Required orifice area and verified capacity per API 520 / GB/T 12243 critical flow formula
Formula: Ac = Wr / (10 · C · Kdr · Kb · Pdr · √(M/(Z·T))) where C = 3.949 · √(k · (2/(k+1))^((k+1)/(k-1))) (k=1.4 → C=2.70). Pdr = 1.10·Ps + Pa (10% overpressure). Kdr = 0.8 derated coefficient for gas service. Kb = 1.0 for conventional valves with near-atmospheric back pressure; consult API 520 for higher back pressure. This is the critical (sonic) flow formula; valid when P2/P1 ≤ critical pressure ratio.