Cv is the valve’s flow capacity: Q = Cv·√(ΔP/SG). Sizing means choosing a Cv that gives the duty flow at a sensible opening — not wide open, not nearly shut.
The inherent characteristic (linear, equal-percentage, quick-opening) is the flow-vs-travel curve at constant ΔP. Equal-percentage is the usual choice because, once piping pressure losses are added, its installed characteristic comes out roughly linear — giving even loop gain.
Valve authority (the valve’s share of the system ΔP) is what distorts inherent into installed. Low authority — an oversized valve in a high-loss system — ruins controllability. And dropping a lot of pressure can trigger cavitation/flashing that destroys the trim.
1 · Cv — the capacity number
Every control valve is rated by its flow coefficient Cv (or metric Kv) — the flow it passes for a given pressure drop:
Sizing is a balancing act. Choose the valve so the normal duty falls comfortably in its controllable mid-range — roughly 20–80% open. Too small and it can’t pass max flow; too large and it does all its work in the first crack of travel, where control is coarse and unstable. A valve that runs nearly shut to hold normal flow is the classic oversizing mistake — the same trap as oversizing a pump.
2 · The inherent characteristics
The inherent characteristic is the relationship between flow and valve travel at a constant pressure drop — a property of the trim shape. Three are standard:
| Characteristic | Shape | Use when… |
|---|---|---|
| Linear | Flow ∝ travel | Most of the system ΔP is across the valve (high authority); level & some flow loops. |
| Equal-percentage | Each equal travel step changes flow by an equal percentage (small at the bottom, large at the top) | Pressure drop varies a lot with flow (most piping systems). The default choice. |
| Quick-opening | Most flow in the first part of travel | On/off and some pressure-relief duties, not modulating control. |
Equal-percentage dominates because real systems aren’t constant-ΔP — and that is the whole point of the next idea.
3 · Valve authority & the installed characteristic
In a real line, the valve shares the available pressure with the pipe, fittings, exchangers and the pump curve. As the valve opens and flow rises, the system losses grow as flow² and eat into the pressure left for the valve — so the valve’s actual flow-vs-travel curve, the installed characteristic, is not its inherent one.
How much it distorts is set by the valve authority N — the fraction of total system ΔP taken by the valve when fully open:
This is why equal-percentage is the default: its inherent curve bows the opposite way, so at moderate authority the installed characteristic straightens out to roughly linear — constant loop gain across the range. Move the authority slider and watch a well-chosen equal-% valve stay linear while a linear valve at low authority turns into an unusable quick-opener:
Interactive — Inherent vs installed characteristic
Live modelFlow vs valve travel
q = f/√(N+(1−N)f²), normalised so q=1 at full travel. “Gain” is the local slope dq/dh; the best controllability has a flat gain (spread near 1:1) across the range.4 · Cavitation & flashing
A control valve works by converting pressure into turbulence and heat. As the fluid accelerates through the restriction, its static pressure dips — at the vena contracta, just downstream of the throttling point, it reaches its lowest. If that local pressure falls below the liquid’s vapour pressure, the liquid boils into bubbles, exactly as in a cavitating pump:
- Cavitation — pressure recovers downstream above vapour pressure, so the bubbles collapse violently against the trim and body, hammering the metal with implosions that sound like gravel and erode the trim in weeks. The cure: anti-cavitation trim (staged pressure letdown), more back-pressure, or splitting the drop across two valves.
- Flashing — downstream pressure stays below vapour pressure, so the bubbles don’t collapse; the two-phase jet erodes the body by high-velocity impingement instead. Different damage, harder geometry to fix; choose erosion-resistant materials and body styles.
Both also choke the valve — once flashing begins, increasing ΔP no longer increases flow (the gas analogue is the compressor’s stonewall). Sizing for liquids therefore checks the cavitation/choking limit, not just Cv.
The control valve closes the loop with the rest of the plant. It is where the throttling-vs-VFD energy question is decided, its cavitation is the same physics as a pump’s, and worn or sticking trim shows up in process variability and in valve-signature diagnostics. A sluggish loop is often a sized-and-characterised-wrong valve, not a tuning problem.
Key takeaways
- Cv sets capacity (Q = Cv·√(ΔP/SG)); size so normal duty sits at ~20–80% open.
- Equal-percentage is the default because its installed characteristic comes out roughly linear in real, lossy piping.
- Valve authority distorts inherent into installed — low authority (oversized valve, lossy system) wrecks loop gain; aim for N ≳ 0.25–0.5.
- Cavitation and flashing destroy trim when the vena-contracta pressure drops below vapour pressure — size for it and use anti-cavitation trim.