risk logic inc

9/22/2015 RiskLogicInc.

http://www.risklogic.com/articles/jan2003.html 1/3

RiskLogicInc..48DimmigRoad,UpperSaddleRiver,NJ07458.Phone:(201)9300700.Fax:(201)9308795.www.risklogic.comJanuary2003:FireProtectionforAirPreheaters

Although fires in air preheaters (ap's) ofUS power plants are infrequent, the result can be extensivedamage and downtime of severalmonths.Here,we consider steps that can be taken tominimize theincidenceandconsequencesoftheseevents.

Basicallywe are dealingwith a heat exchanger that uses hot flue gas to heat primary and secondaryboiler combustion air to increase efficiency of fuel combustion. Although there are two basic types,RecuperativeandRegenerative,the"rotaryplateregenerativetype"isbyfarthemostcommonlyusedintheUSAutilityindustryandisthetypeweshallfocusonhere.

Thistypeofunithasabearingsupportedrotorshaftinahousingtowhichsegmentalbasketscontainingtheheating surface (plates) are attached.As the rotor turns,heating surfacespass through thehotgasstreamabsorbingheatwhichisgivenuptowarmincomingcombustionair.

Ap's are subject to severe fire damage when ignition of combustible deposits in the baskets occurs.Thesedepositsareusuallytheresultofcarryoverunburnedcarboninthefluegasesinoilorcoalfiredunits.This occursmost duringperiodsof poor firing conditionsor start up,whenplates are cold andbelowthedewpointoftheproductsofcombustion.Thisisacceleratedbythescalewhichformsontheplates in the corrosive flue gas stream and moisture laden air stream, and by the effect of alternateheatingandcooling.Unburnedcoalcanbecometrappedinthescale,mostlyonthehotendoftheunitwherehotfluegasenterstheheattransferelement.

Afireinvolvingoildepositswouldbecharacteristicofanyordinaryoilfire.Coaldepositscanignitetoform a hot ember type fire. The fire is aggravated by the air velocity through the unit aswell as theexcess oxygen it provides.Fire propagation is a concerndue to depositswhichmayhave adhered tosurfaces of ductwork and the precipitator unit. Undetected, an ap fire can develop temperaturesexceeding 3,000F, and become a metal fire involving the ap structure itself. Long before this,elevated temperaturesseverely reduce thestructural steel strengthandshouldbeconsidereda limitingparameter.

Whathumanelementconsiderationscanbeusedtomitigatethefirehazard?

Operators need to be aware of (1) the possibility of creating a fire hazardwith offratio firing of oilburnersandigniters,(2)thepossibilityofoilcarryoverduringcoldstart,(3)theimportanceofregularscheduledsootblowingpermanufacturerinstructions,(4)theneedtomonitorpressuredropacrosstheap todetectplugging, (5)closemonitoring throughobservationports inallducts,strategically locatedforeasyviewingofthebaskets,and(6)theimportanceofastoppedrotoralarmtothecontrolroom.

Whatelsecanbedone?

Equipment suppliersoftenprovideair inlet andoutletmonitoring thermocouples.Somecanprovideadedicated fire detection and/or suppression system. Units usually have steam or air systems for sootblowingandhavewaterspraynozzlesforbasketwashdownwhentheboilerisdown.



Thermocouplesarenottooreliableforfiredetectionbecauseofheatdilutionduetoairandgasflow,thedifferentialneededbetweenactualoperatingtemperatureandthealarmsetpointstoavoidfalsealarms,poorhotspotdetectioncapability,andthelargenumberneededtocoverthecrosssectionoftheairandfluegasducts.Similarproblemsarefoundwhenusingspottypefiredetectors.Productsofcombustiondetectors are not suitable due to the dust atmospheres and high particulate environment. Continuouswiretypedetectorswouldgivebettercrosssectioncoverage,buthavemanyofthesameweaknessesasthermocouples.

Infrareddetectorsprovidesomeadvantages.Theyrespondtoconstantlevelsofradiation,suchaswouldbe emittedby a rotor hot spot. Several, placed across the cross sectionof the air inlet duct only, anddirectedwiththelenspointedatthebasketswouldnotbeasaffectedbytheheatofdilutionfromgasorair flows. The differential set points between the operating and fire condition are eliminated, and indetectinghotspots,therotorrevolutionsgivesacompletescanoftheentirerotorbasketsysteminshorttime(rotorrevolutionisaboutoneperminute).Inadditiontheyhavequickresponsetime.

This is thepreferredchoice.Andwhen located in the air inletduct, there is lowerbasket temperaturewhich minimizes background radiation, more favorable atmosphere and less airborne particulate.However, there are somedisadvantages, primarily cost each detectorwould require awater coolingsystemandanairsootblowingsystemtooptimizetheoperationalcapability.

Severalsuppressionalternativesareavailable.

Water spray, with open nozzles inside the ap, connected to a water supply through a deluge valveopenedbythefiredetectionsysteminstalledprovidescooling,andsteamproducedaswater impingeson hotmetal surfaceswill have a smothering effect. There are disadvantages: (1)water collection ordrainage is needed to prevent excessive loads on ductwork, (2) open spray nozzles opposing the airflow tend to clog without nozzle protection, and (3) possible thermal shock to rotor and housingdependsonthemagnitudeofthefireandtheassociatedmetaltemperatures.

Gaseous systems operate best as a total flooding systemwith a soak time.Ap's duct dampers cannotnormally be relied on to provide a tight shutoff.With the high leak rate, gaseous flooding becomesinappropriate.

Foamswouldreducethenecessaryquantityofwaterandhavegoodsuppressanteffectiveness,butarenotaseffectiveaswaterasacoolingmedia.

Conclusion

High velocity air and gas through ap's provide excess oxygen, rapid fire intensification, high metaltemperatureandultimatestructural failure.Remedialactivities tominimize theprobabilityofa fire,aswell as reducing combustible deposits upon which fire can propagate are essential. Early detection,quickactionbyoperatorsandeffectivemeansofextinguishmentcombinetoavoidmajordamageandlengthydowntime.Infraredtemperaturemonitoringisconsideredthebetteroftheavailablechoicesinthesuggestedlocation.Thiscanbesupplementedbyfixedtemperatureheatsensorsinthesealareaoftherotorbasketwheretherearenothighflowratesofairorgas.ThesedetectionsystemsshouldalarmtotheControlRoom.

Uponreceiptofalarms,operatorsmustinitiateimmediateactiontoprovidevisualconfirmationofafirecondition.Athreetofiveminuteresponseshouldbethegoal.Oncefireisconfirmed,emergencyboilertrip procedures should be initiated to reduce air and flue gas flow rates. Water spray suppressionsystems should be activated. This can be local or control roommanual action, with a 5minute autoinitiationprovidedfromthetimeoftheinitialdetectionalarm.



Suggestedwaterspraydesignis0.3gpmpersqftovertheentirecombustionairandfluegasductcrosssection fromboth sides. Fire hose stations formanual fire fighting should be available in the generalareaoftheap.

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