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metal to metal face seal 2

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pedrof

Mechanical
Oct 19, 2004
56
I have a standard fitting from a supplier which we need to fit to an aluminium jacket, once it is in it is staying there.
Their fitting is stainless steel and has a viton o-ring in a groove to face seal against whatever body it is threaded into. The o-ring stretches over an M10 thread into a groove on the fitting body. There is a reasonable flat surface area that a copper washer could seal on.
We need to rate the seal we design to 250°C so I want to use a copper washer as a gasket to overcome temperature rating.
It is a static low pressure application, the fluid it is sealing is silicon oil. I think it should be OK but does anyone think different thermal expansion coefficients of the stainless and aluminium will be a problem?
 
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Hi Pedrof,
I think you will have pb.
We had a press fit assembly between SS and Al and it failed after sterilization@130C.
Not only that but we had corrosion pb.
We redesigned th system and isolated both materials by anodiszing the Al and chroming the SS.

Please add skecth if possible.
Good luck.
 
Yes, you will have a problem.

Metal-to-metal seals rely on a high axial force at the sealing interface. This coupled with a softer metal partnership implies a certain amount of component deformation as a necessary requirement in achieving that seal. By adding in thermal variances, you effectively add tolerance in the ability of such metals to achieve that required deformation.

This is the principle behind metal seated ball valves, either floating or trunnion mounted.

Regards,
Cockroach
 
Thanks for the comments. Perhaps you guys could look at the attcahed sketch, looks like a 3 year old did it but thats the snipping tool for you.
Not sure if you are thinking this is a more complex problem than I am.
All I am doing is replacing a viton o-ring with a copper sealing washer.
If you are saying the aluminium expanding more than the stainless steel fitting will mean the fitting will loosen and cause the copper washer to leak then I understand.
I think up to 230°C a PFA encapsulated silicon oring or kalrez will be best to stop the silicon oil leaking out.
All comments welome.
 
 http://files.engineering.com/getfile.aspx?folder=bd01de94-c427-49c0-8a8d-18eb024691a6&file=Copper_seal.JPG
Hi Pedrof,

The assembly may work in term of functionality. For long term, this assembly looks not reliable.
you have three different alloys (Cu, SS and AA) in the same media, you are prevailing galvanic corrosion.So I am not sure how long the copper gasket will 'live'.

Regards,

 
Just get a better o-ring.
Kalrez 4079 is good to +316deg C and Kalrez 7075 is good to +327deg C.

I'd agree with the other comments re galvanic corrosion though.
 
You also have too small of a groove for the o-ring. Once the o-ring is compressed and fills the groove to 100%, the rubber won't compress any more. The copper gasket is probably not being fully energized. Also, the o-ring will expand with temperature increases.

Check out this website to help you design your hardware and o-ring.

 
The fluid is silicone oil so the galvanic corrosion point is not accurate.

The problem you will potentially have is the one brought up by cockroach. Your assembly was designed to rely on an elastomeric o-ring, and hence likely will not be able to generate the axial force necessary to seal the copper washer reliably over time. ISO-parallel thread assemblies designed to work with copper washers tend to have a greater ratio of thread area to gasket area than your assembly shows, and greater strength through the "neck" of the part (the reduced diameter section between the male threads and the face normally sealed by the o-ring) than your assembly seems to offer.

Beware the high temperature claims made by some elastomer manufacturers. Your service is a mild one (silicone oil, low pressure) so you might be OK with Kalrez- check the compatibility tables though. Our experience has been that the o-rings fail even in static seating applications at temperatures far lower than those claimed by the elastomer manufacturers unless you're sealing something benign like an inert gas. Steam is particularly hard on these o-rings.
 
Thanks for the comments.

One other issue here is that the o-ring is stretched over the male thread before seating in the groove.
I suspect that a PFA encapsulated silicon oring may withstand the temperature (although there is some scepticism about the claimed ratings), but may be easily damaged by the stretching.
It looks like isolast/kalrez/markez direct swap for the o-ring is probably the best bet.
 
It will create a galvanic cell as illustrated as there are dissimilar metals outside wetted area.

To compress the copper and effectively seal with your assembly will probably strip the aluminum threads or shear off the fitting head at the threads' undercut. Also, surface finish of the sealing faces will require to be very fine, mirror like.

I have done this, a 1000psi+ gas tight metal-metal seal, using copper foil rings, cobalt foil and lap-polished stainless steel hardened bodies. Sealing compression force required was high enough for the copper rings to faintly embed into the stainless steel. While this design worked, over time it leaked due to the non-wetted, non-polished stainless steel-copper interface rusting.
 
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