Neither of the two (OPE) cases can fully describe the loading conditions. A new static case, in the form of W+P3+T3+F1 (OCC) will be more indicative of a relief situation.

Notice that the vent stack (or the discharge line after a relief valve can still see considerable temperature (T3) and pressure (P3)--though often marked as “ambient” on the Line List in T1, T2, P1 and P2.

Using the Equivalent Pressure Method (EPM) to qualify the flange design of a relief valve calls for a deeper understanding of the factors in play, beyond a facile exercise of this new feature of C2v5.1. Not to be presumptuous, I would think that before embarking upon external sources for answers, it is prudent to acknowledge first what the B31 Piping Codes have to say about it. Take one reference, the Nonmandatory Appendix II-4.2.3 of B31.1 Piping Code does narrate somewhat about EPM; by and large its “B” part has been generally regarded in doing a leakage stress evaluation.