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Cryogenic Sealing

Cryogenic Sealing

Cryogenic Sealing

(OP)
I need some help/advice on the methods used in cryogenic sealing.  I am in the process of designing a cryogenic connector (40 bar) for use with liquid oxygen, nitrogen and argon, and need some advice on the best sealing interface to use.  My all brass connector has a 'nut' on a series of ball bearings being pulled up on a flange via a special spanner.  At present i have used an equilateral triangle cross section virgin ptfe seal clamped between two taper's which works ok at ambient but leaks at cryogenic - when the seal gets cold!! What other cross sections are used in cryogenic sealing to better allow for the shrinkage and are there any other materials i can use?  Are there standard seals out there that i can incorporate into my design?

RE: Cryogenic Sealing

I take it you're refering to some kind of piping connection, ie: tube to tube or pipe to pipe joint?  Or are your refering to something else?  I'm confused by the part about ball bearings.  How large is the connector?

It sounds like you're using a triangular Teflon gasket, probably inside a gland, similar to an O-ring gland.  Is this correct?  If so, I'd suggest using a copper gasket.  Anneal it first.  Ensure contact stress is on the order of 10,000 - 15,000 psi so that the copper gasket deforms when installed.  That's just a rough number, something well above the 6000 psi yield strength of annealed copper.  You'll have to calculate pressure loads and bolt forces of course.  The contact width should be about 0.030" to 0.050", and be tapered so it's only that thin right where it makes contact with the flanges, but gets thicker behind this contact face.

Another option is to use a spring loaded Teflon U-cup seal.

If it's pipe, you can use pipe threads.

For tubing, most tube fittings are acceptable, ie: Swagelok, Parker, Flared fittings, etc...  

Hope that helps.  Perhaps a bit more info would help me to understand what you're doing.

Thanks,
Dave.

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