Trunnion Ball Valves in Vacuum Service

[aggieengineerrmb]

Can a standard, soft seated trunnion ball valve be used in a vacuum service? I hear that these types of valves often have trouble sealing bubble tight at relatively low pressures, say 80 PSI, and if true it seems like vacuum service would only compound the problem.

The other side of this would be while the upstream seat that normally seals with positive pressure might be compromised in a vacuum, it seems that the downstream seat would be helped (vacuum would pull it into the ball making an even tighter seal).

Just curious if anyone has some real world experience, or knows of manufacturers who have been successful in the past.

 

[zdas04]

I think you've been seriously misinformed. Floating ball valves seal by the dP shifting the ball into the downstream seat. Floating ball valves can occasionally have trouble with less than 1 psid (not 80 psi).

On trunion ball valves, the only seating mechinism is the springs that push the seat into the ball. I've used them in very low dP service and vacuum service and I can't point to a valve that has a better chance of long-term success in vacuum service.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[aggieengineerrmb]

zdas04 - Thanks for the reply, but I don't think I agree with you 100%. While it's true that most trunnion valves seal upstream initially as a result of a force pushing the seat into the ball exterted by a compressed spring, the seat seal is also affected by the dP across the seat. The higher the dP, the harder the seat is forced inot the ball. In fact, at full rated pressure of the valves, the force exterted by the compressed springs is only a tiny fraction of the force exterted by the dP.

I still have the concern about sealing bubble tight at low pressures (less than 80 PSI). Once you get above 80 PSI, the seat presses so hard into the ball it will almost always make a tight seal unless it is seriously damaged, but below that it may be tough to get a perfect seal, and you will have a slow leak across the seat. My concern is a vacuum will only make this worse by couteracting to a degree even the small force of the compressed spring.

 

[zdas04]

The arithmetic I've seen from two different ball-valve manufacturers has indicated that the proportion of seating force provided by dP is negligable compared to spring force. I've verified this by measuring a surrogate for torque (i.e. gas pressure on the actuator when the valve started moving) on big trunion valves with zero dP and with nearly max dP and the difference was a very small proportion of the total torque.

You have to keep in mind that the dP on a trunion ball valve is actually the difference between the upstream piping and the trapped gas within the ball, and between the trapped gas and the downstream piping. One is helping the springs and the other is opposing the springs. The net can be pretty small.

On the other hand I've had situations where I had a high pressure on both sides of the valve with the body cavity vented. This should yeild maximum dP across both seats, and in that case the gas required to start the ball turning was about the same as any other time.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"