MethodologyforAssessingaBoilingLiquidExpandingVaporExplosion

(BLEVE)BlastPotential

ChrisP.KeddyNASATestandEvaluationContractNASAWhiteSandsTestFacility

LasCruces,NewMexico

https://ntrs.nasa.gov/search.jsp?R=20120014185 2018-12-27T03:07:26+00:00Z

Introduction

• CompositeVesselsarenowusedtostoreavarietyoffluidsorgasesincludingcryogenicfluidsunderpressure

• Suddenfailureofthesevesselsundercertainconditionscanleadtoapotentiallycatastrophicvaporexpansionifthermalcontrolisnotmaintainedpriortofailure

• Thiscanleadtoa“BoilingLiquidExpandingVaporExplosion”orBLEVE

Scope• BLEVEs

– Definition– “SuperheatEnergy”and“SuperheatLimit”– ThermodynamicsofBLEVEs– WorkAvailableforBlast

• ReversibleAdiabatic• IrreversibleIsentropic

– Step‐by‐stepmethodologyforestimation• CryogenicBLEVES

– NitrogenExample(ComparisonofBlastPotentials)• Hydrostatic• Pneumatic• BLEVE

– OtherCryogens,CurrentWork,andSafety

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BLEVE• BoilingLiquidExpandingVaporExplosion(BLEVE)

– Anyheatedfluidundersufficientpressurethatissuddenlyexposedtolowerpressures(ex.ambient)can‘flash’tovaporifthefluidtemperatureisaboveacertainvalueknownasthe‘superheatlimit’temperature(Tsl)

– ThemechanismatoraboveTsl isahomogeneousnucleationprocessthroughouttheentireliquidmassandvaporizationproceedsinthemillisecondtimeframe.Thisprocessissimilartorapidcombustioninsolidsthatconvertsolidstogasinthesub‐milliseconddomain(i.e.explosives).

– Theprocesscreatesaco‐volumeofliquidandagasnearthedensityoftheoriginalliquidactinglikeahighlypressurizedgasvolumewithinthevesselatapressuretypicallywellinexcessoftheoriginaldesignburstpressure.

– Theendresultisablastthatisverysimilartoanon‐idealgaspneumaticbursteventandcancreatesignificantoverpressuresposingarisktolifeandproperty.

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TypicallytheTsl forawiderangeofcompoundshasbeenfoundtobe:Tsl ~0.89Tcto0.90TcTc =CriticalTemperatureofFluid

PropertiesofInterest• StateVariables

– T=Temperature(K)– P=Pressure(N/m^2)– M=Mass(kg)– U,u=Internalenergy(kJ),Specificinternalenergy(kJ/kg)– H,h=Enthalpy(kJ),Specificenthalpy(kJ/kg)– V, =volume(m^3).Specificvolume(m^3/kg)– S,s=entropy(kJ/K),specificentropy(kJ/kgK)

• StatesSubscripts– Subscript1referstotheinitialstate– Subscript2referstotheexpandedstate(ambient)– Subscriptgreferstostateofsaturatedvaporatambientconditions(state2)– Subscriptfreferstostateofsaturatedliquidatambientpressure(state2)

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Thermodynamics• Statecalculationscanbeusedtoestimatetheavailableenergy(work)

availabletogenerateablastwave• Twoboundingvaluesbrackettherangeofavailablework

– Maximum:ReversibleAdiabaticExpansion(isentropicwork)=Wi =U1 – U2

– Minimum:IrreversibleExpansionworkagainstatmosphericpressure(Wo =Po∆V)

• Typicallythemaximumisentropicworkvalueisusedtoboundthe‘worse’casescenarioforhazardassessment(Wi =∆U)

• Theliquid’sinitialinternalenergy,U1,canbefoundfortheinitialstateusingtablesorgraphs.Sincemosttablesorgraphsonlysupplyh,,andsthevalueofU1 canbefoundfromu=h– p andthesystemmass

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Step‐by‐StepMethodology

• Firstdeterminetheinitialstate– Ideallytheexacttemperatureofthefluidisdesirable– AlternativelythepressurejustpriortoBLEVEcanbeusetodeterminethemaximumtemperatureofthefluidbyassumingsaturatedconditions

Example:LiquidNitrogen– FindInitialstate– DetermineifatoraboveTsl– Solveforinitialandfinalstates

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NitrogenCurve

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77.347 K,0.101325 Mpa,14.7 psia, 1 atm

0

100

200

300

400

500

0

0.5

1

1.5

2

2.5

3

3.5

4

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.

0

105.

0

110.

0

115.

0

120.

0

125.

0

130.

0

Pres

sue

(psi

a)

Pres

sure

(MPa

)

Temperature (K)

Nitrogen Saturation Curve

Liquid

Gas

Tsl =112.3 K, 1.825 mPa, 264.7 psia

126.1 K, 3.4 mPa, 493.1 psia

PrimaryMethodReversibleAdiabaticExpansion(Isentropic)(IsentropicWork,∆u)(Subscript1indicatesinitialstate)• Findu1 (findh1 and1 atP1 andT1)Whereu1 =h1‐p11 (eq.1)

• Findu2 basedon:

u2 =(1‐X)hf +Xhg ‐ (1‐X)p2f‐ Xp2g (eq.2)

whereX=VaporRatio=(s1‐sf)/(sg‐sf) (eq.3)– Subscript1referstotheinitialstate– Subscript2referstotheexpandedstate(ambient)– Subscriptgreferstostateofsaturatedvaporatambientconditions(state2)– Subscriptfreferstostateofsaturatedliquidatambientpressure(state2)

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IsentropicWork

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Calculated Values are readily available for various compounds(Table Right)Table 6.12 Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVES, Center for Chemical Process and Safety, American Institute of Chemical Engineers, 1994Shows values of isentropic work Wi expressed as Eex (Energy of Explosion) for a range of temperatures

(Table below)Table 1: Casal, J. and Salla B., “Using Liquid Superheating Energy for the a Quick Estimation of Overpressure in BLEVEs and Similar Explosions”, Journal of Hazardous Materials, Vol. 137, Issue 3, 10-2006, pg 1321-1327SE = Total Superheat Energy of System

ComparisonofVariousFluids

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Casal, J. and Salla B., Using Liquid Superheating Energy for the a Quick Estimation of Overpressure in BLEVEs and Similar Explosions, Journal of Hazardous Materials, Vol. 137, Issue 3, 10-2006, pg 1321-1327

Comparisonofrelativeisentropicwork(Wi)potentialforvariousfluidsnearTsl

BlastCharacterization

12Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVES, Center for Chemical Process and Safety, American Institute of Chemical Engineers, 1994

BLEVEBasicMethod

ComparisonofBlastPotential(Nitrogen)

• Example:1liter(0.001m^3)ofNitrogen,P1 =500psi(3.45MPa)

• StoredEnergyComparison– Hydrostatic:Theapproximatestoredenergyinpressurizedliquidsisrelativelysmall,(basedonabulkmodulusofliquidnitrogenof~13Gpa)1literofnitrogenat500psistores

• ~0.5J– Pneumatic:Nitrogengas(assumedideal)at500psiand1litercanstore

• ~5.5kJ– BLEVE:1literSuperheatedLiquidNitrogenat500psi(T=126K)canstoreupto

• ~30kJ

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MPaPMPaPPPVPU 1014.0,45.3,4.1,1

1 21

1

1

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EquivalentPneumaticSystem• Thepreviouscalculationsshowedthe‘work’availabletoaBLEVEcanbeseveraltimesthatofrelativelylowpressure(500psi)gaseouspressurevesselorhydrostaticcase– BLEVEvs.Pneumaticupto~6timesgreater(inthiscase)– BLEVEvs.Hydrostaticupto~60,000timesgreater(inthiscase)

• To‘match’pneumaticandBLEVEpotentialsapneumaticallychargednitrogenvesselwouldbeat~2,250psi

• FinallyitshouldbenotedthatnitrogenBLEVEs(orpneumaticreleases)generatealocalasphyxiationhazard

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OtherCryogens• LiquidOxygen(LOX)isverysimilartoliquidnitrogen(LN2)initsBLEVEbehaviorbut

hastheaddedhazardofanoxidizerandpromotedcombustionrisks• LiquidHydrogen(LH2)

– ExaminationofCharacteristics– BoilingPoint(1atm)=20.37K– CriticalPoint=32.97K– Tsl (estimateRedlich‐Kwong equationofstate)=29.51K– ThefinalresultisLH2BLEVEshaveanestimatedblastpotentiallessthanLN2

(basedonmass)butcanstillbecatastrophicandhaveanadditionalvaporcloudexplosionhazardinair(combustion,deflagration,ordeflagrationtodetonation)

• CurrentWSTFeffortsinclude:– VerifyingBLEVEthreshold(Tsl)valueforNitrousOxide(N2O)– DeterminingBlastPotentialofNitrousOxideBLEVEs

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OtherHazards

• AdditionalSafetyNotes– Cryogenicsystemscancondense‘Air’andformaliquidconsistingof~50%liquidoxygenand~50%liquidnitrogenasitdripsoffthecoldsurfaces

– Liquidoxygenwhenincontactwithhydrocarbonsorproductscontaininghydrocarbons(ex.oil,grease,asphalt,leathergoods,etc.)canformimpactorshocksensitiveexplosivecompoundsrivalingthestrengthofsimilarsolidhighexplosives

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