Size a safety relief valve for gas or vapor service per API 520 / GB/T 12243 using the critical (sonic) flow formula. The C coefficient is derived automatically from the ratio of specific heats k. Formula: Ac = Wr / (10·C·Kdr·Kb·Pdr·√(M/(Z·T))).
Open Calculator →
Sonic (choked) flow at the orifice throat — valid when P2/P1 ≤ critical pressure ratio.
C = 3.949·√(k·(2/(k+1))^((k+1)/(k-1))); k=1.4 → C=2.70. Auto-calculated, no manual lookup.
Kdr = 0.8 for gas service (higher than liquid 0.4) reflects certified capacity.
Pdr = 1.10·Ps + Pa (10% overpressure, absolute) drives the mass flux.
Gas molar mass M and compressibility Z set the density term in the flow equation.
T (K) at the relief condition scales the √(M/(Z·T)) density factor.
Input required relief capacity Wr and set pressure Ps (gauge).
Enter molecular weight M, compressibility Z, temperature T and k.
Set Kdr (0.8), Kb and Kc for the valve and installation.
Read required area Ac, diameter do and verified capacity W.
Enter your parameters and get instant sizing and pass/fail results.
Open Calculator →