12621FeatherwoodDrive•Suite120•Houston,Texas77034Tel:(281)922-8324•Fax:(281)922-4362www.SIS-Tech.comINTRODUCTIONTOLAYEROFPROTECTIONANALYSISAngelaE.Summers,Ph.D.,P.E,President,SIS-TECHSolutions,LP“IntroductiontoLayersofProtectionAnalysis,”MaryKayO’ConnerProcessSafetyCenterSymposium,TexasA&MUniversity,CollegeStation,Texas,October2002.“IntroductiontoLayersofProtectionAnalysis,”JournalofHazardousMaterials,104(2003).“IntroductiontoLayersofProtectionAnalysis,”AngelaSummersandScottSandler,ISAEXPO2003,ResearchTrianglePark,NC:Instrumentation,Systems,andAutomationSociety,Houston,Texas,October20-23,2003.KeywordsLayersofProtectionAnalysis,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.June15,2007Page2of612621FeatherwoodDrive,Suite120Houston,Texas77034WhatisLOPA?Layersofprotectionanalysis(LOPA)isasemi-quantitativemethodologythatcanbeusedtoidentifysafeguardsthatmeettheindependentprotectionlayer(IPL)criteriaestablishedbyCCPS1in1993.WhileIPLsareextrinsicsafetysystems,theycanbeactiveorpassivesystems,aslongasthefollowingcriteriaaremet:Specificity:TheIPLiscapableofdetectingandpreventingormitigatingtheconsequencesofspecified,potentiallyhazardousevent(s),suchasarunawayreaction,lossofcontainment,oranexplosion.Independence:AnIPLisindependentofalltheotherprotectionlayersassociatedwiththeidentifiedpotentiallyhazardousevent.Independencerequiresthattheperformanceisnotaffectedbythefailureofanotherprotectionlayerorbytheconditionsthatcausedanotherprotectionlayertofail.Mostimportantly,theprotectionlayerisindependentoftheinitiatingcause.Dependability:TheprotectionprovidedbytheIPLreducestheidentifiedriskbyaknownandspecifiedamount.Audita...
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