methodology for assessing a boiling liquid expanding vapor
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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
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500
0
0.5
1
1.5
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3.5
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60.0
65.0
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90.0
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105.
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110.
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115.
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120.
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130.
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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
211
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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