Valve sizing is arithmetic plus discipline about which pressure drop belongs to the valve. Compute Cv_req = Q x sqrt(SG / dP) from the design flow and the drop across the valve alone, convert to Kv where datasheets are metric, add 15-20% margin, round up one trim size, and recheck the operating drop plus noise and cavitation. The worked default, Q 25 at dP 1 bar, needs Cv 25.00.
Formulas
Required Cv
Cv_req = Q x sqrt(SG / dP)
Q in m3/h, dP in bar, SG relative to water.
Cv to Kv
Kv = 0.865 x Cv
Use before comparing metric datasheets.
Worked example
Water duty 25 m3/h at 1 bar
Inputs: Q 25; dP 1; SG 1.
Calculation: Cv_req = 25.00; with 20% margin select Cv 30 and up.
Lesson:
Frequently asked questions
Which dP do I use?
The drop across the valve at flowing conditions, excluding pipe and fittings.
How much margin?
Add 15-20% and round up one size so control stays in 20-80% open.
Cv or Kv?
Kv = 0.865 x Cv. Convert before comparing datasheets.
Sources and further verification
- Flow coefficient Wikipedia
- Control valves Engineering ToolBox
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