Pressure spike when cycling gate valve

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ChristofferBraestrup
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Pressure spike when cycling gate valve

Post by ChristofferBraestrup »

I recently bought a cute little Leybold/VAT kf40 gate valve.

Condition looks pristine, and I'm sure the gate seals fine. There is however, a pressure spike every time I open or close it, jumping from about 10^-6 to 10^-5 torr and back.

The gate is actuated by a linear rod being modern in or out by the handle, so I assume it is just o-ring sealed, and no bellows.

Could one lubricate this shaft with a few drops of diffusion pump oil? It seems to have been oiled before, unless that oil residue is just contamination.

What are your experiences with manual gate valves? Should I try disassembling the rod seal, or accept it as it is?
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Dennis P Brown
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Re: Pressure spike when cycling gate valve

Post by Dennis P Brown »

I have a manual gate valve and the shaft uses no oil or vacuum grease; these are generally a bad idea since they are just contamination. So if the o-ring seals are good, oil/grease is of no use. If the o-rings are bad then vacuum grease can improve the performance but at a cost to how clean the system remains. A jump in that range is trivial for a fusor.
ChristofferBraestrup
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Re: Pressure spike when cycling gate valve

Post by ChristofferBraestrup »

Getting to the axle seals would require punching out multiple pins. I was given a bit of Santovac-5, applied to the shaft, and this easily gave me an order of magnitude in the better direction.

When a shaft is slid from outside air directly into vacuum, I guess there will always be a pressure increase, as by definition the surface moisture needs to flash off.

It's also difficult to diagnose, as the valve casing is a large and strange shape, so it will take quite some time to pump out anyway.

Thanks for the advice anyway!
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Joe Gayo
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Re: Pressure spike when cycling gate valve

Post by Joe Gayo »

If the valve seat is biased to one side, you could try putting that side to the main chamber and have the shaft side towards the pump. That might help.
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Joe Gayo
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Re: Pressure spike when cycling gate valve

Post by Joe Gayo »

I think bellows sealed is the only way to go...
Peter Schmelcher
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Re: Pressure spike when cycling gate valve

Post by Peter Schmelcher »

I have a Varian butterfly valve that uses an o-ring shaft seal. I never rotate it because if I do it looks like a virtual leak for a day or so. Vacuum grease reduces the problem but a no o-ring system is better if you want a high vacuum.
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