Butterfly Valve Cv & Flow Coefficient: Sizing for Flow, Pressure Drop and Cavitation
Written by
Allen Zhang · Senior Application Engineer, LAUX VALVE
Torque sizing tells you which actuator turns the valve; flow sizing tells you whether the valve will actually pass the flow you need without choking it or tearing itself apart with cavitation. The number at the centre of flow sizing is the flow coefficient — Cv in US units or Kv in metric — and getting it right is what stops engineers from oversizing a valve so much that it only ever cracks open, or undersizing one so it screams and pits. This guide explains Cv and Kv, the pressure-drop equation, how a butterfly valve's flow characteristic shapes control, the cavitation limit, and a worked example you can copy.
Cv and Kv: what the flow coefficient means
The flow coefficient is simply a measured capacity rating: Cv is the number of US gallons per minute of 60 °F water that flow through the fully open valve when the pressure drop across it is exactly 1 psi. Kv is the metric twin — cubic metres per hour of water at a 1 bar drop. A bigger Cv/Kv means a freer-flowing valve. Because a butterfly valve's disc stays in the stream, its Cv at full open is high but not as high as a same-size full-bore gate or ball valve. Crucially, Cv is not a single number: it changes with disc angle, so manufacturers publish a Cv (or Kv) value for each opening from about 10° to 90°.
| Disc angle | Approx. Cv | Approx. Kv | % of full Cv |
|---|---|---|---|
| 20° | 55 | 48 | ~7% |
| 40° | 180 | 156 | ~23% |
| 60° | 430 | 372 | ~55% |
| 70° | 620 | 536 | ~79% |
| 90° (full) | 785 | 679 | 100% |
The pressure-drop equation
For incompressible liquids well away from cavitation, the relationship between flow, pressure drop and coefficient is the basic valve equation: Q = Cv × √(ΔP/SG), where Q is flow in GPM, ΔP is pressure drop in psi and SG is the fluid specific gravity (1.0 for water). Rearranged to find the coefficient you need: Cv = Q / √(ΔP/SG). In metric terms, Q (m³/h) = Kv × √(ΔP_bar / SG). Two habits keep you out of trouble: always work at the flow and ΔP that actually occur at the control point, and confirm the chosen valve reaches that Cv well before 90° so you keep controllable travel in reserve.
Flow characteristic and why oversizing kills control
Oversized valve (controls at 10–30°)
- All the control happens in the first sliver of travel — twitchy, unstable
- Disc parked nearly closed — high velocity jet, prime cavitation zone
- Seat and disc edge erode quickly from the throttling jet
Correctly sized (controls at 50–70°)
- Control spread across the responsive mid-band of travel
- Moderate disc angle — lower velocity, less cavitation risk
- Reserve travel left to handle future flow increases
The cavitation limit
When a liquid accelerates through the throttled gap, its local pressure falls. If it drops below the fluid's vapour pressure, vapour bubbles form and then collapse violently as pressure recovers downstream — that is cavitation, and it sounds like gravel, erodes the disc and seat, and can destroy a valve in weeks. The usual guard is the cavitation index σ = (P1 − Pv) / (P1 − P2), where P1 and P2 are upstream and downstream pressures and Pv the vapour pressure. Compare your σ against the manufacturer's published incipient/choking limits for that valve at that opening; if you are below the limit, open the valve more (lower ΔP per valve), split the drop across two valves in series, or fit an anti-cavitation trim.
Worked example: sizing for a chilled-water line
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1. State the duty
Water at 7 °C, design flow Q = 250 m³/h, allowable valve drop ΔP = 0.4 bar, SG ≈ 1.0. We want this flow at a controllable mid-travel angle, not wide open.
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2. Compute the required Kv
Kv = Q / √(ΔP/SG) = 250 / √(0.4/1.0) = 250 / 0.632 ≈ 395 m³/h. This is the Kv the valve must deliver at the chosen control angle, not at full open.
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3. Pick the valve from its Kv curve
From the manufacturer's curve, a DN200 valve reaches Kv ≈ 395 at roughly 62° open — right in the controllable band. A DN250 valve would hit it near 45° (still fine); a DN150 valve would need ~80° (too far open, little reserve).
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4. Check velocity and cavitation
Confirm line velocity stays within ~3–4 m/s for water, then compute σ from system pressures and compare with the valve's cavitation limit at 62°. With 0.4 bar drop and ample downstream pressure, σ is comfortably above the limit — no cavitation expected.
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5. Confirm DN200 and lock the spec
DN200 controls at ~62°, keeps velocity in range, avoids cavitation, and leaves travel in reserve. Document the design flow, ΔP, control angle and σ on the data sheet so the choice is auditable.
Frequently asked questions
What is the difference between Cv and Kv?
They are the same concept in different units. Cv is imperial: US gallons per minute of 60 °F water through the fully open valve at a 1 psi pressure drop. Kv is metric: cubic metres per hour of water at a 1 bar drop. Convert with Cv ≈ 1.156 × Kv, or Kv ≈ 0.865 × Cv. Use whichever matches your data sheet, but never mix them in one calculation — a value labelled Kv plugged into a Cv equation will be off by about 15%.
Why shouldn't I size a butterfly valve to run at 90% open?
Because near full open the flow characteristic is flat — large angle changes barely change flow, so you have almost no controllability and no reserve for higher flow later. You also can't trim out errors in the system curve. Sizing the design flow to occur around 60–70% keeps the valve on the responsive part of its characteristic, leaves headroom for future demand, and avoids forcing it to operate wide open where it adds little control value. The opposite error — sizing so flow needs only 10–30° — is just as bad because it parks the disc in the cavitation-prone, erosion-prone zone.
How do I know if my butterfly valve will cavitate?
Compute the service cavitation index σ = (P1 − Pv) / (P1 − P2) from your upstream and downstream pressures and the fluid's vapour pressure, then compare it against the manufacturer's published cavitation coefficient for that valve at the operating angle. If your σ is above the incipient limit, you are clear; if it falls between incipient and choking, expect noise and gradual damage; below choking, expect rapid erosion. The fixes are to reduce the per-valve ΔP (open the valve more or upsize), stage the pressure drop across two valves, raise downstream pressure, or use anti-cavitation trim. Always check σ at the worst real operating point, not just the design point.
Does a butterfly valve have a higher pressure drop than a ball or gate valve?
When fully open, yes — slightly. A butterfly valve's disc stays in the flow path, so its full-open Cv is a bit lower (and its pressure drop a bit higher) than a same-size full-bore ball or gate valve, whose bore is unobstructed. The difference is usually small and rarely decisive: the butterfly valve's huge advantages in weight, cost and footprint at large diameters normally outweigh the modest extra loss. Where pumping energy over the life of a continuously-open large line truly dominates the cost, a full-bore gate valve's near-zero loss can be worth its higher first cost — but for most water, HVAC and process duty the butterfly valve wins overall.
References & further reading
- IEC 60534-2-1 — Control valve sizing equations (incompressible flow)
- ISA-75.01.01 — Flow Equations for Sizing Control Valves
- Crane Technical Paper 410 — Flow of Fluids Through Valves, Fittings, and Pipe
- AWWA Manual M49 — Butterfly Valves: Torque, Head Loss, and Cavitation Analysis
- IEC 60534-8-2 — Laboratory measurement of noise generated by hydrodynamic flow
