12621FeatherwoodDrive,Houston,Texas77034281-922-8324(phone)�281-922-4362(fax)www.SIS-TECH.comINTRODUCTIONTOLAYEROFPROTECTIONANALYSISAngelaE.Summers,Ph.D.,P.E,President,SIS-TECHSolutions,LPTobepresentedattheMaryKayO’ConnerProcessSafetyCenterSymposium,TexasA&MUniversity,October2002.PublishedinJournalofHazardousMaterialsKeywordsLayersofProtectionAnalysis,LOPA,hazardanalysis,protectionlayers,IEC61511,ANSI/ISA84.01-1996,riskmitigationAbstractLayersofprotectionanalysis(LOPA)isapowerfulanalyticaltoolforassessingtheadequacyofprotectionlayersusedtomitigateprocessrisk.LOPAbuildsuponwell-knownprocesshazardsanalysistechniques,applyingsemi-quantitativemeasurestotheevaluationofthefrequencyofpotentialincidentsandtheprobabilityoffailureoftheprotectionlayers.ThispaperwillprovideanoverviewoftheLOPAprocess,highlightingthekeyconsiderations.IntroductionTheprocessindustryisobligatedtoprovideandmaintainasafe,workingenvironmentfortheiremployees.Safetyisprovidedthroughinherentlysafedesignandvarioussafeguards,suchasinstrumentedsystems,procedures,andtraining.DuringaHAZOP,theteamisresponsibleforassessingtheprocessriskfromvariousprocessdeviationsanddeterminingtheconsequenceofpotentialincidents.Theteamidentifiesthesafeguardsusedtomitigatethehazardousevent.Iftheteamdeterminesthatthesafeguardsareinadequate,theteamwillmakerecommendationsforfurtherriskreduction.Theteamisinstructedtolistallsafeguards.Theteamoftenlistssafeguardsthatonlypartiallymitigatetheprocessrisk.Theteamalsodoesnotaddresswhetherthesafeguardsareindependentfromoneanother.Thisoftenresultsintheteamassumingmoreriskreductionfromthesafeguardsthanispossiblebasedontheintegrityoftheindividualcomponents.Furthermore,ateam’sperceptionoftheintegrityofaspecificsafeguardimpactstheassumedriskreductionforthatsafeguard,resultingininconsistencyinthenumberofrequiredsafeguardsforsuccessfulmitigationoftheprocessrisk.Unfortunately,theinconsistencycanresultinover-andunder-protectedprocessrisk,dependingontheteamcomposition.Consequently,theremustbeanindependentengineeringassessmentofthesafeguardstoensurethatadequateriskreductionisbeingprovided.12621FeatherwoodDrive,Houston,Texas77034281-922-8324(phone)�281-922-4362(fax)www.SIS-TECH.comWhatisLOPA?Layersofprotectionanalysis(LOPA)isasemi-quantitativemethodologythatcanbeusedtoidentifysafeguardsthatmeettheindependentprotectionlayer(IPL)criteriaestablishedbyCCPS1in1993.WhileIPLsareextrinsicsafetysystems,theycanbeactiveorpassivesystems,aslongasthefollowingcriteriaaremet:Specificity:TheIPLiscapableofdetectingandpreventingormitigatingtheconsequencesofspecified,potentiallyhazardousevent(s),suchasarunawayreaction,lossofcontainment,oranexplosion.Independence:AnIPLisindependentofalltheotherprotectionlayersassociatedwiththeidentifiedpotentiallyhazardousevent.Independencerequiresthattheperformanceisnotaffectedbythefailureofanotherprotectionlayerorbytheconditionsthatcausedanotherprotectionlayertofail.Mostimportantly,theprotectionlayerisindependentoftheinitiatingcause.Dependability:TheprotectionprovidedbytheIPLreducestheidentifiedriskbyaknownandspecifiedamount.Auditability:TheIPLisdesignedtopermitregularperiodicvalidationoftheprotectivefunction.ExamplesofIPLsareasfollows:•Standardoperatingprocedures,•Basicprocesscontrolsystems,•Alarmswithdefinedoperatorresponse,•Safetyinstrumentedsystems(SIS),•Pressurereliefdevices,•B...
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