PSV leaking by?

[kmg1983]

Hey guys...For Crude oil application, we have a pilot operated pressure relief valve, set pressure of 235 psi, which has recently been "leaking by" with only 14-15 psi on the line. What are possible causes of this? There is a filter screen installed in the pilot sense line which may be plugged or clogged with debris, this is my thought...any other thoughts as to what may be causing this? The main valve has been sent in for service and cleaning once before after a similar issue.

 

[JAlton]

It may be leaking because the pressure is so low. There needs to be a pressure differential to keep a PORV Main Valve closed. The Dome side (or back of the Piston) has a 30% larger area than the seat side. This difference in area creates the force required to keep the Main Disc (Piston) closed. However, at start up, when the pressure is low, the Main Valve may not seat at first. Once the pressure builds up, the differential will take over and shut the Main Valve.

There are however, three possible leak paths through the main valve. If the Pilot is leaking, you probably have bad seals (o-rings). However, if the Main Valve is leaking, you may be leaking beneath the nozzle, across the seats or through the dome (down along the sides of the piston (disc). If you put the valve on a test bench and pressurize the body to about 50% of set, then you can do a test to determine the leak path. Cover the outlet partially and pour water in the bowl of the body (just enough to cover the connection between the body and the nozzle). If you get bubbles there, you have a leak beneath the nozzle. Tough to fix. Sned to factory for repair. If no bubbles, then fill to above seat level. If bubbles appear, then disassemble and replace seat O-Ring. Problem solved. If no bubbles at seat, continue to fill body above where Piston and sleeve intesect. Bubbles there mean the dome pressure is getting past the piston rings. Disassemble and replace piston rings. Problem solved. That is a quick explanation. I hope it helps.

JAC

 

[kmg1983]

Could you elaborate a little more on the differential pressure conditions? Wouldn't the dome be seeing the same pressure as the inlet? and if so, the larger area would provide more force to keep the main valve closed.

 

[JAlton]

Yes, the larger area does increase the force holding the disc closed. However, when the pressure is first applied the differential is very small (i.e. 2 psi inlet versus 2.6 psi dome pressure) Some PORV manufacturers use a spring to hold the main disc closed until the pressure gets higher. HOwever, sometimes the initial differential is insufficient to keep the main disc on its seat. Thus seat leakage is detected.

JAC