1.What are the emergencies that are expected to happen in boiler operation?
Emergencies that can happen
in boilers operation are:
- Boilers tubes leakage
- Failure of feed water control station
- Unbalanced draft in furnace
- Furnace explosion
- Blow down valve failure
- Feed water pumps failure
- Secondary combustion in super heaters and ESP
2.List out the
potential reasons for boiler tubes failure.
Reasons for boiler tubes
failure:
- Failure due to overheating
- Failure due to internal scales
- Failure due to aging
- Failure due to uneven expansion
- Failure due to ash and flue gas erosion
- Failure due to material defects
- Failure due to internal and external tube corrosion
- Improper or effected circulation due to opening/passing of low point drains
3.List down
the sequential action taken after boiler tube leakage to restore the boiler.
Following activities are
done to restore the boiler:
- After tube leakage, Shutdown the boiler as soon as possible
- Allow to cool the boiler continuously by circulating feed water
- If necessary start fast cooling by keeping ‘ON’ ID and FD fans
- After cooling the boiler, identify the leakage tube/coil
- Drain the water up to the leakage point
- Find out the root cause of leakage
- If leakage is minor like pin hole, crack then weld the tube (follow welding procedure)
- If leakage is major like fish mouth damage, then replace the damaged part
- Allow to cool the joint for two hours
- Fill up the boiler and carryout hydraulic test at 1.25% of maximum operating pressure
- After ensuring the joint is free from leakage, drain the boiler up to normal drum level
- Start the boiler as per start up procedure
4.What are the
reasons for boiler drum level fluctuation?
Possible reasons for drum
level fluctuations are:
- Sudden loading and unloadingof Boiler
- Fluctuation in steam pressure
- Safety valve sudden pop up
- Priming and foaming
- Failure of drum level controller
- Leakage or passing of blow down valves
- Uncontrolled firing rate
- Leakages in tubes or coils
5.How do you
handle a boiler, if steam drum safety valve lifts before lifting of super
heater line safety valve?
Drum safety valve lifting
may cause failure of super heater coils due to less steam flow to coils. In
such cases maintain sufficient steam flow to super heater coils by maintaining
drum level normal by allowing more feed water flow into drum. And reduce the
firing rate and drum pressure for re-seating drum safety valves quickly.
6.What are the
possible reasons for furnace draft fluctuations?
Following are the some
reasons for furnace draught fluctuation:
- Unbalanced draft
- Fuel moisture variation
- Sudden load change
- Failure of ID, FD fan
- Flue gas path choke up generally in APH
- Air and flue gas leakages
- Loose or improperly fitted fans guide vanes.
7.What action
you will take if boiler blow down valve fails?
Following immediate
actions shall be taken during boiler blow down valve failure:
- Allow more feed water flow and maintain drum level normal.
- Reduce the boiler load and check the quantity of water flowing through the blow down line.
- If the water loss is less and acceptable run the boiler till upcoming planned shutdown, If it is more than acceptable, shutdown the boiler and get done the repair work of valve.
8.What are the
main causes for boilers explosion?
Major causes of boilers
explosion:
- Unbalanced draft i.e. high positive pressure
- Secondary combustion
- Bypassing the boiler protections and interlocks
- Overheat
- Scale and corrosion of pressure parts
9.What
are the causes of higher flue gas outlet temperature of boiler? And how do you
troubleshoot?
- Improper
Heat Transfer in Economizer, APH: Clean
Economiser coils and APH tubes regularly, maintain flue gas temperature as per
manufacturer guide.
- Accumulation
of Soot on Super Heater Coils, Economizer: Operate soot blowers regularly.
- Burning
of Higher Moisture Content Fuel: Limit
the moisture content of fuel.
- Higher
Economizer Feed Water Inlet Temperature: Reduce steam flow to HP/LP heaters.
- Excess
Heating Surface of Boiler: Modify
the boiler pressure parts design in consultation with manufacturers and
designers.
10.What are the
problems you do come across during sudden load cutoff on boiler? How do you
troubleshoot?
During sudden load cut off
following abnormal operation may take place:
- Drum
Level Low: Increase
feed water flow proportionally.
- Safety
Valve Lift: Drum
level starts to rise, so control feed water flow. In case of drum safety valve
lift there may be chances of failure of super heater coils due to less steam
circulation (starvation of coils), so utmost care must be taken to maintain
drum water level.
- Furnace
Draft May Become Unbalance: Maintain
furnace draft by controlling ID and FD fan speed manually.
- Higher Attemperation: Reduce firing rate and attemperator water flow.
More fuel combustion is
manly is due to:
-
Incomplete Combustion: Achieve proper and complete combustion, limit excess air and fuel moisture. Maintain 3Ts of combustion.
- Low Economizer Inlet Feed Water Temperature: Increase bleed steam to HP and LP heaters.
- Formation of Soot on Heat Transfer Surfaces: Operate soot blowers regularly at rated pressure and temperature.
12.Why does the
boiler backfire?
- Back firing of boiler is due to:
- Unbalanced draught
- Reduced speed of ID fan or more speed of FD fan
- Improper locking of fans damper (which may disturb in fan running due to air pressure)
- Sudden trip of ID fan or closing of damper
- High moisture fuel
- More excess air
- Jamming of APH
13.List out the
reasons for more unburnt in ash.
Excess unburnt is due to:
- Incomplete combustion
- Insufficient combustion air
- More moisture content fuel
- Not meeting 3T’s of combustion
- Not working of cinder reinjection system
- Lower GCV fuel
- High ash content in fuel
- Low over fire air
14.What
are the trouble shoots during boiler start up?
- Boiler
Flue Gas Ducts Explosion: Before
light up purge the boiler properly.
- Trouble
to Achieve Furnace Temperature: Maintain
balanced draught (slightly positive).
- Fluctuation
in Drum Level: Maintain
drum level by taking feed water control valve in manual. Do not allow the drum
level to go high (>90%) or less (<20%).
- Tripping
of ID Fan or FD fans Due to Draught Fluctuation: Maintain furnace draft in manual mode
until boiler load reaches more than 30% of MCR.
- High/Low
Steam Temperature: Maintain
attempteration in manual and keep open attemperator isolation valves.
- Flue gas condensation in APH: By pass the APH for FD air flow to avoid flue gas condensation.
15.How
do you troubleshoot the formation of clinker?
Clinker formation can be
avoided by following ways:
- Using low moisture coal
- Maintaining proper air fuel mixture
- Achieving proper combustion
- Using coal with no foreign materials
- Avoiding frequent startups and stops
- Use coal with high ash fusion temperature
17.How
do you reduce the auxiliary power consumption of Boiler?
- Maintain proper air fuel mixture so that speed of ID, FD fans and fuel feeding system can be optimized.
- Avoid using high moisture coal as it requires more air for combustion.
- Use VFDs to fuel feeding system.
- If boiler is running on load less than <30% shutoff the stand by fans.
- Tune the feed water control valve for maximum opening to reduce the speed of Boiler feed pump.
18.What
are the possible reasons for higher main steam temperature?
- Failure of Attemperator control valve
- More boiler load
- More heat transfer in super heater zone due to more velocity of flue gas
- More firing rate
- Low Economiser inlet feed water temperature
- High moisture fuel
- More excess air
- Low drum level
- Leakage of super heater coil
19.What are the
effects of high and low drum levels?
High drum level:
- Results into carryover of water into super heater coils which may damage coils due to high thermal shock.
- Can damage safety valve seats.
- Disturbs circulation ratio.
- Carry over with sharp fall in super heater temperature.
- Water may enter turbine and serious damages to turbine may occur
- Super Heater deposit and fouling.
- Turbine Deposit and increase in specific steam consumption.
Low drum level may results
less steam flow to super heater coils.
20.How
do you control boiler corrosion rate?
- Maintain pH of water in the desired range to control corrosion.
- O2 in feed water should be controlled within 0.007 ppm, primarily by good deaeration, supplemented by Oxygen scavenger dozing.
- Condenser leak should be immediately detected and acted upon.
- DM water storage tank should be properly sealed.
- Condensers and heaters should be air-tight.
- Proper wet lay-up of units during shut downs.
21.How do you
identify the leakage of economizer tube? What is its effect?
An economizer tube leakage
can be detected by sound or increased make up water requirements. The leak
should be investigated at the earliest possible time and the unit to be shut
down in a normal manner. Water leaks in the economizer can cause considerable
erosion damage to adjacent tubes. Water carried over from an economizer tube
leak may lead to cold end corrosion also.
22.What are the
reasons for lifting of steam drum safety valve and how do you bring the boiler
to normal operation?
Drum safety valve may lift
due to following reasons:
Excessive
fuel firing rate.
Sudden
decrease in steam load.
Faulty setting
of safety valve.
The following actions have
to be taken to bring boiler into normal operation:
Reduce
fuel-firing rate.
Open
the start up vent valve.
Observe
the steam flow of the super heater. If there is no flow, trip the boiler.
Maintain the
steam drum level.
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