Equal Percentage vs Linear vs Quick-Opening: Control Valve Flow Characteristics Compared

Why does a valve that works perfectly in one loop hunt badly in another? Very often the answer is the flow characteristic — the relationship between travel and capacity. This article compares the three inherent characteristics, shows how each behaves once installed in a real system, and gives a practical selection method.

1. What a flow characteristic is

The inherent flow characteristic is the curve of flow rate versus valve travel (plug position) measured at a constant pressure drop — typically the drop the test stand holds at each point. It is a property of the trim geometry: the contour of the plug or cage windows, or the port shape of a rotary element.

Three inherent characteristics matter in practice:

Linear — capacity proportional to travel.
Equal percentage — equal increments of travel produce equal percentage changes of flow.
Quick-opening — a large flow change near closed travel; used mostly for on-off service and relief-like duties.

Inherent flow characteristics at constant pressure drop. Equal percentage (R = 50 rangeability) plotted per its exponential definition; quick-opening shown as the common square-root approximation.

2. The equations behind the curves

Linear: Q = Q_max × h — flow is directly proportional to travel.

Equal percentage: Q = Q_min × Rᵌ — each 1 % of travel multiplies flow by a constant factor. R = Q_max/Q_min is the rangeability (50:1 is typical for globe trim).

Quick-opening (common form): Q ≈ Q_max × √(1−(1−h)²) — most of the capacity is reached in the first half of travel.

The practical difference is loop gain — dQ/dh, the change in flow per unit travel. A linear valve has constant gain. An equal-percentage valve has low gain where it spends most of its travel (small openings) and high gain when wide open. A quick-opening valve has very high gain just off the seat and almost none afterwards.

3. Why the installed characteristic is what matters

Real systems rarely hold a constant pressure drop across the valve. Pumps, heat exchangers and pipework consume more of the available head as flow rises, so the valve's share of the pressure drop is largest at low flow and smallest at high flow. The curve the loop actually sees — the installed characteristic — is therefore the inherent curve reshaped by this falling ΔP.

The strength of this distortion is expressed by the pressure-drop ratio ΔP_valve/ΔP_total:

Valve takes most of the drop (ratio ≥ ~0.7): installed ≈ inherent. A linear valve stays linear; the characteristic is essentially what the datasheet shows.
Valve takes little of the drop (ratio ≤ ~0.3): the installed curve bends sharply. A linear valve becomes quick-opening-like — huge gain at small travel, dead above mid-travel. An equal-percentage valve bends toward linear.

This reshaping is the single most important fact in characteristic selection: you compensate in the trim for what the hydraulics will do to the curve.

4. Selection in practice

Choose equal percentage when

The valve operates at small openings much of the time (wide throttling range).
The valve takes a small, varying share of the total system pressure drop — the classic case of a valve on a long process line fed by a centrifugal pump.
Stable control is required over a wide load range: low gain at low flow keeps a possibly unstable small-signal process (e.g. heat transfer with a steep temperature/flow curve) from oscillating.

Choose linear when

The valve takes most of the system pressure drop and that drop is fairly constant — e.g. liquid pressure-dropping stations, recirculation loops, and many level-control applications where the process itself has near-constant gain.
The process gain is roughly independent of load, so the loop wants a valve whose gain is too.

Choose quick-opening when

The duty is essentially two-position (on-off) with throttling only at the ends — batch drains, bypasses, startup vents.
A large, fast flow change is wanted from a small actuator travel, and fine control near the set point is not required.

5. Common misconceptions

"Equal percentage is more accurate." No characteristic is intrinsically more accurate; equal percentage is a compensation for falling valve ΔP and rising process gain. In a high-drop application it will control worse than linear.
"The datasheet curve is what the loop sees." Only when the valve holds most of the drop. Always sketch the installed curve before blaming the controller tuning.
"Rotary valves are always equal percentage." S-ball and characterized-ball elements are close to equal percentage, but many butterfly and V-port designs are intermediate; some are adjustable by changing the characterizing plate.
"Rangeability equals turndown." Rangeability is the trim's inherent Q_max/Q_min capability; installed turndown is smaller because leakage, actuator resolution and the installed ΔP reduce the usable span.

6. A worked feel for the numbers

Take an equal-percentage valve with R = 50 moving from 40 % to 50 % travel. Each 10 % of travel multiplies flow by 50^0.10 ≈ 1.42 — a 42 % flow increase. The same 10 % step taken at 90 % travel also gives 42 % — that self-similarity is what makes the characteristic "equal percentage". A linear valve's 10 % step always adds the same absolute flow; at low flow that can be a 200 % change of process input, at high flow only a few percent.

Related reading: the valve's contribution to the loop starts with how it throttles — see How a Control Valve Works. For converting your process data into a valve coefficient, use the Liquid Cv Calculator. Field symptoms caused by a mismatched characteristic (hunting at some loads, sluggish at others) are covered under Troubleshooting.

Editorial note. The characteristic equations follow IEC 60534-2-1 and ISA-75.01.01 conventions; installed-curve behaviour follows the standard constant-volume system model. Engineering content is educational — verify selection against the applicable standard and manufacturer data before use in actual installations.