How does control valve sizing differ for gas/steam service?

Gas/steam sizing considerations: compressibility (gas expands through valve, affecting capacity), critical flow (choked flow when velocity approaches sonic, Mach 1), specific heat ratio (affects expansion), and upstream temperature. ISA sizing equation: W = N*Cv*P1*Y*sqrt(X*M/(Z*T)), where Y is expansion factor, X is pressure ratio, M is molecular weight, Z is compressibility. At critical flow (typically P2/P1 < 0.5), flow is limited regardless of downstream pressure. Use appropriate Y factor equation. Consider noise prediction (high velocities cause aerodynamic noise).