October 2015 Flange Leakage

Things have gone from bad to worse in the Petrochemical Plant Design business. Now is the time to hibernate with an operating company who are still busy maintaining and upgrading their facility.

The Power game is still chugging along especially in the U.S. where they are trying to initiate programs that will reduce carbon emissions.

Flanges are designed to remain leak free under hydro-static test pressure when cold and operating pressure when hot. The normal design practice for flanges takes no account of bending moments in the pipe.

Although a flange can transmit some bending moment before it opens up, it is as well to realize that this moment is often very small.

At present there are no published analytical methods that will assure that flange leakage will not occur. B31.1 (see Table 112) and SC VIII, Div. 1, Appendix 2 does offer some guidance that will help the designer with materials, flange types, bolting, and gaskets but the assurance that a selected flange configuration will not leak is beyond these sources. A major source of uncertainty is the flange gasketing. Gasket manufacturers often have guidance on their product's proper installation and may have performance data, based on testing, as well.

If the assurance that no leakage will occur is required, then, short of the designer performing testing, working with a reputable gasket manufacturer will be necessary.

But from a careful reading of B16.5 - 2001, Para. 2.4, one can discover that in addition to pressures and temperatures the "forces and moments developed in the connected piping" must be taken into consideration. Another authoritative set of flange rules exist in the ASME BPV Section VIII, Div. 1, Appendix 2 which also requires the external forces and moments on the flange to be considered.