Chapter33Vessel ContentsReaScenarioStreamRulesFlashOptionsMaximumvessel pressurePercent ofMAwPCUser specifiedbarInletpipepressureloss3Maximum loss (percent)Tail pipe pressure lossUse97%ruleCMaximum loss (percent):CDonotmodel tail pipepressurelossValvedifferentialsetpressureDifferential set pressure changes as back pressure changesExceptfor the limit on maximum vessel pressure,these rules apply only whenthe relief device is selected to be a PSV (safety relief valve) or PSV incombination with a PSD (rupture disk) for gas or two-phase service.These rules have been included as an aid for good design practice. Pressure Reliefwill generate warnings if any of the rules are violated. However, any design orsafety analysisdecision involvingthese rules should bebased on your owninterpretationoftherelevantcodesanddesignpractices.Each section of the Rules sheet is summarized below. See Aspen Plus UnitOperation Models for a detailed discussion of design rules.The following table summarizes theapplication of the design rules:RuleDeviceApplicationAt 10% over-pressuretInlet pressure lossGas/2phase servicePSVA1t 10% overprssurettGas/2 phaseservicePSVTailpressure loss97% ruleAtorabove10%over-pressuretGas/2 phase service PSVAwaysAll devicesMax vessel pressureIf 10% over-pressure is not reached, the highest pressures are scaled to 10% over-pressure. Ifallpressures are above 10% over-pressure, these rules are not applied and a warning is issued.Aspen Plus User Guide33-11Version 10.1-0
Aspen Plus User Guide Version 10.1-0 33-11 Chapter 33 Except for the limit on maximum vessel pressure, these rules apply only when the relief device is selected to be a PSV (safety relief valve) or PSV in combination with a PSD (rupture disk) for gas or two-phase service. These rules have been included as an aid for good design practice. Pressure Relief will generate warnings if any of the rules are violated. However, any design or safety analysis decision involving these rules should be based on your own interpretation of the relevant codes and design practices. Each section of the Rules sheet is summarized below. See Aspen Plus Unit Operation Models for a detailed discussion of design rules. The following table summarizes the application of the design rules: Rule Application Device Inlet pressure loss At 10% over-pressure† Gas/2 phase service PSV Tail pressure loss At 10% over-pressure†† Gas/2 phase service PSV 97% rule At or above 10% over-pressure† Gas/2 phase service PSV Max vessel pressure Always All devices † If 10% over-pressure is not reached, the highest pressures are scaled to 10% over-pressure. If all pressures are above 10% over-pressure, these rules are not applied and a warning is issued
PressureReliefCalculationsMaximumVesselPressureFor the dynamic scenarios of fire or heat input,you must provide the maximumvesselpressure.ThisvaluecanbeexpressedasanabsolutevalueintheUserSpecified field, or as a percentage of the maximum allowable working pressure(MAWP)entered on the Design Parameters sheet of thePressureReliefDynamicInputform.When using the Percent of MAWP specification, MAWP is converted to gaugepressure before AspenPlus applies thepercentage entered.Inlet PipePressureLossUse this field to enter the maximum inlet piping pressure loss as a percentage ofdifferential set pressure.This is calculated at 10% over-pressure or maximumpressure if 10% over-pressure is not reached. Aspen Plus generates a warning ifthe total pressure loss in the inlet piping is greater than or equal to the specifiedpercentage of differential set pressure.The specified value is usually3 (thedefault),and thisruleis often called the"3%rule."Tail Pipe Pressure LossUse this framefor specifying the methodfor setting theallowabletail pipepressure loss.You can do one of the following:Use the 97% RuleEnter themaximum loss expressed as apercentageof differential setpressure (known as the X% Rule)Specify that Pressure Relief not model tail pipe pressure lossWhen the 97% rule is used, Aspen Plus generates a warning if the valve pressuredropislessthan97%ofthevalve'sdifferential setpressure.WhentheX%ruleisused, Aspen Plusgenerates a warning if thepressure loss after the valve is equalto or greater than X% of the valve differential set pressure.The following tables suggests which rule should be used for the most commontypes of safety valves:Valve TypeSuggested Tail Piping RuleStandard spring loaded97% Rule or X% with X=1097% Rule or X% with X=10 Pop action pilot with unbalanced pilot vented to dischargeBalanced bellows spring loadedX% with X=30Modulating pilot withbalancedpilotsorpilots ventedtoX% with X=40atmosphere33-12Aspen Plus User GuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-12 Pressure Relief Calculations Maximum Vessel Pressure For the dynamic scenarios of fire or heat input, you must provide the maximum vessel pressure. This value can be expressed as an absolute value in the User Specified field, or as a percentage of the maximum allowable working pressure (MAWP) entered on the Design Parameters sheet of the Pressure Relief DynamicInput form. When using the Percent of MAWP specification, MAWP is converted to gauge pressure before Aspen Plus applies the percentage entered. Inlet Pipe Pressure Loss Use this field to enter the maximum inlet piping pressure loss as a percentage of differential set pressure. This is calculated at 10% over-pressure or maximum pressure if 10% over-pressure is not reached. Aspen Plus generates a warning if the total pressure loss in the inlet piping is greater than or equal to the specified percentage of differential set pressure. The specified value is usually 3 (the default), and this rule is often called the “3% rule.” Tail Pipe Pressure Loss Use this frame for specifying the method for setting the allowable tail pipe pressure loss. You can do one of the following: • Use the 97% Rule • Enter the maximum loss expressed as a percentage of differential set pressure (known as the X% Rule) • Specify that Pressure Relief not model tail pipe pressure loss When the 97% rule is used, Aspen Plus generates a warning if the valve pressure drop is less than 97% of the valve's differential set pressure. When the X% rule is used, Aspen Plus generates a warning if the pressure loss after the valve is equal to or greater than X% of the valve differential set pressure. The following tables suggests which rule should be used for the most common types of safety valves: Valve Type Suggested Tail Piping Rule Standard spring loaded 97% Rule or X% with X=10 Pop action pilot with unbalanced pilot vented to discharge 97% Rule or X% with X=10 Balanced bellows spring loaded X% with X=30 Modulating pilot with balanced pilots or pilots vented to atmosphere X% with X=40
Chapter33ValveDifferentialSetPressureYou can specify whether the differential set pressure (DSP), changes when backpressure changes (that is, whetherthe valve is balanced or vented).The following tables indicates whether the DSP changes for the most commontypes of safety valves:Valve TypeDoes DSP change?YesStandard spring loadedYesPop action pilot with unbalanced pilot vented to dischargeNoBalanced bellows spring loadedNoModulating pilot with balanced pilots or pilots vented toatmosphereSpecifying the Venting SystemA Pressure Relief venting system can consist of the following components:A vessel neck (nozzle)Up to two lengths (segments) of inlet pipe connecting the vessel neck to thepressure relief deviceThepressure relief device (safety valve, rupture disk, relief vent, orcombinationrupturediskandsafetyvalve)Up to two lengths (segments) of tail pipe from the relief device to theatmosphereortoanotherpieceofequipmentYou do not have to include all these components in a Pressure Relief calculation.But the block must include at least a pipe section or a relief device.Usethe Configuration sheet onthe ReliefDeviceformto specify:Relief device type, including service for safety relief valvesNumber of inlet and tail pipe sectionsWhether the vessel neck is to be specifiedWhether the vessel neck and piping should be ignored during dynamic runsIf you choose Open Vent Pipe as the device, then:NoreliefdeviceisallowedYou must specifya vessel neck,inletpipe,ortail pipeFor the Steady StateFlowRating of Relief Valve scenario, the venting systemconsists only of a safety valve. No piping is allowed.Aspen Plus User Guide33-13Version 10.1-0
Aspen Plus User Guide Version 10.1-0 33-13 Chapter 33 Valve Differential Set Pressure You can specify whether the differential set pressure (DSP), changes when back pressure changes (that is, whether the valve is balanced or vented). The following tables indicates whether the DSP changes for the most common types of safety valves: Valve Type Does DSP change? Standard spring loaded Yes Pop action pilot with unbalanced pilot vented to discharge Yes Balanced bellows spring loaded No Modulating pilot with balanced pilots or pilots vented to atmosphere No Specifying the Venting System A Pressure Relief venting system can consist of the following components: • A vessel neck (nozzle) • Up to two lengths (segments) of inlet pipe connecting the vessel neck to the pressure relief device • The pressure relief device (safety valve, rupture disk, relief vent, or combination rupture disk and safety valve) • Up to two lengths (segments) of tail pipe from the relief device to the atmosphere or to another piece of equipment You do not have to include all these components in a Pressure Relief calculation. But the block must include at least a pipe section or a relief device. Use the Configuration sheet on the ReliefDevice form to specify: • Relief device type, including service for safety relief valves • Number of inlet and tail pipe sections • Whether the vessel neck is to be specified • Whether the vessel neck and piping should be ignored during dynamic runs If you choose Open Vent Pipe as the device, then: • No relief device is allowed • You must specify a vessel neck, inlet pipe, or tail pipe For the Steady State Flow Rating of Relief Valve scenario, the venting system consists only of a safety valve. No piping is allowed
PressureReliefCalculationsSpecifyingthe ReliefDeviceThe following types of relief devices are available:Safety relief valve (both liquid and gas/2-phase)Rupture diskEmergency relief ventOpenventpipe (pipesectionorvessel neck)Relief valve / Rupture disk combinationOn theConfiguration Sheet of the ReliefDevice form,choose the type of reliefdeviceyouwish tosimulatebyclicking on oneof the optionsabove.The defaultreliefdeviceisasafetyreliefvalve.Configuration SafetyValveRupture Disk ReliefVentVesselNeckReliefdevice@ Safety relief valveService:CRupture diskC Emergency telief ventCOpen ventpipeCRelief valve/Rupture disk combinationInletpipes/ Tail pipe10一No. of tail pipes:No. of inlet pipes:OptionalModelvesselneckTenrevesselneck and ioine duringdunamierunDepending onthetypeofrelief deviceyouchooseontheConfiguration sheetoftheReliefDeviceform,oneormoreadditionalsheets(tabs)ontheReliefDeviceformmaybecomeactiveforfurtherspecificationofthedevice.Forexample,ifyou select Rupture disk as your relief device on the Configuration sheet, theRuptureDisktabwillbecome active and displayan incomplete symbol indicatingthatfurtherspecificationsarerequired onthissheet.Each of these additional sheets is explained later in this section.Built-in tables within Aspen Plus contain:.Several standard commercially available valves,rupture disks,andemergencyreliefventsAllthemechanicalspecificationsandcertified coefficientsneededinthereliefcalculationsYou can customize Aspen Plus by modifying or adding tables ofvalve,disk, andvent characteristics.For more information, see AspenPlus System Management.33-14AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-14 Pressure Relief Calculations Specifying the Relief Device The following types of relief devices are available: • Safety relief valve (both liquid and gas/2-phase) • Rupture disk • Emergency relief vent • Open vent pipe (pipe section or vessel neck) • Relief valve / Rupture disk combination On the Configuration Sheet of the ReliefDevice form, choose the type of relief device you wish to simulate by clicking on one of the options above. The default relief device is a safety relief valve. Depending on the type of relief device you choose on the Configuration sheet of the ReliefDevice form, one or more additional sheets (tabs) on the ReliefDevice form may become active for further specification of the device. For example, if you select Rupture disk as your relief device on the Configuration sheet, the Rupture Disk tab will become active and display an incomplete symbol indicating that further specifications are required on this sheet. Each of these additional sheets is explained later in this section. Built-in tables within Aspen Plus contain: • Several standard commercially available valves, rupture disks, and emergency relief vents • All the mechanical specifications and certified coefficients needed in the relief calculations You can customize Aspen Plus by modifying or adding tables of valve, disk, and vent characteristics. For more information, see Aspen Plus System Management
Chapter33Youcandooneofthefollowing:ChooseadevicefromthetablesEnteryour own specificationsand coefficientsFor liquid service valves, you can specify the full-lift over-pressure factor. Thisallows you to simulate some of the older-style valves, which do not achieve fulllift until 25% over-pressure is reached.Safety ValveIfyouselectSafetyReliefValveorReliefValve/RuptureDiskCombinationasthetype of reliefdevice,you must complete the SafetyValve sheet to specifythesafetyreliefvalve(PSV)tobeusedinthesimulation.DefinethevalveintheManufacturer's Tables frame. Once the Type, Manufacturer, Series, and NominalDiameter have been selected, a unique valve is described and Aspen Plus fills inthefollowing data in the Valve Parameters frame:InletDiameterThroat DiameterOutletDiameterDischargeCoefficientIf you want touse a valve not listed in the Manufacturer's Tables,you must typeinvaluesfortheValveParameterslistedabove.Notethatif you selectavalvefromthetablesandthenoverwriteanyoftheValveParameters,allManufacturer's Tables fields will be blanked out.To complete the form, enter the differential setpoint for the valve.Thisrepresents thepressuredifference across thevalve which is neededfor thevalveto start opening.Rupture DiskIf youselectRuptureDiskorReliefValve/RuptureDiskCombinationas thetypeof relief device, you must complete the RuptureDisk sheet to specify the rupturedisk (PSD) to be used in the simulation.Define the rupture disk in theManufacturer's Tables frame.Once the Manufacturer,Style,and NominalDiameter have been selected, a unique PSD is described and Aspen Plus fills inthefollowingdata in theRuptureDisk Parametersframe:DiameterDischarge CoefficientIf youwant touseaPSDnot listed intheManufacturer'sTables,youmust typeinvaluesfortheparameterslistedabove.Notethatifyouselectarupturediskfrom the tables and then overwrite any of the rupture disk parameters, allManufacturer'sTables fieldswillbeblankedout.33-15AspenPlusUserGuideVersion10.1-0
Aspen Plus User Guide Version 10.1-0 33-15 Chapter 33 You can do one of the following: • Choose a device from the tables • Enter your own specifications and coefficients For liquid service valves, you can specify the full-lift over-pressure factor. This allows you to simulate some of the older-style valves, which do not achieve full lift until 25% over-pressure is reached. Safety Valve If you select Safety Relief Valve or Relief Valve / Rupture Disk Combination as the type of relief device, you must complete the Safety Valve sheet to specify the safety relief valve (PSV) to be used in the simulation. Define the valve in the Manufacturer's Tables frame. Once the Type, Manufacturer, Series, and Nominal Diameter have been selected, a unique valve is described and Aspen Plus fills in the following data in the Valve Parameters frame: • Inlet Diameter • Throat Diameter • Outlet Diameter • Discharge Coefficient If you want to use a valve not listed in the Manufacturer's Tables, you must type in values for the Valve Parameters listed above. Note that if you select a valve from the tables and then overwrite any of the Valve Parameters, all Manufacturer's Tables fields will be blanked out. To complete the form, enter the differential setpoint for the valve. This represents the pressure difference across the valve which is needed for the valve to start opening. Rupture Disk If you select Rupture Disk or Relief Valve / Rupture Disk Combination as the type of relief device, you must complete the Rupture Disk sheet to specify the rupture disk (PSD) to be used in the simulation. Define the rupture disk in the Manufacturer's Tables frame. Once the Manufacturer, Style, and Nominal Diameter have been selected, a unique PSD is described and Aspen Plus fills in the following data in the Rupture Disk Parameters frame: • Diameter • Discharge Coefficient If you want to use a PSD not listed in the Manufacturer's Tables, you must type in values for the parameters listed above. Note that if you select a rupture disk from the tables and then overwrite any of the rupture disk parameters, all Manufacturer's Tables fields will be blanked out