written by: Dr V T Sathyanathan
• edited by: Lamar Stonecypher
• updated: 7/9/2010
The damage due to air pre-heater fire is very severe depending upon the intensity of the fire. In many cases it has made the regenerative air pre-heater elements a molten mass. The boiler availability loss will be high if a fire takes place in the air heater.
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Air pre-heaters are heat recovery systems used in boilers to improve boiler operating efficiency.There are two major types of air pre-heaters: the recuperative and the regenerative.For higher sizes boilers, say 350 t/hr and above, designers adopt the regenerative type. This is mainly due to the compactness of regenerative air pre-heaters for higher heat duty. In both cases, air heater fires are not uncommon in boilers, but more common in the regenerative type.When a boiler is operated with pulverized coal with a higher load, then the probability of air heater fire is low. An air pre-heater fire can happen during startup or shutdown when there is a possibility of a large volume of unburnt particles reaching the air pre-heater. This unburnt gets deposited in the air pre-heaters elements and gets ignited when the temperature rises and particles that are still burning reach this area.
The plant operator will have to quickly respond to symptoms that show a tendency for air pre-heater fire to start.The possibility of detecting a fire in the air pre-heater in a boiler is very difficult from the operating parameters of boiler. However, indications of an air pre-heater fire can be verified from a certain parameter changing its trend suddenly. When both air and flue gas temperature leaving the air pre-heater increase suddenly with a steep gradient, then the reason for it is to be verified. The specific causes for an air pre-heater fire, the plant response, and the immediate operator response required during this are discussed.
The specific causes
Heavy Unburnt Oil / soot deposits in air pre-heater due to
Poor atomization of oil being fired
Worn-out oil burner tips leading to coarser particles of oil
Low air flow through the burner leading to unburnt particles
Improper air distribution between primary, secondary etc
Wrong tip position with respect to diffuser or spinner
Inadequate frequency of operation of APH Soot blowers
Plugged Nozzles of APH Soot blowers
Low blowing steam pressure in APH Soot blowers
Sharp rise in air temp at APH outlet will be noticed
Increase in flue gas temperature at air pre-heater outlet
Immediate Operator Action
Trip unit on emergency
Stop all fans immediately
Close all dampers on the air and flue gas side
Flood APH with water through air heater water washing system or otherwise
Rotate APH to avoid buckling of the air pre-heater rotor
Adopting some of the good operating practices of boiler can prevent air pre-heater fire totally during the life of the boiler.
Reduce number of startups of boiler to a bare minimum.
Avoid operating boiler in low loads requiring oil support for a prolonged duration.
It is a good practice to check oil viscosity characteristics on a periodic basis and whenever there is a change in supplier.
Clean oil gun tips and lap the internals to satisfy the design requirements.
Never restart the oil burners without purging during startup.
Boiler startup operation must be avoided when air pre-heater soot blowers are not available.
Always make it a practice to soot blow all the air pre-heaters, the non working air pre-heaters also must be soot blown by keeping the isolation damper marginally open to allow the soot to be blown out.
During startups, low load operation, and shutting down, make it a practice to watch the trend of air and gas temperature leaving the air pre-heaters.
Operate air pre-heater soot blowers regularly.
Any fire detecting device provided by supplier has to be kept active and tested as and when possible.
To repair the air pre-heater damaged due to fire takes large amount of time and needs spares for replacements. Some of the spares of large air heaters will not be available off-the-shelf which can further delay the process of repair. The boiler availability loss will be high if a fire takes place in an air heater. Responding quickly to suspected air pre-heater fire immediately and taking corrective action can save a lot of money, downtime, and generation loss.