Fraction of Gasket Load Remaining after Relaxation, ϕg, for Sheet Gaskets

Tightening your bolted flange assembly and walking away for good may not work well for all gaskets. In fact, gaskets with a low Fraction of Gasket Load Remaining after Relaxation, ϕg, may need to be retightened after relaxing. 

A 0.7 default value for the Fraction of Gasket Load Remaining after Relaxation, ϕg, is often used in ASME PCC-1 Appendix O calculations. This default value may not accurately reflect the true Gasket Relaxation Fraction of the gasket material, especially for ⅛” thick sheet gaskets. Many times thick sheet gaskets relax more than anticipated. 

Some gasket materials, like PTFE, may retain only around 50% of their original gasket stress a week after assembly. This is why retightening back to the intended Target Assembly Gasket Stress,SgT is a great idea. Retightening a week after original assembly can help minimize the chances of the gasket “walking” during operation and reduce the occurrence of leaks.