Q & A > Question Details
How should we size a PSV outlet line when we are considering liquid relief as the determining factor?
Our understanding is that if vapor is relieved then for PSV inlet line size, pressure drop is the design criteria and for outlet line size sonic velocity is the design criteria.
25/08/2009 A: Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
To add to my answer above higher pressure drop may be allowed for PSV outlet line if PSV is bellows or pilot type PSV. For bellows type PSV the pressure drop may be up to 30% of the set pressure and for pilot type PSV, pressure drop may be up to 50% of the set pressure.
24/08/2009 A: Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
PSV outlet line sizing for liquid relief should also be based on pressure drop. For a conventional relief valve the pressure drop in the outlet line should not be more than 10% of the set pressure.
The same criterion is applicable for vapor relief also. However the difference is that in case of liquid relief, actual required relief flow is used for calculating the pressure drop while in case of vapor the maximum capacity of the relief valve is used to calculate the pressure drop.