Control valve Cv and Kv calculator
Cv is the flow of water in US gallons per minute at one psi of pressure drop. Kv is the metric equivalent, in cubic metres per hour at one bar.
m3/h
bar
bar
Required Kv
28.284
- Required Cv
- 32.697
- Valve authority
- 0.2500 Too low — the valve will control badly
- Flow your valve would pass
- 22.627 m3/h
- Pressure drop at the required flow
- 0.3906 bar
- Share of the valve capacity used
- 88.4 %
- Valve looks oversized
- 0
- Flow in litres per minute
- 333.3
Show the working
- Flow rate = 20
- Pressure drop across the valve = 0.5
- Specific gravity = 1
- Pressure drop across the rest of the system = 1.5
- Valve Kv rating = 32
- Required Kv = 20 * sqrt(1 / 0.5) = 28.2843
- Required Cv = 28.2843 * 1.156 = 32.6966
- Valve authority = (0.5 + 1.5) > 0 ? 0.5 / (0.5 + 1.5) : 0 = 0.25
- Flow your valve would pass = 32 * sqrt(0.5 / 1) = 22.6274
- Pressure drop at the required flow = 32 > 0 ? 1 * (20/32)^2 : 0 = 0.390 625
- Valve looks oversized = 32 > 28.2843 * 1.5 ? 1 : 0 = 0
- Share of the valve capacity used = 32 > 0 ? 28.2843/32*100 : 0 = 88.3883
- Flow in litres per minute = 20 * 1000 / 60 = 333.333
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- Kv = Q sqrt(SG / dP), with Q in m3/h and dP in bar
- Cv = 1.156 Kv
- valve authority = valve drop / total circuit drop
An oversized control valve does all its work in the first fraction of its travel and hunts. Authority below 0.25 makes any tune poor.
These are liquid equations. Gas and steam sizing needs compressibility and choked-flow terms.