Presentation on theme: "“Grace under pressure” – Ernest Hemingway"— Presentation transcript:
1 “Grace under pressure” – Ernest Hemingway Pressure Relief“Grace under pressure”– Ernest HemingwayHarry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil)and Professor Emeritus Art Sterling
2 What is the Hazard? Despite safety precautions … Equipment failuresHuman error, andExternal events, can sometimes lead to …Increases in process pressures beyond safe levels, potentially resulting in …OVERPRESSURE due to a RELIEF EVENT
3 What are Relief Events? External fire Flow from high pressure source Heat input from associated equipmentPumps and compressorsAmbient heat transferLiquid expansion in pipes and surge
4 Potential Lines of Defense Inherently Safe DesignPassive ControlActive ControlLow pressure processesOverdesign of process equipmentInstall Relief Systems
5 What is a Relief System? A relief device, and Associated lines and process equipment to safely handle the material ejected
6 Why Use a Relief System?Inherently Safe Design simply can’t eliminate every pressure hazardPassive designs can be exceedingly expensive and cumbersomeRelief systems work!
7 Pressure Terminology MAWP Design pressure Operating pressure Set pressureOverpressureAccumulationBlowdownPage 1Operating Pressure:Pressure usually subjected to during normal operationDesign pressure:Set by process conditionsPressure which will provide a suitable margin above the operating pressure in order to prevent nuisance tripsNormally set at greater of 25psi or 10% of set pressure above the operating pressureMAWPMaximum Allowable Working PressureDefined in the construction codes for unfired vessels (ASME)Set by metallurgical conditions based on type of material and its thicknessMust be equal to or greater than the design pressureSet pressureInlet pressure at which the valve is adjusted to openLiquid service valve starts to openVapor service valve popsAccumulationPressure above MAWP that results during a release. Code allows 10%.OverpressureAny pressure above the SP.
8 Code Requirements General Code requirements include: ASME Boiler & Pressure Vessel CodesASME B31.3 / Petroleum Refinery PipingASME B16.5 / Flanges & Flanged FittingsASME Section 8 for unfired vesselsASME B31.3, “Process Piping”, This Code prescribes requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping.ASME B16.5, “Pipe Flanges and Flanged Fittings”, This Standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings.
9 Code RequirementsRelieving pressure shall not exceed MAWP (accumulation) by more than:3% for fired and unfired steam boilers10% for vessels equipped with a single pressure relief device16% for vessels equipped with multiple pressure relief devices21% for fire contingencyGo over bullets.
10 Relief Design Methodology LOCATE RELIEFSCHOOSETYPEDEVELOP SCENARIOSSIZE RELIEFS(1 or 2 Phase)CHOOSE WORST CASEDESIGN RELIEF SYSTEM
11 Locating Reliefs – Where? All vesselsBlocked in sections of cool liquid lines that are exposed to heatDischarge sides of positive displacement pumps, compressors, and turbinesVessel steam jacketsWhere PHA indicates the needLOCATE RELIEFS
15 Superimposed Back Pressure Pressure in discharge header before valve opensCan be constant or variableSuperimposed back pressure is always presentIn the example the suction pressure of the pump is always on the outlet of the pressure relief valve. It can be constant or variable depending on the nature of the system.Superimposed back pressure can give us problems if variable when trying to specify a PRV.What will the back pressure be when the valve is called upon, i.e. will it be lifting alone or as part of a plant wide release?Could affect the type of pressure relief valve chosen.More of a problem for valves with lower set points as pressure variation will be a greater percentage of set pressure.CHOOSETYPE
16 Built-up Back Pressure Pressure in discharge header due to frictional losses after valve opensTotal = Superimposed + Built-upPressure which develops as a result of flow in the discharge header after the pressure relief valve opensIs not present when valve is closed.In the example no built-up back pressure exists until the valve opens. Upon opening the built-up back pressure develops and increases as the flow through the valve increases until full lift.The sum of the superimposed and built-up back pressure will give the total back pressure on the pressure relief valve outlet. The PRV does not care where the back pressure comes from.CHOOSETYPE
18 Picture: Bellows Relief Valve Venting arrangements must be carefully selected and designed to meet the following requirements:1.Manufacturer’s shipping plugs must be removed from the bonnet vent holes before a new valve is commissioned.2.Each PR valve must be installed so that the bonnet vent does not allow released vapors to impinge on lines or equipment,or towards personnel walkways. Where necessary, a short nipple and elbow should be added to direct flow away from such areas. In these cases, the vent piping should discharge horizontally to avoid entry of rainwater and debris, and should terminate in a position which is accessible for leak testing.3.In cases where bellows failure would release flammable, toxic or corrosive liquids through the vent, a short nipple and elbow should be used to direct leakage to an open funnel which is piped to grade and ties into a catch basin or manhole through a sealed inlet connection.4.Although venting to the atmosphere is preferred, an alternative is to tie into a closed low pressure system, if available.CHOOSETYPE
19 Pros & Cons: Conventional Valve AdvantagesMost reliable type if properly sized and operatedVersatile -- can be used in many servicesDisadvantagesRelieving pressure affected by back pressureSusceptible to chatter if built-up back pressure is too highNow that we’ve described these valves, let’s try to summarize the advantages and disadvantages of the various types.Go over bullets.CHOOSETYPE
20 Pros & Cons: Balanced Bellows Valve AdvantagesRelieving pressure not affected by back pressureCan handle higher built-up back pressureProtects spring from corrosionDisadvantagesBellows susceptible to fatigue/ruptureMay release flammables/toxics to atmosphereRequires separate venting systemContinue going over bullets.CHOOSETYPE
23 Conventional Rupture Pin Device The buckling pin concept is derived from Euler’s formula. Euler’s formula relates the force or pressure needed to buckle a long, thin column to the length 2 , diameter 4 , and modulus of elasticity of the column.The key to the accuracy of the buckling pin valve is the repeatability of the buckling pin.CHOOSETYPE
24 When to Use a Spring-Operated Valve Losing entire contents is unacceptableFluids above normal boiling pointToxic fluidsNeed to avoid failing lowReturn to normal operations quicklyWithstand process pressure changes, including vacuumCHOOSETYPE
25 When to Use a Rupture Disc/Pin Capital and maintenance savingsLosing the contents is not an issueBenign service (nontoxic, non-hazardous)Need for fast-acting devicePotential for relief valve pluggingHigh viscosity liquidsCHOOSETYPE
26 When to Use Both TypesNeed a positive seal (toxic material, material balance requirements)Protect safety valve from corrosionSystem contains solidsCHOOSETYPE
27 Relief Event Scenarios A description of one specific relief eventUsually each relief has more than one relief event, more than one scenarioExamples include:Overfilling/overpressuringFireRunaway reactionBlocked lines with subsequent expansionDeveloped through Process Hazard Analysis (PHA)DEVELOP SCENARIOS
28 An Example: Batch Reactor Control valve on nitric acid feed line stuck open, vessel overfillsSteam regulator to jacket fails, vessel overpressuresCoolant system fails, runaway reactionDEVELOP SCENARIOS
32 Fire Scenario CalcsAPI 520 gives all equations for calculating fire relief rate, step-by-stepDetermine the total wetted surface areaDetermine the total heat absorptionDetermine the rate of vapor or gas vaporized from the liquidSIZE RELIEFS(Single Phase)
34 Determine Heat Absorption Prompt fire-fighting & adequate drainage:Otherwise:whereQ is the heat absorption (Btu/hr)F is the environmental factor1.0 for a bare vesselSmaller values for insulated vesselsAwet is the wetted surface area (ft2)SIZE RELIEFS(Single Phase)
35 Determine Vaporization Rate whereW = Mass flow, lbs/hrQ = Total heat absorption to the wetted surface, Btu/hrHvap = Latent heat of vaporization, Btu/lbSIZE RELIEFS(Single Phase)
36 Determine Relief Vent Area Liquid ServicewhereA is the computed relief area (in2)Qv is the volumetric flow thru the relief (gpm)Co is the discharge coefficientKv is the viscosity correctionKp is the overpressure correctionKb is the backpressure correction(r/rref) is the specific gravity of liquidPs is the gauge set pressure (lbf/in2)Pb is the gauge backpressure (lbf/in2)SIZE RELIEFS(Single Phase)
37 Determine Relief Vent Area Gas ServicewhereA is the computed relief area (in2)Qm is the discharge flow thru the relief (lbm/hr)Co is the discharge coefficientKb is the backpressure correctionT is the absolute temperature of the discharge (°R)z is the compressibility factorM is average molecular weight of gas (lbm/lb-mol)P is maximum absolute discharge pressure (lbf/in2)c is an isentropic expansion functionSIZE RELIEFS(Single Phase)
38 Determine Relief Vent Area Gas Servicewherec is an isentropic expansion functiong is heat capacity ratio for the gasUnits are as described in previous slideSIZE RELIEFS(Single Phase)
39 A Special Issue: Chatter Spring relief devices require 25-30% of maximum flow capacity to maintain the valve seat in the open positionLower flows result in chattering, caused by rapid opening and closing of the valve discThis can lead to destruction of the device and a dangerous situationSIZE RELIEFS(Single Phase)
40 Chatter - Principal Causes Valve IssuesOversized valveValve handling widely differing ratesRelief System IssuesExcessive inlet pressure dropExcessive built-up back pressureGo over bullets.We will examine each of these causes and propose solutions.SIZE RELIEFS(Single Phase)
41 Worst Case Event Scenario Worst case for each relief is the event requiring the largest relief vent areaWorst cases are a subset of the overall set of scenarios for each reliefThe identification of the worst-case scenario frequently affects relief size more than the accuracy of sizing calcsCHOOSE WORST CASE
42 Design Relief SystemRelief System is more than a safety relief valve or rupture disc, it includes:Backup relief device(s)Line leading to relief device(s)Environmental conditioning of relief deviceDischarge piping/headersBlowdown drumCondenser, flare stack, or scrubberDESIGN RELIEF SYSTEM
43 Installation, Inspection, and Maintenance To undermine all the good efforts of a design crew, simply …Improperly install relief devicesFail to regularly inspect relief devices, orFail to perform needed/required maintenance on relief devices
46 ?? Discharges Pointing Down Anything wronghere?DischargesPointing DownAnything wronghere?
47 ?? Long Moment ArmLongMoment ArmAnything wronghere?
48 ?? Will these bolts hold in a relief event Anything wronghere?
49 Mexico City DisasterMajor Contributing Cause:Missing Safety Valve
50 Summary Pressure Relief Look forward to … Very Important ACTIVE safety elementConnected intimately with Process Hazard AnalysisRequires diligence in design, equipment selection, installation, inspection and maintenanceLook forward to …Two-phase flow methodology/exercise
51 ReferencesCrowl and Louvar – Chemical Process Safety, Chapters 8 and 9Ostrowski – Fundamentals of Pressure Relief DevicesSterling – Safety Valves: Practical Design, Practices for Relief, and Valve Sizing