What Is Cv? The Valve Flow Coefficient Explained
Every valve datasheet carries a Cv number, yet the definition is often half-remembered. This article pins down exactly what Cv measures, how it relates to Kv, and why one number can describe a valve across a huge range of services.
1. The definition
Cv is the flow of water, in US gallons per minute, at 60 °F, that produces a pressure drop of 1 psi across a fully open valve. A valve with Cv = 100 passes 100 gpm of water with 1 psi of drop. Double the Cv and the same flow needs a quarter of the drop — because drop scales with the square of flow:
Q = Cv × √(ΔP / Sg) ⇒ Cv = Q × √(Sg / ΔP)Q in US gpm · ΔP in psi · Sg = specific gravity relative to water. For water, Sg = 1 and the square root terms collapse.
The square-root relationship is the whole story: flow is proportional to the square root of the driving drop. To pass twice the flow through the same valve you need four times the ΔP.
2. Kv — the metric twin
Metric practice uses Kv: the flow of water, in m³/h, that produces a pressure drop of 1 bar across a fully open valve (reference water temperature is defined in the range 5–30 °C depending on which standard edition applies). The two numbers are pure unit conversions of the same idea:
Kv = 0.865 × CvCv = 1.156 × Kv
A "Cv 50" valve is a "Kv 43" valve. Datasheets listing both are the same capacity stated twice.
3. What Cv does not include
- It assumes the valve is fully open. The Cv on a datasheet is the rated maximum. A valve at 50 % travel passes far less — see flow characteristics.
- It assumes water-like, turbulent, single-phase flow. Gases, steam, viscous liquids and flashing liquids all need correction factors (expansion factor Y, viscosity correction FR, the choked-flow limits).
- It assumes the standard test piping. Fitting-induced geometry losses (Fp) can knock several percent off the effective capacity — see the step-by-step sizing procedure.
So Cv is the starting point of sizing, not the whole calculation. IEC 60534-2-1 is essentially a book of corrections around this one number.
4. A feel for the numbers
| Service | Flow (water) | ΔP | Required Cv |
|---|---|---|---|
| Sample line | 5 gpm | 10 psi | ≈ 1.6 |
| Small utility water | 50 gpm | 10 psi | ≈ 16 |
| Process line | 500 gpm | 25 psi | ≈ 100 |
| Header line | 2,000 gpm | 10 psi | ≈ 632 |
Typical catalogue: a 1 in globe trim spans Cv ≈ 5–20; a 6 in globe spans Cv ≈ 150–400; full-bore ball valves of the same 6 in can exceed Cv 900 — which is exactly why balls make poor throttling valves (huge capacity, almost no resolution near the seat).
5. The two questions Cv answers
- "Is the valve big enough?" — required Cv vs rated Cv at the maximum case, with choked-flow checked.
- "Where will it run?" — the ratio required-Cv/rated-Cv, mapped through the characteristic curve, tells you the operating travel. Keep it between ~20 % and ~80 % at normal load.