# Cv Flow Coefficient

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/cv-flow-coefficient/
> Language: en-US
> Category: Flow & valves

## Definition

Cv is a conventional flow coefficient used to express valve flow capacity under a defined test and calculation basis. A larger Cv generally indicates a less restrictive valve, but Cv alone does not state the compressed-air flow at every pressure condition. Gas sizing must account for upstream absolute pressure, downstream pressure, temperature, and whether flow is choked.

## Key facts

- **Use:** Comparative valve flow capacity
- **Not equivalent to:** A fixed SCFM or NL/min value
- **Related conventions:** Kv, sonic conductance C, and critical ratio b

## Why Cv is not a universal flow rate

Cv originated as a coefficient rather than a direct volumetric-flow rating. For compressible air, calculated flow changes with pressure ratio and reference conditions. A catalog's nominal flow test may use another standard or another upstream/downstream condition, so identical-looking numbers are not automatically comparable.

## Compare coefficients on the same basis

Cv and metric Kv are related but not numerically equal. ISO 6358 commonly reports sonic conductance C with critical pressure ratio b. Use an explicit, documented conversion only when the source definitions, units, and reference states are known; otherwise compare manufacturer curves at the intended operating point.

## Select for the complete pneumatic function

Flow capacity is only one valve requirement. Confirm valve function, normal state, actuation method, minimum pilot pressure, voltage, port thread, media, pressure range, leakage, response time, and environmental approvals before treating a Cv match as a replacement match.

## What to verify before selection

- Coefficient convention and units
- Upstream absolute pressure
- Downstream pressure or pressure ratio
- Reference temperature and air condition
- Required actuator fill or exhaust time
- Valve function and pilot limits

## Common mistakes

- Reading Cv as a direct SCFM value
- Treating Cv and Kv as numerically identical
- Selecting a replacement valve from coefficient and port size only

## Frequently asked questions

### Is a valve with twice the Cv guaranteed to deliver twice the air flow?

No. Actual compressible flow depends on the pressure ratio, upstream conditions, connected restrictions, and the sizing equation used.

### Can I compare Cv directly with NL/min?

Not without the pressure and reference conditions used to produce the NL/min value and a compatible calculation basis.

## Technical references

- [ISO 6358-1:2013 — Pneumatic flow-rate characteristics](https://www.iso.org/standard/56612.html): Steady-state test methods and reporting conventions for compressible-fluid components.
- [IEC 60534-2-1:2011 — Control-valve sizing equations](https://webstore.iec.ch/en/publication/2461): Sizing equations for compressible and incompressible flow through industrial-process control valves.