In order to eliminate oversizing and the potential for valve instability (chatter) the smallest vent size obtained by considering all vapor venting evaluated at the selected relief set pressure well below MAWP is recommended. In this case, if two-phase flow should occur (may be likely), only a modest overpressure will occur before turnaround in pressure (Fauske, 2006).
The required relief rate for all vapor venting is given by