air pre-heater fires in large capacity boilers.pdf
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Air Pre‐heater Fires in Large Capacity Boilers
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.
Air pre‐heaters are heat recovery systems used in boilers to improve boiler operating efficiency. There are two major types of air pre‐heaters, generally used : recuperative type and regenerative type. For higher sizes boilers, say 350 t/hr and above, designers generally adopt the regenerative one. This is mainly due to the compactness of regenerative air pre‐heaters for higher heat duty.
The regenerative type air pre‐heaters are more prone to catch fire. When a boiler is operated with pulverized coal with a higher load, then the probability of air heater fire is low unless it stops rotating and control room operator fails to notice it. 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 the related operating parameter when it changes its trend sharply. Though there is a provision of APH Fire Alarm at DCS, operator should get alert when both air and flue gas temperature leaving the air pre‐heater increase suddenly with a steep gradient.
The specific causes for an air pre‐heater fire, the plant response, and the immediate operator response required during this, are discussed below :
The specific causes
Heavy Unburnt Oil / soot deposits in air pre‐heater due to o Poor atomization of oil being fired o Worn‐out oil burner tips leading to coarser particles of oil o Low air flow through the burner leading to unburnt particles o Improper air distribution between primary, secondary etc o 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
Plant Response
Sharp rise in air temp at APH outlet will be noticed Increase in flue gas temperature at air pre‐heater outlet
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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 Keep APH rotating 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. Clean oil gun tips and lap the internals to satisfy the design requirements. Boiler startup operation must be avoided when air pre‐heater soot blowers are not
available. Air Soot Blowing must be done regularly as per schedule. Local checking of APH soot
blowing is also very essential to ensure the effectiveness of soot blowing system. 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.
The trend of air and gas temperature leaving the air pre‐heaters should be closely monitored during startups, low load operation and shut‐down.
Any fire detecting device provided by OEM 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.
Adopting good operating practice always leads to improved availability and efficiency.
Seal clearances should be checked at regular intervals as per OEM recommendation Boiler parameters to be maintained as per or close to design APH leakage should be checked periodically and corrective action to be taken, if
necessary Primary and Secondary combustion air loss to be minimized for effective heat recovery Pressure drop across the APH should be monitored in shiftwise Operating concerns of APH are mainly,
o Corrosion o Plugging and cleaning of air pre‐heater elements o Erosion of heat transfer elements o Fire in the air pre‐heater