Valve Flow Characterization: Equal Percentage vs. Linear

Control valve flow characterization defines the relationship between valve travel (stem position) and flow coefficient (Cv). The two primary inherent flow characteristics—linear and equal percentage—determine how a control valve responds to changes in input signal. Selecting the appropriate flow characteristic is critical for achieving stable, well-behaved control loop response across the operating range of the process. Mismatch between the valve characteristic and process gain leads to poor control performance, hunting, and potential process upsets.

Linear Flow Characteristic

A valve with a linear inherent characteristic produces a directly proportional relationship between valve travel and Cv: a 10% increase in valve travel produces a 10% increase in Cv regardless of position. Linear characteristics provide constant valve gain (change in Cv per unit of travel) throughout the valve stroke. Linear characteristics are appropriate when the process gain is constant—typically when the pressure drop across the valve is a large fraction of the total system pressure drop and does not vary significantly with flow. Globe valves with V-port plugs are commonly supplied with linear characteristics.

• Linear: equal incremental Cv change per percent of travel

• Constant valve gain: uniform control response at all positions

• Best suited: high valve pressure drop ratio (>0.5 of total system drop)

• Applications: pressure control loops, constant pressure drop systems

• Disadvantage: poor control at low flow if process gain decreases with flow

Equal Percentage Flow Characteristic

An equal percentage valve produces an equal percentage change in Cv for each equal increment of valve travel: opening from 40% to 50% and from 80% to 90% both produce the same percentage increase in Cv (typically 25-50% per 10% travel increment). The equal percentage characteristic compensates for decreasing valve pressure drop at higher flows in typical piping systems, where pumping pressure is largely consumed by pipe friction. As flow increases, pipe friction losses increase, reducing the pressure available across the valve. The decreasing valve pressure drop reduces valve gain, which the equal percentage characteristic offsets by providing increasing Cv gain at higher travels.

Installed vs. Inherent Characteristic

The inherent characteristic describes valve behavior at constant pressure drop. The installed characteristic describes behavior in an actual piping system where valve pressure drop varies with flow. In most process systems, the installed characteristic of an equal percentage valve approaches a linear characteristic, making equal percentage the preferred default specification for most flow, level, and temperature control applications. The installed characteristic should be verified by calculating valve Cv across the operating flow range using the actual system pressure drop variation to confirm adequate controllability at minimum and maximum flow conditions.