The flow characteristic is the relationship between valve travel and flow rate. It’s arguably the most important spec on a control valve data sheet that doesn’t involve materials or pressure rating — and one of the most commonly defaulted to “equal percentage” without anyone thinking about why.
The three standard characteristics are linear, equal percentage, and quick open. They’re achieved by physically shaping the plug, drilling the cage, or both. Picking the right one is the difference between a control loop that tunes once and stays put, and one that oscillates or hits the stops every time the operating point shifts.
What the three characteristics actually do
The characteristic is measured at constant ΔP across the valve. That’s an idealization — in real piping, ΔP changes with flow — but it’s how the trim is designed and rated. Here’s the shape of each at constant ΔP:
| Characteristic | Shape (flow vs travel) | Key trait |
|---|---|---|
| Linear | Straight line. 50% travel = 50% Cv. | Equal change in flow for equal change in travel. Intuitive. |
| Equal percentage | Exponential curve. 50% travel might be 15–20% Cv; 80% travel might be 50% Cv; most capacity in the last 20%. | Equal percentage change in flow for equal change in travel. Compensates for piping pressure drop. |
| Quick open | Steep ramp up to 30–40% travel, then flat. | Most flow with the first bit of stroke. Then nothing. |
The reason equal percentage exists at all is that installed characteristic differs from the trim characteristic. Once a valve is in a piping system, the ΔP across the valve decreases as flow increases (more pressure drop in the pipe). An equal-percentage trim in a typical piping configuration tends to look approximately linear once installed. That’s the design intent.
Linear: the right call when ΔP stays constant
Linear trim is the right choice when the valve operates with roughly constant ΔP across it — meaning the piping doesn’t dominate the pressure-drop budget. Common cases:
- Level control on a tank where head pressure is roughly constant
- Liquid flow control with a parallel valve or pump bypass setting a constant upstream pressure
- Steam pressure regulation with the valve sized so it’s the dominant restriction
- Any service where pressure drop ratio (valve ΔP / total ΔP) is > 0.5 across the range
Linear gives a control loop that’s easy to tune: the loop gain stays roughly constant across the operating range, so a single PID tuning works at 30%, 60%, and 90% flow.
Equal percentage: the default for piping-dominated systems
Equal percentage is the right choice when the valve has to share the pressure-drop budget with significant piping, heat exchangers, or other downstream restrictions. That covers most refinery and chemical-plant control loops.
The reason: as flow increases, more of the available pressure drop is consumed by pipe friction, leaving less for the valve. A linear trim in this situation would have low gain at low flow (valve is throttling but not much ΔP change) and high gain at high flow (valve is almost open but the piping is choking the loop). The loop becomes unstable across the range.
Equal percentage compensates by giving you less capacity per percent of stroke at the bottom and more at the top — matching the rising piping resistance to keep the installed gain approximately constant.
When equal percentage is the right call
- Most temperature control loops on heat exchangers
- Pressure control where the valve is upstream of significant piping
- Flow control loops with parallel-path piping or significant elevation
- Pressure drop ratio (valve ΔP at max flow / total ΔP) below 0.3
Quick open: it’s really an on/off characteristic
Quick-open trim isn’t meant for modulating control. It’s used where you need most of the flow with minimal stroke — effectively for two-position service that needs slightly more than a slam-shut on/off valve.
Use cases are narrow:
- Relief valve trim where you need full opening on overpressure events
- Bypass and recirculation valves that are either mostly closed or mostly open
- Pressure relief recirculation on positive-displacement pumps
- Bottoms pump-around where the duty is on/off
If you find yourself spec’ing quick open for a PID-controlled loop, stop. You want linear or equal percentage. Quick open will make the loop unstable above 30% travel because the valve has effectively no remaining authority.
How the characteristic is actually achieved
Two construction styles produce the characteristic:
Contoured plug (V-port or characterized)
The plug is machined with a profile — a V-shaped notch or a contoured taper — that opens progressively as the plug retracts from the seat. This is the classic single-seated globe valve construction. The characteristic is built into the plug shape.
Characterized cage (multi-hole or slotted)
The cage surrounding the plug has a pattern of holes or slots. As the plug retracts, more of the holes are exposed. The hole pattern is calculated to produce the desired characteristic.
Cage trim has advantages: quieter, easier to maintain (replaceable cage rather than full plug), and the same valve body can take linear, equal-percentage, or even noise-attenuating cages depending on the inserted cage. For modern control valves, characterized cage is the more common construction.
What changes the installed characteristic from rated
The rated characteristic is at constant ΔP. The installed characteristic is what you actually see in service. Three factors distort it:
- Rising piping ΔP with flow. The main reason equal percentage exists. The higher the piping’s share of total drop, the more the installed characteristic moves from rated toward linear (equal percentage installed) or toward quick-open (linear installed).
- Cavitation or choked flow. Above the choked-flow limit, additional plug travel adds no capacity. The installed characteristic flattens at the top.
- Trim wear. An eroded plug or cage shifts the characteristic toward quick-open over time. A loop that was tuned six years ago may have drifted significantly — not because the controller changed but because the trim shape did.
How to spec the right characteristic
Three steps:
- Calculate the installed pressure drop ratio at min, normal, and max flow. This is the single most important data point.
- Match the characteristic to the ratio (linear if > 0.5; equal % if < 0.3; ask the vendor for either if 0.3–0.5).
- Plot the installed characteristic — vendor sizing software will do this. If it’s within ±25% of linear across the operating range, the loop will tune easily. If not, reconsider the trim.
Run the ΔP ratio in our Cv calculator
Plug in your flow conditions and the calculator returns required Cv, choked-flow indication, and a recommended valve size. Use the result to confirm the installed characteristic is what you spec’d.
The mistakes that cause loop instability
- Defaulting to equal percentage on a constant-ΔP service. The loop gain rises at the top of stroke and the controller hunts. Switch to linear and the oscillation goes away.
- Linear in a piping-dominated system. Loop is sluggish at low flow, sensitive at high flow. The fix is equal percentage, not retuning.
- Oversized linear valves. Operating at the bottom 20% of stroke amplifies the “low-flow sluggishness” problem; the loop never settles. Resize the valve, don’t change the characteristic.
- Spec’ing characteristic without checking the installed plot. The rated curve looks fine, the installed curve looks like a step function. Always plot installed at min, normal, and max.
- Assuming the characteristic stays the same after trim replacement. A vendor-substituted cage with different hole pattern can change the installed characteristic. Verify the part number after every PM.
The bottom line
Three characteristics, two real choices (linear or equal percentage; quick open is on/off-adjacent), one decision rule: installed pressure drop ratio. Above 0.5, linear. Below 0.3, equal percentage. In between, either — default to equal percentage unless the piping budget says otherwise.
Most loop-tuning problems on globe-valve control loops aren’t controller-tuning problems. They’re characteristic-mismatch problems that look like tuning problems. If your loop hunts at the top of stroke and tunes fine at the bottom (or vice versa), suspect the characteristic before you retune.
If you’ve got a loop that won’t behave or a new service where the characteristic isn’t obvious from the piping, send us the conditions. We’ll run the installed-curve analysis and recommend the trim style and size that gives the controller a fighting chance.