Valve Flow Coefficients: Cv, Kv, and Flow Capacity Rating

The flow coefficient (Cv for US units, Kv for metric units) quantifies the flow capacity of a valve: it is defined as the flow rate of water at 60°F that produces a 1 psi pressure drop across the valve. The flow characteristic describes how the valve flow capacity changes as the valve moves from closed to open position. Understanding flow coefficients, flow characteristics, and the standards for measuring them enables correct valve sizing and selection for control applications.

Cv and Kv Definitions and Conversion

Cv is the flow coefficient in US units: gallons per minute (GPM) of water at 60°F that flows through the valve with a pressure drop of 1 psi. Kv is the metric equivalent: cubic meters per hour (m³/h) of water at 20°C that flows through the valve with a pressure drop of 1 bar. The conversion between Cv and Kv is: Kv ≈ Cv × 0.865. Valve manufacturers publish Cv (or Kv) values for each valve size, type, and trim configuration. The Cv value varies with valve position: for a linear globe valve plug, the Cv increases linearly with stem lift; for an equal percentage plug, the Cv increase per unit stem lift is greater at higher lifts.

• Cv: US flow coefficient, GPM of water at 60°F with 1 psi pressure drop

• Kv: metric flow coefficient, m³/h of water at 20°C with 1 bar pressure drop

• Conversion: Kv ≈ Cv × 0.865, Cv ≈ Kv × 1.156

• Cv varies with valve position: linear, equal percentage, or quick-open

• ISA 75.01: standard method for sizing control valves

Flow Characteristics: Linear, Equal Percentage, Quick-Open

The flow characteristic describes the relationship between valve position and flow capacity (Cv). Linear characteristic provides equal increment of flow per equal increment of stem travel, making it suitable for processes where the control loop requires constant gain across the control range. Equal percentage characteristic provides equal percentage change in flow per equal increment of stem travel, making it suitable for processes with large gain variation (such as heat exchangers and chemical reactors). Quick-open characteristic provides maximum flow capacity at very low stem lift, making it suitable for on-off service but not for modulating control.

Valve Sizing and Cv Selection

Correct control valve sizing requires calculating the required Cv from the process flow conditions (flow rate, inlet and outlet pressures, fluid properties) and then selecting a valve whose Cv range matches the calculated requirement with appropriate margin. The ISA 75.01 standard provides equations for calculating Cv for liquid, gas, and steam services, accounting for fluid compressibility, pressure recovery, and critical flow conditions. Oversizing a control valve causes poor control because the valve operates in the region of very low opening where flow is unstable. Undersizing causes insufficient flow capacity. The selected valve should have a Cv at maximum opening approximately 1.2 to 1.5 times the calculated Cv.