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. PSV2  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?