Flange stresses are calculated in accordance with ASME VIII div 1. The method is same in PD5500. ASME VIII div 2 gives different equations but similar results to div 1. Div 2 includes external loads directly without need for the 'Kellogg' equivalent pressure method.

It is common to find that certain B16.5 flanges are over-stressed in the seating case but ok operating. 150# to 400# are the worst, particularly when a spiral wound agasket is used. This is an incompatability between B16.5 and ASME VIII. Clearly there are many thousands of these joints successfully in operation so it is not a real problem.

If you are dealing with low pressure flanges, you might consider the CEN EN 1591 code.