Traditionally, analysis of flanges under piping loads has been done by converting piping loads to an "equivalent pressure", which can then be added to the actual system design pressure, the sum of which is then compared to the ANSI Standard B16.5 "Pipe Flanges and Flanged Fittings" allowable pressures at temperature (see Section 2.1.3). Piping loads are converted to equivalent pressure by applying the pipe forces and moments over an equivalent gasket area and an equivalent gasket section modulus respectively, as follows:
This method is widely recognized to give highly conservative results, indicating failure in flanges which actually have quite a bit of reserve strength. However, it has long been used due to the lack of any easy method for evaluating flanges under piping loads.
One alternative is to perform a flange analysis according to the requirements of Section VIII, Division I of the ASME Boiler and Pressure Vessel Code. However, these calculations are only intended to assure that the flange will not be over stressed by the necessary joint tightening loads that are required for a leak tight joint.
The problem with these stress calculation methods is that they hold piping loads, which usually cause leakage failure in flanges, to standards for pressure and tightening loads, which usually have a different failure mode—stress failure. Stress calculation methods fail to address the deformation of the flange and its effect on gasket compression and leakage. The basic problem of flange leakage is a complex one not readily availing itself to analysis. Facing selection, gasket type, operating temperature, and initial fitup loads are all factors that are either difficult or impractical to evaluate analytically.
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