Body cavity presure relieving

[TransMech]

Hi all,
I'm looking for some instruction or idea regarding body cavity pressure relieving in ball valve. what kind condition is requested body pressure relieving? If the valve is buried, be below ground, whether it is necessary to apply relieving body cavity pressure in liquid service?
Thanks.

 

[pennpiper]

Oleyln,
What is your background and experience?
What is your title, assignment, or position in this situation?
What kind of plant is this valve in?
Where is this plant?
What is the commodity in this line/valve?
What is the Operating Pressure & Temperature?
Will this valve in fact be buried or is that an alternate choice?




prognosis: Lead or Lag

 

[TransMech]

Thanks,Pennpiper.
I have a few years pipeline design experiences, the valve will be used to transport crude oil in Alberta, be buried due to low TEMP.
Operating Pressure and Temperture is 1200PSI/45C.
Thanks.

 

[Kreot]

Hi,
Could you say abit more about the purpose of the valve? Pure on/off? Use it for maintenence of equipment? Size of the valve?

 

[LittleInch]

Two things:

1) Unless your valve is a double piston valve, the body cavity should relieve via the lower pressure side through the seats so you don't need to worry about it.
2) Buried valves don't normally change in temperature enough for a liquid to reach a pressure that causes any problem on the basis that your valve is actually sealing from pressure on the inside of the valve when closed.




My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TransMech]

Thanks guys, very appreciated.
The valve will be used to isolate pump station deal with Emergency shout down and maintenance, 36inch. I plan to apply a SPE type(DIB-2)for this purpose, it is DPE X SPE. I am nervous to apply DPE x DPE C/W external pressure relieving valve for buried. But the body cavity over-pressure will be relieved to high pressure side (main line) if valve closed. Any comments please.

 

[LittleInch]

That should be Ok. Your valve pressure rating will be equal or higher than your pipeline design so will not cause an issue. DPE seals both side do cause issues with respect to potential cavity overpressure, but if it's buried, where is the extra heat coming from to create a thermal pressure increase?? If you decide you want a TRV, make sure you bring it to the surface before fitting the TRV - makes it easier to deal with / repair / monitor.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Kreot]

I agree with littleinch.

We use the DIB2 valves to enable maintenence of equipment, but i always use the body cavity to monitor the first barrier(SPE) leakage as we do have a strict requirement for a double barrier. In addition i sometimes use the cavity drain to pump some pressure to the cavity for a safer barrier of the DPE seat.
A few weeks ago we had a small leakage on a SPE/DPE valve, i then used the cavitydrainvalve to shock the valve because of dirt behind the floating seats. It worked just fine.
But of course, i see a problem doing this with the valve buried in the ground.

 

[TransMech]

You guys both gave exhaustive description, thanks.
The valves must be buried, I could not imaging any possible reason of potential cavity overpressure, otherwise must be applied with DPExSPE.

 

[TransMech]

Stiaf,
Would you please advise how did you use cavity drainvalve to shock the valve?
Thanks.

 

[Kreot]

Hi,
Yes, the valve was still leaking a bit after cleaning and lubrication of the seats.
To shock the valve you need to rapidly realive the cavitypressure through the cavitydrain valve. This might loosen the seat that still not loosened from the seatpocket.
Just note that it doesn't always work

 

[Dean8573]

Stiaf,

when you mean shock the valve are you referring to applying a differential pressure test? i would have thought that venting the cavity quickly could a can lead to o-ring / lipseal extrusion issues. The cavities on DPE seat x DPE seats should be fitted with a cavity PSV to prevent cavity over pressurisation, obviousally if the valve is burried this can be an issue however i have seen the cavity port fitted with a PSV which then is piped to the outlet.

I would imaginr that DPE seats are not required and as per design the seats will be cavity releif however if you are still worried you could look at drilling a cavity releif hole from your ball to the body.

 

[Kreot]

Hi,
We do not have systems requiring DPE X DPE. Always use SPE X DPE where the SPE seat work as a overpressure cavity reliev. Not necessary with a separate PSV from the cavity.
Not excactly sure what you mean by differential pressure test, but when i schock the valve i normally use 15-20barg cavity pressure (a bit dependent on the upstream pipeline pressure, but at least 50% lower than the pipeline pressure).
The lipseals do not have any problem(never experienced) to withstand such shocking, in case i would say it is a bad design.