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How to Select a Control Valve Flow Characteristic

Most control valve selection arguments are really about one question: which inherent characteristic should the trim have? The answer follows from valve authority, not from habit.

The Question, Stated Plainly

Which flow characteristic should I specify — linear, equal percentage, or quick opening?

The short answer: choose the characteristic that makes the installed characteristic close to linear across the range you actually operate in. The inherent characteristic is what the trim does in a laboratory with constant pressure drop. The installed characteristic is what the loop sees in your pipework, where pressure drop changes as the valve moves. Only the installed characteristic matters to the controller.

The Three Inherent Characteristics

Characteristic	What it does	Typical trim	Best when
Linear	Flow rises in direct proportion to travel: 50% travel gives about 50% flow	Slotted or windowed cage, V-port ball	Pressure drop across the valve is roughly constant, or a large part of the system drop
Equal percentage	Each equal increment of travel multiplies flow by the same factor	Contoured cage, most standard globe trims	Most of the system pressure drop is elsewhere and varies — the common case
Quick opening	Most of the flow arrives in the first third of travel	Disc or single-seat on/off trims	On/off duty, bypass, or relief where you want fast initial capacity

The Rule That Decides It: Valve Authority

Valve authority (sometimes written N) is the fraction of the total system pressure drop that the fully open valve takes:

Authority = ΔP across the wide-open valve ÷ ΔP across the whole system at design flow

The practical guidance is well established in control valve practice:

• Authority above roughly 0.5 — a linear trim behaves close to linear when installed, and equal percentage becomes increasingly distorted.
• Authority between about 0.2 and 0.5 — an equal percentage trim tends to give the most nearly linear installed behaviour. This is why equal percentage is the default in so many specifications.
• Authority below about 0.2 — the valve is small relative to system losses; installed behaviour becomes very non-linear and no inherent characteristic fully fixes it. Re-examine the sizing rather than the characteristic.

How to Check Your Choice Against the Datasheet

The claim above should be verifiable, so here is how to test it on a real submittal rather than taking it on faith.

1. Get the rated Cv (or Kv) from the datasheet. Note whether it is the full-open value and which travel it corresponds to.

2. Get the system pressure drop at design flow with the valve wide open. If the datasheet only gives the valve’s own ΔP, ask the process engineer for the loop total.

3. Compute authority. Divide. If the answer is near 0.3, an equal percentage trim is defensible; if it is near 1.0, a linear trim is.

4. Plot the specified flow at 25%, 50% and 75% travel using the manufacturer’s characteristic table, and check that the loop has usable resolution where the process needs control. A loop that does all its work between 85% and 100% travel will never control well, whatever the characteristic.

A Short Worked Example

A cooling water loop needs 40 m³/h at design. With the control valve wide open, the valve takes 12 m of the loop’s 40 m total head; the rest sits in the exchanger, strainer and pipework.

Authority = 12 ÷ 40 = 0.30.

That lands in the middle band, so an equal percentage trim is the right starting point. Specifying a linear trim here is the classic mistake: the installed curve would be badly bowed, and the loop would be over-sensitive at high travel and sluggish at low travel.

Two Cases Where the Rule Is Deliberately Broken

Temperature control on a heat exchanger with a large turndown. Where the process gain varies strongly with load, a modified or custom characteristic may outperform any of the three standard shapes. That is a conversation with the valve supplier, not a catalogue pick.

Small valves in a wide loop. Below roughly DN25 the choice of characteristic is often secondary to resolution and to whether the actuator can actually position that finely. Check the actuator before agonising over the trim.

What to Put in the Enquiry

Send the supplier four things and the characteristic question largely answers itself: design flow, the pressure drop available across the valve at that flow, the total loop pressure drop, and the turndown you need. With those, a supplier can calculate rather than guess — and you can check their answer using the method above.

Frequently Asked Questions