What Is Cv (Flow Coefficient)?
The flow coefficient (Cv) is a dimensionless number that describes the flow capacity of a valve or fitting. Specifically, Cv equals the number of US gallons per minute of water at 60 degrees F that will flow through the valve with a pressure drop of 1 psi. A higher Cv indicates greater flow capacity. The Cv is determined experimentally by the valve manufacturer and published in valve specification sheets.
Cv is the most widely used flow coefficient in North American practice. The European equivalent is Kv, which uses metric units (m³/h of water with 1 bar pressure drop). The relationship is Cv = 1.156 × Kv. Flow coefficient is essential for valve sizing in process control, HVAC, plumbing, and industrial fluid systems to ensure adequate flow capacity without excessive pressure drop.
Cv Formula
Typical Valve Cv Values
| Valve Type | Size | Full Open Cv |
|---|---|---|
| Ball valve | 1" | 30-40 |
| Ball valve | 2" | 120-160 |
| Globe valve | 1" | 10-15 |
| Butterfly valve | 4" | 500-800 |
| Gate valve | 2" | 100-140 |
Valve Sizing
- Calculate the required Cv from the maximum expected flow rate and allowable pressure drop.
- Select a valve with a full-open Cv at least 20% larger than calculated to ensure adequate capacity.
- For control valves, the Cv at normal operating position should be 60-80% of the full-open Cv for good controllability.
- Consider the Cv at minimum flow to ensure the valve can still control at low flow rates without hunting.
Frequently Asked Questions
How do I convert between Cv and Kv?
Cv = 1.156 × Kv, or Kv = 0.865 × Cv. Cv uses US gallons per minute with psi pressure drop; Kv uses cubic meters per hour with bar pressure drop. Both represent the same physical concept but in different unit systems. When reading European valve specifications, multiply Kv by 1.156 to get the Cv value.
Does Cv change with valve position?
Yes, for control valves. A control valve's Cv varies from zero (fully closed) to its maximum rated Cv (fully open). The relationship between valve position and Cv is called the valve characteristic: linear (Cv proportional to position), equal percentage (each increment produces equal percentage change in Cv), or quick-opening (rapid Cv increase near closed). The characteristic is chosen to match the process control requirements.
Can I use the Cv formula for gases?
The liquid Cv formula shown here does not apply to gases because gases are compressible. For gas flow, a modified formula accounting for upstream pressure, temperature, and specific heat ratio is required. For small pressure drops (less than about 10% of upstream pressure), the liquid formula provides a rough approximation, but for accurate gas valve sizing, use the ISA/IEC gas sizing equations.