Valve Flow Coefficient (Cv) Calculator

Valve Flow Coefficient (Cv)

The flow coefficient $C_v$ characterizes how much fluid a control valve can pass for a given pressure drop. Engineers use it to size valves for process piping, HVAC systems, and hydraulic circuits.

Cv Formula

$$C_v = Q \sqrt{\frac{SG}{\Delta P}}$$ $$Q = C_v \sqrt{\frac{\Delta P}{SG}}$$

Here $Q$ is volumetric flow rate in US gallons per minute (GPM), $\Delta P$ is pressure drop in psi, and $SG$ is the specific gravity of the fluid relative to water. Rearranging the formula gives flow rate from a known $C_v$.

Practical Use

A valve with a higher $C_v$ allows more flow at the same pressure drop. When fluid density differs from water, specific gravity adjusts the calculation so the same valve rating applies across liquids.

Frequently Asked Questions

What units does Cv use?

In US engineering practice, $C_v$ is defined with flow in GPM and pressure drop in psi. The result is dimensionless in the sense that it combines those standard units.

What is specific gravity?

Specific gravity is the ratio of a fluid's density to water at standard conditions. Water has $SG = 1$. Denser fluids have values greater than 1.

How does pressure drop affect Cv?

For a fixed flow rate, a larger pressure drop reduces the calculated $C_v$. Valves with higher $C_v$ ratings need less pressure drop to pass the same flow.

Is Cv the same as Kv?

No. $K_v$ is the metric flow coefficient using m³/h and bar. Conversion between $C_v$ and $K_v$ requires a unit factor; they are not identical numbers.

Does this apply to gases?

This form is most accurate for incompressible liquid flow. Compressible gas flow through valves may need additional corrections for choked flow and expansion.

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Valve Flow Coefficient