Required orifice area and verified capacity per API 520 / GB/T 12243 liquid service formula
Formula: Ac = 0.1964 · Wr / (Kdr · Kb · Kp · Kv · √(ρ · (1.25·Ps − Pb))). The 1.25 factor assumes 25% accumulation (liquid service). Kdr = 0.4 is the derated coefficient per API 520 liquid service. Kb = 1.0 for conventional valves with near-atmospheric back pressure; consult API 520 Figure 31 for higher back pressure. Kv = 1.0 for water-like viscosity; reduce per API 520 viscosity correction for viscous fluids.