Boiler Pressure Parts & Tube Failure
Content
BOILER PRESSURE PARTS
AND TUBE FAILURES
By
Sri A.Prabhakar Rao
12.03.2010
P > 1 Kg / cm2
Economizer
Super
Heaters
Re-Heaters
Water Walls
Safety valves
De-Super heaters and
Boiler Drum
TO PREHEAT FEEDWATER
TO RECOVER HEAT FROM FG LEAVING BOILER
Finned – staggered
Bare tube – inline
CARBON STEEL
Coal can be saved from 15% to 20%.
Increase in 1% Thermal Efficiency for every
6 C change in temperature.
Feed water changes to steam quickly.
Increases Boiler Life.
Decreases thermal stress of Boiler Internal
Parts.
Decrease in combustion rate.
FUNCTION:
It increases the temperature of Main
steam with the help of temperature of
flue gases to get Saturated Steam
admitted to the HPT.
PENDANT SPACED SECTION-located behind the
screen section – heat transfer by convection.
PLATEN SECTION- located above the furnace-
REAR HORIZONTAL SECTION- located in second
heat absorption by radiation.
pass- convective counter flow.
STEAM COOLED WALL- second pass enclosure.
ROOF SECTION- second pass roof
Super heats the steam from Boiler before
admitting it to turbine.
Removes the moisture contents from the
steam to avoid the corrosion and breaking
of turbine blade tips.
FUNCTION:
It heats the temperature of steam outlet
from HPT with the help of Flue gas
temperature.
Re-heats the steam to increase the thermal
efficiency.
Increases the energy in the steam to
perform additional work before exhausting
into condenser from LPT.
FUNCTION:
Water walls carry feed water from ring
headers to Boiler Drum through raiser
tubes.
Increase in efficiency.
Better heat transfer.
Easy and quick erection.
Increased availability of Boiler.
FUNCTION
These are used to safe guard the equipment
in case of emergencies.
FUNCTION:
It controls the main steam temperatures to
safe limit.
FUNCTION:
It separates the steam from steam-water
mixture.
It houses all equipments used for
purification of steam, after being separated
from water.
Stores the DM Water.
Limits the Solid contents.
Facilitate in adding chemicals to maintain
pH value.
To facilitate Blow down.
Max. Temp. C
(Oxidation Limit)
ASTM
SA 210 Gr. A1
Carbon Steel C0.27% Mn
0.93%P.035 S0.10%
425
SA 209
T1
480
SA 213
T11
½ % Mo steel C 0.10to
0.20%Mn
0.30to0.80%P0.025Si0.025
s
1 % Cr. ½ % Mo
SA 213
T22
2 ¼ % Cr. 1 % Mo
580
SA 213
TP 304 H
18 % Cr. 8 % Ni
(Stainless steel)
700
SA 213
TP 347 H
18 % Cr. 10 % Ni
700
550
By
Sri A. Prabhakar Rao
Very
high expectations on
performance
Fuel quality deterioration
New / imported fuels coming to
use
Cheaper power to public – Govt
Availability based tariff
Achieving
higher availability
Optimal performance of the
units
Ability to vary load & meet
varying targets
Optimizing combustion for
varying fuel characteristic
Better
heat rate and low cost
power
Lower pollution levels
Better position in ABT
Low gestation period
Enhancing
life of the plant
Better
heat rate and low cost
power
Lower pollution levels
Better position in ABT
Low gestation period
Enhancing
life of the plant
Making
the optimal plant heat
rate
Handling the fuel quality change
Operational deviations
Reducing the damage
mechanisms
•Availability and reliability of boiler
decreases with increased tube failures.
•Tube failure results in forced outages and
hence direct impact on availability.
Boiler Tube Failures - main cause of
forced outages in electric utility steam
generating boilers
Single tube Failure in a 500 MW
Rs. 6
to 7 Cores (replacement ,power charges
for 3-4 days ,to repair) besides affecting
Plant Morale.
Stress Rupture
Short Term Overheating
High Temperature Creep
Dissimilar Metal Welds
Water-side Corrosion
Caustic Corrosion
Hydrogen Damage
Pitting
Stress Corrosion Cracking
Fire-side Corrosion
Low Temperature
Waterwall Coal Ash Oil Ash
Fatigue
Vibration
Thermal
Corrosion
Erosion
Fly Ash
Falling Slag
Soot Blower
Coal Particle
Lack of Quality Control
Maintenance cleaning damage
Chemical excursion damage
Material Defects
Welding Defects
- indicates that such problems have not been reported in India
Steam / Water
cooled tubes
Plugged by debris,
scale etc.
High Heat
Transfer /
Improper firing
Low water/steam
flow due to poor
circulation /
upstream leak
Corrective Action
Prevent Blockage
Maintain Drum
level
Assure Coolant
flow
Reduce over firing
Redesign tubing
to promote flow
Relocation of
horiz. / inclined
tubes to avoid
film boiling
Typical Locations
Steam cooled
Tubes
Partially choked
Radiant Heat Zone
Gas Blockage
Incorrect Material
Material Transition
Higher stress due
to
◦ weld attachment
◦
◦
◦
◦
◦
◦
Corrective Action
RLA/
IOT
Fluid flushing
Material upgrades
Typical Locations
Corrective Action
At SH / RH dissimilar
weld joints :
Temperature / Stress
excursions
Repair/Replacement
Relocating the weld
Use of Ni-base filler
Frequent inspection
Mechanism : 1. The formation of carbon depleted zone on the ferritic side of the
transition from the ferritic to austenitic structure is the initial step and any
treatment which enhances the formation of this zone will enhance the failure
probability.
2. The carbon depleted soft feerritic zone is constrained by the sorrounding harder
and stronger material and is subjected to strains induced by thermal expansion
mismatch, bending, vibration and pressure.
3. The strain accumulation in the carbon-depleted zone is relieved by creep at
elevated temperature.
4. Creep damage in the form of cavitation, grain boundary sliding and tearing
results in cracking in the carbon depleted zone along and adjacent to the weld
interface
Damage may result from high pH corrosion reaction.
NaOH removes protective magnetite iron oxide layer Fe3O4.
Iron react with water or NaOH eating away the parent metal.
It is also called caustic gouging or ductile gouging.
Typical Locations
Corrective Action
Water-cooled Tubes:
◦
◦
◦
◦
At flow disruptions
Horiz / inclined tube
High Heat flux zone
Flame impingement zone
Probable Root Cause
Concentration of NaOH from
boiler water chemicals
Feed water system corrosion
deposits
Condenser leakage
Temp. increase due to
internal deposits
Control Boiler Water
Chemistry
Reduce corrosion
product ingress
Chemical cleaning
Reweld irregular
welds
Use T11 type steel or
rifled tube
Hydrogen damage may occur where corrosion reaction results
in the production of atomic hydrogen. Damage may result from
Low pH corrosion reaction.
NaOH removes protective magnetic iron oxide layer Fe3O4
Iron react with water or NaOH liberating atomic hydrogen
Atomic hydrogen diffuses into Iron carbide producing methane
gas.
Methane or Atomic H2 cannot diffuse, it accumulates resulting in
cracks at grain boundaries.
Longitudinal burst occur with thick lip
Typical Locations
Water-cooled Tubes:
◦
◦
◦
◦
At flow disruptions
Horiz / inclined tube
High Heat flux zone
Flame impingement zone
Probable Root Cause
Concentration of acidic salts
and low pH water chemistry
Condenser leakage and
ingress of corrosion products
Feed water system corrosion
deposits
Chemical cleaning
contamination
Corrective Action
Control Boiler Water
Chemistry
Check corrosion
product ingress
Chemical cleaning
Replace affected tubes
Pitting corrosion is a localized accelerated attack, resulting in the
formation of cavities around which the metal is relatively
unattached. Thus, pitting corrosion results in the formation of
pinholes, pits and cavities I the metal. Pitting is, usually, the result
of the breakdown or cracking of the protective film on a metal at
specific points. This gives rise to the formation of small anodic and
large cathodic areas
Typical Locations
SH, RH-at regions of :
◦ conc. of chlorides,
sulphates or hydroxide
◦ stressed in fabrication,
service etc. like bends,
attachment welds
Probable Root
Cause
Corrosive conc. From
drum carry-over or
attemperator spray
SS Tube material
sensitized
Stresses
Corrective Action
Replacement
Surveillance for
carry-over
Heat Treatment of
bends
Care during chemical
cleaning
Use of 347H tube
Typical Locations
Gaps between tube banks
and duct walls.
Gas by-pass channels
Protrusions of rows.
Areas close to large ash
accumulation.
Probable Root Cause
Non-uniform, excessive gas
flow with fly ash particles.
High ash coal with -quartz.
Tube misalignment.
Corrective Action
Changing operating
conditions like reduced
load, low excess air etc.
Protections like shields,
baffles etc.
Flow Model study
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.
Design stage:
Selection of material,
• Compatible for working pressure / temperature
• Steam flow and the velocity / pressure
• Heat transfer characteristics/ surface effectiveness
• Metal temperature/Thermal expansion / constraints
• Radius of bends
• Attachments/ Weldments
• Manufacturing aspects
• Transportation / Handling
• Storage
• Erection
• Commissioning
• Operation and Maintenance
Lower flue gas velocity over tube banks
Plain tube in-line arrangement of heat
transfer surface
Optimum end gaps to avoid preferential
gas flow
Erosion shields / cassette baffles
Erosion allowance for leading tubes
Higher flexibility in SH / RH nipples
Redesigned flexible connectors for pendant
type SH coils
Improved supports for LTSH / Eco. Coils
Improved seal plate connection for bottom
hopper
Modified LTSH inlet tube connection
FLUE GAS (FLY ASH) EROSION
Extensive inspection of Economiser / LTSH / screen tubes/
re-heater for erosion prone areas,Verifying the condition of
existing shields and baffles, LTSH supply tube refractory
conditions.
Mapping of thickness and identifying areas / locations for
repair / replacement.
Baffling / shielding at the points of erosion prone areas to the
maximum extent possible.
Changing operating conditions like reduced load, low excess
air etc.
Flow model studY
SB STEAM EROSION
Ensure condensate free steam with a minimum
superheat of 15 C at blower.
Necessary gradient/downward slope of the SB piping
is to be ensured. 1 per meter length of pipe is to be
maintained.
Through thickness survey of WW in the SB location for
three meter radius and replacing the eroded tubes.
Ensuring wall blower nozzle alignment.
Temporary shielding / spraying.
Installing thermal drain system if not available.
FALLING SLAG EROSION
Check
the fuel characteristics for fouling.
Change in fuel if warranted.
Tuning the boiler air regime for optimised
combustion to avoid fouling.
Welding of wear bars at the bottom
S-panel tubes to break the ash boulders
and to avoid direct hitting of the tubes.
Increase tube wall thickness.
LONG TERM OVERHEATING (CREEP)
Maintaining & monitoring the metal
temperature within limits.
Ensuring adequate flow through tubes.
Following the start up curves for rate of
firing.
Assessing the life thro' oxide scale thickness
/CRT by sampling to understand the extent
of overheating.
Tuning of boiler viz. Excess air, temp. Etc.
Mateiral up-grades.
Strict
quality
control
during
tube
replacements to avoid foreign material
entry.
SHORT TERM OVERHEATING
Ensuring adequate flow thro’ tubes.
Preventing blockage.
Maintaining the drum level.
Reduce over firing.
Avoid foreign material entry while
maintenance.
DMW JOINT FAILURE
Replacement of DMW Joints with life
1,00,000 hrs of operation.
Replacement of DMW joints with spool
pieces fabricated at shop.
Relocating welds away from highly
stressed points.
>
HYDROGEN EMBRITTLEMENT
WATER SIDE CORROSION / CAUSTIC GOUGING.
Maintaining / monitoring the water chemistry
guidelines.
Avoiding corrosion products ingress.
Controlling copper deposition thro’ condenser
leakage.
Chemical cleaning of the boier whenever deposit
quantity is > 40 mg /cm2
Carrying out the in-situ hydrogen embrittlement
survey.
Replacing the affected tubes
INTERNAL / O2 PITTING
Ensuring proper operation of de-aerator.
Control feed water oxygen levels to <10 ppb at
CEP out let.
Preservation of the boiler during short /long
outages by dry / wet preservation methods.
FRETTING / RUBBING.
Ensuring
proper expansion gaps
Avoiding sharp corners of the support
lugs.
Ensuring proper positioning of flexible
connectors.
Giving adequate provision for relative
movement of pressure part tubing like
steam cooled spacer and SH/RH tubes.
FATIGUE
Ensuring the proper flexibility.
Removal of tie welding if it is done wrongly
Redesign of attachment to reduce restraints
to thermal expansion.
Restriction on cyclic operation.
Heat treatment and contouring of welds.
WELDING DEFECTS
Ensuring the qualified welder.
Adopting proper quality control procedures.
Process controls.
Replacement of defective joints.
Preventive actions.
RLA of pressure parts / boiler.
Planning for replacement of pr. Parts sections in
total for Economiser,LTSH,Re-heater, etc.
Carrying out IOT / CRT for SH & RH tubes.
H2- embrittlement survey of WW tubes.
Installing advance system like Smart wall
blowing & Acoustic steam leak detection system
etc. for reducing wall blowing frequency and to
avoid secondary damages.
Determination of the correct failure mechanism is a complex
process which can involve many individuals and organizations.
Technical specialists in metallurgy, chemistry, combustion, and
boiler design are often called in to assist in a failure
investigation. The plant’s personnel must provide the initial
information on the failure and boiler conditions prior to the
failure. The plant’s operating records and failure histories
must be in order so that pertinent data may be extracted. The
plant’s management and technical staff must follow up on the
failure investigation and implement the corrective actions
required to correct the problem.
By incorporating joint Task force between the plant owner and
boiler designer / manufacturer/metallurgists/Experts the tube
failures
can
be
prevented/reduced
and
the
availability/reliability can be increased.
Thank You
AND TUBE FAILURES
By
Sri A.Prabhakar Rao
12.03.2010
P > 1 Kg / cm2
Economizer
Super
Heaters
Re-Heaters
Water Walls
Safety valves
De-Super heaters and
Boiler Drum
TO PREHEAT FEEDWATER
TO RECOVER HEAT FROM FG LEAVING BOILER
Finned – staggered
Bare tube – inline
CARBON STEEL
Coal can be saved from 15% to 20%.
Increase in 1% Thermal Efficiency for every
6 C change in temperature.
Feed water changes to steam quickly.
Increases Boiler Life.
Decreases thermal stress of Boiler Internal
Parts.
Decrease in combustion rate.
FUNCTION:
It increases the temperature of Main
steam with the help of temperature of
flue gases to get Saturated Steam
admitted to the HPT.
PENDANT SPACED SECTION-located behind the
screen section – heat transfer by convection.
PLATEN SECTION- located above the furnace-
REAR HORIZONTAL SECTION- located in second
heat absorption by radiation.
pass- convective counter flow.
STEAM COOLED WALL- second pass enclosure.
ROOF SECTION- second pass roof
Super heats the steam from Boiler before
admitting it to turbine.
Removes the moisture contents from the
steam to avoid the corrosion and breaking
of turbine blade tips.
FUNCTION:
It heats the temperature of steam outlet
from HPT with the help of Flue gas
temperature.
Re-heats the steam to increase the thermal
efficiency.
Increases the energy in the steam to
perform additional work before exhausting
into condenser from LPT.
FUNCTION:
Water walls carry feed water from ring
headers to Boiler Drum through raiser
tubes.
Increase in efficiency.
Better heat transfer.
Easy and quick erection.
Increased availability of Boiler.
FUNCTION
These are used to safe guard the equipment
in case of emergencies.
FUNCTION:
It controls the main steam temperatures to
safe limit.
FUNCTION:
It separates the steam from steam-water
mixture.
It houses all equipments used for
purification of steam, after being separated
from water.
Stores the DM Water.
Limits the Solid contents.
Facilitate in adding chemicals to maintain
pH value.
To facilitate Blow down.
Max. Temp. C
(Oxidation Limit)
ASTM
SA 210 Gr. A1
Carbon Steel C0.27% Mn
0.93%P.035 S0.10%
425
SA 209
T1
480
SA 213
T11
½ % Mo steel C 0.10to
0.20%Mn
0.30to0.80%P0.025Si0.025
s
1 % Cr. ½ % Mo
SA 213
T22
2 ¼ % Cr. 1 % Mo
580
SA 213
TP 304 H
18 % Cr. 8 % Ni
(Stainless steel)
700
SA 213
TP 347 H
18 % Cr. 10 % Ni
700
550
By
Sri A. Prabhakar Rao
Very
high expectations on
performance
Fuel quality deterioration
New / imported fuels coming to
use
Cheaper power to public – Govt
Availability based tariff
Achieving
higher availability
Optimal performance of the
units
Ability to vary load & meet
varying targets
Optimizing combustion for
varying fuel characteristic
Better
heat rate and low cost
power
Lower pollution levels
Better position in ABT
Low gestation period
Enhancing
life of the plant
Better
heat rate and low cost
power
Lower pollution levels
Better position in ABT
Low gestation period
Enhancing
life of the plant
Making
the optimal plant heat
rate
Handling the fuel quality change
Operational deviations
Reducing the damage
mechanisms
•Availability and reliability of boiler
decreases with increased tube failures.
•Tube failure results in forced outages and
hence direct impact on availability.
Boiler Tube Failures - main cause of
forced outages in electric utility steam
generating boilers
Single tube Failure in a 500 MW
Rs. 6
to 7 Cores (replacement ,power charges
for 3-4 days ,to repair) besides affecting
Plant Morale.
Stress Rupture
Short Term Overheating
High Temperature Creep
Dissimilar Metal Welds
Water-side Corrosion
Caustic Corrosion
Hydrogen Damage
Pitting
Stress Corrosion Cracking
Fire-side Corrosion
Low Temperature
Waterwall Coal Ash Oil Ash
Fatigue
Vibration
Thermal
Corrosion
Erosion
Fly Ash
Falling Slag
Soot Blower
Coal Particle
Lack of Quality Control
Maintenance cleaning damage
Chemical excursion damage
Material Defects
Welding Defects
- indicates that such problems have not been reported in India
Steam / Water
cooled tubes
Plugged by debris,
scale etc.
High Heat
Transfer /
Improper firing
Low water/steam
flow due to poor
circulation /
upstream leak
Corrective Action
Prevent Blockage
Maintain Drum
level
Assure Coolant
flow
Reduce over firing
Redesign tubing
to promote flow
Relocation of
horiz. / inclined
tubes to avoid
film boiling
Typical Locations
Steam cooled
Tubes
Partially choked
Radiant Heat Zone
Gas Blockage
Incorrect Material
Material Transition
Higher stress due
to
◦ weld attachment
◦
◦
◦
◦
◦
◦
Corrective Action
RLA/
IOT
Fluid flushing
Material upgrades
Typical Locations
Corrective Action
At SH / RH dissimilar
weld joints :
Temperature / Stress
excursions
Repair/Replacement
Relocating the weld
Use of Ni-base filler
Frequent inspection
Mechanism : 1. The formation of carbon depleted zone on the ferritic side of the
transition from the ferritic to austenitic structure is the initial step and any
treatment which enhances the formation of this zone will enhance the failure
probability.
2. The carbon depleted soft feerritic zone is constrained by the sorrounding harder
and stronger material and is subjected to strains induced by thermal expansion
mismatch, bending, vibration and pressure.
3. The strain accumulation in the carbon-depleted zone is relieved by creep at
elevated temperature.
4. Creep damage in the form of cavitation, grain boundary sliding and tearing
results in cracking in the carbon depleted zone along and adjacent to the weld
interface
Damage may result from high pH corrosion reaction.
NaOH removes protective magnetite iron oxide layer Fe3O4.
Iron react with water or NaOH eating away the parent metal.
It is also called caustic gouging or ductile gouging.
Typical Locations
Corrective Action
Water-cooled Tubes:
◦
◦
◦
◦
At flow disruptions
Horiz / inclined tube
High Heat flux zone
Flame impingement zone
Probable Root Cause
Concentration of NaOH from
boiler water chemicals
Feed water system corrosion
deposits
Condenser leakage
Temp. increase due to
internal deposits
Control Boiler Water
Chemistry
Reduce corrosion
product ingress
Chemical cleaning
Reweld irregular
welds
Use T11 type steel or
rifled tube
Hydrogen damage may occur where corrosion reaction results
in the production of atomic hydrogen. Damage may result from
Low pH corrosion reaction.
NaOH removes protective magnetic iron oxide layer Fe3O4
Iron react with water or NaOH liberating atomic hydrogen
Atomic hydrogen diffuses into Iron carbide producing methane
gas.
Methane or Atomic H2 cannot diffuse, it accumulates resulting in
cracks at grain boundaries.
Longitudinal burst occur with thick lip
Typical Locations
Water-cooled Tubes:
◦
◦
◦
◦
At flow disruptions
Horiz / inclined tube
High Heat flux zone
Flame impingement zone
Probable Root Cause
Concentration of acidic salts
and low pH water chemistry
Condenser leakage and
ingress of corrosion products
Feed water system corrosion
deposits
Chemical cleaning
contamination
Corrective Action
Control Boiler Water
Chemistry
Check corrosion
product ingress
Chemical cleaning
Replace affected tubes
Pitting corrosion is a localized accelerated attack, resulting in the
formation of cavities around which the metal is relatively
unattached. Thus, pitting corrosion results in the formation of
pinholes, pits and cavities I the metal. Pitting is, usually, the result
of the breakdown or cracking of the protective film on a metal at
specific points. This gives rise to the formation of small anodic and
large cathodic areas
Typical Locations
SH, RH-at regions of :
◦ conc. of chlorides,
sulphates or hydroxide
◦ stressed in fabrication,
service etc. like bends,
attachment welds
Probable Root
Cause
Corrosive conc. From
drum carry-over or
attemperator spray
SS Tube material
sensitized
Stresses
Corrective Action
Replacement
Surveillance for
carry-over
Heat Treatment of
bends
Care during chemical
cleaning
Use of 347H tube
Typical Locations
Gaps between tube banks
and duct walls.
Gas by-pass channels
Protrusions of rows.
Areas close to large ash
accumulation.
Probable Root Cause
Non-uniform, excessive gas
flow with fly ash particles.
High ash coal with -quartz.
Tube misalignment.
Corrective Action
Changing operating
conditions like reduced
load, low excess air etc.
Protections like shields,
baffles etc.
Flow Model study
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.
Design stage:
Selection of material,
• Compatible for working pressure / temperature
• Steam flow and the velocity / pressure
• Heat transfer characteristics/ surface effectiveness
• Metal temperature/Thermal expansion / constraints
• Radius of bends
• Attachments/ Weldments
• Manufacturing aspects
• Transportation / Handling
• Storage
• Erection
• Commissioning
• Operation and Maintenance
Lower flue gas velocity over tube banks
Plain tube in-line arrangement of heat
transfer surface
Optimum end gaps to avoid preferential
gas flow
Erosion shields / cassette baffles
Erosion allowance for leading tubes
Higher flexibility in SH / RH nipples
Redesigned flexible connectors for pendant
type SH coils
Improved supports for LTSH / Eco. Coils
Improved seal plate connection for bottom
hopper
Modified LTSH inlet tube connection
FLUE GAS (FLY ASH) EROSION
Extensive inspection of Economiser / LTSH / screen tubes/
re-heater for erosion prone areas,Verifying the condition of
existing shields and baffles, LTSH supply tube refractory
conditions.
Mapping of thickness and identifying areas / locations for
repair / replacement.
Baffling / shielding at the points of erosion prone areas to the
maximum extent possible.
Changing operating conditions like reduced load, low excess
air etc.
Flow model studY
SB STEAM EROSION
Ensure condensate free steam with a minimum
superheat of 15 C at blower.
Necessary gradient/downward slope of the SB piping
is to be ensured. 1 per meter length of pipe is to be
maintained.
Through thickness survey of WW in the SB location for
three meter radius and replacing the eroded tubes.
Ensuring wall blower nozzle alignment.
Temporary shielding / spraying.
Installing thermal drain system if not available.
FALLING SLAG EROSION
Check
the fuel characteristics for fouling.
Change in fuel if warranted.
Tuning the boiler air regime for optimised
combustion to avoid fouling.
Welding of wear bars at the bottom
S-panel tubes to break the ash boulders
and to avoid direct hitting of the tubes.
Increase tube wall thickness.
LONG TERM OVERHEATING (CREEP)
Maintaining & monitoring the metal
temperature within limits.
Ensuring adequate flow through tubes.
Following the start up curves for rate of
firing.
Assessing the life thro' oxide scale thickness
/CRT by sampling to understand the extent
of overheating.
Tuning of boiler viz. Excess air, temp. Etc.
Mateiral up-grades.
Strict
quality
control
during
tube
replacements to avoid foreign material
entry.
SHORT TERM OVERHEATING
Ensuring adequate flow thro’ tubes.
Preventing blockage.
Maintaining the drum level.
Reduce over firing.
Avoid foreign material entry while
maintenance.
DMW JOINT FAILURE
Replacement of DMW Joints with life
1,00,000 hrs of operation.
Replacement of DMW joints with spool
pieces fabricated at shop.
Relocating welds away from highly
stressed points.
>
HYDROGEN EMBRITTLEMENT
WATER SIDE CORROSION / CAUSTIC GOUGING.
Maintaining / monitoring the water chemistry
guidelines.
Avoiding corrosion products ingress.
Controlling copper deposition thro’ condenser
leakage.
Chemical cleaning of the boier whenever deposit
quantity is > 40 mg /cm2
Carrying out the in-situ hydrogen embrittlement
survey.
Replacing the affected tubes
INTERNAL / O2 PITTING
Ensuring proper operation of de-aerator.
Control feed water oxygen levels to <10 ppb at
CEP out let.
Preservation of the boiler during short /long
outages by dry / wet preservation methods.
FRETTING / RUBBING.
Ensuring
proper expansion gaps
Avoiding sharp corners of the support
lugs.
Ensuring proper positioning of flexible
connectors.
Giving adequate provision for relative
movement of pressure part tubing like
steam cooled spacer and SH/RH tubes.
FATIGUE
Ensuring the proper flexibility.
Removal of tie welding if it is done wrongly
Redesign of attachment to reduce restraints
to thermal expansion.
Restriction on cyclic operation.
Heat treatment and contouring of welds.
WELDING DEFECTS
Ensuring the qualified welder.
Adopting proper quality control procedures.
Process controls.
Replacement of defective joints.
Preventive actions.
RLA of pressure parts / boiler.
Planning for replacement of pr. Parts sections in
total for Economiser,LTSH,Re-heater, etc.
Carrying out IOT / CRT for SH & RH tubes.
H2- embrittlement survey of WW tubes.
Installing advance system like Smart wall
blowing & Acoustic steam leak detection system
etc. for reducing wall blowing frequency and to
avoid secondary damages.
Determination of the correct failure mechanism is a complex
process which can involve many individuals and organizations.
Technical specialists in metallurgy, chemistry, combustion, and
boiler design are often called in to assist in a failure
investigation. The plant’s personnel must provide the initial
information on the failure and boiler conditions prior to the
failure. The plant’s operating records and failure histories
must be in order so that pertinent data may be extracted. The
plant’s management and technical staff must follow up on the
failure investigation and implement the corrective actions
required to correct the problem.
By incorporating joint Task force between the plant owner and
boiler designer / manufacturer/metallurgists/Experts the tube
failures
can
be
prevented/reduced
and
the
availability/reliability can be increased.
Thank You
