Cv vs Kv: Flow Coefficient Conversion
Cv and Kv both express valve flow capacity, but they use different units. Cv is the US/imperial system coefficient; Kv is the metric/SI coefficient. The conversion factor is exact: Cv = 1.156 x Kv (and Kv = 0.865 x Cv). Mixing the two without conversion causes a 15.6% sizing error.
Quick Conversion
| Direction | Formula |
|---|---|
| Kv to Cv | Cv = Kv x 1.156 |
| Cv to Kv | Kv = Cv x 0.865 |
The factor 1.156 derives from the ratio of measurement units: 1 US gallon = 3.785 liters, 1 psi = 0.0689 bar. When these conversions are combined in the flow coefficient equations, the result is 1.156.
Side-by-Side Definition
| Parameter | Cv | Kv |
|---|---|---|
| Full name | Flow coefficient (imperial) | Flow coefficient (metric) |
| Definition | US GPM of water at 60 degF through a valve at 1 psi delta-P | m3/h of water at 5-30 degC through a valve at 1 bar delta-P |
| Flow unit | US gallons per minute (GPM) | Cubic meters per hour (m3/h) |
| Pressure unit | psi | bar |
| Temperature | 60 degF (15.6 degC) | 5-30 degC |
| Standard | ISA/IEC 60534, ANSI/ISA 75.01 | IEC 60534, EN 60534 |
| Primary region | North America | Europe, international |
Conversion Table
| Kv (m3/h) | Cv (US GPM) | Kv (m3/h) | Cv (US GPM) |
|---|---|---|---|
| 0.1 | 0.12 | 100 | 115.6 |
| 0.5 | 0.58 | 150 | 173.4 |
| 1.0 | 1.16 | 200 | 231.2 |
| 2.5 | 2.89 | 300 | 346.8 |
| 5.0 | 5.78 | 500 | 578.0 |
| 10 | 11.56 | 750 | 867.0 |
| 25 | 28.9 | 1,000 | 1,156 |
| 50 | 57.8 | 2,500 | 2,890 |
Where Each Is Used
| Context | Coefficient | Reason |
|---|---|---|
| API valve datasheets | Cv | API standards use US customary units |
| European EN project specs | Kv | EN standards use SI/metric units |
| International EPC projects | Both (verify which) | Mixed standards in global projects |
| ISA control valve datasheets | Cv (Section 1) | ISA 75.01 uses Cv |
| IEC control valve datasheets | Kv or Cv (both supported) | IEC 60534 defines both |
| Japanese projects (JIS) | Cv (common) | Historically adopted US practice |
| Middle East projects | Cv (dominant) | Oil and gas follows API/ANSI |
Common Sizing Errors
| Error | Consequence | How to Avoid |
|---|---|---|
| Using Kv value in Cv formula | Valve undersized by 15.6% | Always check the coefficient label on the datasheet |
| Using Cv value in Kv formula | Valve oversized by 15.6% | Convert before inserting into sizing equations |
| Mixing Cv and Kv on same datasheet | Incorrect comparison between valves | Standardize to one coefficient per project |
| Confusing Kv with Kvs | None (Kvs is the rated/maximum Kv) | Kvs = Kv at full opening; same units |
Practical Example
A European project specifies a control valve with required Kv = 87. An American manufacturer offers a valve with Cv = 100.
Is the valve adequate?
Convert: Kv of offered valve = 100 x 0.865 = 86.5
The offered Kv (86.5) is slightly below the required Kv (87). The valve is marginally undersized. A valve with Cv = 101 or higher (Kv = 87.4) would be needed.
Without conversion, the specifier might assume Cv 100 > Kv 87 and accept the valve; but this would be incorrect.
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