Control Valve Cv Calculator (Liquid)
Size a control valve for liquid service. The calculator finds the required Kv and Cv and checks whether the flow is choked (cavitating or flashing) using the IEC 60534-2-1 method.
- ΔP = P1 − P2 = 3 bar
- FF = 0.96 − 0.28 × √(Pv ÷ Pc) = 0.9566, ΔPchoked = FL² × (P1 − FF × Pv) = 8.075 bar
- Kv = Q × √(SG ÷ ΔP) = 50 × √(1 ÷ 3) = 28.87, Cv = 1.156 × Kv = 33.37
Liquid Valve Sizing Equations
Cv = 1.156 × Kv
ΔPchoked = FL² × (P1 − FF × Pv), FF = 0.96 − 0.28 × √(Pv ÷ Pc)
FL is the liquid pressure recovery factor of the valve (from the manufacturer), Pv the vapour pressure and Pc the critical pressure of the liquid. If the actual ΔP exceeds ΔPchoked, the flow no longer increases and ΔPchoked is used for sizing. This calculator assumes the valve is the same size as the pipe (Fp = 1).
Worked example
50 m³/h of water at 25 °C, P1 = 10 bar(a), P2 = 7 bar(a), FL = 0.9. ΔP = 3 bar, which is below the choked limit of 8.1 bar. Kv = 50 × √(1 ÷ 3) = 28.9 and Cv = 33.4.
Frequently Asked Questions
What is the difference between Cv and Kv?
Cv is US gallons per minute of water at 1 psi pressure drop; Kv is m³/h of water at 1 bar drop. Cv = 1.156 × Kv.
What is choked flow in a control valve?
When pressure inside the valve falls to the vapour pressure, bubbles form and the flow stops increasing with more pressure drop. This is choked flow and usually comes with cavitation damage.
Where do I find FL for a valve?
FL is published by the valve manufacturer for each valve type and travel. Typical values are about 0.9 for globe valves and 0.5 to 0.7 for butterfly and ball valves at full opening.
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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