air pockets and pressure surges

AirPocketsandPressureSurges:TheInvisibleThreat

Ifairispresentinthesystemandgaugesarenotcarefullymonitored,pressurescanexceedthePSIratingofthefireprotectionsystem,causingseriousdamageorhinderingtheeffectivenessofthesystem.

Manyofthecatalyststhatresultinproblemswithaplantssprinklersystemgounnoticeduntilitistoolate.Oneofthemostcommoncausesforsystemdamagethatisoftenoverlookedisthepresenceofairpocketswithintheundergroundsupplypiping.Thesesymptomsareoftenimperceptibleunlesspressuregaugesarecloselymonitored;allowingtheseproblemstheopportunitytointensifyuntilsystemsexperienceafalsetriporsustainmajordamage.Fortunately,therearesimplemeasuresthatcanbetakentopreventsupplypipingairpocketsfromthwartingsystemfunctionality;itisjustamatterofexecutingbestpracticesandrecognizingproblemsearlybeforetheyescalateintobrokenequipment. HowDoAirPocketsFormWithinaSprinklerSystem?

Thereareanumberofwaysinwhichairpocketscandevelopwithinawatersupplysystem.Whenanewsystemisinstalledatafacility,itisinitiallyfilledwithairatatmosphere.Itistheactionsthataretakenbeforethesystemisputintousethatdeterminewhetherornotthosepocketsremain.Similarly,whenrepairsoroutagesoccur,airisallowedtoenterthesystem,whichcancreatethesameproblemsaswhenasystemisfirstinstalled.Anotherwayinwhichaircanaccumulateinthesystemisfromthewateritself.Overtime,oxygencanseparatefromthewaterandformairpocketsthroughoutthehighpointsinthesystem.HowDoesTrappedAirBecomeaThreattoSprinklerSystems?

Regardlessofhowtheairentersthesystem,thesepocketswillalwaysnaturallygravitatetowardsthehighpointswithinthesupplysystem.Astheairaccumulatesatthesehighpoints,itbecomesmoredifficultforwatertopassthroughtheseareasbecausetheairactsasarestriction.Asthewaterisforcedtopassthroughrestrictions,itsvelocitywillsubsequentlyincrease,untilithasreachedahighenoughvelocitytodislodgetheair.Asthewateristhenstoppedagainbyanotherhighpointortheendofamain,theabruptchangeinvelocitycanresultinpressuresurgesorwaterhammer.Anotherscenariothatcanresultinwaterhammerisatthetimeofsystemstartup.Ifwaterenterstheemptypipeataratetooquicklyforairtoescape,asurgeofairpressurecausedbytherushofwatercancreateawaterhammer.

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WhatAretheConsequencesofaPressureSurge?

Ifcaughtearly,minorpressuresurgeswillhavelittleeffectonafacilitysfireprotectionsystem.Gaugescanindicatethatapressuresurgeisoccurringandtheappropriatemeasurescanbetakentorectifytheoriginoftheproblembeforeitcausesdamagetothesystem.However,ifasurgegoesunnoticedandtheissueescalates,theconsequencescanbedestructivetothesystemanditseffectivenesscompromised.Whenapressuresurgeoccurswithinawatersupplysystem,pressurelevelscanreachupwardsof300poundspersquareinch(PSI)andbecometrappedonthesystemsideofasprinklersystemvalve.Thestandardequipmentassociatedwithsprinklersystemsinpowergeneratingandchemicalprocessingplantstypicallyhavearatingof175,inaccordancewithNFPAstandards.Whensprinklerequipmentissubjectedtopressuresthatexceedtheirrating,equipmentcanfailandcomponentssuchasvalvesandpumpsaresusceptibletodamage.Evenifpressuresurgesofalowermagnitudeoccurrepeatedly,therepeatedstresscyclescanweakenthesystemcomponents.Fireprotectionsystemsthatsufferdamageasaresultofpressuresurgesarerenderedineffectivewhilerepairsarebeingmade,puttingthefacilityatriskuntilthesystemisfunctionalagain.

Alternatively,whendamageisnotimmediatelyrecognized,facilitiesaresubjectedtoanevengreaterthreatbecauseitispossiblethatthefaultyequipmentwillnotberealizeduntilprovenineffectiveintheeventofafire.Perhapsthemostprevalentconsequencerelatedtopressuresurgesinsprinklersystemsistheoccurrenceoffalsetrippingofdryvalvescausedbysurges.Falsedischargethatistriggeredbyapressuresurgeisburdensomeforplantstaffandalsotakesacriticalcomponentofthefireprotectionsystemoutofservice.Oncedischargehasoccurred,plantstaffmustshutthesystemdownanddrainit,exposingthefacilitytoriskwhilethesystemisbeingserviced.Furthermore,ifanaccidentaldischargeoccursinfreezingconditionsanditisnotimmediatelyrecognized,thereisapossibilitythatpipescouldfreeze,causinglongtermimpairmentstothesystem.

Whenafireprotectionsystemsustainsdamageasaresultofapressuresurge,systemsaretakenoutofservice,leavingfacilitiesvulnerabletofireswhilerepairsaremade.

Beyondtheissuesrelatedtodamagedequipmentandfalsetrips,theoverallperformanceofadrysprinklersystemcanbeimpededbythepresenceofair.Ifthedifferentialvaluesofairpressurewithinthesystempipesareskewedtocompensateforsurges,itcantakelongerforwatertodischargeintheeventofafire.Excessiveairpressureandrestrictionscausedbypocketsofaircanresultinslowerdeliveryofwatertotheopenhead,allowingfirestogrowinintensity,makingthemmoredifficulttocontrol.


SafeguardingFireProtectionSystemsfromPressureSurges

Takingintoconsiderationallofthepotentialproblemsthatcanstemfrompressuresurgescausedbytrappedairinawatersupplysystem,thebestsolutionispreventingpressuresurgesfromoccurringaltogether.Therearedifferentmethodsforpreventingairfrombecomingtrappedwithinawatersupplysystemandthemosteffectivesolutioninvolvescarefulplanningthatbeginsinthedesignphaseofaproject.Becauseairinherentlymigratestothehighpointsofasystem,undergrounddesignsthatincludemanychangesinelevationaremoreproblematicthanleveldesigns.Oneofthemostcriticalmeasuresthatmustbetakentopreventairfrombecomingtrappedwithinthesystemtakesplaceafterthesystemhasbeeninstalledandisbeingfilledwithwater.Linesshouldbefilledslowly,allowingairampleopportunitytoescapefromthepiping.Additionally,hydrantsandleadinsshouldbeflushedinasystematicmanner,beginningwiththebackofthesystemandmovingtowardsthefront,ensuringthateverysectionofthesystemisthoroughlyflushedbeforebeingputintouse.Airshouldbebledfromallofthehighpointsofthesystemtoexpelanyremainingairthatexists.And,afterthesystemisoperational,theseactionsshouldbeperformedonaregularbasistopreventanyaccumulationofairwithinthesprinklerpipes.DiligentMaintenanceMitigatesRisk

Thenegativeeffectsofneglectingtrappedairwithinfireprotectionsystemscanbedirewhentheperformanceofthesystemisinhibitedasaresult.Damagedequipmentcannotonlybeacostlyandtimeconsumingproblem,butthesafetyofaplantisdiminishedwhenvitalcomponentsofafireprotectionsystemarenotoperatingatpeakperformance.Lessobviousimpediments,suchasalinerestriction,canalsohaveharmfuleffectsifnotadequatelyaddressed.Diligentexecutionofsimple,yeteffectivepractices,suchasventingpocketsofairfromthehighpointsinthesystemcanhelpensurethatfireprotectionsystemsarefunctioningattheiroptimalcapabilityintheeventofafire.


air pockets and pressure surges

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