Pressure Safety Valve Design

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?