Using oversized PSV
Using oversized PSV
(OP)
I'm sizing PSV's for fire case on two heat exchangers located near an upstream vessel for which a PSV exists (isolation valves are installed in connected piping). By inspection the surface area of the heat exchangers is about two thirds of the vessel for which the PSV exists. I've been asked if we could use relief valves in the heat exchangers identical to the vessel PSV to minimize spares and to eliminate having to perform the calculations. Is there a problem with using slightly oversized relief valves? Thanks for your input!





RE: Using oversized PSV
Fire case for vessel, there is liquid filled, 2/3-phase in vessel or gas filled vessel. Similarly, there is liquid filled gas filled for HX (if cooler or heater) but 2-phase in HX (if condenser or evaporator). Comparing surface area of vessel and HX can not justify same PSV to be used.
For example, vessel is having 2 phase (Vapor & Liquid). It will based on wetted surface area (majority this is governing case). However, the HX may be a heater on the vapor outlet which is completely filled with gas. Gas expansion will occur. The load will be much smaller (normally).
So, it is difficult to judge from the "surface area" only.
JoeWong
Chemical & Process Technology
RE: Using oversized PSV
Correct me if I'm off base, but here is my line of thinking. The second unit seems like wetted area is the way to proceed, the first unit would see a smaller load as there would not be vapor generation from heat input.
To my previous last question. If it is determined the relieving capacities required are less than that of the vessel, would there be a problem using the oversized valves?
Again, thanks very much for the input.
RE: Using oversized PSV
One of the problem link to oversized PSV is chattering. Chatterng could lead to seat damage, leakage, etc.
PSV normally can take load from 25-100% of it design load (subject to PSV type - only manufacturer (not vendor) can advise). So again, without knowing the relief load (especially the condenser), it is hard to say if the PSV will experience chattering.
I feel that you have high chance to use same size PSV. Just need some figure to justify it.
JoeWong
Chemical & Process Technology
RE: Using oversized PSV
I2I
RE: Using oversized PSV
RE: Using oversized PSV
RE: Using oversized PSV
Cheers!
MechBRR
RE: Using oversized PSV