Pilot PSVs Lifting Due to Backpressure Surges?
Pilot PSVs Lifting Due to Backpressure Surges?
(OP)
I have a 2"x3" pilot operated PSV (AGCO model 4731023/S/NACE) set at 100 psig, with a normal system operating pressure of ~35 psig. The PSV discharges into a common flare header, which has several other PSVs and blowdown valves tie-ing into the header in close vicinity of the 2x3 valve discharge. The flare header discharge point is several hundred feet of pipe away from these PSVs/BD-valves.
The PSV protects the shell side of Shell (MEG, MAWP of 220 psig, Op P ~35 psig) and Tube (HP Fuel Gas, Op Press ~600psig) HX for a tube rupture scenario.
When the plant has an ESD there is a blowdown valve that "spikes" the common flare header, which can/has seen anywhere from 50 - 300 psig (estimated from various models and observation). During these ESD events we have had several incidents where the PSV has lifted and effectively emptied the MEG system volume, without the system side (shell) pressure ever coming near the PSV setpoint. We think the PSV might be lifting due to the pressure spike on the discharge side.
My questions are:
Is our theory of backpressure surges lifting a pilot operated PSV valid?
Why the PSV does not reseat, is another mystery as well - perhaps a siphoning effect?
Are there any modifications we can make to a pilot operated valve that will prevent these incidents from happening again? We can't "afford" another "not-for-cause" MEG release.
Thanks!
PBAK
The PSV protects the shell side of Shell (MEG, MAWP of 220 psig, Op P ~35 psig) and Tube (HP Fuel Gas, Op Press ~600psig) HX for a tube rupture scenario.
When the plant has an ESD there is a blowdown valve that "spikes" the common flare header, which can/has seen anywhere from 50 - 300 psig (estimated from various models and observation). During these ESD events we have had several incidents where the PSV has lifted and effectively emptied the MEG system volume, without the system side (shell) pressure ever coming near the PSV setpoint. We think the PSV might be lifting due to the pressure spike on the discharge side.
My questions are:
Is our theory of backpressure surges lifting a pilot operated PSV valid?
Why the PSV does not reseat, is another mystery as well - perhaps a siphoning effect?
Are there any modifications we can make to a pilot operated valve that will prevent these incidents from happening again? We can't "afford" another "not-for-cause" MEG release.
Thanks!
PBAK





RE: Pilot PSVs Lifting Due to Backpressure Surges?
Why it is not reseating, I can't help you other than to say if you are getting the pressure below the blowdown setting, they should be (fat lot of good that is I know).
We had pilot operated valves on the booster pumps for our reinjection water pumps. They would occasionally lift during start-up although the shut-off head of the booster pumps via calculations were well below set pressure and would not reseat for whatever reason (pilots were clean when we disassembled them and worked fine in the shop when they arrived in the field). I worked with the vendor for quite some time and were unable to explain why the PSVs were not reseating. We finally changed them out to iso-domes.
RE: Pilot PSVs Lifting Due to Backpressure Surges?
http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1996/in96061.html
RE: Pilot PSVs Lifting Due to Backpressure Surges?
However, just try to slow down the closing time of your XXV/SDV (if you have one)to see if you can avoid the PRV lifting.