Control Valve Flow Characteristics: Linear, Equal Percentage, Quick Opening

Control valve flow characteristics define the relationship between valve travel and flow rate at constant pressure drop. Selecting the correct flow characteristic is essential for stable and accurate process control over the full operating range.

Inherent Flow Characteristics

• Linear: flow is directly proportional to valve travel; Cv increases linearly from 0 to 100% travel

• Equal percentage: each equal increment of valve travel produces the same percentage increase in flow; logarithmic relationship

• Quick opening: large increase in flow at small valve travel; concave-up characteristic curve

• Modified parabolic: intermediate between linear and equal percentage

Installed Flow Characteristics

The inherent characteristic is the flow-travel relationship measured at constant pressure drop across the valve. In actual installations, pressure drop varies as a function of flow, modifying the installed characteristic. The installed characteristic is what the controller actually sees and must be evaluated for control stability.

Characteristic Selection Guidelines

• Equal percentage: appropriate when pressure drop varies significantly with flow (typical for most process loops); provides consistent control gain across the range

• Linear: appropriate when pressure drop is relatively constant across the flow range; provides constant process gain

• Quick opening: used for on/off service, pressure relief bypass, and applications needing maximum flow quickly

Valve Sizing and Rangeability

Equal percentage valves have high rangeability (typically 50:1 or better), meaning they can control accurately from 2% to 100% of rated flow. This wide range allows the valve to handle load variations without operating in the unstable low-travel region.

Characterization in Smart Positioners

Modern smart valve positioners allow electronic flow characterization to modify the valve's input-output relationship. This capability can compensate for non-ideal process characteristics or customize the valve response for specific control requirements.

Practical Recommendations

Most process control applications perform best with equal percentage trim. Linear trim is appropriate for liquid level control with constant head and for some pressure control applications. Quick-opening trim should be limited to on/off and startup/bypass service.