Control Valve Cv Calculator (Gas)
Size a control valve for gas or vapour service with the IEC 60534-2-1 mass flow equation, including the expansion factor Y and choked flow limit from xT.
- ρ1 = P1 × M ÷ (Z × R × T) = 11.13 kg/m³
- x = ΔP ÷ P1 = 0.2000; Fγ × xT = 0.7000; Y = 1 − x ÷ (3 Fγ xT) = 0.9048
- Kv = W ÷ (27.3 × Y × √(x × P1 × ρ1)) = 8.582, Cv = 9.921
Gas Valve Sizing Equations (IEC 60534-2-1)
Y = 1 − x ÷ (3 × Fγ × xT), Fγ = γ ÷ 1.40
x = (P1 − P2) ÷ P1, choked when x ≥ Fγ × xT
W is mass flow (kg/h), P1 inlet pressure (bar absolute), ρ1 inlet density (kg/m³) and xT the valve pressure drop ratio factor from the manufacturer. When choked, x is limited to Fγ × xT and Y falls to 0.667.
Worked example
1000 kg/h of air at 40 °C, 10 to 8 bar(a), xT = 0.7. ρ1 = 11.13 kg/m³, x = 0.2, Y = 0.905, so Kv ≈ 8.6 and Cv ≈ 9.9.
Frequently Asked Questions
What is xT in gas valve sizing?
xT is the pressure differential ratio at which flow chokes for a gas with γ = 1.4. It is a valve characteristic, typically 0.6 to 0.8 for globe valves and lower for ball and butterfly valves.
Why is the expansion factor Y needed?
Gas expands and its density falls as it accelerates through the valve. Y corrects the liquid style equation for this; it ranges from 1 at very low pressure drop to 0.667 at choked flow.
Can I size with standard flow instead of mass flow?
Yes, convert first: mass flow = standard flow × standard density, where standard density = 101325 × M ÷ (8314.46 × 273.15) for 0 °C and 1.013 bar.
Learn more on the blog
Control valve flow coefficient (Cv) explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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