Is the Pilot on a Pilot-Operated PSV Supposed to Leak? 1

[KernOily]

Morning guys. Question: Is the pilot on a pilot-operated PSV supposed to leak during normal operation of the process?

Is the pilot supposed to vent if the system/process pressure is fluctuating? I would think so, to maintain the correct actuating pressure on the dome, yes?

Thanks guys! Pete

 

[The Obturator]

What Pilot Operated Pressure-relief Valve are you discussing and on what application/fluid?
POPRV designs come in either pop/snap acting or modulating action pilots.
Also most designs are non flowing types, but there are also some flowing designs about (meaning the pilot is still passing the fluid during main valve relief).
Modern POPRV designs are generally 'bubble tight' up to an average of 95% set pressure.
What operating to set pressure do you have?
Without due care and regular maintenance, any valve will leak.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[KernOily]

TO - Thanks for your reply.

I do not know anything about this valve. I'll provide answers to your questions as best I can.

What Pilot Operated Pressure-relief Valve are you discussing and on what application/fluid? (Valve mfg/type/model are unknown. The fluid service is raw natural gas (oilfield produced gas), ambient temperature, with specific gravity about 0.85.)

What operating to set pressure do you have? (Actual values are unknown at this time. For the moment, let's assume operating pressure is less than 90% of set pressure.)

 

[The Obturator]

As absurd it may sound, and again depending on the actual design, some POPRV's may leak where operating pressures are less than 90% of set. A POPRV is a design that requires system pressure to keep it closed and tight. Main selection of a POPRV is to enable the user to work closer to the set pressure without leakage. If that pressure is not high enough, the maximum sealing action diminishes and the valve leaks. Once a leak starts, it does not go away. It simply gets worse. POPRV's have elastomeric seals in construction. Ultimately these need replacing at service intervals. We can look at other issues that can cause leakage but full information is required.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[MortenA]

All safety valve leak a little i believe. Especially if the valve has lifted then the seat may get damaged.

--- Best regards, Morten Andersen

 

[KernOily]

Thanks guys. Just to clarify, the leak sniffer/camera is seeing a leak coming from the pilot and not from the valve body or seat. The valve itself is not leaking. I am working on getting a photo of the valve and I'll post it upon receipt. Thanks.

 

[shvet]

Pilot in pilot-operated valve is just a tiny spring-loaded valve. It behaves the same as all other spring-loaded valves do and starts leaking when force balance across seat becomes too low.
As per API 527 all PSVs (including pilot-operated) are allowed to leak somehow. Leakage arbitrary limit for PSVs (including pilot-operated) has been defined at 90% of set pressure. This 90% point does not mean that leakage is not able to occur below 90%. See API 527 for details.
Some manufacturers can assure higher seat tightness class but you should consult a particular manufacturer. Note that for this reason bursting disks exist.

All fittings/valves/seals are leaking. Even weld joints/seams are leaking. The leak rate is the only question. What are you looking for? Absolute tightness?

 

[KernOily]

shvet - thanks. Not sure I 100% agree with 'all fittings leak' based on 36 years of working in the process industries, but we'll set that aside since it's not germane to this situation.

A leak survey via camera/sniffer revealed a leak from this pilot of a few thousand ppm. I need to either (1) fix the leak or (2) prove to the authorities that the leak is part of normal operation of the device; hence my question as to whether a pilot is supposed to leak by design. I have a lot of experience with conventional PSVs but not so much with pilot-operated. Thanks.

 

[The Obturator]

If you would tell us the type of pilot, then we could answer you better. Either that or ask the vendor. Note that pop/snap acting pilots will vent to atmosphere at time of overpressure relief (ie., the fluid inside the pilot and on top of the piston/dome, keeping the valve closed, will be vented to atmosphere). As mentioned earlier, some designs are also flowing pilots. I suspect your pilot has elastomers that need replacing, but without the info requested can't comment more.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[KernOily]

I'm trying to get the info on the PRV. I'll post back here when I get it. Thanks.

 

[shvet]

Not sure I 100% agree with 'all fittings leak'

If so then vacuum generators would not needed, correct? All those ejectors, vacuum pumps etc. Barometric condensers and proper fittings would be enough.
The same for VOC air pollution around a process side - there are no sources of major leaks but pollution is continuously being reported.

Leak rate is the only question.

 

[Latexman]

Leak rate is the only question.

Yep. My brother, a diesel mechanic, says, "everything leaks, but only those that puddle up, make a mess, or cause problems get attention".

Good Luck,
Latexman

 

[KernOily]

Touché. What's a few ppm between friends, anyway.