Hello,
I have ran into a problem choosing a set pressure for a PSV on a KO vessel. The vessel has a demister pad, so a relief device is suggested as per API standards.
We have ruled out an overfilling scenario due to the presence of liquid level alarms and drainage, and the only possible overpressure scenario is a blocked-in flow.
Operating P = 55 psig
Operating T = 125 F
MAWP = 270 psig
Now it would make sense to have a set pressure at the MAWP of 270 psig. However, the fluid to be relieved condenses at approximately 205 psig. At this point, the presence of liquid in the vessel would activate liquid level alarms and shut the process down (the process would shut down before ever reaching anything close to the set pressure).
Should the PSV still be sized at a set pressure of 270 psig for a flashing fluid, despite the fact that it would realistically never see that scenario? Or should the set pressure be lowered so that it can be sized for a more realistic overpressure scenario (vapor phase only), even if it is below the vessel MAWP.
Thanks in advance.