Sealing of ButtWeld ends for Cryogenic Testing

[NGiLuzzu]

Dear All,
I would like to know if anybody has ever faced the problem in object.
Having to perform cryogenic tests on butt-welding valves, it is not easy to obtain a good sealing of the test caps, due to the particular shape of the ends...
Is it possible to have, somehow, a good (flat) sealing surface, on which the gasket can be centered and compressed properly?
In addition to this, what kind of material would you use (see also Thread408-82258)?

Thanks and Regards, 'NGL

 

[BVMan]

I got involved in doing some Cryo Tests a few years back using a Spanish Manufacturer, they welded Dished Ends onto the valves to do the Type Testing