Fusor Forums

My gas admittance system consists of some copper tubing and Swagelok fittings connected to (from the chamber side of things) a micrometer needle valve and then a Swagelok/Nupro SS-4H valve. This valve has a stainless-on-stainless seat and a stainless bellows bonnet, and AFAIK it is welded together.

I discovered that even when tightened this valve has a slow inlet to outlet leak, just enough to keep my chamber in the annoying fractional micron range.

Is there anything I can do about this valve, or must I buy a new one? Vacuum grease as temporary solution?

If the bellows is leaking (likely because that is a common failure point) then the item is junk. If the o-ring is damaged, that can be replaced if there is sufficient room to access that seal.

It's not a leaking bellows. it's leaking past the seat. There are no O-rings in this part.

(It's a small 1/4 inch valve.)

Have you tried compressed air to clear the valve seats? If that alone does not do it - maybe try aggressive and warm solvents (maybe soak over night) that are then followed by compressed air to try and clean the seating surfaces?

I tried compressed air, need to test it again though.

I must admit, I have never seen a metal on metal valve seat in a belows valve. There is always an elastomer O ring seal.

Richard Hull

The swagelok SS-4H is a top end, all metal valve, with through leak rates at 4e-9 cc/s when closed for a new valve. Used valves on ebay typically have leak rates still in the 1e-7 cc/s or lower. Eventually the seat does wear out; due to the welded construction, there is really no way to fix the valve.

It's definitely a good choice in valve though, I use one on my fusor to seal off the gas admit and vent to atmosphere ports. I'd recommend just getting another one.

Datasheet

I was poking around one of my 20 year old Lesker catalogs today and found the offering then of two all metal to metal seal valves. They claimed it used a copper bonnet seal ring and that it was replacable. Not a cheap valve. Back in 1999 the vlave was about $900.00. Ouch.

I'll stick with my fully refurbed Duniway varian elastomer gasket seal bellows valves at the bargain basement price.... I paid $375.00 each and bought three back in 2004.

I did pick up a precision sapphire, gas leak valve a this year's HEAS. 20 years ago Lesker was asking $1,145 and when new and claimed a shutoff operating pressure of 10e-12 torr. Mine is mounted on a 2.75 conflat with the inlet on a 1.33 conflat. Got it for $25.00!!! I really made out at the HEAS in vacuum gear. Others apparently failed to see at sight. (Something I have written about here before.)

Richard Hull

By "bellows valve", are you thinking of these things:





Because this is an SS-4H:





This valve has a body made of ordinary SS 316, a valve stem tip made of hardened stainless steel, and the valve bonnet (including both sides of the bellows) is all welded together. After further examination I do not think it possible to disassemble without "creative" (i.e. destructive) methods. List price of this valve is a not-too-crazy $180 USD.

The only metal-metal seal is the hardened stem tip pressing into the non-hardened body seat feature.

With a combination of compressed air (to remove particles), mechanical over-torquing (to try to brinell the seat into better condition) and vacuum grease (the sealant of scoundrels) I managed to get it to seal up tightly. Who knows if it will work repeatedly...

The valve in the photo appears as most such bellows valves, easily disassembled. The top screws allows for removal of the bellows unit and top half of the valve. This allows the vacuumist to clean the seat within the body of the valve and replace the seal gasket on the base of the bellows. A cracked or defective bellows is, often, the end of the valve.... The valve shown, looks pneumatic.

Richard Hull

The valve I have is the one in the second photo.

It has a bellows, but no screws.

Glad it is sealing again with vacuum grease (better than throwing it out!) This tends to indicate the seat surface might have issues but if the grease works and you can tolerate its presence, why not, then?