Fire detection and fire fightingsystems in regenerative heatersprotect availability of the powerstation.

Figure 1: Basket damaged by fire in an Air Preheater.

Figure 2: Rotor damaged by fire in an Air Preheater.

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Fire detection andFire fighting equipment

Air preheaters can be vulnerable to fires when operatedunder adverse conditions. Fire in regenerative heaters coulddramatically affect the availability of the Power Station

Air preheaters can be vulnerable to fires whenoperated under adverse conditions. Suchproblems are generally caused by periods offiring heavy fuel oil in combination with poorcombustion this often leads to soot and oilresidue being deposited on the heaterelements.

The risk of fire is even higher in coal-fired unitsduring subsequent transient operatingconditions, when the coals mills are brought online. At that time, the carryover of incandescentparticles from incomplete combustion providesa potential ignition source of oily depositsformed during start up with lighter fuels (gasoilor natural gas firing).

Given the risk of severe damage to the airpreheater, it is important to detect any fires assoon as possible. Therefore, on the basis ofmany years of design and operating experiencewith earlier versions of fire detection equipment,Howden has developed a Fire DetectionSystem for rotary regenerative air preheatersusing the most up-to-date technology.

Introduction

Howden’s Fire Detection System consists ofthree main components specially designed forregenerative heaters.

Howden Fire detection and firefightingequipment consist of:

- Fast response fire detection probes.

- Fire detection monitoring equipment.

- Fire Extinguishing System

Fast response fire detection probes

The air preheater is installed with a set of firedetection probes that are independently wiredback to the Fire Detection Panel. One probe isused for each ring of elements in the heater.The probes fit into tubes, which pass throughthe heater wall into the air outlet ductimmediately after the elements baskets rotatefrom the hot gas side to the air side of thepreheater.

Each probe tip is positioned close to the rotor,where it responds to the air temperature exitingfrom each basket as it passes underneath.Indeed, the response time of both the firedetection probes and the associated FireDetection Monitoring Equipment has beendesigned to be fast enough to identify firesoccurring in single baskets before they develop

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Fire detection and Firefightingequipment

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Figure 3: Fast response fire detection probes and manifold

Figure 4: Fire detection monitoring equipment.

Figure 5: Rotor damaged by fire in an Air Preheater

far enough to produce a large enough fire thatcan be detected by bulk gas temperaturemeasurements.

The probes consist basically of open-endedthermowells with type-K thermocouples. Thesehave a reduced diameter tip to give a fastresponse to changes in temperature. This lighttip is protected against mechanical damage bya perforated stainless steel shroud that allowsthe passage of gas.

Fire detection monitoring equipment.

The thermocouple signals are wired back toModular Fire Detection Monitoring Equipmentdesigned to detect both high temperature andhigh rate of change of temperature at eachprobe. The “high temperature” setting isconfigured to suit the operating temperature ofthe heater.

The “rate of change” feature is consideredessential to early detection of fires in the heater.A fire in a single basket will produce a rise intemperature of both the gas and air passingthrough it. The column of hot air will rotate withthe element baskets concerned. Themonitoring points are usually in the secondaryair duct so the hot air passing over the probetip will subject the probe to this temperaturetransient once per revolution.

If the fire detection system is to generate analarm, it must be able to pick up the rapidchange in temperature as the fire sweepsquickly under the probe. The Fire DetectionSystem is designed to generate alarms on eachchannel if a predetermined rate of change oftemperature is exceeded.

Fire Extinguishing SystemA set of curved manifolds with multiple nozzleswill be installed to cover the entire area of therotor from gas, primary air and secondary airsides of the air preheater. A very thin titaniumprotective disc prevents each nozzle from beingplugged by flying ashes. In case of a fire, thisdisc is ruptured by the water ensuring fullavailability of the system.