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floater ball valve for vacuum service

floater ball valve for vacuum service

floater ball valve for vacuum service

(OP)
I searched for similar questions but i couldn't find a clear answer.

I have a 1" floater ball valves size and the operating pressure will be (-15) psig on one side and atmospheric on the other side

is this going to work? thanks in advance.

seats are teflon
and stem seals type is packing

RE: floater ball valve for vacuum service

You need to be careful with vacuum as you rightly say there isn't a lot of differential pressure to play with.

I believe there are specific valves used for vacuum service which are not standard floating ball design.

http://www.sharpevalves.com/pdfs/Engineering/SPD01...
http://valveproducts.metso.com/documents/jamesbury...

It does seem to vary on exactly how low your vacuum is.

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

RE: floater ball valve for vacuum service

(OP)
the manufacturer said the valve was qualified for 10 millibar differential, does that mean the lowest differential pressure it can operate under is .145 psig ?

so if the operating pressure is -15 psig absolute it doesn't matter, all it matters is the differential pressure because that's how the seat gets energized?

RE: floater ball valve for vacuum service

That is correct, dP rules. Having said that, I've had a lot of problems with infrequently operated floating ball valves in vacuum service. It seems like the seat wants reach a lower entropy level and acts like the seat is rotating very very slowly against the ball. Any trash on the ball starts them leaking for no apparent reason. With the price of 1 and 2 inch ball valves, I tend to install two in series--for initial depressurization I leave the inner valve open and shut the outer valve. Once the system is about half way to a vacuum I shut the inner valve and put a plug in the line. This process has always worked. When I couldn't cap the line the process has worked about 3/4 of the time. A single valve has worked about 1/4 of the time.

Vacuum with dP actuated floating ball valves is a very counter-intuitive undertaking. If you see a situation that just makes no sense (and you likely will), don't ask "how did this happen" ask "where can I apply a(nother) band-aide. I've spent days and days of my life asking "how" and always ended up asking "can I patch this mess and get on with my life".

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: floater ball valve for vacuum service

(OP)
i am wondering how the floating ball valve will seal at operating differential pressure of .145 psig ?
basically the difference between inlet and outlet pressures is the .145 psi ? but in this case there will hardly be any flow? and is that enough pressure to push the ball against the downstream seat?

another thing is if the guage pressure at inlet is .145 psi = -14.5 psia , then the pressure is reversed?

RE: floater ball valve for vacuum service

I'll just add my two pence worth. I do not have the experience with using but making valves. If you are looking at 1 & 2" valves, I would suggest that you are looking at 3 piece, bolted construction? These are generally rated anywhere from 60 to 138 bar, so the pressures in a vacuum system are not going to get the floating bit happening to any great degree, you will really be relying on the crush of the seats. Therefore I would be more concerned about the environmental seals, these are generally designed to stop the pressure getting out rather than in. The same goes for the stem seals, generally the pressure is pushing up on the skirt of the stem, often assisting with the sealing. Not in this instance, it isn't! Therefore I would suggest that you look for pure PTFE seats and seals - I assume that fire safe is not a requirement? Ideally the body seals should be a 'tongue and groove' type to ensure adequate compression on the seal and to stop it moving (in!). I would also suggest that a stem with a stem nut and spring seal energizer would assist, this means that you can ensure the the lower stem seals remain in contact with body and stem, no pressure related forces required. Lastly the surface finish - and fit - on the stem is quite critical, the more highly polished the better, but not at the risk of taking it out of round.

I take the experience that ZDAS04 gave, viz once you loose surface finish on the ball, game over. So this gets back to how often you operate the valve. In a high cyclic operation I think that you will be changing out valves on a regular basis. The crush on the seats will not last for ever. You could look at harder seat materials but that will also make getting the seal more difficult.

Good luck, be interested to hear how you go!

RE: floater ball valve for vacuum service

PeterIgg,
When we first took a natural gas field on vacuum, stem seal leaks were a major problem. The only way we were able to identify them was with a "bee smoker" (you know the devices that bee keepers use to quiet the hive while collecting honey), and we found that many stem seals on older valves leaked. New valves often had to have the stem seal adjustments tightened, but almost all of them could be made to hold. With the older valves, we replaced the packing/o-rings and were generally able to make them tight. Virtually all of the valves in our field were three piece threaded construction. We didn't ever see a leak in the body threads.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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