PressureReliefCalculationsThe scenario you choose on the Setup Scenario sheet determines which of theremainingPressureReliefformsandsheetsyouneedtocompletetodefineyoursystem.Therequiredformswillbedisplayed as incomplete intheleftpaneoftheDataBrowserFormsandsheetsthatdonotapplytoyourchosenscenarioareinactive.Use Next to guide you through the required forms.Specifying the Inlet Stream for Steady State ScenariosFor the steady state scenarios of relief valveor relief systemratingyoumustprovide the composition of the stream entering the relief valve or system.ScenarioStreamRulesFlashOptionsReVessel ContentsStreamcompositionReference streamReferencestreamcompositionBasis:Mole-fraction口FractionComponentReference streamtemperatureH20BeferencestreampressurMETHANOLReferencestream vaporfractianCAACIDUserflashspecificationsMCACETTemperature:Ic1Pressure:barVap.fraction:IYou can:Specify the composition of a stream or vessel by referencing a stream from theflowsheetSpecify the stream composition directly in the pressure relief blockThese specifications areentered onthe Stream sheet ofthePressureRelief Setupform.To specify the composition of a stream or vessel by referencing a stream from theflowsheet:1.Select the Stream tab from thePressureRelief Setup form33-6Aspen Plus User GuideVersion10.1-0
Aspen Plus User Guide Version 10.1-0 33-6 Pressure Relief Calculations The scenario you choose on the Setup Scenario sheet determines which of the remaining Pressure Relief forms and sheets you need to complete to define your system. The required forms will be displayed as incomplete in the left pane of the Data Browser Forms and sheets that do not apply to your chosen scenario are inactive. Use Next to guide you through the required forms. Specifying the Inlet Stream for Steady State Scenarios For the steady state scenarios of relief valve or relief system rating you must provide the composition of the stream entering the relief valve or system. You can: • Specify the composition of a stream or vessel by referencing a stream from the flowsheet – or – • Specify the stream composition directly in the pressure relief block These specifications are entered on the Stream sheet of the Pressure Relief Setup form. To specify the composition of a stream or vessel by referencing a stream from the flowsheet: 1. Select the Stream tab from the Pressure Relief Setup form
Chapter332.OntheStreamsheet,clicktheReferenceStreamCompositioncheckbox3.Inthefieldbelowthecheckbox,selectthedesired stream.To specify the stream compositiondirectlyin the pressurerelief block:1.SelecttheStreamtabfromthePressureRelief Setupform,toopen thatsheet.2.In the Stream Composition frame of the Stream sheet, choose a compositionbasisfromtheBasislist.3.EnterthecomponentfractionsintheFractionfieldsnexttoeachcomponentpresent in the stream.In addition tothe composition,youmust specifythethermodynamic conditions ofthe inlet stream. This involves specifying some combination of the three statevariables: temperature, pressure, and molar vapor fraction. These values caneither be entered directly on the Stream sheet, or referenced from theflowsheetif you have referenced a stream composition.Toreferenceaninletstreamstatevariablefromaflowsheetstream:1.On the Stream sheet, first ensure that you have checked the Reference2StreamCompositioncheckbox.Youcannotreferencestatevariablesfromastream if you have not also referenced the composition.2.IntheReferenceStreamframeoftheStreamsheet,clickthecheckboxindicating which stream variable you wish to reference:ReferenceStreamTemperatureReferenceStreamPressureReferenceStreamVaporFractionTo specifya state variable for the inlet stream directly in the pressure reliefblock:On the Stream sheet, enter a value in one of the fields of the User Flash1.Specifications frame:TemperaturePressureVap.Fraction2.Select the appropriate units for the entered value.If you wish, you can reference one state variable, and enter the other directly33-7AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-7 Chapter 33 2. On the Stream sheet, click the Reference Stream Composition check box. 3. In the field below the check box, select the desired stream. To specify the stream composition directly in the pressure relief block: 1. Select the Stream tab from the Pressure Relief Setup form, to open that sheet. 2. In the Stream Composition frame of the Stream sheet, choose a composition basis from the Basis list. 3. Enter the component fractions in the Fraction fields next to each component present in the stream. In addition to the composition, you must specify the thermodynamic conditions of the inlet stream. This involves specifying some combination of the three state variables: temperature, pressure, and molar vapor fraction. These values can either be entered directly on the Stream sheet, or referenced from the flowsheet if you have referenced a stream composition. To reference an inlet stream state variable from a flowsheet stream: 1. On the Stream sheet, first ensure that you have checked the Reference Stream Composition check box. You cannot reference state variables from a stream if you have not also referenced the composition. 2. In the Reference Stream frame of the Stream sheet, click the check box indicating which stream variable you wish to reference: • Reference Stream Temperature • Reference Stream Pressure • Reference Stream Vapor Fraction To specify a state variable for the inlet stream directly in the pressure relief block: 1. On the Stream sheet, enter a value in one of the fields of the User Flash Specifications frame: • Temperature • Pressure • Vap. Fraction 2. Select the appropriate units for the entered value. If you wish, you can reference one state variable, and enter the other directly
PressureReliefCalculationsSpecifying Initial Vessel Contents for DynamicScenariosFor the dynamic scenarios of fire or heat input, you must provide the initialconditions of the vessel on the Vessel Contents sheet of the Pressure Relief Setupform.ScenarioStreamVessel ContentsRulesFlashDptionsReaReference streamVessel illageReferencestreamcompositionFillage:MPadgas一componentReference streamtemperatureFefVessel compositionReference streamvaporfractionBasis:Mole-fraction-Userflash specificationsFractionComponentCTemperature:H20Pressure:[bar一METHANOLVap.fraction一CAACIDMCACETThe initial conditions of the vessel can be specified in terms of:CompositionThermodynamicconditions (temperature,pressure orvaporfraction)FillagePadgascomponentThe composition of thevesselmust bespecified for allpressure relief blocks,howeverthe remaining specification canbe specified using one of thefollowingthree combinations:Twooftemperature,pressure,andmolarvaporfractionTwo of temperature,pressure,andfillageAll of temperature,pressure,fillage,and pad gas componentThe pressure relief model uses this information and the volume you define on thePressureReliefReliefDeviceformtocalculatetheinitialmassinthesystem.Specifying Vessel Composition for Dynamic ScenariosThe vessel composition can be provided by either:.ReferencingaflowsheetstreamcompositionSpecifying values directly on the Vessel Contents sheet33-8Aspen Plus User GuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-8 Pressure Relief Calculations Specifying Initial Vessel Contents for Dynamic Scenarios For the dynamic scenarios of fire or heat input, you must provide the initial conditions of the vessel on the Vessel Contents sheet of the Pressure Relief Setup form. The initial conditions of the vessel can be specified in terms of: • Composition • Thermodynamic conditions (temperature, pressure or vapor fraction) • Fillage • Pad gas component The composition of the vessel must be specified for all pressure relief blocks, however the remaining specification can be specified using one of the following three combinations: • Two of temperature, pressure, and molar vapor fraction • Two of temperature, pressure, and fillage • All of temperature, pressure, fillage, and pad gas component The pressure relief model uses this information and the volume you define on the Pressure Relief ReliefDevice form to calculate the initial mass in the system. Specifying Vessel Composition for Dynamic Scenarios The vessel composition can be provided by either: • Referencing a flowsheet stream composition • Specifying values directly on the Vessel Contents sheet
Chapter33To specify the composition of thevessel by referencing a stream from theflowsheet:1.SelecttheVesselContentstabfromthePressureRelief Setupform2.OntheVesselContentssheet,clicktheReferenceStreamCompositioncheckbox.3.In thefield belowthe check box,selectthe desired streamIDTo specify the stream composition directly in the pressure relief block:1.Select the Vessel Contents tab from the Pressure Relief Setup form2.In the Vessel Composition frame of the Vessel Contents sheet, choose acompositionbasisfromtheBasislist.3.Enterthecomponentfractions intheFractionfieldsnexttoeachcomponentpresent in the vessel.Defining Thermodynamic Conditions for DynamicScenariosTodefinethethermodynamic conditions,youmust specify somecombinationofthe state variables temperature,pressure, or vapor fraction.The requiredcombination depends on whether you specify fillage and pad gas component.When specifying temperature, pressure, and molar vapor fraction, you can either:Enter a valuedirectly on theVessel Contents sheetReferenceaflowsheetstreamfromwhichtoretrievethevalueToreferenceavesselstatevariablefromaflowsheetstream:1.OntheVesselContentssheet,firstensurethatyouhavecheckedtheReference Stream Composition check box.You cannot reference statevariablesfromastreamifyouhavenotalsoreferencedthecomposition2.In the Reference Stream frame of the Vessel Contents sheet, click the checkbox indicating which stream variable you wish to reference:Reference Stream TemperatureReferenceStreamPressureReferenceStreamVaporFractionTo specify a state variable for the vessel directly on the Vessel Contents sheet:1.OntheVessel Contentssheet,enteravalueinoneofthefieldsoftheUserFlashSpecificationsframe:TemperaturePressureVap.Fraction2. Select the appropriate units for the entered value.Aspen Plus User Guide33-9Version 10.1-0
Aspen Plus User Guide Version 10.1-0 33-9 Chapter 33 To specify the composition of the vessel by referencing a stream from the flowsheet: 1. Select the Vessel Contents tab from the Pressure Relief Setup form. 2. On the Vessel Contents sheet, click the Reference Stream Composition check box. 3. In the field below the check box, select the desired stream ID. To specify the stream composition directly in the pressure relief block: 1. Select the Vessel Contents tab from the Pressure Relief Setup form. 2. In the Vessel Composition frame of the Vessel Contents sheet, choose a composition basis from the Basis list. 3. Enter the component fractions in the Fraction fields next to each component present in the vessel. Defining Thermodynamic Conditions for Dynamic Scenarios To define the thermodynamic conditions, you must specify some combination of the state variables temperature, pressure, or vapor fraction. The required combination depends on whether you specify fillage and pad gas component. When specifying temperature, pressure, and molar vapor fraction, you can either: • Enter a value directly on the Vessel Contents sheet • Reference a flowsheet stream from which to retrieve the value To reference a vessel state variable from a flowsheet stream: 1. On the Vessel Contents sheet, first ensure that you have checked the Reference Stream Composition check box. You cannot reference state variables from a stream if you have not also referenced the composition. 2. In the Reference Stream frame of the Vessel Contents sheet, click the check box indicating which stream variable you wish to reference: • Reference Stream Temperature • Reference Stream Pressure • Reference Stream Vapor Fraction To specify a state variable for the vessel directly on the Vessel Contents sheet: 1. On the Vessel Contents sheet, enter a value in one of the fields of the User Flash Specifications frame: • Temperature • Pressure • Vap. Fraction 2. Select the appropriate units for the entered value
PressureReliefCalculationsIf you wish,you can reference one statevariable,and enter another directly.FillageFillage is the initial fraction of the vessel volume filled with liquid (liquidholdup). The fillage must be greater than zero and less than 0.994.If you specify thefillage but not apad-gas component, the temperature orpressure specification will be used to determine the bubble point (vapor pressureforpurecomponents)oftheinitialmass.To specify fillage:> On the Vessel Contents sheet, enter a value in the Fillage field of the VesselFillage frame.Pad Gas ComponentPad Gas Component represents the component being added to bring the pressureup to the specified level. For example, nitrogen is often used as a pad gas inhydrocarbonstoragetanks.To specify a pad gas component:> On the Vessel Contents sheet, choose a component from the Pad GasComponentlistintheVessel Fillageframe.Design RulesUse the Rules sheet of the Pressure Relief Setupformto specify rulesregarding:Maximumvessel pressure (dynamicscenarios only)Inlet pipe pressure lossTail pipe pressure lossValvedifferentialsetpressure33-10Aspen Plus User GuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-10 Pressure Relief Calculations If you wish, you can reference one state variable, and enter another directly. Fillage Fillage is the initial fraction of the vessel volume filled with liquid (liquid holdup). The fillage must be greater than zero and less than 0.994. If you specify the fillage but not a pad-gas component, the temperature or pressure specification will be used to determine the bubble point (vapor pressure for pure components) of the initial mass. To specify fillage: ➤ On the Vessel Contents sheet, enter a value in the Fillage field of the Vessel Fillage frame. Pad Gas Component Pad Gas Component represents the component being added to bring the pressure up to the specified level. For example, nitrogen is often used as a pad gas in hydrocarbon storage tanks. To specify a pad gas component: ➤ On the Vessel Contents sheet, choose a component from the Pad Gas Component list in the Vessel Fillage frame. Design Rules Use the Rules sheet of the Pressure Relief Setup form to specify rules regarding: • Maximum vessel pressure (dynamic scenarios only) • Inlet pipe pressure loss • Tail pipe pressure loss • Valve differential set pressure