Calculating the flow rate of a gas pressure regulator is essential for sizing equipment in industrial and HVAC applications. The flow rate through a regulator depends on the orifice size (represented by the Flow Coefficient, Cv), the pressure drop across the device, and the properties of the gas being regulated.
Key Variables in the Calculation
Cv (Flow Coefficient): A relative measure of its efficiency at allowing fluid flow. It describes the number of US gallons in a minute of 60°F water that will flow through a valve with a pressure drop of 1 PSI.
P1 (Inlet Pressure): The pressure of the gas entering the regulator.
P2 (Outlet Pressure): The regulated pressure leaving the regulator.
Specific Gravity (G): The ratio of the density of the gas to the density of air. For air, G = 1.0. For natural gas, this is typically around 0.6.
Temperature: Gas volume is highly sensitive to temperature. Calculations are typically converted to Standard Cubic Feet per Hour (SCFH).
Sub-critical vs. Critical (Choked) Flow
When the outlet pressure (P2) is significantly lower than the inlet pressure (P1), the gas reaches the speed of sound at the regulator's orifice. This is known as Critical Flow or Choked Flow. Beyond this point, further reducing the outlet pressure will not increase the flow rate.
In most gas applications, critical flow occurs when the absolute outlet pressure is less than approximately 53% of the absolute inlet pressure.
Example Calculation
If you have a regulator with a Cv of 0.5, an inlet pressure of 100 PSIG, an outlet pressure of 80 PSIG, and you are flowing Air (G=1.0) at 60°F: