O-Ring for penstock bulhead (dish-end)

[flamby]

Hi folks,

I have to provide a 1500mm dia hemispherical bulkhead to close a penstock unit of the same size. I have already designed and fabricated a hemispherical bulhead with flanges. I specified the material of O-ring as NBR/ EPDM/ Delrin/ Peek.

The whole assembly was to be tested in shop with 130MPa of test pressure. AT 105 MPa pressure, the leakage started. On opening we noticed that NBR seal has cold-flown and completely gone out-of-shape.

What material do you suggest for such high pressure application? This was first intercourse with O-Rings and it ended up in a failure. Actually I asked the suppliers and they said NBR will meet the requirement. What am I missing?

I should have asked earlier...but still not too late.

Thanks.

 

[Metalguy]

Assuming you're sealing water, a good grade of EPDM should work fine. Just get a grade that has a relatively low amount of oil-fill.

Your problem is most probably related to the gap the O-ring must seal-the higher the pressure, the smaller the gap must be. O-rings will seal thousands of PSI, but the gap must be small or the elastomer will flow. This is elementary O-ring sealing. Get Parker's O-ring manual!

 

[EdDanzer]

Check to see if part deformation is changing the gap you are sealing. If there is much change sealing is difficult.

 

[flamby]

OOPS! A mixup of units in my original post. The test pressure is 130Kg/ cm2 (13 MPa). Similarly 105 MPa should have been 10.5 MPa. Sorry.

Yes, I see a gap of around 2mm on 10.5MPa pressure, through which the seal is flowing. Will any workaround like metallic backing strip work?

 

[EdDanzer]

Yes a backup of some sort should help, but the seal movement will cause wear and premature failure. Stiffing the area of deformation would be a better choice.

 

[unclesyd]

Greene Tweed offers several different cross sections of rings that may help.

http://www.gtweed.com/85256D3C0038D8DA/vwContentByKey/N25PFPN9221ELETEN

The spring energized gasket may also be considered.

http://www.helicoflex.com/products.htm

A square cross section with 2 backup rings may work. Also using backup rings with your existing O-rings may be viable solution. The also have spring energized o-rings.