FKM Oring for CNG

[raheeli]

I am using a VM835-75 oring for a high pressure CNG application. Problem is at -40 C temperature oring is unable to hold pressure and leaking. Failure happens only if i soak the part at -40 C under 1000 Psi pressure than after one hour increase pressure from 1000 to 2000 psi. If i soak it at 2000 psi to begin with it doesnt fail. whats going on? VM835-75 is supposed to be good for -40 C.

Thanks

Raheel
Product Engineer

 

[hydtools]

It may be that the gland surface finish is not good enough.

At 2000 psi the o-ring is forced into the gland surface irregulaities, whereas at 1000 psi it is not and the seal material is too stiff at -40C to further conform and will not pass the subsequent 2000 psi test.

Ted

 

[RolMec]

As I understand it: if you soak firstly under 1000 psi your application is working tight, but then when pressure increases the Oring fails. So as hydtools said: Oring "sets" at 1000 psi and can't follow up the increase of gap etc., esp. at -40°C. This would call for a pulsating pressure application of Oring , you would like to check back with your supplier. Think it could be done by choosing harder material of Oring + do finer machining of the mating parts.
Regards

Ron

 

[raheeli]

Thankyou for your replies. I repeated the test by soaking the oring at no pressure at -40. After one hour i increased the pressure to 1000 Psi and it still failed. I repeated the test with nitrile orings and it passed. Parker says VM835 is good for -40 but looks like its not the case.

 

[dgallup]

Are you familiar with the ASTM D2000 or SAE J200 classification for rubber materials? It is the best way to explicitly state what you need from an elastomer rather than calling out some proprietary compound that may or may not be up to the task. For example:

SAEJ200 M4HK710A1-11B38EO78F17

That is about as good as you are going to get for low temperature sealing from FKM. It is rated for -40C but I still doubt it would take a change from 1000 to 2000 PSI.

I think you should really consider using FMVQ, it is easy to get good sealing well below -40. I would look at specifying something like:

SAEJ200 M2FK606A19EF31E036F19Z1Z2
Z1 = 70±5 Shore A Durometer Hardness
Z2 = 8 MPa min. Tensile Strength

The F19 gets you -55C.