PTFE Seat Ball valve problems

[daveb9]

We manufacture 2" and 3" 2 piece ball valves with a maximum pressure of 500PSI for the Freon gas market

My problem is that we have had some seal failures, the failures have occurred in 0.02% or less of the valves that we have produced. However, I would like to eradicate the problem completely

The seals in the valves are a 25% Glass filled seal and the maximum temperature is 80°C

The failure mechanics appears to be the seal turning inside out for approximately 5mm at the point where the ball bore first contact the seal when the valve is being closed.

Hope my description is adequate.

We are currently evaluating other seal material, however, we can not get our standard material to fail in laboratory conditions making if difficult to achieve definite solution.

Thank you in advance
Dave

 

[MikeHalloran]

I have limited experience making 2-piece ball valves in 1/4" size. I associate 'eyebrow' failures, such as I think you've described, with extrusion of the seal material into the fluid bore.

I suspect that your design does not support the i.d. of the seal with a metal cylindrical feature extending in from the body and cap almost to the ball, and that the glass loading is used to compensate for that.

I.e., PTFE, even with glass loading, is still a plastic material in the strictest rheological sense of the word, and in response to the stress imposed at assembly, will _slowly_ move into the bore unless it's physically restrained from doing so.

Use more glass or support and trap the seal better, and it will extrude more slowly.







Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[daveb9]

Thanks Mike

But our valve does have a inner support and appears not to have extruded into the valve bore, the seal has been turned inside out for a 5mm section from the outside of the seal into the bore.

My latest theory which we will test, is that the valve has only been opened 90% the small of area seal that is not in contact with the ball may have extruded due to temperature or product flow. Then when the valves is closed the ball edge catches the seal and pulls it out of its housing.


Dave B

 

[g1a1]

Daveb9,

Sounds like the seat ID is smaller than the Ball ID. During closing the ball catches the seat lip and plastically deforms it. I have experience with liguid nitrogen seat designs and have seen seats react in this manner due to shrinkage at low temps (-320F). Excessive assembly forces can also cause this problem at low temps.
GDC