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 outlet pressure drops below the critical pressure ratio, and further reducing downstream pressure no longer increases flow. For natural gas, choked flow typically occurs when the outlet absolute pressure is less than about 53% of the inlet absolute pressure.
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.