ball valves BS6364 with cavity provision relief

[fcochet]

Hi,

One of our customer ask us ball valves for cryogenic service according to BS 6364 and he mentionned that valves must have a cavity provision relief.

Does anybody know what it is because manufacturer doesn't know.

Tahnk you for your help

 

[gerhardl]

Being no cryogenic expert I am fairly sure that this is to ensure that gas (fluid) is not trapped in the body cavity behind the seals and giving overpressure and gas leakage after the line is else emptied or under lower pressure.

I am not sure about the practical solution (block and bleed or inside automatic over/past seals ?). I am sure, however, that if your manufacturer has not supplied for this type of cryogenic service before, and has inadequate experience or knowledge to check out this requirement himself, I would'nt go with this supplier for such a project.

 

[Gator]

It is possible that cryogenic liquid gets trapped in the ball cavity when the valve is in the closed position. This can cause big problems when the line warms up, so often a vent hole is drilled in the ball on the low pressure side.

 

[SteveWag]

Very common on systems using chlorine gas, vent to the low side. Just a small hole drilled and champfered.
Steve

 

[dcasto]

True DDB valves have a hole drilled and tapped into the body between the two sealing surfaces. There is another tap drilled to the open side of one of the seals. A piece of tubing with a check valve in it is used to connect the two taps. If the valve is shut pressure build up between the two seats is relieved via the check valve see:

http://www.c-a-m.com/cam/search/showdocw.cfm?DOCUMENT_ID=7345

 

[Compositepro]

Cryogenic ball valves have a vent hole in the ball and an extended bonnet so that the valve handle does not get stuck in the ball of ice that forms on the valve. Since ball valves are designed to seal against the downstream seats the vent hole should be on the upstream side of the ball.

 

[Gator]

<i>"Since ball valves are designed to seal against the downstream seats the vent hole should be on the upstream side of the ball."</i>

Could you elaborate on that?

 

[Compositepro]

When a ball valve is in a closed position the upstream pressure is pressing the ball away from the upstream seat and toward the downstream seat. Seals can be designed to compensate for this but in ball valves the down stream seat will always seal more reliably than the upstream seat, particularly after some wear. A vent hole bypases one of the two seals in a ball valve. It is not the downstream seal that should be bypassed. Below is a link to a brochure on cryogenic ball valves.

http://www.controlandpower.com/catalog/PDFs/Worcester/Worcester Cryogenic BV (SB 442-19).pdf

 

[Gator]

Compositepro, what you write makes sense but weren't we discussing venting of the ball cavity and not seating/sealing issues. I think the original asker was wondering about this, not how well a valve closes.

After more than a few years' cryo design (cold boxes, warm end, customer sites) experience I've never seen a ball valve specified by the provider. It's all extended stem (as you correctly state, but forgot to mention that these valves are buried in perlite or under 3"+ of insulation if not VJ-ed), bonnet pressurized globes for control on the liquid side and butterflies for the "warm" gases.