Pilot PSVs Lifting Due to Backpressure Surges?

[PrudhoeBayAK]

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

 

[TD2K]

Yup, AGCO pilot operated valves are known for this, can't remember which models off the top of my head and I'm away from my office (and I'm not implying this is unique to their valves) to refresh my failing mind with my catalogues. Talk to your PSV vendor, there is a 'back flow preventer' (I think is the name AGCO uses) that is installed in the line between the top of the piston and the PSV discharge line to prevent this, I'm sure any other vendor will also have them.

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.

 

[talee]

Try this link

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1996/in96061.html

 

[tad123]

I have seen a similar incident that the PRV started lifting though the pressure in the system did not reach the set point. If you have a XXV/SDV at the discharge line, and during ESD the XXV/SDV close too quick; it could create a transient effect and pressure in the system may reach the set pressure and cause the PRV lift. When the PRV lifts agaisnt a high back pressure (50-300 psig); the seat may be damaged and coud not reseat. The only thing that I don't understand is your PRV should be reseated before the pressure is built up in the flare header due to blowdown.
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.