Valve Cv vs Kv
Detailed Explanation
The flow coefficient quantifies how much fluid a valve can pass at a given pressure drop. It is the single most important parameter for sizing control valves and comparing the flow capacity of different valve types and sizes.
Cv (US standard) is defined as the number of US gallons per minute of water at 60°F that will flow through a fully open valve with a pressure drop of 1 psi across the valve.
Kv (metric standard) is defined as the number of cubic meters per hour of water at 15°C that will flow through a fully open valve with a pressure drop of 1 bar.
The difference between Cv and Kv is purely a unit conversion. They describe the same physical property.
Conversion Table
| Cv Value | Kv Value | Valve Example (approx.) |
|---|---|---|
| 1.0 | 0.865 | 1/4” needle valve |
| 10 | 8.65 | 1” ball valve |
| 50 | 43.3 | 2” globe valve |
| 100 | 86.5 | 3” ball valve |
| 250 | 216 | 4” globe valve |
| 500 | 433 | 6” ball valve |
| 1,000 | 865 | 8” butterfly valve |
| 5,000 | 4,325 | 16” butterfly valve |
| 10,000 | 8,650 | 24” ball valve |
Conversion formulas:
- Cv = 1.156 x Kv
- Kv = 0.865 x Cv
These factors derive from the ratio of US gallons to cubic meters and psi to bar.
When Each Is Used
| Context | Coefficient Used |
|---|---|
| US, Canada, Middle East projects | Cv |
| Europe, Asia, Africa projects | Kv |
| ISA/ANSI standards | Cv |
| IEC 60534 standard | Kv |
| API 6D valve datasheets | Cv (typically) |
| EN standards | Kv |
| Manufacturer catalogs | Both (dual-listed) |
Most international valve manufacturers publish both Cv and Kv in their datasheets. When only one value is given, apply the conversion factor above.
Practical Considerations
Valve sizing. Control valve sizing software (e.g., Fisher, Emerson, Flowserve tools) accepts either Cv or Kv as input. Verify which unit the datasheet provides before entering values.
Reduced bore effect. Published Cv/Kv values assume a fully open valve. For reduced-bore ball valves, the actual Cv is lower than a full-bore valve of the same size. Always check the manufacturer’s tested Cv, not a generic estimate based on nominal size.
Rangeability. For control valves, the ratio of maximum to minimum controllable Cv (the rangeability) determines the turndown capability. Typical globe control valves offer 50:1 rangeability; ball valves offer 100:1 or more.
For more on valve specifications and pressure-temperature ratings, refer to ASME B16.34.
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