Linear vs Equal Percentage Valve Characteristic Explained

The Short Answer

Valve characteristic describes how flow changes as the valve opens. A linear characteristic produces a flow change roughly proportional to the opening change, while an equal percentage characteristic produces a small flow change at low openings and a large change at high openings, following a fixed percentage relationship. The choice depends on the process: linear suits processes with constant pressure drop, while equal percentage generally suits processes where the pressure drop across the valve falls as flow increases.

What a Characteristic Curve Tells You

Every control valve has an inherent characteristic defined by the trim geometry, usually published by the manufacturer as a curve of flow versus opening. Two curves dominate industrial practice: linear and equal percentage. A third, modified parabolic shape is sometimes used for special cases. The installed characteristic, which includes the effect of the piping system and varying pressure drop, can differ from the inherent curve, which is why valve selection software models the whole system rather than just the valve.

Linear Characteristic

With a linear characteristic, equal increments of stem travel produce approximately equal increments of flow, assuming constant pressure drop. This behavior is useful when the pressure drop across the valve is relatively stable, such as level control with a nearly constant supply, or when the loop gain needs to stay predictable. Linear trim is also common in some on-off applications, although most on-off service does not require a specific characteristic.

Equal Percentage Characteristic

An equal percentage characteristic gives the same percentage change in flow for each equal change in opening. At low openings the flow change is small, providing fine control; at high openings the change is large, which compensates for the falling pressure drop typical of pump-fed systems. This makes equal percentage the general-purpose choice for flow and pressure control loops where the available pressure drop decreases as flow increases, which is the most common situation in process plants.

Which to Choose

Condition	Typical Choice
Pressure drop roughly constant across the stroke	Linear
Pressure drop falls as flow increases (pump-fed process)	Equal percentage
Fine control needed at low flow rates	Equal percentage
Level control with stable supply pressure	Linear or modified
General flow/pressure regulation	Equal percentage (default)

Common Misconceptions

A common misconception is that an equal percentage valve is “more precise” everywhere. In fact, at high openings its flow change per stroke is large, so if the process operates mostly wide open, a linear valve may give better control. Another misconception is that the characteristic can be ignored for on-off valves; for on-off service it rarely matters, but for any throttling duty the installed characteristic should be reviewed with the loop design.

Practical Recommendation

Select the characteristic based on the shape of the system pressure-drop curve, not on habit. For the typical pump-fed process loop, equal percentage is the safe default. If the pressure drop is flat across the stroke, linear may give more stable control. When in doubt, model the installed characteristic with the piping system before finalizing the trim. For related selection logic, see our isolation vs control valve discussion and the broader valve selection guides.

Conclusion

Linear and equal percentage characteristics serve different pressure-drop environments. Choosing the right trim shape is a control loop decision that affects stability and rangeability. Matching the characteristic to the installed system behavior is more important than choosing by convention.

This article provides general educational knowledge only and is not professional engineering advice.

Tags: Valve Selection