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January 2003: Fire Protection for Air Preheaters

Although fires in air preheaters (ap's) of US power plants are infrequent, the result can be extensive damage and downtime of several months. Here, we consider steps that can be taken to minimize the incidence and consequences of these events.

Basically we are dealing with a heat exchanger that uses hot flue gas to heat primary and secondary boiler combustion air to increase efficiency of fuel combustion. Although there are two basic types, Recuperative and Regenerative, the "rotary plate regenerative type" is by far the most commonly used in the USA utility industry and is the type we shall focus on here.

This type of unit has a bearing supported rotor shaft in a housing to which segmental baskets containing the heating surface (plates) are attached. As the rotor turns, heating surfaces pass through the hot gas stream absorbing heat which is given up to warm incoming combustion air.

Ap's are subject to severe fire damage when ignition of combustible deposits in the baskets occurs. These deposits are usually the result of carryover unburned carbon in the flue gases in oil or coal fired units. This occurs most during periods of poor firing conditions or start up, when plates are cold and below the dew point of the products of combustion. This is accelerated by the scale which forms on the plates in the corrosive flue gas stream and moisture laden air stream, and by the effect of alternate heating and cooling. Unburned coal can become trapped in the scale, mostly on the hot end of the unit where hot flue gas enters the heat transfer element.

A fire involving oil deposits would be characteristic of any ordinary oil fire. Coal deposits can ignite to form a hot ember type fire. The fire is aggravated by the air velocity through the unit as well as the excess oxygen it provides. Fire propagation is a concern due to deposits which may have adhered to surfaces of ductwork and the precipitator unit. Undetected, an ap fire can develop temperatures exceeding 3,000°F, and become a metal fire involving the ap structure itself. Long before this, elevated temperatures severely reduce the structural steel strength and should be considered a limiting parameter.

What human element considerations can be used to mitigate the fire hazard?

Operators need to be aware of (1) the possibility of creating a fire hazard with off-ratio firing of oil burners and igniters, (2) the possibility of oil carryover during cold start, (3) the importance of regular scheduled soot blowing per manufacturer instructions, (4) the need to monitor pressure drop across the ap to detect plugging, (5) close monitoring through observation ports in all ducts, strategically located for easy viewing of the baskets, and (6) the importance of a stopped rotor alarm to the control room.

What else can be done?

Equipment suppliers often provide air inlet and outlet monitoring thermocouples. Some can provide a dedicated fire detection and/or suppression system. Units usually have steam or air systems for soot blowing and have water spray nozzles for basket wash down when the boiler is down.

Thermocouples are not too reliable for fire detection because of heat dilution due to air and gas flow, the differential needed between actual operating temperature and the alarm set points to avoid false alarms, poor hot spot detection capability, and the large number needed to cover the cross section of the air and flue gas ducts. Similar problems are found when using spot type fire detectors. Products of combustion detectors are not suitable due to the dust atmospheres and high particulate environment. Continuous wire type detectors would give better cross section coverage, but have many of the same weaknesses as thermocouples.

Infrared detectors provide some advantages. They respond to constant levels of radiation, such as would be emitted by a rotor hot spot. Several, placed across the cross section of the air inlet duct only, and directed with the lens pointed at the baskets would not be as affected by the heat of dilution from gas or air flows. The differential set points between the operating and fire condition are eliminated, and in detecting hot spots, the rotor revolutions gives a complete scan of the entire rotor basket system in short time (rotor revolution is about one per minute). In addition they have quick response time.

This is the preferred choice. And when located in the air inlet duct, there is lower basket temperature which minimizes background radiation, more favorable atmosphere and less airborne particulate. However, there are some disadvantages, primarily cost - each detector would require a water cooling system and an air soot blowing system to optimize the operational capability.

Several suppression alternatives are available.

Water spray, with open nozzles inside the ap, connected to a water supply through a deluge valve opened by the fire detection system installed provides cooling, and steam produced as water impinges on hot metal surfaces will have a smothering effect. There are disadvantages: (1) water collection or drainage is needed to prevent excessive loads on ductwork, (2) open spray nozzles opposing the air flow tend to clog without nozzle protection, and (3) possible thermal shock to rotor and housing depends on the magnitude of the fire and the associated metal temperatures.

Gaseous systems operate best as a total flooding system with a soak time. Ap's duct dampers cannot normally be relied on to provide a tight shutoff. With the high leak rate, gaseous flooding becomes inappropriate.

Foams would reduce the necessary quantity of water and have good suppressant effectiveness, but are not as effective as water as a cooling media.

Conclusion

High velocity air and gas through ap's provide excess oxygen, rapid fire intensification, high metal temperature and ultimate structural failure. Remedial activities to minimize the probability of a fire, as well as reducing combustible deposits upon which fire can propagate are essential. Early detection, quick action by operators and effective means of extinguishment combine to avoid major damage and lengthy down time. Infrared temperature monitoring is considered the better of the available choices in the suggested location. This can be supplemented by fixed temperature heat sensors in the seal area of the rotor basket where there are not high flow rates of air or gas. These detection systems should alarm to the Control Room.

Upon receipt of alarms, operators must initiate immediate action to provide visual confirmation of a fire condition. A three to five minute response should be the goal. Once fire is confirmed, emergency boiler trip procedures should be initiated to reduce air and flue gas flow rates. Water spray suppression systems should be activated. This can be local or control room manual action, with a 5 minute auto initiation provided from the time of the initial detection alarm.

Suggested water spray design is 0.3 gpm per sq ft over the entire combustion air and flue gas duct cross section from both sides. Fire hose stations for manual fire fighting should be available in the general area of the ap.