GEETANJALI INSTITUE OF TECHNICAL STUDIES
(Affiliated to Rajasthan Technical University Kota, Rajasthan)
A Practical Training Report On Types of Boilers & Mounting In Mechanical Engineering Session 2012 From 12june-11 july Training at HINDUSTAN ZINC LTD., DEBARI
Mechanical Department Geetanjali Inst. Of Technical Studies Dabok Udaipur
Bhupendra Singh Jhala B-Tech IV Year VII Semester
I wish to acknowledge the encouragement received from Mr A.Raman( Principal Of G.I.T.S., Udaipur) ,Mr Vishnu Agarwal (HOD Mech. Department) and special thanks to Mr. Vijendra Sankhala and Mr. Pankaj Ahir for initiating my interest in training. Their mastery & work helped me in covering out this work smoothly. I am also grateful to all the faculty of mechanical departments who helped me to improve my thinking as well as the practical knowledge
Bhupendra Singh Jhala
1. Introduction 2. Preface 3. Classification Of Boilers 4. Types Of Boilers 5. Boilers Mounting And Accessories
Boiler is a metal container in which a liquid is heated and changed into a vapor. Most boilers change water into the vapor steam. Steam is used to heat buildings and processes. It changes from vapor to liquid form as it delivers heat into a room or building, giving off even more heat as a result. Some heating systems, called hydronic systems, circulate hot water rather than steam. However, the heat source in these systems is still referred to as a boiler. Steam produced in boilers is also used in steam turbines and for refining oil or drying paper.The process of heating a liquid until it reaches it's gaseous state is called evaporation Heat is transferred from one body to another by means of (1) radiation, which is the transfer of heat from a hot body to a cold body through a conveying medium without physical contact, (2) convection, the transfer of heat by a conveying medium, such as air or water and (3) conduction, transfer of heat by actual physical contact, molecule to molecule.
The heating surface is any part of the boiler metal that has hot gases of combustion on one side and water on the other. Any part of the boiler metal that actually contributes to making steam is heating surface. The amount of heating surface a boiler has is expressed in square feet. The larger the amount of heating surface a boiler has the more efficient it becomes.
The heat required to change the temperature of a substance is called its sensible heat. In the teapot illustration to the left the 70 oF water contains 38 Btu’s and by adding 142 Btu’s the water is brought to boiling point.
In the illustration to the left, to change the liquid (water) to its gaseous state (steam) an additional 970 Btu’s would be required. This quantity of heat required to change a chemical from the liquid to the gaseous state is called latent heat. The saturation temperature or boiling point is a function of pressure and rises when pressure increases. When water under pressure is heated its saturation temperature rises above 212 oF. This occurs in the boiler. In the example below the boiler is operating at a pressure of 100 psig which gives a steam temperature of 338 oF or 1185 Btu’s.
Latent Heat When heat is added to saturated steam out of contact with liquid, its temperature is said to be superheated. The temperature of superheated steam, expressed as degrees above saturation, is referred to as the degrees of superheat.
Practical training is a way to implement theoretical knowledge to practical use to become a successful engineer it is necessary to have a sound practical knowledge because it is only way by which one can acquire proficiency & skill work successfully different industries.
It is proven fact bookish knowledge is not sufficient because things are not as ideal in practical field as they should be.
Hindustan Zinc Ltd. Is one of the best examples to understand the production process & productivity in particular of Zinc.
This report is an attempt made to study the overall production system & related action of Zinc Smelter, Debaria HZL.It is engaged in production of high grade zinc metal & other byproducts viz. Cd, sulphuric acid etc. Since 1968 by adopting Hydro metallurgical technology.
Classification of Boilers:
Boilers can be classified as follows:
1. According to the flow of water and hot gases – fire tube (or smoke tube) and water tube boilers. In fire tube boilers, hot gases pass through tubes which are surrounded with water. Examples: Vertical, Cochran, Lancashire and Locomotive boilers. There may be single tube as in case of Lancashire boiler or there may be a bank of tubes as in a locomotive boiler. In water tube boilers, water circulates through a large number of tubes and hot gases pass around them. Eg.bobcock& Wilcox boiler. 2 According to the axis of the shell – vertical and horizontal boilers. 3. According to location or position of the furnace. Externally and internally fired boilers. In internally fired boilers, the furnace forms an integral part of the boilers structure. The vertical tubular, locomotive and the scotch marine boilers are well known examples. Externally fired boilers have a separate furnace built outside the boiler shell and usually below it. The horizontal return tube (HRT) boiler is probably the most widely known example of this type. 4. According to the application – stationery and mobile boilers. A stationary boilers is one of which is installed permanently on a land installation. A marine boiler is a mobile boiler meant for ocean cargo and passenger ships with an inherent fast steaming capacity. 5. According to steam pressure – low, medium and high pressure boilers.
Types of Boilers:
Firetube boilers consist of a series of straight tubes that are housed inside a waterfilled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As the hot gases flow through the tubes, they heat the water surrounding the tubes. The water is confined by the outer shell of boiler. To avoid the need for a thick outer shell firetube boilers are used for lower pressure applications. Generally, the heat input capacities for firetube boilers are limited to 50 mbtu per hour or less, but in recent years the size of firetube boilers has increased.
Firetubeboilers are subdivided into these groups. Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubes with a separate combustion chamber. Scotch, Scotch marine, or shell boilers have the firetubes and combustion chamber housed within the same shell. Firebox boilers have a water-jacketed firebox and employ at most three passes of combustion gases. Most modern firetube boilers have cylindrical outer shells with a small round combustion chamber located inside the bottom of the shell. Depending on the construction details, these boilers have tubes configured in either one, two, three, or four pass arrangements. Because the design of firetube boilers is simple, they are easy to construct in a shop and can be shipped fully assembled as a package unit.
These boilers contain long steel tubes through which the hot gases from the furnace pass and around which the water circulates. Firetube boilers typically have a lower initial cost, are more fuel efficient and are easier to operate, but they are limited generally to capacities of 25
tonnes per hour and pressures of 17.5 kg per cm .
Watertube boilers are designed to circulate hot combustion gases around the outside of a large number of water filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In the older designs, the tubes were either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, watertube boilers can be fabricated in larger sizes and used for higher-pressure applications. Small watertube boilers, which have one and sometimes two burners, are generally fabricated and supplied as packaged units. Because of their size and weight, large watertube boilers are often fabricated in pieces and assembled in the field. In watertube or “water in tube” boilers, the conditions are reversed with the water passing through the tubes and the hot gases passing outside the tubes. These boilers can be of a single- or multiple-drum type. They can be built to any steam capacity and pressures, and have higher efficiencies than firetube boilers.
Almost any solid, liquid or gaseous fuel can be burnt in a watertube boiler. The common fuels are coal, oil, natural gas, biomass and solid fuels such as municipal solid waste (MSW), tire-derived fuel (TDF) and RDF. Designs of watertube boilers that burn these fuels can be significantly different. Coal-fired watertube boilers are classified into three major categories: stoker fired units, PC fired units and FBC boilers. Package watertube boilers come in three basic designs: A, D and O type. The names are derived from the general shapes of the tube and drum arrangements. All have steam drums for the separation of the steam from the water, and one or more mud drums for the removal of sludge. Fuel oil-fired and natural gas-fired watertube package boilers are subdivided into three classes based on the geometry of the tubes. The “A” design has two small lower drums and a larger upper drum for steam-water separation. In the “D” design, which is the most common, the unit has two drums and a largevolume combustion chamber. The orientation of the tubes in a “D” boiler creates either a left or right-handed configuration. For the “O” design, the boiler tube configuration exposes the
least amount of tube surface to radiant heat. Rental units are often “O” boilers because their symmetry is a benefit in transportation. “D” Type boilers “D” type boilers have the most flexible design. They have a single steam drum and a single mud drum, vertically aligned. The boiler tubes extend to one side of each drum. “D” type boilers generally have more tube surface exposed to the radiant heat than do other designs. “Package boilers” as opposed to “field-erected” units generally have significantly shorter fireboxes and frequently have very high heat transfer rates (250,000 btu per hour per sq foot). For this reason it is important to ensure high-quality boiler feedwater and to chemically treat the systems properly. Maintenance of burners and diffuser plates to minimize the potential for flame impingement is critical.
“A” type boilers
This design is more susceptible to tube starvation if bottom blows are not performed properly because “A” type boilers have two mud drums symmetrically below the steam drum. Drums are each smaller than the single mud drums of the “D” or “O” type boilers. Bottom blows should not be undertaken at more than 80 per cent of the rated steam load in these boilers. Bottom blow refers to the required regular blowdown from the boiler mud drums to remove sludge and suspended solids.
“O” types boilers
“O” design boilers have a single steam drum and a single mud drum. The drums are directly aligned vertically with each other, and have a roughly symmetrical arrangement of riser tubes. Circulation is more easily controlled, and the larger mud drum design renders the boilers less prone to starvation due to flow blockage, although burner alignment and other factors can impact circulation.
Electric boilers can use electric resistance heating coils immersed in water and are normally very low-capacity units. Other types of electric boilers are electrode-type units that generate saturated steam by conducting current through the water itself. Boiler water
conductivity must be monitored and controlled. If the conductivity is too low, the boiler will not reach full operating capacity. When the conductivity is too high, over-current protection will normally shut off the power.
Proper conductivity and high-quality water as well as effective water treatment is required. Solids from the saturated steam tend to accumulate slowly on the insulators supporting the electrodes from the grounded shell. The unit must be shut down periodically so that the insulators can be washed off to prevent arcing. Finally, voltages of up to 16 kV may be used. Protection is needed for ground faults, over-current and, for three-phase systems, loss of phase. The main electrical disconnect switch must be locked out before performing maintenance on the boiler.
Cast Iron boilers
Cast iron boilers are fabricated from a number of cast iron sections that are bolted together. The design of each section includes integral water and combustion gas passages. When fully assembled, the interconnecting passage create chambers where heat is transferred from the hot combustion gases to the water. These boilers generally produce low-pressure steam (15 psig) or hot water (30 psig) and burn either oil or natural gas. Because of their construction, cast iron boilers are limited to smaller sizes. Because the components of these boilers are relatively small and easy to transport, they can be assembled inside a room with a conventional size doorway. This feature means that cast iron boilers are often used as replacement units, which eliminate the need for temporary wall removal to provide access for larger package units. They consist simply of a firebox surrounded by a water chamber for heat to be transferred directly from the firebox to the boiling water or to
tube-type water heaters, while there are no boiler tubes. There is minimal need for feedwater, and the boiler water does not concentrate.
Another boiler type that is sometimes used to produced steam or hot water is the tubeless boiler. The design of tubeless boilers incorporates nested pressure vessels with water located between the shells. Combustion gases are fired into the inner vessel where heat is transferred to water located between the outside surface of the inner shell and the inside surface of the outer shell. For oil-fired and natural gas-fired vertical tubeless boilers, the burner is typically located at the bottom of the boiler. Some special applications of boilers require specific designs and operating procedures. These include waste to steam (trash to steam), waste heat recovery and heat recovery steam generators (HRSG).
Boilers are occasionally distinguished by their method of fabrication. Packaged boilers are assembled in a factory, mounted on a skid, and transported to the site as one package, ready for hookup to auxiliary piping. Shop assembled boilers are built up from a number of individual pieces or subassemblies. After these parts are aligned, connected, and tested, the entire unit is shipped to the site in one piece. Field erected boilers are too large to be transported as an entire assembly. They are constructed at the site from a series of individual components. Sometimes these components require special transportation and lifting considerations because of their size and weight. The packaged boiler is so called because it comes as a complete package. Once delivered to the site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Packaged boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection.
The features of packaged boilers are:
•Small combustion space and high heat release rate resulting in faster evaporation.
• Large number of small diameter tubes leading to good convective heat transfer. • Forced or induced draft systems resulting in good combustion efficiency. • A number of passes resulting in better overall heat transfer. • Higher thermal efficiency levels compared with other boilers.
Boiler Mountings and Accessories:
Boiler mountings are the machine components that are mounted over the body of the boiler itself for the safety of the boiler and for complete control of the process of steam generation.
Various boiler mountings are as under:
1) Pressure gauge 2) Water Level Indicator 3) Fusible plug 4)Safety Valve i) Lever Safety Valve
ii) Spring Loaded safety Valve 5) Steam stop valve 6) Feed check valve 7) Blow off cock
1) Bourdon's pressure gauge:
Function: 1.To record the steam pressure at which the steam is generated in the boiler. 2. A bourdon pressure gauge in its simplest form consists of elliptical elastic tube bent into an arc of a circle. 3. This bent up tube is called as BOURDON’S tube. 4. One end of tube gauge is fixed and connected to the steam space in the boiler. The other end is connected to a sector through a link. 2) Water Level Indicator: • The function of water level indicator is to indicate the level of water in the boiler constantly. • It is also called water gauge. • Normally two water level indicators are fitted at the front end of every boiler. 3) Fusible plug: • Function: To extinguish fire in the event of water level in the boiler shell falling below certain specified limit. • It protects fire tubes from burning when the level of the water in the water shell falls abnormally low and the fire tube or crown plate which is normally submerged in the water, gets exposed to steam space which may not be able to keep it cool. • It is installed below boiler's water level. • When the water level in the shell falls below the top of the plug, the steam cannot keep it cool and the fusible metal melts due to over heating. Thus the copper plug drops down and is held within the gunmetal body by the ribs. Thus the steam space gets communicated to the firebox and extinguishes the fire. Thus damage to fire box which could burn up is avoided. • By removing the gun metal plug and copper plug the fusible plug can be put in position again by interposing the fusible metal usually lead or a metal alloy.
4) Safety Valve:
Function : The function of safety valve is to release the excess steam when the pressure of steam inside the boiler exceeds
the rated pressure. There are 4 types of safety valves:
i) Lever Safety Valve
The disadvantage of this valve is that it admits of being tempered with, and the effect of a small addition to the weight is magnified considerably in its action on the value.
ii)Spring Loaded safety Valve • For locomotives and marine engines both the lever and dead weight types are unsuitable for obvious reasons, and the valve must be spring loaded, as such valve is unaffected by vibration or deviation from the vertical. • Disadvantage : One disadvantage of this valve is that the load on the valve increases as the valve lifts, so that pressure required just to lift the valve is less than that required to open it fully.
iii)Dead Weight Safety Valve • It is mainly used for low pressures, low capacity, stationary boilers of the Cornish and Lancashire types. • Merits: 1)Simplicity of design 2)Gives quite a satisfactory performance during operation. 3)It cannot be easily tempered from the pressure adjustment view. • Demerits: 1)Unsuitable for use on any boiler where extensive vibration and movement are experienced( e.g. locomotive and marine work). 2)It is not suitable for high pressure boilers because a large amount of weight is required to balance the steam pressure. iv)High steam and low water safety valve • It serves the following purposes. (i) The steam automatically escapes out when the level of water falls below a certain level. (ii) It automatically discharges the excess steam when the pressure of the steam rises above a certain pressure. Use: It is generally used on Lancashire or Cornish boiler. It cannot be used in mobile boilers.
5) Steam stop valve:
• A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. • Function: to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam pipe to the engine. • When the hand wheel is turned, the spindle which is screwed through the nut is raised or lowered dependingupon the sense of rotation of wheel. The passage for flow of steam is set on opening of the valve.
5) Feed check valve:
Function: The function of a feed check valve is to control the supply of water to the boiler and to prevent the escaping of water from the boiler when the pump pressure is less or pump is stopped. i) To allow the feed water to pass into the boiler. ii) To prevent the back flow of water from the boiler in the event of the failure of the feed pump. The feed check valve is fitted in the water space of the boiler slightly below the normal level of the water.
6) Blow off cock:
Function: To drain out the water from the boiler for internal cleaning, inspection, repair or other purposes. • It may discharge a portion of water when the boiler is in operation to blow out mud, scale or sediments, periodically. • It is fitted on the boiler shell directly or to a short branch pipe at the lowest part of the water space. Manhole and mud box: Manhole provides opening for cleaning, inspection and maintenance purpose. Mud box is a collection chamber (as shown in Babcock and Wilcox boiler) for collecting the Mud Thus, in such a way Types of Boilers & their Mountings and Accessories are determined.