Control Valve Characteristics: Why Equal Percentage Usually Wins
9 min read · updated 2026-08-15
A loop that hunts at low flow and crawls at high flow rarely has a tuning problem. It usually has a gain problem — and gain was decided by the trim in the valve and the pressure drop allocated to it, long before anyone opened the controller faceplate.
Two curves, and only one is on the datasheet
The inherent characteristic describes how flow coefficient varies with travel at constant pressure drop. It is a bench measurement, determined purely by the shape of the plug and seat, and it is what the vendor plots.
The installed characteristic is what happens in a real pipe. Open the valve, flow rises, friction losses everywhere else rise with the square of flow, and those losses steal pressure drop from the valve. The valve gets less differential exactly when it is trying to pass more flow, so the curve flattens at the top.
The distortion always runs the same direction: toward quick opening. Lots of flow early, then saturation.
Valve authority decides how bad it gets
At N = 1 the valve owns the entire pressure drop and installed equals inherent. At N = 0.1 the pipework dominates and the valve does nearly all its work in the first third of travel, whatever trim is fitted. Between those, everything is a matter of degree.
Authority is not something you discover later — it is decided at sizing time, by how much pressure drop you allocate to the valve. Allocating generously costs pumping energy forever. Allocating meanly saves that energy and buys a control problem that also lasts forever. The common compromise is 25–33% of total dynamic loss at maximum flow, and our valve sizing guide covers how that fits into the wider sizing exercise.
The three trims
Linear — Cv proportional to travel. Sensible where the valve genuinely sees near-constant pressure drop: high authority systems, and level loops where the valve discharges to roughly fixed conditions.
Equal percentage — each equal increment of travel changes flow by an equal percentage of the current flow, not by an equal absolute amount. Cv follows R raised to the power of fractional travel minus one, where R is the rangeability, typically 50. The curve starts shallow and steepens toward the top.
Quick opening — most of the capacity arrives in the first part of travel. Suited to on-off and relief duty, not modulating control.
Why equal percentage is the default
Here is the whole argument in one sentence: system friction bends the curve toward quick opening, equal percentage trim bends it the opposite way, and the two roughly cancel.
Put equal percentage trim into a system with authority around 0.3–0.5 and the installed characteristic comes out close to a straight line. Put linear trim into the same system and it comes out distinctly quick-opening — steep early, flat late. You can see both in the valve characteristic calculator, which plots inherent and installed curves together as you change authority.
What you are really buying: constant gain
Installed gain is the slope of that installed curve — percent flow change per percent travel change. It matters because it multiplies directly into loop gain, and a controller can only be tuned for one gain at a time.
A loop whose gain varies two-fold across the operating range can be tuned once and will behave acceptably everywhere. A loop whose gain varies five-fold cannot: tune it for stability at high flow and it is sluggish at low flow; tune it for response at low flow and it oscillates at high flow. Operators then reduce the gain until it stops oscillating, and the loop is slow everywhere. No amount of PID adjustment fixes a mechanical gain problem — which is worth knowing before reaching for the tuning calculator.
How this shows up in the field
- Hunting near the seat, sluggish when open — classic low authority with linear trim, or simply an oversized valve.
- Valve sits below 20% travel in normal operation — oversized. It is working where installed gain is steepest and mechanical resolution is worst, which is the least controllable part of its range.
- Loop was tuned during commissioning and has been retuned three times since — usually a gain-variation signature rather than a process change.
- Digital positioner characterisation can partially compensate a mismatched trim in software. Useful as a retrofit when replacing the valve is impractical, but it cannot manufacture pressure drop that was never allocated.
The practical sequence
- Allocate valve pressure drop at sizing — this sets authority, and it is the decision that matters most.
- Choose trim to suit that authority: linear if authority is high, equal percentage if it is moderate or low.
- Size so maximum flow lands at 70–80% travel and normal flow at 50–70%.
- Check installed gain variation across the working range before tuning.
- Then tune — and if it will not tune, revisit steps 1 to 3 rather than the controller.
Calculators used in this guide: valve characteristic & installed gain · liquid Cv sizing · gas & steam Cv sizing · PID tuning