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?