Chapter3333Pressure ReliefCalculationsThis chapter describes how to use the Pressure Relief (Pres-Relief) features ofAspen Plus to:Determinethesteady-stateflowrating ofpressurerelief systemsDynamically model vessels undergoing pressure relief due to a fire or heatinput specified by the user.Topics in this chapter include:Aboutpressurerelief calculationsSetting-up pressure relief calculationsSpecifyinginformationforthescenariosSpecifying the composition and conditionsDefining the venting systemSpecifying chemical reactionsSpecifying when to stop the simulation (stop criteria)Specifying the over pressurefactor and piping pressuredroprulesExaminingcalculationresults33-1AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-1 Chapter 33 33 Pressure Relief Calculations This chapter describes how to use the Pressure Relief (Pres-Relief) features of Aspen Plus to: • Determine the steady-state flow rating of pressure relief systems • Dynamically model vessels undergoing pressure relief due to a fire or heat input specified by the user. Topics in this chapter include: • About pressure relief calculations • Setting-up pressure relief calculations • Specifying information for the scenarios • Specifying the composition and conditions • Defining the venting system • Specifying chemical reactions • Specifying when to stop the simulation (stop criteria) • Specifying the over pressure factor and piping pressure drop rules • Examining calculation results
PressureReliefCalculationsAbout Pressure Relief CalculationsUse PressureRelief to simulate a vesselundergoingpressure relief or for simplevalve rating. Pressure Relief uses the same physical property models and data asotherAspenPlusflowsheetmodels.The modeling equations for nozzle flow, and for bubbly and churn-turbulentdisengagement are based on technology developed by the Design Institute forEmergencyRelief System (DIERS)UsersGroup of theAIChE.Thistechnologyisconsideredthebestavailableforpressurerelief systemdesign.TheAspenPlusPipeline model simulates flow through the inlet and tail pipes in the reliefsystem.Pressure Relief always operates in rating mode. This means that the programwill calculate the pressure profile in the vessel and piping, given the size of therelief device. In addition, you must specify the:Dimensions of theequipment being protected anda connecting nozzle ifpresentPressure relief scenarioDimensions of inlet and tail piping, if presentDimensionsofthereliefdeviceEachPressureReliefblockmodelsonescenarioandonevessel.Tomodelmorethan one scenario or pressurized vessel in an Aspen Plus run,include more thanone PressureReliefblock in the simulation.Pressure Relief blocks are not part ofthesimulationflowsheet(thereisnoiconneeded),buttheycanreferencesimulationstreams.Pressure Relief analyzes the specified scenario and reports:RatedcapacityResults profiles (temperature,pressure,vapor fraction)Whether the system meets design rules that you select or that applicablecodes (such as ASME) requireCreating aPressureReliefBlockTocreatea PressureRelief block:1. From the Data menu, point to Flowsheeting Options, then Pres-Relief.2.In the PressureRelief Object Manager,click New.3.In the CreateNewID dialogbox,enteran ID (name)or accept thedefaultID.33-2Aspen Plus User GuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-2 Pressure Relief Calculations About Pressure Relief Calculations Use Pressure Relief to simulate a vessel undergoing pressure relief or for simple valve rating. Pressure Relief uses the same physical property models and data as other Aspen Plus flowsheet models. The modeling equations for nozzle flow, and for bubbly and churn-turbulent disengagement are based on technology developed by the Design Institute for Emergency Relief System (DIERS) Users Group of the AIChE. This technology is considered the best available for pressure relief system design. The Aspen Plus Pipeline model simulates flow through the inlet and tail pipes in the relief system. Pressure Relief always operates in rating mode. This means that the program will calculate the pressure profile in the vessel and piping, given the size of the relief device. In addition, you must specify the: • Dimensions of the equipment being protected and a connecting nozzle if present • Pressure relief scenario • Dimensions of inlet and tail piping, if present • Dimensions of the relief device Each Pressure Relief block models one scenario and one vessel. To model more than one scenario or pressurized vessel in an Aspen Plus run, include more than one Pressure Relief block in the simulation. Pressure Relief blocks are not part of the simulation flowsheet (there is no icon needed), but they can reference simulation streams. Pressure Relief analyzes the specified scenario and reports: • Rated capacity • Results profiles (temperature, pressure, vapor fraction) • Whether the system meets design rules that you select or that applicable codes (such as ASME) require Creating a Pressure Relief Block To create a Pressure Relief block: 1. From the Data menu, point to Flowsheeting Options, then Pres-Relief. 2. In the Pressure Relief Object Manager, click New. 3. In the Create New ID dialog box, enter an ID (name) or accept the default ID
Chapter33Click OK.4.ThePressureReliefSetupformappears.See the remaining sections of this chapter for information on completing thePressure Relief specifications.About Pressure Relief ScenariosA Pressure Relief scenario is a situation that causes venting to occur through therelief system.There are four types of generic scenarios to choose from:Steady-state flow rating of relief system (valve and piping)Steady-stateflowrating of reliefvalve (nopiping)Dynamic run with vessel engulfed by fireDynamic run with specified heatflux intovesselEach scenario is described briefly below.Steady-State Flow Rating of Relief System ScenarioUse this scenariotofind theflowratethroughan emergencyrelief system,giventheconditionofthestreamflowingintoitandtheupstreamanddownstreampressures. The relief system may include a relief valve a vessel neck, and twosegments each, of inlet and tail pipes, as well as any number of block valves andfittings.Inthis scenariothepressurereliefmodel calculates thesteady-stateflowrate through the specified system.Steady-State Flow Rating of Relief Valve ScenarioUse the valve rating scenario when:.Youknowthepressure,temperature,and streamcompositionYou wanttofind out thevalvecapacityThe differencesbetween theValverating scenario andthe Relief SystemRatingscenario are that in the Valve Rating scenario:No piping is allowedThe relief devicemustbe aProcess SafetyValve (PSV)oraProcess SafetyValve-ProcessSafetyDisk(PSV-PSD)33-3AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-3 Chapter 33 4. Click OK. The Pressure Relief Setup form appears. See the remaining sections of this chapter for information on completing the Pressure Relief specifications. About Pressure Relief Scenarios A Pressure Relief scenario is a situation that causes venting to occur through the relief system. There are four types of generic scenarios to choose from: • Steady-state flow rating of relief system (valve and piping) • Steady-state flow rating of relief valve (no piping) • Dynamic run with vessel engulfed by fire • Dynamic run with specified heat flux into vessel Each scenario is described briefly below. Steady-State Flow Rating of Relief System Scenario Use this scenario to find the flow rate through an emergency relief system, given the condition of the stream flowing into it and the upstream and downstream pressures. The relief system may include a relief valve a vessel neck, and two segments each, of inlet and tail pipes, as well as any number of block valves and fittings. In this scenario the pressure relief model calculates the steady-state flow rate through the specified system. Steady-State Flow Rating of Relief Valve Scenario Use the valve rating scenario when: • You know the pressure, temperature, and stream composition • You want to find out the valve capacity The differences between the Valve rating scenario and the Relief System Rating scenario are that in the Valve Rating scenario: • No piping is allowed • The relief device must be a Process Safety Valve (PSV) or a Process Safety Valve-Process Safety Disk (PSV-PSD)
PressureReliefCalculationsDynamicRunWithVesselEngulfedbyFire ScenarioPressure Relief provides three standards for computing the fire exposurescenario:NFPA30API520API2000The chosen fire standard determines which regulations are used to calculate firescenario factors such as vessel wetted area, energy input, and credit factors.Pressure Relief assumes the calculated energy input is constant during the entireventing transient. If appropriate, you can specify individual credit factors fordrainage, water spray, and insulation to reduce the energy input. Alternatively.youcanspecifyanoverallcreditfactor.Dynamic Run With Specified Heat Flux Into VesselScenarioThe heat input scenario is similar to the fire exposure scenario, except:You choose the energy input valueCredit factors are not allowedThere is no cut-off time for the duration of the event. The specified heat flux canbe constant, from a constant temperature source, or a function of time.This scenario can beused to model:Full-on electrical heaters or other constant energy sources, by selecting theConstant Duty heat input method.Pressure reliefcaused by runaway reactions,by using the Constant Dutyheatinput method and specifying a value of zero for heat duty.Heat input from a source such as heat exchanged by selecting the Calculatedfrom the Heat Source method and providing a source temperature, a heattransfer coefficient and surface area.33-4AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide Version 10.1-0 33-4 Pressure Relief Calculations Dynamic Run With Vessel Engulfed by Fire Scenario Pressure Relief provides three standards for computing the fire exposure scenario: • NFPA 30 • API 520 • API 2000 The chosen fire standard determines which regulations are used to calculate fire scenario factors such as vessel wetted area, energy input, and credit factors. Pressure Relief assumes the calculated energy input is constant during the entire venting transient. If appropriate, you can specify individual credit factors for drainage, water spray, and insulation to reduce the energy input. Alternatively, you can specify an overall credit factor. Dynamic Run With Specified Heat Flux Into Vessel Scenario The heat input scenario is similar to the fire exposure scenario, except: • You choose the energy input value • Credit factors are not allowed There is no cut-off time for the duration of the event. The specified heat flux can be constant, from a constant temperature source, or a function of time. This scenario can be used to model: • Full-on electrical heaters or other constant energy sources, by selecting the Constant Duty heat input method. • Pressure relief caused by runaway reactions, by using the Constant Duty heat input method and specifying a value of zero for heat duty. • Heat input from a source such as heat exchanged by selecting the Calculated from the Heat Source method and providing a source temperature, a heat transfer coefficient and surface area
Chapter33Selecting a Pressure Relief ScenarioTo specify a pressure relief scenario:1.On the Pressure Relief Setup Scenario sheet,click one of thefour choices ofpressure relief scenario.Vessel Cantents RulesFlashOptionsFerScenarioStreamPressurerelief scenarioSteady stateflowratingofreliefsystemCSteady state flowrating of reliefvalveCDynamic runwith vessel engulfedbyfireCDynamic tun with specified heat flux into vesselSpecificationsCapacity option:-CodePVent discharge pressure:barMEstimated flowrateMolekmol/hrFromthe Capacityoption list,selectCodeorActual.2.Choosethisoption Torun theMeaningThis option is useful forsimulation atCode (default)Code capacityDe-rates capacity of relief deviceDetermining if a pressure relief valve oras specified byASMEcoderupturediskofagivensizeisadeguaterequirements.for the chosen scenario.ActualActual capacityProduces best estimate of reliefChecking whether the inlet or tail pipesystem effluentDoes not de-ratesections meet the code compliantcapacity.requirements.3. Specify the Vent Discharge Pressure. You need to specify this for all pressurerelief scenarios. If there are any tail pipe segments after the relief device thedischargepressurereferstothepressureattheendofpiping.Thisistypicallytheatmospheric pressureof theflareheaderback pressure.4.Ifyouselectedasteadystateflowscenario,entertheEstimatedFlowRate5. The value you enter is used as a starting point to determine the rated flow fortherelief systemor safetyvalve.Youcan enterthisflowinamass,moleorstandard liquidvolumebasis.AspenPlusUserGuide33-5Version 10.1-0
Aspen Plus User Guide Version 10.1-0 33-5 Chapter 33 Selecting a Pressure Relief Scenario To specify a pressure relief scenario: 1. On the Pressure Relief Setup Scenario sheet, click one of the four choices of pressure relief scenario. 2. From the Capacity option list, select Code or Actual. Choose this option To run the simulation at Meaning This option is useful for Code (default) Code capacity De-rates capacity of relief device as specified by ASME code requirements. Determining if a pressure relief valve or rupture disk of a given size is adequate for the chosen scenario. Actual Actual capacity Produces best estimate of relief system effluent. Does not de-rate capacity. Checking whether the inlet or tail pipe sections meet the code compliant requirements. 3. Specify the Vent Discharge Pressure. You need to specify this for all pressure relief scenarios. If there are any tail pipe segments after the relief device the discharge pressure refers to the pressure at the end of piping. This is typically the atmospheric pressure of the flare header back pressure. 4. If you selected a steady state flow scenario, enter the Estimated Flow Rate. 5. The value you enter is used as a starting point to determine the rated flow for the relief system or safety valve. You can enter this flow in a mass, mole or standard liquid volume basis