Part 1 Design Requirements

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Part1DesignRequirements:AComparisonofVapourCloudExplosionModels

andtheImportanceofProperlyAssessingPotentialIncidentImpact

Part2AnalysisoftheBuncefieldOilDepotExplosion:ExplosionModelingand

ProcessSafetyPerspective

NoahL.Ryder,P.E.* ,ChristopherF.Schemel*,SamMannan#

Abstract

Thispaperdiscussesthevariousexplosionmodelsthatarefrequentlyusedinsupportoffacilitylayoutandriskassessmentsandexaminestheresultsthateachmodeltypewillproduceforcomparativepurposes.Itisshownthatwhileempiricalbasedmodels,suchastheTNTequivalencyandTNOMulti-EnergyMethodmaybesimpleinconcept,thatproperapplicationofthemodelcanbecomplex.Inadditiontheuseofthesemodelsprovideresultsthatresultinatotallossofspatialspecificity.Mostempiricalmodelsassumeauniformhemispherethatchangessolelyinaradial

fashion,thespecificsofaplantlayoutandtheimpactthatlocalizedcongestionmayplayinthedevelopmentofaflammablecloudanditsignitioncanbeextremelyimportantintheproperdesignofafacilityorinconductinganappropriatehazardreview.Thedifficultyinassigningappropriatelevelsofcongestionandindeterminingwhetherappropriatespacingbetweenequipmentexistswhenapplyingthemodelcanresultinsignificantunderoroverpredictionofthepotentialimpact.Theseconceptuallysimpleconceptsareoftenquitedifficulttoimplementandhighlightsomeofthecomplexitiesofapplyingempiricalmodelsbroadly.Abriefoverviewonmodelingisprovidedinordertohighlightthepurposeofmodelingandthenecessityofensuringthattheappropriatemodelhasbeenconstructedorusedinassessingthesituation.Recentevents,Buncefield,aswellaspreviousevents,inwhichthesetoolswereapplied,clearlyshowthatincidentsinwhichalargevapourcloudreleasefollowedbyadelayedignitionareforeseeable.

Evenifthespecificchainofeventsisdeemedimprobable,theendresult,namelyavapourcloudreleaseandsubsequentignition,issufficientlypredictablethatanappropriateapplicationofthecommontoolsoftheindustryshouldbeabletoadequatelycapturetheincidentimpact.Areviewofeachofthemodelsispresentedalongwiththeassumptionsthataccompanythemodel.SpecificexamplesfromtheRIGOStestseriesareexaminedtocomparetheMEM,TNT,andtheCFDtoolFLACSinordertoillustratethedifferencesthateachmodelwillproducewhenattemptingtoassessthesamesituation.Thedifferencesintheseresultsisthendiscussedfromthecontextthatachangeinrequirementsspecificationandscenariodevelopmentcanhaveamajorimpactwhenaforeseeableincidentoccurs,buttheinitialtoolsusedtoassessthescenariodontcapturethenecessarylevelofdetailedinformationorthetoolsareusedinappropriately.

Introduction

*Packer Engineering, Inc.

Noah L. Ryder is the corresponding author and may be reached at (301)775-2967 or via email at [email protected]

# Mary Kay OConnor Process Safety Center


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Thespacingofpetrochemicalprocessunitsandbuildingshasbeenacriticalsafetyandoperationsissuesincelargescalerefiningoperationsbegan.Numerousstandardsandguidelinesexistwhichattempttoprovideclarityontheproceduresforassessingfacilitysitingandlayout,howevermanyoftheseguidelinesareinconsistentwitheachotherinoneormoreareasandamajorityoftheguidelines

areprescriptiveinnaturebearinglittlerelevancetorealworldperformance.Overthepast40yearsalargenumberofexplosion/fireincidentshaveoccurredinoilandpetrochemicalfacilities,whichhaveimpactedotheronsiteandoff-siteareas.Theseincidentshavecausedthevariousguidelinesandstandardstobereevaluatedandforchangestobemade,howeverthesechangesarereactionaryinnatureandareoftennotbasedinscience.Thedrivingforcebehindspacingandothersafetycriteriaisthepotentialconsequenceofanincident,todatetheprimarytoolsthathavebeenusedtoassistinmakingthesedeterminationsareempiricallyderivedtoolsthathavebeenshowntobedifficulttoapplyproperlyorimproperfortheapplication.Thiscreatesapotentialfortheconsequencesofaneventtobeseverely

underpredictedandfordamagetobedonefarbeyondthatinitiallyanticipated.OverviewofModeling

Brieflywewilldescribethefundamentalsofamodel,asitwillbeusefultosummarizetheprinciplesofmodelinginageneralfashiontounderstandthebasicapproachesthatmaybetaken.Amodelisapurposefulrepresentationofthephysicalworld,withmanytypesofmodelingexisting:visualmodels(pictures),verbalmodels(speech,articles,andbooks),three-dimensionalmodels(sculptures),andmathematicalmodels.Thepracticeofengineeringoftenrequirestheuseofmodelsinordertogeneratean

engineeringsolutiontoadefinedproblem.Onceconstructed,amodelisverifiedandvalidatedbyaseriesoftestsandexperimentstoindicatehowwellthemodelrecreatesthephenomenamodeled.Thedifferencebetweenverificationandvalidationishoweversignificantandisoftenoverlooked.ASTME1355,definesVerificationas,Theprocessofdeterminingthattheimplementationofacalculationmethodaccuratelyrepresentsthedeveloper'sconceptualdescriptionofthecalculationmethodandthesolutiontothecalculationmethodanditdefinesValidationas,Theprocessofdeterminingthedegreetowhichacalculationmethodisanaccuraterepresentationoftherealworldfromtheperspectiveoftheintendedusesofthecalculationmethod.

Inshorttheverificationprocessensuresthatthemodelsmathematicformulationsarecorrectlyyieldingwhatisintended(i.e.thealgorithmusedtodeterminetheproductof5and4yields20)whilethevalidationprocessensuresthattheresultsofthemodelprovideafairrepresentationoftherealworld.Fromanengineeringperspective,theintentionofamodelistoproduceananalysistoolthatadequatelyapproximatesrealityforthespecificsituationforwhichthemodelwascreated.


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Mathematicalmodeling,whichformsthebasisformostengineeringapplications,beginswithasituation/phenomenathatexistsintherealworldandaprecisedefinitionoftheproblembyreducingextraneousinformationandsimplifyingtheproblem,whilestillenablingthenecessaryinformationtobecaptured,Figure1.Thekeyinthisprocessisthatavalidatedandverifiedmodelmustbeusedinthe

appropriatemanner;similartoanytypeoftoolifusedinappropriatelytheresultsorapplicationoftheresultswillbeincorrect.

Figure1Firstorderviewofmathematicalmodelingi

Forthepurposeoffacilitysitingandlayout,modelsareusedtoevaluatethedispersionofflammableandtoxicgasses,tomodelfiresandtheimpacttostructuresandpersonnel,andtomodelexplosionseverityanditsimpactonstructuresandpersonnel.Itisthislastissuewithwhichwearemostconcerned.Historyhasshownusthatexplosionscanhavedevastatingimpactsbothintermsofemployeeandpublicsafetyandintermsofcontinuityofbusinessoperations.Ofthelargestonshoreindustriallosses(>$150,000,000)asof200283%werearesultofexplosionsandVCEs.iiAftereachincidentoccurreddetailedanalyseswereperformedtotryanddeterminewhatcausedtheincidentandwhatcanbedonetopreventafuturesimilarincident,oftenthisincludedgoingbackandrevisingguidelinesandstandards.

WhatModelsAreUsedFor

Thepurposeofamodelistoobtaindatathatcanbeusedinausefulfashion,thusforexplosionsmostoftenthedatathatisdesiredis:whatthepressureprofilewilllooklike(i.e.whatisthedamagethatmightbeexpectedatvariouslocations);andwhatthenecessaryspacingbetweenprocessunitsorstructuresshouldbetominimizeimpact.Inordertoassesstheimpactofavapourcloudexplosionthegaseousfuelmustbeidentifiedforreactivity,energycontent,theconcentrationanddistributionofthecloudmustbeknown,andtheareasofcongestionandconfinementmustbeidentified.


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ForsimplifiedanalysesmuchoftheaboveinformationisassumedwhileforotherssuchasCFDamoredetailedprofileofthevapourcloudanditsinteractionswiththecongestion/confinementcanberesolved.

Regardlessofthemodelusedthedesiredoutcomeistoobtainpressurecontoursasafunctionofdistancefromthepointofignition.Eachmodelattemptstocapturethisinformationeitherfromcorrelationstoexperimentaldataorbysimulatingthephysicalprocessesofanexplosion.

Models

Explosionmodelsmustbysomemethodaccountforthesizeofthecloud,theenergyofthecloud,andtheefficiencyofcombustionorthecongestionoftheareaofinterest.Acloudwilldisperseandtheempiricalmodelsassumethatthecloudisahomogeneous,stoichiometriccloudthatisdispersedsymmetricallyinaradialfashion.Clearlythisisnotrealisticineventhemostidealizedcaseandthusthe

startingpointformostanalysesthatutilizetheseempiricaltoolsisgoingtobeflawedfromtheoutset.TNTEquivalencyMethod

TheTNTequivalencymethodologyoriginatesfromearlymilitarytestingandisfrequentlyusedbecausethereisextensiveblastdamagedataavailablefromarangeofsources.ThemethodologyassumesthatavapourcloudexplosioncanbeequatedtoanequivalentquantityofTNTusingayield(efficiency)factor.OnceanequivalentamountofTNThasbeenestimatedthecharacteristicsoftheexplosionandexpectedlevelofdamagecanbeestimated.iii

Thebasicmethodologyrequiresthattheweightofthefuelcontainedwithinthevapourcloudbeknownandthatanappropriateyieldfactorisassigned.Thisvolumeoffuelisthenassignedanenergyvaluebasedonthefuelheatofcombustionofthefuelandtheyieldfactor,theequationisbelow:

WTNT =eWfQf

QTNT

whereeistheyield,Wfistheweightofthefuelinthecloud,Qfistheheatofcombustionofthefuel,andQTNTistheheatofcombustionofTNT.Thisisthenusedalongwithaformofthecuberootscalinglawtodeterminethedistanceatwhichtheblastwavepredictsthepeaksideonpressure.

R = Z WTNT( )1

3 whereRisthedistancetopeakoverpressure,Zisthescaleddistance(takenfromFigure2,andWTNTistheequivalentweightofTNT.


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Figure2Peakside-onoverpressurevs.Scaleddistance(Baker,et.al1996)

TheTNTmethoddoesnottakeintoaccountthevaporcloudsizeortheobstaclesandcongestionintheregionofthevaporcloud,andthusiswidelyusedbecauseofitsrelativesimplicityandthevastamountofdatathatexistsfromexperiments(compiledintoFigure2).However,theTNTmethodologyhasbeenshowninnumerousstudiesiv,v,vitohavesignificantlimitationswhenappliedtoVCEsnamely:

ATNTblastwaveisofashorterdurationandproducesahigheroverpressurethananequivalentenergyVCEexplosion

TheTNTblastwaveresultsinadifferentimpactonstructuresthanthatofaVCE

AVCEhasalongerpositivephasedurationwhichinturnresultsinalargerimpulse

Degreeofmixing,ignitionstrength,confinement,congestion,andturbulencearenotconsidered

ThewiderangeofTNTequivalenciesthatarefoundwhenreviewingarangeofVCEsoffuelswithsimilarheatsofcombustion.vii,viii,ix,x

TheoverallconclusionregardingtheTNTmethodologyisthatforcertainfirst-orderapproximationsregardingbuildingvulnerabilityitmaybeacceptable,howeverfortheinterpretationofaVCEitisinappropriate.Multi-energymethodThemulti-energymethodassumesthatonlythevolumeofgasthatiswithinaconfinedorobstructedareacontributestotheoverpressuregeneratedinanexplosion.xiThefuelthatisburnedinunconfinedregionsistreatedasafireballwithnoassociatedoverpressure.Itisassumedthattheflameburnsinahemisphericalshapefromthepointofignitionandthatthereisaconstantflamespeed.Themixturewithintheconfinedareaisassumedtobehomogeneousand


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stoichiometricandthereisnomechanismfordifferentiatingbetweenfuelstypes.xiiTheunderlyingassumptionisthatvaporcloudsarenon-homogenousmixturesandthatintheabsenceofconfinementadetonationcannotsupportitself.xiiiThemethodologyisentirelydependentontheassumedlevelof

congestion/confinementwithinaparticularregionorsetofregions.Themethodreliesonsummingorseparatingcongestionzonesinordertodeterminewhetherignitionwouldresultinasingleormultipleblasts.Theoverpressureisthendeterminedasafunctionofthecombustionenergyandthescaleddistancefromignitiontoatarget.Theblaststrengthisassignedavaluefrom1-10.Theequationstodeterminetheoverpressureareasfollows:

R = R PO/ E( )

13, P

S= P

S/ P

O

whereR istheenergyscaleddistance,Risthedistancefromthecenterofthe

assumedhemisphere,Poistheambientpressure,Eistheavailableenergy,PS isthe

dimensionlesspeakoverpressure,andPSisthepeakside-onoverpressure.

Figure3Dimensionlessoverpressurevs.EnergyScaledDistance(vandenBerg,1989)

TheBaker-Strehlowmethodologyusesessentiallythesamemethod,howeverinsteadofspecifyinganexplosionstrengththismethodrequirestheusertoassumeaflamespeed,andthecurvesarerepresentedasafunctionoftheflamespeed.Thereremainmanyquestionsastowhenanareacontributestothepressureandwhencongestionzonesshouldbelumpedtogetherintoasinglelargerarea.Inparticularthesafeminimumclearancebetweenzonesisatopicofgreatinterestand


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hasyettobeclearlydefined.TheMEMmethodologyhassomemajorobstaclestoitsuse:

Themodeldoesnotaccountforfuelreactivity Themodelassumesahomogeneous,hemisphericalcloud,withspherically

symmetricalisopleths Themodeldoesnottakeintoaccounttheobstaclesthatmayaffectthepressurecontours Determiningtheminimumseparationdistanceisnon-trivial Determiningtheinitialblaststrengthisnon-trivial

CFD

Acomputationalfluiddynamicsmodelsimulatesthephysicsinthousandsormillionsofcells.CFDmodelsthebasicconservationequationsusingtheNavier-Stokesequationsgoverningfluidflow.Submodelsthensimulateturbulenceandcombustion.MostmodelsuseaPorosityDistributedResistance(PDR)tomodelsmall-scaleobjects(suchasgratingonanoffshoreplatform).Mostmodelscan

simulatethedispersionofagassubsequentlyafterthedispersionhasbeenmodeleddirectly,theexplosionmaybesimulated.Thesimulationofthegasdispersionwillmodelthegasflowthroughthecomputationaldomaininandaroundthestructures,vegetation,etc.thatexistaswellastakingintoaccountanyenvironmentalfactors.Thisthenproducesacloudthatisvariedinconcentration,isnon-symmetric,andhasapreexistinglevelofinducedturbulence.

Akeyaspectofexplosionsistheextentandconcentrationofthevapourcloud,thusaccuratedispersionmodelingcanplayasignificantrole.AswasshownintheBPTexasCityincidentreport1,usingCFDtomodelthedispersionandaccountingforwind,obstacles,andthetwo-phaseflowgeneratedaflammablecloudinthesame

areaswheredamagewasobserved,whiletheempiricalmodelsdidnotaccuratelycapturetheextentofthevapourcloud.Thusinordertoaccuratelymodelanexplosionthemodelmustbeabletosimulatethereleaseandflowofgastakingintoaccounttheobstacles,structures,andtheambientweatherconditions.Afterthecloudhasdispersedanignitionsourceisintroduced.Withcfdthelocationandenergyoftheignitionsourcecanbemodeledtoseetheimpactthatvaryingignitionconditionsmayhaveonthedevelopmentofthevapourcloudexplosion.Theturbulenceismodeledusingak-turbulencemodelwhilethecombustion

chemistryismodeledusingaflameandburningvelocitymodel.Therehavebeennumerousarticlesandbooksauthoredontheimplementationofthesemodelsandit

issuggestedtorefertotheseifadditionalinformationisrequired.xivCFDmodelshavelimitationsinthatthey:

Requiresignificantlymoreinformationregardingthegeometry1Appendix 17 of the BP Texas City Fatal Accident Report details the cfd dispersion modeling and provides a comparison of the areasof the plant that were damaged with the extent of the vapour cloud.

http://www.bp.com/liveassets/bp_internet/us/bp_us_english/STAGING/local_assets/downloads/t/final_appendix_17.pdf


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Mayrequiresubstantiallymoreresourcestoutilize Mayrelyonsub-gridmodelstomodelphysicalphenomenathatcantbe

resolvedatthemodelscale

RequirementsSpecification/ScenarioDevelopment

Eachcorporationorplanthasitsownlevelofrisktolerancethatisgeneratedbasedontheplantlocation,proximitytootherprocessunits,off-sitepersonnelandstructures,regulations,etc.Oftenthisrisktolerancemaynotbeformallycodifiedinthecorporatestructure,howeverthisisthedeterminingfactorinspecifyingthedesignofaprocessunitandsitespacing.Nosingleprescriptiveguidelineisappropriateforallapplications,insteadananalysisneedstobeperformedtodeterminewhatthepotentialconsequencesmaybeasaresultofanincidentandwhethertheoutcomeistolerableornot.Whatneedstobedoneistoassesseachsitebasedonitsownsite-specificconditionsandinordertodothisthetoolsutilizedmustbeappropriateandallow

fortheproposedscenariostobemodeledcorrectly.CCPSGuidelinesforEvaluatingProcessPlantBuildingsforExternalExplosionsandFiresstatesmanyofthesebuildingdesignandsitingcriteriaarebaseduponbroadplantdesignguidelinesand

notuponanevaluationofspecificmaterials,releaseconditions,orplantgeography.Whileeffectiveinmanyapplications,thisapproachcanleadtodesignsthatareoverly

conservativeinsomeinstancesorthatfailtoprovidethedesireddegreeofprotection

inotherinstances.Therecommendedframeworkassetoutintheguidelineallowsforengineeredsolutionstobearrivedatthatmeetallthestakeholdersrequirements.

CurrentGuidanceonSpacing

Theprocessofdesigningthelayoutandspacingofequipmentinanewrefineryorothercomplexoperationisadifficulttask.Manyfactorscompeteagainsteachotherandoftensafetymaysuffer.Ifissuessuchascost,schedule,sitesecurity,andoperationalefficiencycouldbeignoreditwouldbesimpletoplaceunitprocessesandequipmentfaroutsideoftherangeofpotentialharmtootherstructures,equipment,orpersonnel.Thisclearlyisnotthecase,andthusadetailedassessmentmustbedonetodeterminetheoptimalcombinationthatresultsinanoperationthatcanrunefficientlychemically,financially,securely,andyetstillposeaminimalrisktotheplantitselfaswellastoanyneighboringstructuresandpersonnel.Withexistingsitesthatareexpandingorrevisingoperationstheoptionsareevenmorelimitedandthusclearlydeterminingwhatthepotentialexplosionscenariosare,

whatanacceptablelossis,andwhatthepotentialrepercussionsarebecomeevenmoreimportant.InfollowingtheprocessassuggestedbyCCPS,Figure4,aftertheinitialsiteischosenallofthehazardsmustbeidentifiedandtheconsequencesofeachincidenttypemustbeevaluatedinordertoproperlyassessthenecessaryspacingofequipment.


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Figure4Sitelayoutflowchart(afterGuidelinesforFacilitySitingandLayout,CCPS)xv

Spacingtablesoriginallyweredevelopedforfireriskandwerebasedonengineeringjudgmentandhavebeenadjustedperiodicallybasedonincidents,regulations,andadvancesinengineeringanalysis.CommonspacingtablessuchasthoseprovidedinGuidelinesforFacilitySitingandLayout,CCPSwerecompiledfromvariousindustrysourcesandhistoricalprecedent.Bothspacingtablesortheuseofsimplifiedmethodologiestotryanddetermineappropriatespacingaimsatachievingonegoal,minimizingtheextentofdamageassuminganincidentoccurs.Inthecaseofexplosionsthegoalistospaceequipmentfarenoughawayfromeachotherthatanexplosioninoneareaorprocessunitdoesnotpropagateandcauseacontributingorsecondaryexplosioninanotherareaorprocessunit.Inadditionwe

wouldliketominimizethedamagetofacilitiesasaresultofthepressurewavegeneratedduringtheexplosionincident.Theminimumspacingisoftenreferredtoasthecriticalseparationdistanceandtheproperidentificationofthisdistanceisessentialinproperlyapplyinganyofthesimplifiedempiricalapproachesusedtoassistinassessingthepotentialimpactofanexplosion.Theexistingguidanceonseparationdistancevariesgreatly,theYellowBooksuggestsspacingequalto10obstaclediameters,whileCCPShascompiledalistofspacingfromnational,industry,andinternalcorporatestandardswhichisshowninTable1.MuchoftheCCPSspacingwasoriginallyforfirescenariosbutoftenisappliedtoexplosionriskassessmentsaswell.Themostextensivetesting

performedtodatetodeterminetheseparationdistanceindicatesthattheguidancesuggestedbytheYellowBookmaybeinsufficient.TheRIGOStestingshowedthatthedeterminationoftheseparationdistanceiscomplexandthatitemssuchaspiperackswhichmightnormallybeignoredplayasignificantroleincontributingtoanexplosionandpushingwhatwouldnormallybealow-pressureeventintothehighexplosionoverpressurerange.


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Table1ComparisonofSelectedOilandChemicalPlantMinimumSpacingCriteriaxvi

Betweenprocessunits 50-200ft(15-60m)

Betweencontrolhouse&processunit 50-1200ft(15-365m)

Betweencontrolhouseandstoragetanks 50-350ft(15-105m)

Betweenotherbuildingsand:

Processunits 100-400ft(30-120m)

Pumps,compressors 0-100ft(0-30m)Storagetanks 5-400ft(2-120m)

Betweenmotorcontrolcentersand:

Processunits 0-200ft(0-60m)

Pumps,compressors 0-100ft(0-30m)

Storagetanks 0-250ft(0-75m)

Thisisofimportancebecausemostoftheexistingguidanceandcurrentpracticeistypicallyfarlessthanthe10obstaclediameterssuggestedbytheYellowBookandthelargestpoolofexperimentaldata,nottomentiontheincidentsthathaveoccurred,indicatethattherecommendedseparationdistanceisinsufficientintheeventofanexplosion,evenwhensignificantlyhighoverpressuresarenotinvolved

asinthecaseofBuncefield.TheSkikdaLNGfacilityisanotherexampleofafacilitywheresimplespacingruleswereappliedandtheresultingincidentshowedtheinadequacyofthedeterminedspacing.Multipleexplosionsinvolvingmultipleprocesstrainsoccurredfollowedbyafire.Anadditionalfactorregardingspacingisthattheminimumspacingforanexplosionorfirescenarioisnotthesame.IntheBuncefieldincident,whiletheexplosioncauseddamage,itwasthefiresthatragedfordaysandwereabletospreadbetweentanksandunitsthatcausedthemostdamageonsite.

ExistingResearchonCongestion

Thedynamicsofanexplosionareinfluencedgreatlybyseveralfactorsnamely:cloudprofile,ignitionstrength,confinement,andcongestion.ThesefactorswilllargelydeterminethecharacteristicsoftheVCE.Theconfinementofthecloudcanbeviewedaswhethertherearehorizontalorverticalbarriersthatconfinethecloudtoaparticularvolume,whereasthecongestionisthepercentofthespacethatisoccupiedbyobstructions(vegetation,equipment,piping,etc.).ThelevelofcongestionistypicallymorerelevantinonshoreVCEsthanconfinement.Onceignitionoccursthelevelofcongestionplaysacrucialroleinthedevelopmentofthepressurewave.Intheabsenceofcongestionthefuelinthevaporcloudwillcombustintheformofafireball/flashfirethattypicallyproducesminimaloverpressuresontheorderofseveralmillibars.Ascongestionistheprimarydriverofturbulenceandturbulenceissubstantialinthedevelopmentoftheexplosionthisinteractioniskey.


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Alargenumberofexperimentalandtheoreticalstudiesxvii,xviii,xixhavebeenundertakentoexaminetheimpactofcongestiononthedevelopmentofanexplosionandthegeneralconclusionsarethatincreasingtheVolumeBlockageRatio(VBR,percentofthevolumeblockedbyobstacles)willincreasetheflamespeedandthusincreasetheoverpressure.Theexactmechanismshoweverarenotwellunderstood

astheprocessisacomplexinteractionoftheacceleratingflamefrontandtheobstruction.Thisinteractionoftheflameandtheobstaclecausesbothturbulencebyvortexsheddingandlocalwakerecirculationwherebytheflamecanbewrappedin

onitself.xxTherearebothdirectandindirecteffectscausedbytheobstruction.Indirectlytheobstaclealterstheflowfield,turbulence,andreducestheavailablevolume,andthedirecteffectistheflameinteractionwiththeobstacle.TheMEM,Baker-Strehlow,andTNTmethodologiesallrelyoncharacteristiccurvestodefinetheoverpressure,howeveronlytheMEMandBaker-Strehlowtakeintoaccountcongestion.Themultienergymethodrequirestheusertodeterminethelevelofcongestionandtothenapplyablaststrengthtodeterminetheresulting

overpressure,whiletheBakermethodologyrequirestheusertoestimatetheflamespeed(whichislargelybasedoncongestioninadditiontofuelproperties)inordertodetermineoverpressure.Inbothcasesthedeterminationoftheappropriatecriteriaislargelyajudgmentcall,howeverstudiescomparingtheempiricalblastcurvestoexperimentaldataindicatethatforsubsonicdeflagrationsthecurvesmatchreasonablywellwiththeexperimentaldataxxiindicatingthatifthecorrectblastcurvecanbespecifiedintheriskassessmentthenthemethodologymaybesound.Thedifficultyhoweveristhatthereisnosolidguidanceonchoosinganappropriatecombinationofblastcurvesthathasbeenshowntobeanaccurateportrayalofthe

realworldphenomena.Alsotheempiricalcorrelationswerederivedbasedonalimitedsetofexperimentsandthusanysituationwhichisnotthesameastheexperimentsislikelygoingtoresultinadifferentblastprofilewhichmaynotfallonthederivedblastcurves.Aseriesofexperiments(MERGE&EMERGE)xxii,xxiiiwereperformedinthe1990stoattempttoderiveacorrelationbetweenthesizeoftheoverpressurewiththesizeanddensityofobstructionsinacongestedarea.Theresearchallowedsomelevelofdirectguidancetobeusedbypractitionerswhendeterminingtheblockageinacongestedarea,theobstaclediameters,andtheflamepathlength.Thisinformationhoweverislargelylimitedinitsusefulnessasthedataonlytranslatestosimilar

geometriesandnobroadaccuratecorrelationhasbeenfoundforgeneraluse.Morerecentlytherehasbeenextensiveworkusingcomputationalfluiddynamics(CFD)toolsforriskassessments.Inthiswaytheimpactofcongestionontheexplosiondevelopmentcanbedirectlyobservedgivenawiderangeofpotentialscenarios.Thisresearchhasshownthatamuchgreaterlevelofdetailcanbeobtainedusingthismethodologyandthatamorerealisticunderstandingofthe


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hazardsofcongestionandtheresultingseparationdistancescanbeobtained.xxiv,xxv,xxvi

TNOxxvii,Mercxxxviii,HSExxix,andKinsellaxxx(seeTable2)haveprovidedguidelinesforinterpretingfieldconditionsandapplyingthemforusewiththeMultiEnergyMethodology.Theseguidelineshoweverarenotconsistentwitheachother,still

leavemuchopenendedandallowforsignificanterror.Theconclusionsofmanypeer-reviewedarticles,includingthosethatsetouttheguidelines,isthattheappropriatedeterminationisnon-trivial.InKinsellasarticle,whichisoftencitedasthejustificationforthechoiceofanSTvaluehestates,itshouldbenotedthatthedataissomewhatlimitedandthatstructuraldamagereportsofincidentsarefraught

withuncertaintiesandassumptions.Inthelightoftheseuncertaintiesthemethod

proposedinthispapershouldberegardedmoreasascreeningtoolratherthana

detailedexplosionassessmentmethod.xxxi

Table2Kinsellaguidelinesontheselectionofinitialblaststrengths.

InadditionthemostrecentversionoftheYellowBookrecognizesthelimitationsintheguidanceandthefactthatusingthisguidanceasthebasisforananalysiscan

leadtoanunderpredictionoftheoverpressurebyafactorofupto30.xxxiiTheYellowBookstates,itisthereforerecommendedtobeconservativeinthechoiceofasourcestrengthfortheinitialblastandnottousethisguidanceatall.FurthertheYellowBooksuggeststhatwithoutadditionalinformationanyobstructedregionshouldbegivenaninitialblaststrengthof10.

Thusatitsmostbasiclevelandwiththosewhoaremostfamiliarwiththemethodologyandguidelinestheimplicationisthatthereissignificantroomforerror.IfoneweretofollowtheguidanceprovidedinKinsellaorfromothersourcesandtousearangeofSTvaluesfrom5-7,asKinsellasuggests,thepeakoverpressurewithintheinitialblaststrengthsvariesbyafactorof5.FinallyamongstalltheguidancethereislittletonoactualphotographsorconcreteexamplesofwhatwouldconstituteaparticularSTcurve,inessenceitbecomesaneducatedguess.

ComparisonofModelResults

ToattempttocomparedirectlythemodelsdiscussedinthisarticleanumberoftheRIGOSxxxiiiexperimentsweremodeledusingtheMEM,TNT,andtheFLACScfdcode.


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Theseexperimentswerewellcharacterizedandcaneasilybeclassifiedusingtheexistingguidanceforboththespacingoftheunitsaswellastheappropriatesourcestrength.Thepressureprofilewascomparedforthemodelsandcomparedagainsttheexperimentaldata.AlsotheinfluenceofthelocationoftheignitionsourcerelativetothevapourcloudwasexaminedusingtheFLACScode.

AbriefsummaryoftheRIGOSexperimentsisprovidedhere,howeverfulldetailsmaybefoundinHSEResearchReport369:Researchtoimproveguidanceonseparationdistanceforthemulti-energymethod(RIGOS).TheRIGOSexperimentswereaseriesofexperimentsdesignedtomeasuretheoverpressureandflamelocationofaVCEinordertodevelopaguidelinefordeterminingthecriticalseparationdistancebetweenprocessunits/areasinordertodiscriminatebetweenasingleormultipleblasts.Theexperimentscontainedtwoconfigurationsofstructures,arepresentationofwhichisseeninFigure5,adonorstructureandanacceptor.Theacceptorsizeand

densityoftheacceptorwerekeptconstantforallexperiments.

Figure5RepresentationofonetestconfigurationofRIGOStests(donorleft,acceptorright)

Thedimensionsofthedonor,theseparationdistance,thedonorVBR,andthefuel(ethyleneandmethane)werevaried.Theentiregeometrywasencapsulatedinplasticsheetingandastoichiometricfuel/airmixturewasintroducedintothevolumeatwhichtimethecloudwasignitedinthecenterofthedonor.TheAEtestseriesusedethylenewhiletheAMtestseriesusedmethane.

TheguidanceinKinsella,withregardstotheMEMblaststrength,suggestedthattheRIGOSstructureswerelowenergyignition,lowcongestion(


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BasedontheguidanceintheYellowBooktheinitialblaststrengthwaschosenasanST10.DetailedpressuremeasurementsusingFLACSandcomparedwiththeexperimentsfortheAE08,AE09,AM01,andAM05RIGOStestseriesareshownbelowinFigure9-Figure12.

Figure6SliceofthepressureprofileforAEseriesDD2donorrigconfiguration

WhencomparingtheresultsoftheexperimentswiththeFLACSresultsitisevidentthatthemodeldoesarelativelygoodjobofreproducingboththepositiveandnegativephasealongwiththepressuresobservedatthesensorlocations.BasedonthepositiveresultsofthemodeladditionalpressuredatawasextractedatotherlocationsfromthecfdresultstocomparethepressuresatspecificdistancesusingtheTNTandMEMmethodologies.TheresultsforseveraloftheRIGOSmodelsusingbothsimplyadonorconfigurationandadonor/acceptorconfigurationareshowninTable3.Whatisclearisthattheresultsvarywidelydependingonthemodelandwithinamodelbasedontheguidancechosen.

OnelimitationthatisclearimmediatelywiththeMEMmethodologyisthatthereisnowaytoaccountforfuelreactivity.GuidancefromFMRCfortheTNTmethodallowssomeaccountingforreactivitybyincreasingtheyieldbasedontheclassificationofthefuel,howeversimplyincreasingtheyielddoesnttrulysolvetheproblem.Whilemosthydrocarbon/airstoichiometriccombustionresultsinapproximately3.5MJ/m3thefuelreactivitycanvaryandthiswillhaveanimpactontheoverpressuredevelopment.ThisbecomesveryobviousastheAE09andAM05structuresarethesameonlythefuelhasbeenalteredfromethylene(AE09)tomethane(AM05),areductionoftheheatofcombustionfrom3.64to3.23MJ/m3.Thepressurecontourslooksignificantlydifferenthoweverifyourelyontheempiricalmodelstheywillproducethesameoverpressurevalueandthussignificantlyunderpredicttheoverpressureforethylene.Additionallytheinabilitytotakeintoaccounttheignitionlocationisevidentandpotentiallyimportant.TheAE/AMtestserieswereignitedinthecenterofthedonorrig,howeverusingtheMEMorTNTmethodologyignitionisassumedinthecenterofthecloud.Forthesamesizecloudanendignitionwillresultin


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significantlydifferentpressureprofiles.ThisisclearlyshowninacomparisonoftheignitionlocationsinFigure7andFigure8.

Figure7AE09testwithdonorcenterignition

Figure8AE09testwithignitionbetweendonor/acceptor


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Figure9AE09ExperimentalResults

Figure10AE09FLACSResults


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Figure11AE08ExperimentalResults

Figure12AE08FLACSResults


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Conclusions

Thispaperalongwithextensiveresearchbeforeitclearlyshowsthatthesimpleempiricaltools,suchastheMEM,TNT,andBaker-StrehlowareinsufficientfordetailedanalysesofVCEseitherforriskassessmentpurposesorforreconstruction.Amplepeerreviewedliterature,whichhasbeencitedhere,indicatesalackof

knowledgeregardingappropriateselectionofinitialblaststrengths,specifyingcongestionlevels,ignitionstrength,etc.Thesehoweverarecriticalpiecesofinformationifthesemethodologiesaretobeproperlyapplied.Fromascientificperspectivetheempiricalapproachesareinterestingandworthfurtherpursuit,howeverfromanengineeringstandpointtheyhaveseriousdeficiencieswhenbeingemployedtoassessscenariosthataffectsafetyandbusinesscontinuity.Partoftheproblemwithcomparingcorrelationmodels,oranymodel,totherealworldisthattheyareoftencomparedagainstdamagefromincidentsandthedamageobservedinanincidentisusedtodeterminetheoverpressuresthatoccurredintheincident.Thiscanbeaninherentlyflawedsystemasmostofthe

experimentsexaminingdamageweredoneusingpointsourceexplosivesandidealstructures,thiscombinationwillrespondsignificantlydifferentthanaVCEandanimperfectobject(i.e.newcaroffthelotvs.10yearoldvehicle).StudiessuchasthatdonebyJiangetalthatcomparethecorrelationmodelstopastincidentsstartwiththeunderlyingassumptionthattheoverpressureobservedcanbeaccuratelydeterminedmerelyfromthephysicalobserveddamage.Thesimplecomparisonofawell-documentedsetofexperimentsusingthevariousmethodsclearlyshowstheinabilityofthemodelstoadequatelycapturethepotentialimpactofanincident.ShortofchoosinganST10valueforallapplications,whichwillmostlikelyresultinextremelyconservativeresults,theonlywayto

capturetherelevantinformationandtogenerateresultsofsufficientdetailistouseatoolthatadequatelysimulatesthereal-worldphenomena.TheFLACStoolhasbeenshowninnumerousstudiesincludingthisonetobeabletoadequatelymatchexperimentaloverpressuresandtoprovideamoreaccurateoverpressureprofileforuseinariskassessment.Inordertobeabletoconductathoroughassessment,thespecificsiteconditionsmustbeabletoberesolvedinsufficientdetail.Inmostcasesthislevelofdetailcannotbeobtainedusingempiricalmodelsrathertheanalystmustrefertoacfdtool,orresorttooverlyconservativeassumptions.Withempiricaltoolssincethephysicalphenomenaarenotbeingmodeleditisimpossibletoaccuratelypredict

whatwillhappeninanevent,theestimatewilleitherunderoroverpredictthepressurecontours.


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Table3ComparisonofTNT,MEM(St2,3,&10),andFLACSresultsofRIGOSExperiments

RIGOSTest# CongestedVapour

CloudVol(m3)

Distancefrom

IgnitionSource(m)

MEMST2

Pressure(barg)

MEMST3

Pressure(barg)

MEMST10

Pressure(barg)

TNTPressure2

(barg)

FLACSPressure

(barg)

AE09 2.8 .704 .02 .050 20 40.7 .713

SD/DD=0.25 1.35 .02 .050 8.19 11.8 1.48

1.76 .02 .050 3.72 7.10 .639

2.46 .02 .050 1.16 3.75 1.14

3 .02 .050 .897 2.57 4.469

6.16 .010 .024 .356 .650 .158

9.16 .006 .016 .214 .305 .103AE08 2.8 .704 .02 .050 20 40.7 .478

SD/DD=1.50 1.35 .02 .050 8.19 11.8 .751

1.76 .02 .050 3.72 7.10 .376

2.46 .02 .050 1.16 3.75 .149

3 .02 .050 .897 2.57 .086

6.16 .010 .024 .356 .650 .008

9.16 .006 .016 .214 .305 .005

AE08/09 1.4 .704 .02 .050 20 26.2 .713

DonorOnly 1.35 .02 .050 4.18 7.58 1.47

1.76 .02 .050 1.55 4.57 .639

2.46 .02 .050 .859 2.41 .176

3 .016 .040 .665 1.65 .086

6.16 .08 .019 .265 .418 .008

9.16 .05 .013 .159 .196 .005

2

Factory Mutual Research Corporation assigns classifications to fuel based on reactivity and assigns a yield to the fuel, methane is a Class I fuel which has a 5%

yield, ethylene is a Class II fuel which has a 10% yield


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Figure13AE09donoronly:Comparisonofmodelresults,peakTNTandST10valuestruncated

Figure14AE09ComparisonofmodelresultsPeakTNT(40barg)&MEMSt10(20barg)valuestruncated

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Part 1 Design Requirements