Valve characteristic selector
The trim characteristic determines how flow changes with valve travel, and it interacts directly with tuning. A mismatched characteristic makes the process gain vary across the operating range, which means no single tuning set can hold.
| Service | Usual characteristic | Why |
|---|---|---|
| Flow control, roughly constant pressure drop | Linear | Keeps the installed gain flat across the range, so one tuning set holds. |
| Flow control, pressure drop falls sharply with flow | Equal percentage | The trim compensates for the falling drop, linearising the installed response. |
| Level control | Linear | Level is already integrating; a nonlinear valve makes it harder, not easier. |
| Temperature via steam or coolant flow | Equal percentage | Matches the typically nonlinear heat transfer relationship. |
| Pressure reduction across a large drop | Equal percentage | Better resolution where the valve spends most of its time, near closed. |
| On/off or relief service | Quick opening | Most of the capacity is delivered early in the travel. Not for throttling. |
This is the usual starting point, not a specification. What actually matters is the installed characteristic in your system, which depends on how the pressure drop across the valve varies with flow.
The three characteristics
- Linear — flow proportional to travel. Suits services where the pressure drop across the valve stays roughly constant, which includes most flow control and most level control.
- Equal percentage — each increment of travel changes flow by a fixed percentage of the current flow. Suits services where the pressure drop falls substantially as flow rises, which the characteristic compensates for.
- Quick opening — most of the flow change happens early in the travel. Mainly an on/off and relief application; rarely appropriate for throttling.
Why a mismatch shows up as a tuning problem
What matters is the installed characteristic — the relationship between travel and flow once the valve is in the actual system, with the actual pressure drop. That can be very different from the inherent characteristic stamped on the plate.
An equal-percentage valve in a system with constant pressure drop gives an installed characteristic that is strongly nonlinear, so the controller sees a process gain that changes several-fold across the range. The loop is stable where it was tuned and unstable elsewhere, which is usually reported as an intermittent tuning problem.
Related
Glossary
Last reviewed 2026-08-01.