Fix the duty point first
Write down the design flow and the pressure drop available across the valve at flowing conditions, excluding pipe and fittings. Most sizing errors start here: using system total head instead of valve drop undersizes the trim.Compute required Cv and convert units
Cv_req = Q x sqrt(SG / dP) with Q in m3/h and dP in bar. Convert with Kv = 0.865 x Cv before comparing metric datasheets. Add 15-20% margin and round up one size so normal control sits between 20 and 80% open.Recheck drop, noise, and cavitation
Recompute the valve drop at the selected Cv, confirm the pump still has head to spare, and screen velocity and cavitation at the vena contracta. Gas service additionally needs the choked-flow check before any number is trusted.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
Next step