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.

Control Valve Sizing Online - Lapar Control Valve

Control Valve Sizing and Selection

Control Valve Sizing and Selection

Enter up to six operating conditions — minimum / normal / maximum are included by default, plus three optional alternatives (tick include to add them) — and read the required Cv, choked-flow status and resulting valve travel of each enabled case off one selected valve, so the opening range stays inside a controllable band. Every result and chart the single‑case calculator gives is available per condition.

Cv · Kv · travel % | up to 6 cases · choked flow · cavitation · noise · turndown check · liquid‑gas mixtures

Service

Units

Export

Valve Specification

Valve style

FL and xT are set automatically from the valve type. Replace with the certified trim datasheet for final selection.

pipe in

Sch

pipe out

sch

Piping geometry factor F_p 1.000

Cv safety margin

Inherent characteristic

Rangeability R

Globe ≈ 50, ball ≈ 100.

Target travel from %

to %

Capacity curve

Rated Cv

Body size

Inlet size

Outlet size

Process data

Process fluid

Basis of calculation.

Viscous liquids: Cv = Cv_turbulent / F_R, with F_R from the valve Reynolds number (ISA‑75.01.01 Annex A). Turbulent at Re_v ≥ 40 000, so water duties are uncorrected; heavy oils raise the required Cv.
Liquid: Cv = (Q / (Fp · N1)) · √(Gf / ΔP), with choked limit ΔP_choked = FL² · (P1 − FF · Pv), FF = 0.96 − 0.28√(Pv/Pc). Flashing uses the isenthalpic steam-table flash across the outlet; the expanded two‑phase volume sets the outlet velocity.
Gas / steam: Cv = W / (N8 · Fp · P1 · Y · √(x·M / (T1·Z))), Y = 1 − x / (3 · Fk · xT), choked when x ≥ Fk·xT.
Liquid‑gas mixtures use Fisher Catalog 12 §2 (eq. 1): Cvr = (Cvl + Cvg)(1 + Fm) — both phases are sized separately through the valve, and Fm comes off Figure 1 at the inlet gas volume ratio Vr = Vg/(Vl + Vg) (the gas volume carrying the compressibility factor Z). It collapses onto the single‑phase equations at both ends: no gas gives the liquid answer, no liquid gives the gas answer.
Travel from the inherent characteristic Cv/Cv_rated = f(h, R): linear f = h, equal‑percentage f = R^h−1, quick‑opening f = √h.
Valve size is selected once, from the LARGEST case (Cv × margin, keeping the worst travel ≤ max travel). All other cases are then turndown conditions on that valve. Noise (IEC 60534‑8‑3 aerodynamic / ‑8‑4 hydrodynamic), choked flow, flashing and the recommended downstream size are evaluated per condition in the detail panel.
Rated capacities come from the Lapar catalog tables (High precision / High capacity curve). FL and xT follow the owner FL/XT reference table (single-row values, not size-dependent) — 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) · ISO 13703 / API RP 14E (two‑phase erosion)

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.

Address: Via Pantano 2, Milan (MI) 20122, Italy
Postcode: 20122
Phone: +39 028.2000.500
VAT N°: IT08379220968
R.E.A. MI-2021657
E-mail: sales@lapar.eu
Website: https://www.lapar.eu

Lapar Control Valve

Control Valve Sizing and Selection

Control Valve Sizing and Selection

Control Valve Sizing and Selection

Related Articles

Valve Sizing-online

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.