Sealing material for Cryogenic test

[NGiLuzzu]

Dear All,
we need to seal two test caps against the buttweld ends of a 12" class 300 cryogenic valve and cannot use our usual SWGs: the slot for the gasket is 5.5 mm deep, 8.0 mm wide and inclined respect to the flange flat-face plane of about 40°; the relevant mean diameter is about 315 mm.
We are now using a 8.5 mm square cross-section graphite braid as a seal, but it leaks at ambient temperature and relatively low pressures. It should be tight up to 30 bar of internal helium pressure and down to -196°C temperature.

What kind of sealing material and cross section would you suggest?

Some websites give Virgin PTFE, for example, as appropriate for low temperatures down to -160°C, others give it as appropriate for cryogenic temperatures down to -196°C or -268°C... what do you think about it? And what about fiber reinforced or filled PTFE?

Thanks to all, 'NGL

 

[iainuts]

I'm sure there are as many ideas on sealing this as there are people making suggestions.

Generally, any Teflon product is good to temperatures of absolute zero. In fact, I've found many elastomers are also good despite the fact they have very high glass transition temperatures. So long as relative motion between parts being sealed can be removed, most materials should be ok.

The problem with most elastomers and plastics is the relatively large amount of shrinkage at low temperatures when compared to the surrounding metals. The result is a loss of contact stress at the sealing surfaces and leakage. Leakage often causes further drop in temperature and/or erosion of the seal resulting in yet more leakage.

My preference in designing sealed joints has been either copper or other soft metal gaskets or U-cup shaped, spring loaded seals made of filled Teflon.

For metal (copper) gaskets the contact stress is important, as it must be maintained throughout the temperature range the joint experiences. Bolted flange joints are typcially good at maintaining the contact stress.

U-cup seals are probably the easier type for most applications as the seal manufacturer will give you all the relavent information needed to ensure a leak free joint.

I'm looking at what you wrote and it sounds like you have a gland into which a seal will be placed, a fairly large gland. If this is the case, I'd personnaly suggest the copper gasket because the gland is so large. I'd fear that filling the gland with Teflon or other plastic/elastomer material would leak without having the benefit of a spring loaded seal.

 

[marklobo]

ECTFE (Kel-F) is the material of choice for our cryogenic soft-seating valve trims. It exhibits low creep and good properties at very low temperatures (we've applied it at 18 degrees K).

 

[sterl]

Although some inspectors object to it, we have loaded gaskets with pneumatic cylinders instead of bolts and found all sorts of sealant materials work very capably at low temperatures and high pressures...PTFE included. Original clamp and disk arrangement was in fact a modified Butterfly Valve. Cylinder was pressurized with nitrogen, which was also the test fluid. Much easier to handle and devise than spring-loaded seal.

May not suit every pipe end or flange, but has been used to address a whole variety of problems...

 

[sterl]

Although some inspectors object to it, we have loaded gaskets with pneumatic cylinders instead of bolts and found all sorts of sealant materials work very capably at low temperatures and high pressures...PTFE included. Original clamp and disk arrangement was in fact a modified Butterfly Valve. Cylinder was pressurized with nitrogen, which was also the test fluid. Much easier to handle and devise than spring-loaded seal.

May not suit every pipe end or flange, but has been used to address a whole variety of problems...

 

[NGiLuzzu]

About the problem of butt-weld ends sealing, please take also a look to Thread408-89368 within this Forum...
Thanks to all, 'NGL

 

[valveguru]

Sealing at cryogenic temperatures even with teflon or KellF presents issues. You are better off using thin sections instead of thick section. Old ANNIN split body valves used .032" thick sheet gasket compressed to .018/.020" thick and readily passed cryogenic testing at these temperatures. However, it was important to have a metal to metal compression on the metal body components in order to pass without leakage. Surface finish also needed to be 16rms or better.

You did not mention the leakage requirements. If you are trying to pass mass spectrometer testing with leakage tests below 1e-5 CC/sec sniffer rates, you may need to upgrade the seal. Try a company called HELICOFLEX, which make a spring energized/sealing metal C ring. The spring loading provides both enerization and loading effect for seal, and does not rely on pressure. Since it is all metal, differntial expansion problems are eliminated. You can get jacket materials and spring loading as necessary. I have seen them used on 8000 psi liquid hydrogen systems.

 

[NGiLuzzu]

Thanks to all for the interesting answers!!

Just for your information about energized gaskets (like the ones suggested by Valveguru), please take a look at Thread774-82259 within this site.

Regards, 'NGL