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Pressure Safety Valve Design

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.

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Pressure Safety Valve Design

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  1. Pressure Safety Valve Design • Presented to ES-317 at UWO in 1999 • Dick Hawrelak

  2. 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.

  3. PSV Design Methods • NFPA 30 • API-521 • Natural Gas Association.

  4. Problems • Heat input uncertain. • Wetted area versus vessel area. • Protection factors for heat input reduction are variable. • Treatment of physical properties are different.

  5. PSV1 • Single component fluid. • Blocked in vessel. • Fire conditions. • Boiling liquid.

  6. 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.

  7. PSV2f • Single component fluid. • Blocked in vessel. • Fire conditions. • PSV sized for two phase flow.

  8. PSV3 • Vessel with 100% vapor. • Blocked in vessel. • Fire conditions. • Vapor expansion.

  9. PRDV • Rupture disc sizing. • Sonic or sub-sonic flow. • Overpressure for any reason.

  10. PRDL • Rupture disc sizing for flashing liquid. • Homogenius Equilibrium Model. • Landis CRITFLO method.

  11. PSV4 • PSV sizing for liquid expansion. • No phase change.

  12. PSV5 • Sizing relief vents. • Runaway reactions.

  13. 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.

  14. EXPLO • Explosion venting routine. • Scale-up from pilot plant data.

  15. 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.

  16. 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?

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