安全阀计算手册1.docx
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安全阀计算手册1
Description
Thisdocumentbrieflydescribestheonephasereliefvalvecalculationspreadsheet,whichwillbethestandardreliefvalvecalculationtoolofLGN.ThecalculationmethodusedinthisspreadsheetisbasedoncalculationmethodsasgiveninAPI520part1（7thed.,2000,page41-56）.Itisstronglyrecommendedtoreadthesepagesbeforeonestartswiththecalculations.AllcalculationsaredoneinSIunits.
FORMULAS
GASANDVAPORFLOW
∙Flowthroughthereliefvalveiscalledcriticalif
where:
Pb
Maximumbackpressure
kPaa
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
ki
IdealgasheatcapacityratioorCp,ig/Cv,ig
-
Cp,ig
Idealgasheatcapacityatconstantpressure
kJ/kg.K
Cv,ig
Idealgasheatcapacityatconstantvolume
kJ/kg.K
Althoughthisequationisonlyvalidforidealgases,itmaybeusedtocalculatecriticalpressureratio.kicanbeobtainedfromtable7inAPI520part1.Otherwise,thefollowingformulacanbeused:
where:
ki
Idealgasheatcapacityratio
-
Cp,ig
Idealgasheatcapacityatconstantpressure
kJ/kg.K
MW
Molecularweight
-
R
Gasconstant
KJ/kmol.K
Iftheidealgasheatcapacityisnotknown,usetheactualheatcapacityinthesameequation.Errorsarefoundtobeonlysmall.
∙
CriticalFlow（GasorVapor）:
Thefollowingequationcanbeusedtosizeapressurereliefdeviceingasorvaporservicethatoperatesatcriticalflowconditions.
where:
A
Requireddischargearea（forstandardtypesseetable1）
mm2
W
Requiredmassflowthroughthedevice.
kg/hr
C
Coefficientdeterminedfromanexpressionoftheratioofthespecificheats（k=Cp/Cv）ofthegasorvaporatinletrelievingconditions.Thiscanbeobtainedfromfigure32inAPI520orwiththefollowingformulae:
Cp
Heatcapacityatconstantpressureatrelievingconditions
kJ/kg.K
Cv
Heatcapacityatconstantvolumeatrelievingconditions
kJ/kg.K
Kd
Effectivecoefficientofdischarge.Forpreliminarysizing,usethefollowingvalues:
∙Forreliefvalvewithorwithoutrupturedisk:
0.975
∙Forinstallationofrupturediskonly:
0.62
-
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
Kb
Capacitycorrectionfactorduetobackpressure,tobeappliedforbalancedtypereliefvalvesonly;Forpreliminarysizing,thiscanbeestimatedfromfigure30inAPI520（seealsotable2）.
ForconventionalandpilotedoperatedvalvesKb=1
-
Kc
CombinationcorrectionfactorforinstallationswitharupturediskupstreamofthePSV.
∙Kc=1.0whenarupturediskisnotinstalled
∙Kc=0.9whenarupturediskisinstalledupstreamthePSVanddoesnothaveapublishedvalue
-
T
Relievingtemperatureatinletofthereliefvalve.
K
Z
Compressibilityfactoratinletrelievingconditions
-
MW
Molecularweight
Kg/kmol
Theaboveequationshouldalsotobeusedtosizebalancedpressurereliefvalvesinsubcriticalflow（inthiscaseKbshouldbeobtainedfromthemanufacturer）.
∙
SubcriticalFlow:
Requiredreliefvalveareaforconventionalandpilotoperatedreliefvalves
where:
A
Requireddischargearea（forstandardtypesseetable1）
mm2
W
Requiredmassflowthroughthedevice.
kg/hr
F2
Coefficientofsubcriticalflow,seefigure34inAPI520orusethefollowingequation:
-
k
Ratioofspecificheats,Cp/Cv
-
r
Ratioofbackpressuretoupstreamrelievingpressure,Pb/P
-
Cp
Heatcapacityatconstant
kJ/kg.K
Cv
Heatcapacityatconstantvolume
kJ/kg.K
Kd
Effectivecoefficientofdischarge.Forpreliminarysizing,usethefollowingvalues:
∙Forreliefvalvewithorwithoutrupture:
0.975
∙Forinstallationofrupturediskonly:
0.62
-
Kc
CombinationcorrectionfactorforinstallationswitharupturediskupstreamofthePSV.
∙Kc=1.0whenarupturediskisnotinstalled
∙Kc=0.9whenarupturediskisinstalledupstreamthePSVanddoesnothaveapublishedvalue
-
Z
Compressibilityfactoratinletrelievingconditions
-
T
Relievingtemperatureatinletofthereliefvalve
K
MW
Molecularweight
Kg/kmol
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
Pb
Maximumbackpressure
kPaa
STEAMRELIEF
where:
A
Requireddischargearea（forstandardtypesseetable1）
mm2
W
Requiredmassflowthroughthedevice.
kg/hr
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
Kd
Effectivecoefficientofdischarge.Forpreliminarysizing,usethefollowingvalues:
∙Forreliefvalvewithorwithoutrupture:
0.975
∙Forinstallationofrupturediskonly:
0.62
-
Kb
Capacitycorrectionfactorduetobackpressure.Thiscanbeobtainedfromthemanufacturer’sliteratureorestimatedfromfigure30ofAPI520（seealsotable2）.Thebackpressurecorrectionfactorappliestobalancedbellowsvalvesonly.
ForconventionalandpilotedoperatedvalvesKb=1
-
Kc
CombinationcorrectionfactorforinstallationswitharupturediskupstreamofthePSV.
∙Kc=1.0whenarupturediskisnotinstalled
∙Kc=0.9whenarupturediskisinstalledupstreamthePSVanddoesnothaveapublishedvalue
-
KN
CorrectionfactorforNapierequation.
ForP10,339kPaa
KN=1
ForP>10,339andP22,057kPaa
-
KSH
Superheatsteamcorrectionfactor.Thiscanbeobtainedfromtable3（equivalenttotable9ofAPI520）.ForsaturatedsteamatanypressureKSH=1.0
-
LIQUIDRELIEF
where:
A
Requireddischargearea（forstandardtypesseetable1）
mm2
Q
Volumeflowrate
L/min
Kd
Effectivecoefficientofdischargethatshouldbeobtainedfromthevalvemanufacturer.Forapreliminarysizing,aneffectivedischargecoefficientcanbeusedasfollows:
∙Forreliefvalvewithorwithoutrupturediskinliquidservice:
0.65
∙Forinstallationofrupturediskonly:
0.62
-
KW
Correctionfactorduetobackpressure.Ifthebackpressureisatmospheric,useavalueforKwof1.0.Balancedbellowsvalvesinbackpressureservicerequirethecorrectionfactortobedeterminedfromtable2（equivalenttofigure31ofAPI520）.Conventionalandpilotoperatedvalvesrequirenospecialcorrection.
-
Kc
CombinationcorrectionfactorforinstallationswitharupturediskupstreamofthePSV.
∙Kc=1.0whenarupturediskisnotinstalled
∙Kc=0.9whenarupturediskisinstalledupstreamthePSVanddoesnothaveapublishedvalue
-
Gl
Liquidgravityatflowingconditionsreferredtowateratstandardconditions
-
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
Pb
Maximumbackpressure
kPaa
KV
Correctionfactorduetoviscosity.Factorcanbedeterminedfromfigure36inAPI520orfromthefollowingequation:
-
Rn
Reynoldsnumber.
-
Liquidviscosity
cP
Tomakeanestimateforthearea,initiallyKv=1istakenandaninitialareacanbecalculated.FromtheretheReynoldsnumbercanbecalculated.WiththisReynoldsnumberKvcanbecalculatedandfromtherethecorrectestimateforthereliefvalvearea.
RUPTUREDISKSIZINGONLY
Thesizingofarupturediskisdonebyusingtheappropriateequationaslistedabove.Whenusingtheseequations,acoefficientofdischarge,Kdof0.62,shouldbeused.
VALVESELECTION
Whentherequiredsizehasbeencalculated,thevalveisselectedbasedonAPI526（seetable1）.Thevalvetypesarestandardtypes.Forhigh-pressuresystemsadifferentvalvenumberfromtheonesgivenmayberequired.Thismustbecheckedbytheuser.
Iftherequiredareaisgreaterthanthatofa8T10sizemultiplevalveswillbespecified.Alldataconcerningselectedareaandmaximumflowthroughselectedreliefvalvewillthenbebasedononevalve.
TEMPERATUREESTIMATES
Thespreadsheetestimatesadeviceoutlettemperatureunderchapter“DeviceOutputData”.
Theoutlettemperatureofthevalveinsteamservicehasbeenestimatedby
where:
Pb
Backpressure
kPaa
Tsat
Steamsaturationtemperature
°C
Abovepressuresof1barthisisaccuratetoabout2%.
Theoutlettemperatureforvaporsandgasses,undertheassumptionofadiabaticexpansion,isdeterminedby:
where:
Tout
Relievingtemperatureatoutletofthereliefvalve
K
Tin
Relievingtemperatureatinletofthereliefvalve
K
Pb
Maximumbackpressure
kPaa
P
Upstreamrelievingpressure.Thisisthesetpressureplustheallowableoverpressureplusatmosphericpressure.
kPaa
k
Ratioofspecificheatsatrelievingconditions,Cp/Cv
-
TABLES
Table1
OrificeAreasandTypes
（API526,table1,p.2）
Areain²
Areamm²
Orifice
0.110
71.0
1D2or1½D2or1½D3
0.196
126.5
1E2or1½E2or1½E3
0.307
198.1
1½F2or1½F3
0.503
324.5
1½G3or2G3
0.785
506.5
1½H3or2H3
1.287
830.3
2J3or3J4
1.838
1185.8
3K4or3K6
2.853
1840.6
3L4or4L6
3.600
2322.6
4M6
4.340
2800.0
4N6
6.380
4116.1
4P6
11.05
7129.0
6Q8
16.00
10322.6
6R8or6R10
26.00
16774.2
8T10
Table2
GaugePressureRatioin%
CorrectionFactors,KbandKw
Tableisbasedonfigures30and31inAPI520PT1（p.37+38）
Kb（figure30）
Kw（figure31）
Vapors&Gases
Liquids
Accumulation
10%
Accumulation
16%
Pb/Ps（%）
Kb
Pb/Ps（%）
Kb
Pb/Ps（%）
Kw
0
1.000
0
1.000
0
1.000
30
1.000
30
1.000
15
1.000
31
0.990
31
1.000
16
0.997
32
0.980
32
1.000
17
0.990
33
0.970
33
1.000
18
0.985
34
0.960
34
1.000
19
0.980
35
0.945
35
1.000
20
0.975
36
0.930
36
1.000
21
0.970
37
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