There are several methods of assessing ASME B16.5 flange loading in piping systems to check for leakage in a piping system governed by ASME B31.3 piping code.

1. Calculate extraneous force and moment at flange location and compute an equivalent pressure (16m/PI/G^3 + 4F/PI/G^2). Add this pressure to the line pressure and undertake an ASME VIII, BS 5500 flange analysis. ASME B16.5 flanges are accepted although some sizes/classes do not comply with ASME VIII analysis even with basic ASME B16.5 PT rating pressure

Question. Given that this "pressure" is made up of primary and secondary loads is it valid to apply allowable stress values above the basic Sh value from B31.3 (eg 90% SMYS)? Clearly all leakage checks (gasket compression and ASME rigidity) must be met.

2. Compare calculated moment (torsion or bending) at flange location with allowable in ASME III NC 3658.3
Mfs = 3125(Sy/36000)CAb. This is valid for ASME B16.5 flanges only and assumes high strength bolting.
Question. Is this method used by anyone?

3. Calculate extraneous force and moment at flange location and compute an equivalent pressure.
Add this pressure to line pressure and compare it with the line PT rating at design temperature.

Question. Is it reasonable to apply a factor to the PT rating based on the ability to carry 1.5 x PT rating without leaking and the knowledge that flanges operating at pressures equal to the PT rating with reasonable external loads do not leak.