How do I select a control valve?

Select a control valve in nine stages: collect process data, calculate the required Cv or Kv, check for choked flow, flashing and cavitation, choose the valve body type, choose the flow characteristic, specify pressure class and materials, select the actuator and fail position, verify performance, then complete the specification sheet. Bore size follows from the calculated Cv, not from the pipe size.

How is Cv calculated for a control valve?

Cv is the US liquid flow coefficient: the flow in US gallons per minute that produces a 1 psi pressure drop across the valve at fully open with water at about 60 degrees Fahrenheit. For liquids, Cv = 1.96 x Ft x Q divided by N x Fl x Fp x the square root of (P1 minus P2), where Q is flow in gpm, N is the number of flow paths, P1 and P2 are upstream and downstream pressure in psia, and Ft, Fl and Fp are temperature, liquid recovery and piping geometry correction factors.

What is the difference between Cv and Kv?

Cv and Kv measure the same flow capability in different units. Cv uses US gallons per minute at a 1 psi drop, Kv uses cubic metres per hour at a 1 bar drop. Kv is approximately equal to 0.865 times Cv, and Cv is approximately equal to 1.156 times Kv.

What is the difference between linear and equal percentage flow characteristics?

A linear trim has a constant slope, so 50 percent of rated travel gives roughly 50 percent of rated flow. An equal percentage trim has a constant proportional change, so 50 percent travel gives about 50 percent of rated Cv. Use linear trim when process gain is constant, and equal percentage trim when pressure drop or system resistance varies, which is the most common case.

What does ATO and ATC mean on a fail-safe actuator?

ATO means air to open, so loss of air supply closes the valve and the valve fails closed. ATC means air to close, so loss of air supply opens the valve and the valve fails open. Pneumatic spring-return actuators are ATO or ATC as standard. Electric actuators achieve fail position through a return spring, a stored-energy mechanism, or a battery or UPS backed drive.

Should a control valve be sized for maximum flow?

Not necessarily. A valve sized only for peak flow spends most of its life near 10 percent open, where the trim sees high velocity, noise, vibration and rapid wear. Target steady operation between about 20 and 80 percent of rated travel. If maximum flow is rare, consider a split-range arrangement, two valves in parallel, or an interchangeable trim.

How do I prevent cavitation in a control valve?

Cavitation occurs when vapour bubbles form and collapse in the throat. Control it by keeping downstream pressure above the fluid vapour pressure, reducing the pressure drop across a single stage, using multi-stage or staged trim, specifying an anti-cavitation cage, selecting a larger bore and trimming the flow so velocity stays low, and avoiding high recovery factors downstream of the valve.

What is valve rangeability?

Rangeability is the ratio of maximum to minimum controllable Cv. A conventional globe valve typically offers 30 to 50 to 1, while characterized trim can reach 100 to 1 or more. Practically, most process control loops need a turndown of only about 30 to 1 between the minimum controllable and maximum rated stroke.

Single-Case Control Valve Sizing

Single-Case Control Valve Sizing Calculator

Single-Case Control Valve Sizing Calculator

Compute the required flow coefficient for liquid, gas and steam service using the ISA‑75.01.01 / IEC 60534‑2‑1 equations, with choked‑flow and cavitation checks.

Cv · Kv · Av | choked flow · cavitation · noise · valve selection

Service

Units

Process fluid

Specific gravity G_f

Water at 60°F = 1.0.

Volumetric flow

Inlet pressure

Absolute psi. Gauge = psig + 14.7.

Outlet pressure

Enter the pressure that actually exists at the valve outlet.

Inlet temperature

Valve & installation

Valve style

FL and xT are typical published values — replace with the manufacturer’s data for final selection.

Piping geometry factor F_p

1.0 = plain pipe, no reducers.

Cv safety margin

Typically 1.10–1.25. Do not oversize.

Rangeability R

Globe ≈ 50 (select 30 or 100). Other styles use their typical value: ball ≈ 100, butterfly ≈ 50, eccentric plug ≈ 50, angle ≈ 50.

Upstream pipe size NPS (Sch 40)

Drives the pre-valve (inlet) pipe velocity at inlet pressure.

Downstream pipe size NPS (Sch 40)

Internal bore & wall drive the IEC 60534‑8‑3 noise estimate (gas & steam only) and the post-valve (outlet) velocity. When the liquid flashes, the expanded two-phase volume raises the outlet velocity.

Capacity curve Lapar

Rated‑Cv table used for the suggested size.

Rated Cv select installed valve

Pick the actual valve from the catalog; drives travel & characteristic.

Basis of calculation.

Liquid: Cv = (Q / (Fp · N1)) · √(Gf / ΔP), N1 = 1.0 (US units).
Liquid choking: ΔP_choked = FL² · (P1 − FF · Pv), with FF = 0.96 − 0.28√(Pv/Pc). Cavitation index σ = (P1 − FF·Pv) / ΔP, incipient cavitation at σ = 1.
Gas / steam: Cv = W / (N8 · Fp · P1 · Y · √(x·M / (T1·Z))), with N8 = 19.3 for lb/h · psia · °R and N8 = 94.8 for kg/h · bar · K.
Expansion factor Y = 1 − x / (3 · Fk · xT), Fk = k/1.4; choked when x ≥ Fk·xT, then Y = 0.667.
Kv = 0.865 · Cv · Av = 2.40×10⁻⁵ · Cv
Water saturation pressure from the Wagner & Pruss correlation; vapor pressures of other liquids from Antoine coefficients.
Suggested body size comes from the Lapar rated‑capacity tables (High precision curve, Table 2 / High capacity curve, Table 3). The Table 2 lower‑half header reads “Port size(in)” but the values 65–300 are millimetres. FL, xT and Fd remain typical published values — confirm against the certified trim datasheet, plus actuator, materials and shut‑off class.

References: ANSI/ISA‑75.01.01‑2012 · IEC 60534‑2‑1:2011 · IEC 60534‑8‑3:2010 (aerodynamic) · IEC 60534‑8‑4:2015 (hydrodynamic) · Emerson/Fisher Control Valve Handbook 5th ed.

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