Pressure Safety Valve Design Presented to ES-317 at UWO in 1999 Dick Hawrelak.
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Transcript of Pressure Safety Valve Design Presented to ES-317 at UWO in 1999 Dick Hawrelak.
Pressure Safety Valve Design
• Presented to ES-317 at UWO in 1999
• Dick Hawrelak
ASME Code
• Section VIII for Unfired Pressure Vessels requires a PSV to be designed for the worst case scenario (fire or other upset).
• PSV alone does not protect the pressure vessel.
• Flames may impinge on the vapor space causing vessel failure.
• Flammable liquid pressure vessels still require water coverage.
PSV Design Methods
• NFPA 30
• API-521
• Natural Gas Association.
Problems
• Heat input uncertain.
• Wetted area versus vessel area.
• Protection factors for heat input reduction are variable.
• Treatment of physical properties are different.
PSV1
• Single component fluid.
• Blocked in vessel.
• Fire conditions.
• Boiling liquid.
DIERS
• Prediction of two-phase flow for PSVs designed for single phase flow.
• Most often the vessel is filled beyond the “swell” and “foam” heights.
• See example in PSV Folder in Chemical Plant Design.
PSV2
• Single component fluid.
• Blocked in vessel.
• Fire conditions.
• PSV sized for two phase flow.
PSV3
• Vessel with 100% vapor.
• Blocked in vessel.
• Fire conditions.
• Vapor expansion.
PRDV
• Rupture disc sizing.
• Sonic or sub-sonic flow.
• Overpressure for any reason.
PRDL
• Rupture disc sizing for flashing liquid.
• Homogenius Equilibrium Model.
• Landis CRITFLO method.
PSV4
• PSV sizing for liquid expansion.
• No phase change.
PSV5
• Sizing relief vents.
• Runaway reactions.
HP Trip
• ICI method to lower vessel pressure before PSV blows.
• Manual Pressure control station to by-pass PSV.
• Would rather vent manually than allow PSV to blow.
EXPLO
• Explosion venting routine.
• Scale-up from pilot plant data.
PSV Header and Flare Stack Design
• Flare header design - multiple PSVs blow in fire area 50’ x 50’.
• Complex headers for dry HCs, wet HCs, dry liquid and wet liquid blowdowns.
• Flare KO drums.
• Three API-521 flare stack methods.
Possible Exam Questions
• What are the four main problems with the different PSV design methods?
• In a fire situation, does the PSV alone protect the pressure vessel?
• Why would a PSV designed for sigle phase flow under fire conditions relieve a two phase vapor / liquid mixture?