Flanges designed to ASME Sect VIII Div 1 Appendix 2 and those manufactured per ASME B16.5 are accepted based on different criteria. There is no assurance that a B16.5 flange will meet its standard pressure-temperature rating when analyzed per Appendix 2.
To determine the MAWP of a flange designed per Appendix 2 requires the simultaneous solution of multiple equations: those equations for calculated stress defined in Appendix 2.
Practically speaking, it is simpler to have an electronic spreadsheet, MathCAD worksheet, or computer program, etc, each written to calculate the stresses as per Appendix 2. Then manually (or let the computer software do so automatically) iterate to find the pressure that corresponds to one of the calculated stress becoming equal to the allowable stress: by definition the pressure that corresponds to the first calculated stress (Sh, St, Sr, or one of the defined combinations) just meeting its corresponding allowable stress is the MAWP; any higher pressure will result in the allowable stress being exceeded.
Because of the wide variations in geometry, gasket properties, etc there can be few, if any, generalizations made about what may in general be the limiting factor when determining the flange MAWP. Thus there aren't any shortcuts to the process described above.
One generalization that can be made is that many B16.5/B16.42 flanges do not meet their published ratings when analyzed per Appendix 2. On the other hand, a number of these flanges actually exceed their published ratings. In practice it seems that the very small diameter (and proportionately "heavier" than their larger diameter “classmates”) flanges will exceed their rating, while the larger diameter flanges will not meet their ratings.
