Valve Cavity Pressurisation 1

[Bazinga]

We have some ball valves that were leak checked during commissioning by shutting the valve (typically with pressure on one or both sides)by depressurizing the cavity and then checking the cavity vent for if either of the seals are allowing gas to leak through. Now assuming the test went fine and no further gas is detected once the cavity has been vented, the valve was then opened again under pressure. My question is, will the valve cavity become pressurized if you open the ball under pressure? And if so, does that mean we should be venting all the cavities again after opening to prevent trapped pressure?

 

[CptMaaB]

More details please.
What type of ball valve do you have, floating ball, trunnion mounted, single or double piston.
What kind of material is used for ball and seats, and so on...
I assume there is no vent bore in the ball, because of the tests you already made.

 

[Bazinga]

No vent bore in the ball. These are trunnion mounted double piston valves with ptfe seats. So no pressurisation of the cavity should occur when the valve is closed after its been vented.

 

[Danlap]

Your initial questions:
Will the valve cavity become pressurized if you open the ball under pressure? Yes for sure, valves in partial open position will exposed its cavity towards (pressurized) upstream and/or downstream.
And if so, does that mean we should be venting all the cavities again after opening to prevent trapped pressure? This question is a quite vague for me. Why do you need to vent cavity every time? it is designed to handle such pressure. Or maybe there is some details that have not yet shared e.g. subject for temperature fluctuation (for example trapped pressure was in cold state e.g. water and then it will be heated up as superheated steam); subject for polymerization, etc.
If that is the case, then IMO you're using the wrong type/design of valve. Simple solution is add cavity hole on the ball (facing upstream). Not so simple is to add Automatic Bleed valve (ABV) on the cavity's bleed nozzle. ABV is quite common design for DBB Plug valve.

My personal take-off:
- For not so clean medium, PTFE seats would not last long (worn out, scratches, etc.), hence sooner or later there will be direct leak path from upstream to cavity.
- Manual venting (by operator) every time after valve is operated, is not fool proof method. People may forgot, and also in the event seat(s) already compromised then the leakage which is being vented will not stop.

Regards,
Danlap

 

[LittleInch]

Agree with Danlap.

Yes, of course it does - have you ever looked at or considered what happens with the valve 50% open??

Do you vent cavities?

Usually no. Reason is most valves are single piston effect so excess pressure in the cavity higher than the low spring force ( typically 5 bar) just vents to the downstream or lower pressure side.

Even if you've got DPE seals, the pressure rise is really quite low for gas as the temperature increase is based on absolute temperature so the percent increase in pressure from ambient / solar is really quite low.

Trapped pressure is totally normal so no one I know vents these down unless you're trying to use it as an isolation check or check on whether the valve is passing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bazinga]

Ok thanks all! Appreciate the responses.

 

[LittleInch]

Liquid is different for Double Piston Effect (DPE).

Then you need thermal relief.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.