Per AGA Guidelines & 49 CFR 192
Understand regulator station design, Cv calculations, and relief valve sizing per AGA and 49 CFR 192
Regulator sizing calculates the required flow coefficient (Cv) from the design flow rate, inlet and outlet pressures, gas specific gravity, and temperature per ISA 75.01. The Cv determines the regulator body size needed to pass the required flow without excessive noise or instability.
Choked flow occurs when the pressure drop ratio x = (P1 − P2) / P1 reaches the valve's critical value xT (about 0.70–0.75 for regulators), beyond which lowering the downstream pressure no longer increases flow. This calculator declares choked flow when x reaches xT. (The classic isentropic critical pressure ratio for k ≈ 1.3 gas is roughly P2/P1 ≈ 0.53, i.e. x ≈ 0.47, but real valves choke at their geometry-dependent xT.)
Per 49 CFR 192, regulator stations must have overpressure protection. The relief valve must handle the full flow capacity of the regulator in a failed-open scenario to prevent downstream piping from exceeding its maximum allowable operating pressure (MAOP).
Droop is the decrease in outlet pressure as flow increases through a regulator, caused by the spring and diaphragm response characteristics. It is managed by selecting regulators with appropriate droop specifications and using pilot-operated designs for tighter pressure control.