Valve Flow Coefficient Converter
Convert between Cv (US) and Kv (metric) valve flow coefficients for industrial valve sizing and selection.
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Understanding Valve Flow Coefficients
Flow coefficient is the key specification for sizing control valves in industrial piping systems. It quantifies how much fluid can flow through a valve at a given pressure drop. Two systems exist: Cv (used in the United States and the United Kingdom) and Kv (used in continental Europe and most other countries). They measure the same thing but use different units.
Cv (US Flow Coefficient):
Cv is defined as the flow rate of water in US gallons per minute (GPM) at 60F (15.6C) that produces a pressure drop of 1 PSI across the valve.
Cv = Q × √(SG / ΔP)
Where:
- Q = flow rate in US gallons per minute (GPM)
- SG = specific gravity of the fluid (water = 1.0)
- ΔP = pressure drop across the valve in PSI
Kv (Metric Flow Coefficient):
Kv is defined as the flow rate of water in cubic meters per hour (m3/h) at 15C that produces a pressure drop of 1 bar across the valve.
Kv = Q × √(SG / ΔP)
Where:
- Q = flow rate in cubic meters per hour (m3/h)
- SG = specific gravity of the fluid (water = 1.0)
- ΔP = pressure drop across the valve in bar
The Conversion:
Cv = Kv × 1.1560
Kv = Cv × 0.8650
Kv = Cv / 1.1560
This conversion factor accounts for the difference between US gallons per minute and cubic meters per hour, as well as the difference between PSI and bar pressure units.
Typical Valve Cv/Kv Values:
| Valve Size (inches) | Valve Type | Typical Cv | Typical Kv |
|---|---|---|---|
| 1/2" | Globe | 5-10 | 4.3-8.7 |
| 3/4" | Globe | 10-20 | 8.7-17.3 |
| 1" | Globe | 15-35 | 13.0-30.3 |
| 1-1/2" | Globe | 35-70 | 30.3-60.6 |
| 2" | Globe | 50-120 | 43.3-103.8 |
| 3" | Globe | 100-250 | 86.5-216.3 |
| 4" | Globe | 200-500 | 173.1-432.7 |
| 2" | Ball (full port) | 100-150 | 86.5-129.8 |
| 3" | Ball (full port) | 250-400 | 216.3-346.1 |
| 4" | Ball (full port) | 500-750 | 432.7-649.0 |
| 2" | Butterfly | 60-120 | 51.9-103.8 |
Valve Sizing Process:
- Determine the required flow rate (GPM or m3/h)
- Determine the allowable pressure drop (PSI or bar)
- Calculate the required Cv or Kv
- Select a valve with a Cv/Kv equal to or greater than the calculated value
- Verify that the valve can operate within the required range (valves work best between 20-80% of their rated Cv)
Important Notes:
- Flow coefficients assume turbulent flow conditions and incompressible fluid (liquid)
- For gases and steam, modified equations are needed that account for compressibility
- The relationship between valve opening and Cv depends on the valve characteristic (linear, equal percentage, or quick opening)
- Always add a safety margin of 10-20% above the calculated Cv when selecting a valve