3 replies to this topic

[black friday]

Hi All, 

As described above I have a situation where I'm replacing a large PSV on a helium relief tank. I've purchased 3mm viton gaskets as per the material spec for the piping class. Based on the service criteria I think Viton is a perfectly suitable choice. However it appears a teflon gasket is what is actually installed, and this is causing grief for me because we can't possibly install a gasket that's different to what's in the plant, even if it fits the material spec.

So I'm just asking if anyone else could comment on the suitability of viton and teflon for this service. I think there's no reason why I can't use the viton gaskets, with 100/150 NM torque for the 2/3" lines respectively. 

Cheers!

[Art Montemayor]

Whether it's a relief valve, pump, pipe flange, or anything else, when you’re dealing with helium you are dealing with a tough fluid to contain.  It is used as the index fluid in calculating leakage or permeability.  Therefore, you have to identify what are the conditions of the fluid – temperature and pressure.

 

Teflon is one gasket material I gave up on 5 years after it was introduced.  Teflon has no “memory” – once under stress, it will not resume its former shape.  This feature alone defeats it as a desirable gasket material.  What you call Teflon is probably a combination of other materials with it – such as glass fibers, metal, or elastomers.  

 

Whether Viton works depends on the application.  I would expect it to work – at least for a while.  Viton is noted for its applications where you have “aggressive” fluids – corrosives, acids, bases, etc.  For helium, I would apply a metal gasket, such as nickel or stainless steel.  However, the QUALITY of the joint – the flange finish, the alignment, the correct torque, the cleanliness, the temperature, and no vibrations – plays a large role in the ultimate success.  The material is just a small part of the solution.  After all, helium is going to leak or permeate through anything – whether you like it or not.  Just like hydrogen, because of its molecular size, it will ultimately escape.  So careful attention to the quality field application is important.

 

The pipe “class” means nothing to identifying the correct gasket material especially when dealing with a tough actor like helium.  How you arrived at identifying Viton as the correct one is something you haven’t explained.

[black friday]

Hi Art, 

When I refer to "class" of the piping I mean the piping specification we use internally. This listed viton as the appropriate gasket material for all the pipework used for the helium used as a cryogenic process fluid. 

The helium itself in the relief tank is typically ambient but can range down to 8 Kelvin in some parts of the circuit, so conceivably in a relief scenario this cryogenic helium could be discharged through the relief tank. The pressure in the tank is limited to 2 barg by the PSV and a control valve. 

Given it's a low pressure  I was going to go with a 3mm thick gasket as well. Would you advise this or a 1.5 mm gasket?

The other suggestion I've had is to use a trade name Gylon 3510, which appears to have good specs based on the data sheet, but is a PTFE base. 

[Art Montemayor]

When it comes to gaskets, I always apply what my mentor taught me:  use the thinnest gasket you can get by with.  This lessens the radial stress on the gasket and makes the seal more effective and stronger.  It makes sense.  As I stated before, a lot depends on the quality and finish of the flange face and its trueness and alignment.

 

I am uploading a copy of Elastomer information from an old Fisher Controls catalog that I still retain for such information.  Note that Teflon is not even mentioned.  I wouldn't use it.

 

I hope this information helps.

 Elastomer Information - Fisher Catalog 501.pdf   3.48MB   18 downloads

 zruelast-fpm-designing-with-fluoroelastomers (1).pdf   762.6KB   19 downloads

 Tightening Evaluation of New 400A Size Metal Gasket.pdf   462.32KB   14 downloads