Mike, I've never seen ethylene polymerise in our transport piping systems in the way you've described. Ethylene gas service is typically a very clean clean service.
Ok, in my example, the pressure rise was not a spike. It occurred relatively slowly and was above the set pressure for minutes. The Operators had high pressure alarm indication in the control room and were dealing with the overpressure for minutes. One can also assume that the discharge was not blocked - I should have added this was a decent sized PRV, 8 x 6.
Daniel, I agree, once a relieving event is over, there's often no way to tell whether a PRV has lifted if no one was there to notice... a bit like that proverbial tree falling in the forest. There are two other possible indicators though to add to my list: a) If the SV fails open/fails to reseat* OR b) if the SV chatters itself into oblivion... either are pretty strong clues that an SV has lifted
(*an SV will fail to reseat if the operating/system pressure remains ABOVE the blowdown pressure. Factory blowdown settings tend to be between 7 - 12% of set pressure, so it is conceivable that a system running close to it's design pressure could actually have PRV's that wont reseat once lifted.)
Ok, I'll give my best guess as to what I believe happened with this ethylene PRV. The investigation could not identify any mechanical failure or other factor that interfered with PRV operation. The incident had me scratching my head for a while.
The clue is the online leakage. I believe that the gas auto refrigerated due to the pressure drop across the seat (down to flare header pressure). This effectively froze the disc to the nozzle or caused some other cold related interference in the moving parts.
When the PRV was being prepared for removal and then transported to the workshop, the PRV had plenty of time to warm up to atmospheric conditions removing all traces of the "evidence". The PRV operated flawlessly on the test bench - except for the leakage.
So in this case, we had clear indication of an overpressure incident and when the operators were out in the plant trying to manage the situation, they clearly noted that the PRV had not lifted/was not relieving and so raised a safety incident.
Mecha20 - how much higher are we talking?
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