Control Valve Flow (Cv)

Formula

Description

The valve flow coefficient Cv quantifies the flow capacity of a control valve: it is the number of US gallons per minute of water at 60°F that will flow through the valve with a pressure drop of 1 PSI. For fluids other than water, the specific gravity correction adjusts for the difference in density. This sizing equation is fundamental to process control engineering and is used to select valves that can deliver the required flow rate at the available pressure drop. Oversized valves give poor control resolution, while undersized valves limit throughput.

Variables

• Q — Volumetric flow rate (US GPM)
• Cv — Valve flow coefficient (dimensionless)
• ΔP — Pressure drop across the valve (PSI)
• SG — Specific gravity of the fluid relative to water (dimensionless)

Practical Notes

Water at 60°F has SG = 1.0. For gases, a different Cv equation is used that accounts for compressibility. The Kv coefficient used in metric countries relates to Cv by Kv = 0.865 × Cv. Valve sizing should target 60-80% open at normal flow to allow room for control. ISA-75.01 provides the full valve sizing equations including choked flow, two-phase flow, and Reynolds number corrections.