Valve Sizing for Gas Service: Compressible Flow Equations

Sizing control valves for compressible gas flow requires accounting for gas compressibility, pressure ratio, and choked flow conditions. The ISA 75.01 / IEC 60534-2-1 standard provides the accepted methodology.

Fundamental Gas Sizing Equation

For non-choked gas flow, the valve Cv is calculated using the gas flow equation incorporating mass flow rate, inlet pressure P1, pressure drop ratio x (= ΔP/P1), gas expansion factor Y, and specific gravity. When x exceeds the choked flow pressure ratio xT, flow becomes choked and further pressure reduction does not increase flow.

Choked Flow and Terminal Pressure Drop

Choked flow occurs when x ≥ Fγ × xTFγ accounts for actual gas specific heat ratio vs. air referencexT is a valve-specific constant from manufacturer dataAt choked conditions, Cv calculation uses x = Fγ × xT

Compressibility Factor Z

For high-pressure gas streams, the real gas compressibility factor Z (from equations of state such as Peng-Robinson or SRK) corrects the ideal gas sizing result. Z factors below 1.0 reduce required Cv; Z factors above 1.0 (rare at process conditions) increase it.

Practical Sizing Steps

Obtain actual operating conditions: T, P1, ΔP, flow rateCalculate Fp (piping geometry factor) for reducers and expandersCompute Cv at minimum, normal, and maximum flowSelect valve trim to operate between 20% and 80% open at normal flowConfirm selected valve does not cavitate (liquid flash) at minimum flow

Common Errors

Using liquid sizing equations for gas, ignoring choked flow, and neglecting the piping geometry factor Fp all lead to undersized or oversized valves. Always confirm sizing with a second calculation tool or vendor sizing software.