. Avoiding steam side deposition: Internal surfaces of steam side components like superheaters and reheaters are deposited with salts carried over along with steam from drum. These deposits impairs with heat transfer because of its insulating effect and leads to overheating failures. Boiler manufacturers help in limiting the solid carry over from steam drum by proper design and fitment of drum internals. From operational point of view proper blow – down, controlled dosing and proper drum level control will help in avoiding deposits in superheaters / reheaters. Solids carry over by the steam into the turbine also can cause turbine damages. To sum up, good water chemistry with prudent boiler operations with vigil will go a long way in minimizing the tube failures to a great extent. While extensive elaboration on technique of water treatment and water chemistry is outside the purview of this paper, an attempt has been made to give a brief coverage in this paper. Hydrogen Damage: Boiler tube failures caused by hydrogen damage result from fouled heat transfer surfaces and an acidic (low pH) condition of the boiler water. Hydrogen damage is some times referred to as “ hydrogen attack‟ or hydrogen embrittlement”. The tube steel will become brittle from the combination of hydrogen and carbon, which forms gaseous methane (CH4) at the grain boundaries in the tube steel. Hydrogen damage develops from the generation of hydrogen during rapid corrosion of the internal surface of the tube. The hydrogen atmos `migrate through the tube steel where they can react with the iron carbide (Fe3C) to form the methane. The larger methane gas molecules become trapped between the grain boundaries and cause a network of discontinuous internal cracks to be produced. These cracks grow and some will link up to cause a through wall fracture. Causes of Hydrogen Damage : Hydrogen is generated under the following circumstances :
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Learning modules material- Engineering (FB) Operation of the boiler with low pH water chemistry from the ingress of acidic salts through condenser leakage, contamination from chemical cleaning, or malfunction of the chemical control subsystem. Concentration of the corrosive contaminants within the deposits on the internal tube wall, especially in crevices, shallow pits, and under weld backing rings.
Caustic corrosion: Boiler tube failures caused by caustic corrosion result from fouled heat transfer surfaces and an active corrodants in the boiler water. Caustic corrosion is some times referred to as “caustic attack”, “caustic gouging, “or “ductile gouging”. Caustic corrosion develops from deposition of feed water corrosion products in which sodium hydroxide (Na OH) can concentrate to high pH levels. At high pH levels, the tube steel‟s prot ective magnetite oxide layer is solubilized and rapid corrosion occurs. The tube surface deposits accumulate at locations where flow is disrupted such as just downstream of welds with backing rings, at bends, in horizontal tubes heated from above or below, and at high heat input locations. Most of the feed water corrosion products deposit on the heated side of the furnace wall tubes since deposition is heat flux related and will favor the tubes within the highest heat absorption zones. Causes of Caustic Corrosion: Caustic corrosion occurs through: Selective deposition of feed water system or pre-boiler corrosion products at locations of high heat flux. Concentration of sodium hydroxide from boiler water chemicals or from upsets in the water chemistry.
Stress Corrosion cracking: Boiler tube failures have been caused by stress corrosion cracking (SCC) and result from the combined effects of tensile stress, a corrosive environment, and a susceptible material. Stress corrosion cracking failures in a boiler usually occur in the austenitic stainless steel used for superheater and reheater tubing. However, SCC failures have occurred in some ferritic reheater tubing when high levels of caustic were introduced from the desuperheating or attemperator spraying station. Causes of Stress Corrosion Cracking : Conditions for stress corrosion crack initiation and propagation exist under the following circumstances: Contamination of boiler water or steam with chlorides or hydroxides. Introduction of high stresses from service conditions. Production of high residual stresses during fabrication and assembly. 142
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Tube failures indications: During the regular operation of the Steam Generator if there is any tube failure occurs it will be indicated by any one or combination of the following points: Sudden or abnormal loud noise Continuous hissing noise Furnace draft fluctuations / pressurization Flame out Falling drum level Increased quantum of make-up water Steam / Water leakage Uncontrolled Boiler Feed Regulation / Feed Pump Trip
SHORT OVERHEATING
TERM
LONG OVERHEATING
TERM
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STEAMSIDE OXIDE SCALE
OVERHEATING, CREEP INCORRECT MATERIAL
–
OVERHEATING – BULGING, SATELLITE SCALE CRACKING
OVERHEATING DEPOSITS
–
WATERSIDE
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DISSIMILAR METAL FAILURE IN SERVICE
WELD
DISSIMILAR METAL WELD MICRO AT TRANSITION
FLATTENING TEST FAILURE HYDROGEN EMBRITTLEMENT
GRAIN BOUNDARY CRACKSHYDROGEN EMBRITTLEMENT
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HYDROGEN EMBRITTLEMENT ETCHED
ROUNDED DISSOLVED PITTING CORROSION
GAS
LONG TERM OVERHEATING WATERSIDE DEPOSTS
DISSIMILAR METAL WELD FAILURE
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CAUSTIC GOUGING STRATIFICATION
CAUSTIC GOUGING
HYDROGEN EMBRITTLEMENT
HYDROGEN EMBRITTLEMENT
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DISSIMILAR METAL FAILURE IN SERVICE
WELD
DISSIMILAR METAL WELD MICRO AT TRANSITION
FLATTENING TEST FAILURE HYDROGEN EMBRITTLEMENT
GRAIN BOUNDARY CRACKSHYDROGEN EMBRITTLEMENT
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HYDROGEN EMBRITTLEMENT ETCHED
ROUNDED DISSOLVED PITTING CORROSION
GAS
STRESS CORROSION CRACK IN COLD BENT TUBE
TRANSGRANULAR STRESS CORROSION CRACKS
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INTERGRANULAR STRESS CORROSION CRACKS
CORROSION FATIGUE CRACKS
CORROSION FATIGUE CRACKS
SIDE WALL LACK OF FUSION IN A WELD
DISSOLVED OXYGEN PITTING CORROSION
EXTERNAL PITTING IMPROPER STORAGE
–
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STRESS CORROSION CRACK IN SS 304H
LOW TEMPERATURE CORROSION
LOW TEMPERATURE CORROSION
WATER WALL CORROSION
FIRE
SIDE
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COAL ASH CORROSION
OIL ASH CORROSION
FATIGUE CRACK
FATIGUE CRACK
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FATIGUE CRACK – INADEQUATE FLEXIBILITY
THERMAL FATIGUE
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FATIGUE CRACK ATTACHMENT WELD
–
CORROSION FATIGUE CRACK IN COLD BEND
CORROSION CRACKS
FATIGUE
CORROSION FATIGUE CRACKS
DAMAGE DUE TO TUBE INSIDE TUBE
PIPE MANUFACTURING DEFECT LAP
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INCORRECT MATERIAL
TUBE MANUFACTURING DEFECT - FOLD
TUBE MANUFACTURING DEFECT - SEAM
TUBE BENDING CRACK – STRIP HEATED BEND
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START STOP GAP IN PULSED MAG WELDING
BURN-THROUGH IN SMAW
TRANSPORTATION DAMAGE AT WIRE LASHING REGION
INDUCTION PRESURE WELD – PASTY JOINT
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LIGAMENT HEADER
CRACKING IN
CLOSE UP VIEW OF LIGAMENT CRACK
INTERNAL EROSION IN A TUBE BEND
CREEP FAILURE OF A LINK PIPE – IMPROPER SUPPORT
Tube failure investigation: An Investigation into a tube failure in an electric utility steam generating boiler has the potential to determine the root cause of that failure. Determination of the root cause can lead to implementation of corrective actions which could reduce or eliminate the likelihood that a similar type of failure will occur. These economic and morale benefits can only be achieved if an investigation into the tube failure is conducted which correctly identifies the 157
Learning modules material- Engineering (FB) failure mechanism and recommends the corrective actions that will control the root cause for that failure. Such an investigation can be a complex process requiring effective communications between equipment operators, maintenance mechanics, plant management, equipment manufacturers, and technical experts in materials, chemistry, and mechanical engineering. For an investigation to be successful, the following activities must be performed by plant personnel : Information and data concerning the tube failure must be gathered quickly before repair activities can begin. Failure descriptions, operating conditions at the time of failure, historical records, and tube samples must be acquired and transferred to others who will conduct the investigation while repairs are being performed. Immediate corrective actions based on the initial results of the investigation must be approved and implemented before repairs are completed. Follow-up corrective actions based on the complete results of the investigation must be planned and implemented before additional failures are experienced.
The likelihood that a failure investigation will be successful and produce the proper corrective action can be enhanced when the plant personnel have knowledge of the basic failure mechanisms that produce tube degradation, cognizance of the root causes for each failure mechanism, recognition of the ways to verify a root cause, and conviction to follow a planned approach to document the failure with pertinent data. Human errors are also an important factor in boiler tube failures. Errors can occur in the design, manufacture, shipping, storage, construction, operation, and maintenance of the boiler tubing. The wrong material can be installed at a critical location, leading to premature failure of the tube. This error can be the result of lack of quality control at the supplier ‟s factory or in the utility‟s storage and stock disbursement process. Boiler tube failures have been experienced due to lack of quality control in maintenance cleaning, welding, chemical cleaning, and tube manufacturing.
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