Pressure Relief “Grace under pressure” – Ernest Hemingway Harry J. Toups LSU Dep

Pressure Relief “Grace under pressure” – Ernest Hemingway Harry J. Toups LSU Dep www.phwiki.com

Pressure Relief “Grace under pressure” – Ernest Hemingway Harry J. Toups LSU Dep

Epps, Kip, Executive Producer, Whipnotic has reference to this Academic Journal, PHwiki organized this Journal Pressure Relief “Grace under pressure” – Ernest Hemingway Harry J. Toups LSU Department of Chemical Engineering with significant material from SACHE 2003 Workshop presentation by Scott Ostrowski (ExxonMobil) in addition to Professor Emeritus Art Sterling What is the Hazard Despite safety precautions Equipment failures Human error, in addition to External events, can sometimes lead to Increases in process pressures beyond safe levels, potentially resulting in OVERPRESSURE due to a RELIEF EVENT What are Relief Events External fire Flow from high pressure source Heat input from associated equipment Pumps in addition to compressors Ambient heat transfer Liquid expansion in pipes in addition to surge

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Potential Lines of Defense Inherently Safe Design Passive Control Active Control Low pressure processes Install Relief Systems Overdesign of process equipment What is a Relief System A relief device, in addition to Associated lines in addition to process equipment to safely h in addition to le the material ejected Why Use a Relief System Inherently Safe Design simply can’t eliminate every pressure hazard Passive designs can be exceedingly expensive in addition to cumbersome Relief systems work!

Pressure Terminology MAWP Design pressure Operating pressure Set pressure Overpressure Accumulation Blowdown Code Requirements General Code requirements include: ASME Boiler & Pressure Vessel Codes ASME B31.3 / Petroleum Refinery Piping ASME B16.5 / Flanges & Flanged Fittings Code Requirements Relieving pressure shall not exceed MAWP (accumulation) by more than: 3% as long as fired in addition to unfired steam boilers 10% as long as vessels equipped with a single pressure relief device 16% as long as vessels equipped with multiple pressure relief devices 21% as long as fire contingency

Relief Design Methodology LOCATE RELIEFS CHOOSE TYPE DEVELOP SCENARIOS SIZE RELIEFS (1 or 2 Phase) CHOOSE WORST CASE DESIGN RELIEF SYSTEM Locating Reliefs – Where All vessels Blocked in sections of cool liquid lines that are exposed to heat Discharge sides of positive displacement pumps, compressors, in addition to turbines Vessel steam jackets Where PHA indicates the need LOCATE RELIEFS Choosing Relief Types Spring-Operated Valves Rupture Devices CHOOSE TYPE

Spring-Operated Valves Conventional Type CHOOSE TYPE Picture: Conventional Relief Valve Conventional Relief Valve CHOOSE TYPE Superimposed Back Pressure Pressure in discharge header be as long as e valve opens Can be constant or variable CHOOSE TYPE

Built-up Back Pressure Pressure in discharge header due to frictional losses after valve opens Total = Superimposed + Built-up CHOOSE TYPE Spring-Operated Valves Balanced Bellows Type CHOOSE TYPE Picture: Bellows Relief Valve Bellows Relief Valve CHOOSE TYPE

Pros & Cons: Conventional Valve Advantages Most reliable type if properly sized in addition to operated Versatile – can be used in many services Disadvantages Relieving pressure affected by back pressure Susceptible to chatter if built-up back pressure is too high CHOOSE TYPE Pros & Cons: Balanced Bellows Valve Advantages Relieving pressure not affected by back pressure Can h in addition to le higher built-up back pressure Protects spring from corrosion Disadvantages Bellows susceptible to fatigue/rupture May release flammables/toxics to atmosphere Requires separate venting system CHOOSE TYPE Rupture Devices Rupture Disc Rupture Pin CHOOSE TYPE

Conventional Metal Rupture Disc CHOOSE TYPE Conventional Rupture Pin Device CHOOSE TYPE When to Use a Spring-Operated Valve Losing entire contents is unacceptable Fluids above normal boiling point Toxic fluids Need to avoid failing low Return to normal operations quickly Withst in addition to process pressure changes, including vacuum CHOOSE TYPE

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When to Use a Rupture Disc/Pin Capital in addition to maintenance savings Losing the contents is not an issue Benign service (nontoxic, non-hazardous) Need as long as fast-acting device Potential as long as relief valve plugging High viscosity liquids CHOOSE TYPE When to Use Both Types Need a positive seal (toxic material, material balance requirements) Protect safety valve from corrosion System contains solids CHOOSE TYPE Relief Event Scenarios A description of one specific relief event Usually each relief has more than one relief event, more than one scenario Examples include: Overfilling/overpressuring Fire Runaway reaction Blocked lines with subsequent expansion Developed through Process Hazard Analysis (PHA) DEVELOP SCENARIOS

An Example: Batch Reactor Control valve on nitric acid feed line stuck open, vessel overfills Steam regulator to jacket fails, vessel overpressures Coolant system fails, runaway reaction DEVELOP SCENARIOS Sizing Reliefs Determining relief rates Determine relief vent area SIZE RELIEFS (Single Phase) Scenarios Drive Relief Rates Overfill (e.g., control valve failure) Fire Blocked discharge SIZE RELIEFS (Single Phase) Maximum flow rate thru valve into vessel Vaporization rate due to heat-up Design pump flow rate

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