Sealing material for Cryogenic test
Sealing material for Cryogenic test
(OP)
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
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





RE: Sealing material for Cryogenic test
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.
RE: Sealing material for Cryogenic test
RE: Sealing material for Cryogenic test
May not suit every pipe end or flange, but has been used to address a whole variety of problems...
RE: Sealing material for Cryogenic test
May not suit every pipe end or flange, but has been used to address a whole variety of problems...
RE: Sealing material for Cryogenic test
Thanks to all, 'NGL
RE: Sealing material for Cryogenic test
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.
RE: Sealing material for Cryogenic test
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