What the calculator is for
The flow coefficient states how much a valve passes for a given pressure drop. Kv is the flow of water in m³/h at a pressure drop of 1 bar; Cv is the flow of water in US gallons per minute at 1 psi. Datasheets use one or the other, so converting them is everyday work in valve selection and checking.
- Cv = 1.156 × Kv and Kv = 0.865 × Cv
- Liquid sizing: Kv = Q √(SG / ΔP) with Q in m³/h and ΔP in bar; Cv = Q √(SG / ΔP) with Q in US gpm and ΔP in psi.
- Flow through a valve: Q = Kv √(ΔP / SG).
Worked example
A control valve must pass 40 m³/h of a liquid with SG 0.92 at a pressure drop of 0.8 bar. Kv = 40 × √(0.92 / 0.8) = 42.9, which is Cv 49.59.
Select a valve whose rated Cv puts this duty roughly between 20 % and 80 % of its capacity. Here a valve rated Cv 75 would run at about 66 % of rated Cv; enter the rated Cv under Advanced to check yours. Where in its travel that falls depends on the trim characteristic: an equal-percentage trim reaches 50 % of its capacity at roughly 80 % travel, a linear trim at about half travel.
The same equations run backwards for checking an installed valve. If the valve above, Cv 75, sees 1.5 bar at full opening, it can pass about 82.8 m³/h of the same liquid, provided the flow is not choked.
Conversion table
| Kv (m³/h at 1 bar) | Cv (US gpm at 1 psi) |
|---|---|
| 0.1 | 0.1156 |
| 0.5 | 0.578 |
| 1 | 1.156 |
| 2.5 | 2.89 |
| 5 | 5.78 |
| 10 | 11.56 |
| 25 | 28.9 |
| 50 | 57.8 |
| 100 | 115.6 |
| 250 | 289 |
Common mistakes
- Mixing units in the formula. Kv needs m³/h and bar; Cv needs US gpm and psi. Mixing them gives an error of up to 16 %, or far more with L/min.
- Using it for gas or steam. Compressible flow needs the expansion factor and different equations.
- Ignoring choked flow. If the pressure drop approaches FL² × (P₁ − FF·Pv), the liquid flashes or cavitates and more ΔP gives no more flow.
- Forgetting reducers. A valve smaller than the line loses capacity to the fittings; the full IEC 60534 method applies a piping geometry factor FP.
Basis
These are the basic incompressible-flow equations of IEC 60534-2-1 and ANSI/ISA-75.01.01 for turbulent, non-choked flow without piping geometry or Reynolds number corrections. Use the full method, or the valve vendor’s sizing software, for the final selection.