Re: Gasket Creep

From: <Davis>
Date: Fri Aug 24 2001 - 09:47:00 EDT

Be careful about retightenting. There are two potential problems. One is safety related if the retightening occurs after startup and pressure/temp/media are present. The other is gasket related. Rubber bound compressed asbestos or nonasbestos gasket products tend to embrittle after a fairly short time period when in a heated condition. This will cause the physical charateristics of the gasket material to change, especially the compressibility. There is a danger that the gasket will crack after retightening beyond a 12-24 hour period after startup especially when heated beyond about 180 degrees F.

As to studs vs. bolts, our group as always maintianed studs are preferred due to the possibility that bolts can be tightened (turned) from the head as opposed to turning the nut. This greatly increases friction as the whole bolt shaft is turning.

One of the most important things here is determining the gasket load in psi required for that material in that application then utilizing the best means of acheiving that load be it torque wrenched, hydraulic tensioners, etc. Star cross pattern, 3-4 pass method, well lubricated fasteners are essential as well.

>>> "Jim Hubbard" <jhubbard@enercon.com> 08/22/01 11:19AM >>>
Embedment loss, early gasket creep and to some extent, elastic interactions, can be compensated for by retighting the bolts after giving them some time to relax. since bolts will interact during any tighting pass merely retighting them won't eliminate interaction loss, but it can reduce it. A final torquing pass in a reverse sequence can be helpful, for example.

Increasing intial perloads are a common way to compensate for relaxation of various kinds. They still relax, but residual preloads are higher because they relax from a higher initial preload. Higher perload also resist selfloosening or vibration loosening.

Embedment and creep can also be compensated for by tighting the bolts to rated perload, waiting a bit, the loosening and retighting them. This process works the threads and joint together. Some people call this "artifical aging" of the joint.

Hope this helps it was taken from EPRI "Good Bolting Practices"

James Hubbard

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Your use of Yahoo! Groups is subject to <a href="http://docs.yahoo.com/info/terms/">http://docs.yahoo.com/info/terms/</a> Received on Fri Aug 24 09:47:00 2001