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.
PSV2f
•
•
•
•
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?