Double piston effect ball valve 1

[svi]

Ref:

http://www.eng-tips.com/viewthread.cfm?qid=205580

Although the seating configuration appears to be the same in the Velan catalogue. How is that the single piston effect is able to seal from either direction and also be able to be self relieving, whereas the DPE is non-self relieving?

A client spec calls for the following,

Seat for all ball valve shall be of double piston effect seat design.

Safety relief valve for body cavity relief shall not be permitted.

Supplementary cavity relief when required for DPE type seats. What will be the alternative to not having a safety relieve valve for the cavity relief?

 

[tr1ntx]

Most double-piston effect valves will relieve cavity pressure at a certain level. They are not all non-self-relieving. For instance:

-There is a valve in service with a fluid that does generate vapor pressure
-The operating pressure is 150 bar

When the valve is closed, the downstream pressure drops to 10 bar. Many DPE valves will relieve cavity pressure at 15 to 25 bar above the low-side line pressure, so in this case would release cavity pressure down to 35 bar, or possibly as low as 25.

"Safety relief" and "cavity relief" are not necessarily the same thing.

Are you an engineering company who is doing the layout on a plant or are you a valve manufacturer?

 

[svi]

At least 2 of the literature that I have looked in says that DPE seats additional provisions for relieving the cavity pressure. Will that mean that the seats can also do the relieving, if specified.
I am from an engineering company doing the buying specifications for the valve.

 

[tr1ntx]

The seats can do the relieving, if so designed, and if the pressures are acceptable. They won't relieve down to zero pressure in the cavity.

Where they said "Safety relief valve for body cavity relief shall not be permitted" they probably meant no pop-off type relief valve venting to atmosphere. I suspect they would require a valve venting back into the process piping somewhere. Possibly even a controlled valve and not a setpoint relief valve.

 

[Nilsoni]

Look into a trunnion ball valve with one side DPE and one side SPE. This become self reliefing and need no bleed or vent. No extreme pressure build-up in cavity is possible.