国外安全阀技术文档

•Overpressure is the rise in pressure in a pressurised system (vessel, tank, pipe) above a pre-determined “safe” limit or design pressure•Some way to reduce (vent) this excess pressure must be provided to return thesystem to a safe condition What is Overpressure ?Common Causes of OverpressureBlocked DischargeThermal ExpansionExternal FireFailure of other equipment suchas Control Valves etc.(Ref. API 521)Blocked DischargeFlow Outlet Block Valve Closed(from compressor or pump)PRDPressureVesselExternal FireSTORAGE ORPROCESS VESSELPRDThermal Expansion Liquid-Full Pipe or Pressure VesselPRDControl Valve FailureLow Pressure Systemor Vessel High PressureSystemPRD –Consider Worst Case •Equipment can be subject to many potentialfailure modes…Fire, blocked discharge, CV failure etc.•The PRD is sized and selected based on theWorst Case Scenario or Governing CaseWhat is Overpressure Protection ?• Any method of discharging (venting)the fluid (liquids and/or vapors) from a system to atmosphere or to some other system so that the pressure will not exceed a specified safe valuePENTAIRTechnology & Principles of Operation11Overpressure Protection…Why is it needed?• Protection of personnel and property • Prevention of catastrophic events • Mandatory by law– Pressure Vessel Code – Third Party Control – Highly Regulated• Satisfy company internal policies • Satisfy regulatory body - auditsPENTAIRTechnology & Principles of Operation12Overpressure Protection…Why is it needed?PENTAIRTechnology & Principles of Operation13Pressure Relief DevicesPENTAIRTechnology & Principles of Operation14Industries ServedNuclear Oil&Gas ProductionPetrochemical AerospaceRefining Fossil Power Marine Food & Beverage Pulp & PaperTank Cars Pharmaceuticals ChemicalPENTAIRTechnology & Principles of Operation15Pressure Relief Device• A device that opens in an emergency orabnormal conditions• Prevents a rise in pressure in excess of aspecified value• May be designed to prevent excessiveinternal vacuum• Could be…pressure relief valve non reclosing pressure relief device or vacuum relief valvePENTAIRTechnology & Principles of Operation16Pressure Relief Device• Self-actuated devices, with tightoperating limits• Must be able to operate at anytime,without any power supply or manual intervention•The last Line of Defence!PENTAIRTechnology & Principles of Operation17History & DevelopmentPENTAIRTechnology & Principles of Operation18The First Safety Valve?Denis Papin, 1681Weight Loaded Safety ValvePENTAIRCopyright © 2010 Tyco Flow Control. All Rights Reserved19Pressure Relief Valves - HistoryAs systems evolved and pressures increased weight and lever valves became impractical….PENTAIRTechnology & Principles of Operation20Designs continue to improve…Jean Disaqulier, 1781Direct Acting Dead WeightSafety ValveThe development of the steam engine by Watt led to the need for Safety ValvesTimothy Hackworth 1830Spring Loaded Safety ValveSpring Loaded Valves: developed from the middle of the 1800’s to todayThe most important and common single element in all safety valve designs was the use of a spring…Flat Disc-As pressure increases to predetermined set point (Set Pressure) disc lifts :-Spring compresses-Higher downward force-Result :-High Overpressure required to achieve full liftNo control of opening characteristics!F dP1Spring = Accurate Adjustment ofValve Set PressureDiscNozzleF sP1A’SkirtSkirted Area -Rapid Opening -Full LiftNo control of valve closing characteristicsA-High Ring Position…-(Small Chamber)-Sharp Opening-Longer Blowdown(Slower Closing)•Compromise-Low Ring Position...-(Large Chamber)-Slow/Sluggish Opening-Short Blowdown (Rapid Closing)FsP1AA’HuddlingChamberControl Ring(Nozzle Ring)BonnetBodyP1P2BACKPRESSURE•MAWP / MAP:Maximum Allowable Working Pressure.Maximum safest pressure for the equipment‘Design’ pressure•Operating / Service / Working pressureNormal working pressure for the equipment •AccumulationThe pressure rise above the MAWP allowed by the code for exceptional circumstances (e.g. safety valve full opening)•Set PressureThe pressure at which the safety valve starts to open •OverpressureThe pressure rise above the set pressure to enable the valve to open fully (typically 10% of set pressure)•Reseat pressurePressure below the set pressure at which the safety valve is set to reclose•BlowdownDifference in % between Set and Reseat pressuresSafety ValveProtected SystemMax. Allowable Working Pressure(Design Pressure...)Reseat PressureBlowdown (typically around 7%)Overpressure (Full) Opening PressureSet Pressure(Tight) Shut-Off PressureOperating Pressure(Service Pressure.. “money-maker”)Accumulation(usually 10%, depends on code)Where the SV is normally sized +/-3%Tolerance•Operating Gap or DifferenceDifference between service pressure and set pressure •Relieving TemperatureTemperature of the fluid when the valve opens ordischarges•Back-PressurePressure existing at the outlet of the safety valve•Constant Super-imposed•Variable Super-Imposed•Built-Up (always variable)•Relief Valve = opens gradually, mainly for liquid service •Safety Valve = opens suddenly (‘Pops’), mainly for steam, gas service•Safety Relief Valve, SRV= suitable for all services •Pressure Relief Valve, PRV = generic term for all these •Pressure Safety Valve, PSV = another generic term•In European Standards : Safety valveNon Closing PRD –Rupture Disc•Weak point in a pressurised system •Ideal PRD where rate of pressure riseincreases exponentially during “runaway” reaction •Unmatched speed of opening •Non-Reclosing DeviceP 1Atmospheric PressureRupture Disc with PRV•No leakage from PRV seats during normal system operation•Allows the use of standard (less expensive) materials of construction for the PRV,Rupture Disc manufactured from exotic(expensive) material•Down stream RD prevents fouling inside PRV •Space between RD and PRV to be fitted with pressure gauge / telltale device –indicatesdisc ruptureNon Reclosing PRD –Buckling Pin (BPRV)•Weak point in a pressurised system•Rapid opening with full flow•Not susceptible to system generated problems associated with some SRV’s –Chattering,pressure fluctuations, temperature…•Non-Reclosing DeviceRelief Valve (Thermal Relief valve)Opens in proportion to the pressureIncrease…Normally used on non compressiblemedia (liquids)•LIMITATIONSShould not be used generally :In steam , air , gas or vapour serviceIn variable back pressure service•APPLICATIONCharacterised by a rapid opening or “pop”action…Used for compressible media, air, steam,gases...•LIMITATIONSShould not be used :In Liquid ServiceSafety Valve•APPLICATIONOn any service, compressible or not •LIMITATIONS Should not be used…On steam boiler drums or super heaters, where a safety valve is generally preferredVariable back pressure conditionsSafety Relief Valve (Conventional)REMOVE•Bellows balances valve operationagainst back pressure, applications with…..Any back pressure, usually up to 50% max.of the valve set pressure valueBonnet must be vented to atmosphere or safe locationLeak at bonnet vent indicates a failed bellowsVery effective in corrosive or dirty service due to protection of guiding faces by bellowsCrosby Style BP Balanced Piston PRVAnderson Greenwood Style81PBalanced Piston PRVBalanced Piston ExamplesBack Pressure•Definition…•Static Pressure Existing at the Outlet of a Pressure ReliefDevice due to Pressure in the Discharge System•Two Types…•Superimposed•Built-upInlet Back PressureSuperimposed Back Pressure •Superimposed BP adds itself to the spring force…•Actual Valve Opening…Spring set + BP•Can be…•Constant•Variable P1P2Superimposed Constant Back Pressure •When PRV Outlet is connected toa Static pressure source•No change under any conditionsof operation•Conventional PRV can be used ifSpring Setting is reduced byConstant Back Pressure…Opened &FlowingClosed•Present Prior to SV opening •Usually result of one or more SV’s discharging into a common piping•Set pressure will increase by the amount of Back Pressure •Bellows / Balanced Piston valve RequiredSuperimposed Variable Back Pressure•The Balanced Bellows..•Isolates top side of disc from Back-Pressure•Isolates spring, guide and stem fromdown stream (outlet) environment•Typically this type of valve can be used…•With backpressure up to 50% of valve set pressure valueBalanced Bellows DesignDischargeBPBU•Developed AFTER SV opens…•Always Variable!•Due to flow, pressure drop in discharge piping•Can be up to 10% of set pressurevalue on Conventional type valves•Will cause reduced capacity or valveinstability if over 10%…Balanced Valve design should be specifiedBuilt Up Back Pressure•Balanced valves:•Bonnet must be vented toatmosphere•No back pressure inside thebonnet or the bellows•Bellows failure ! …..Balanced piston •Additional safety device•Greatly reduces emissions•Correct opening pressure•No reduction in capacity•Stability!Balanced Bellows / Balanced PistonPilot Operated Safety Valve•Main valve controlled by a self actuated pilot Typical applications….•Small differential required between•Set and Operating pressure•Short or controllable blowdown•required•High back pressure conditions•Large capacity and high pressures。