Control Valve Sizing · Instrument Apps Hub

Quick examples

Fluid

Fluid Steam condition Inlet temperature Densitykg/m³ at flowing temperature Vapour pressure Pvabsolute Critical pressure Pcsame unit as Pv Kinematic viscositycSt (water ≈ 1) Molecular weight Mair = 28.97 Specific heat ratio kCp/Cv Compressibility Z

Operating cases

MinimumNormalMaximum Flow

Flow unit Pressure unit Inlet pressure P1 Outlet pressure P2

Same P1/P2 for all three cases. For a pump system where ΔP falls as flow rises, size the maximum case with its own (lower) ΔP.

Valve

Valve style Inherent characteristic F_L · x_T · F_dtypical for the style; use vendor values if known Valve size Line sizereducers if bigger than the valve Rated Cv (optional)from the vendor's table; blank = typical Body materialfor velocity limits

Animate: —

Required Cv (max)

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Valve size

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Flow regime (normal)

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Checks & advice

Installed travel

Rule of thumb: equal-% valves about 15–20 % open at minimum flow, ~75 % at normal and at most 90–95 % at maximum. Linear trims: keep all cases between 15 % and 95 % of rated Cv.

Which characteristic?

Temperature	Equal %
Liquid pressure	Equal %
Gas pressure, small volume	Equal %
Gas pressure, large volume	Linear if valve ΔP stays > 20 % of system ΔP
Flow and level	Linear if valve ΔP at max flow > 20 % of ΔP at min flow; else equal %
On/off, relief, surge	Quick opening

How it was calculated

Equations used (IEC 60534-2-1 / ISA-75.01.01)

Liquid Cv = Q / (N1·Fp) · √(ρ1/ρ0 / ΔPsizing) N1 = 0.0865 (m³/h, kPa) ΔPchoked = (FLP/Fp)² · (P1 − FF·Pv), FF = 0.96 − 0.28·√(Pv/Pc) ΔPsizing = min(ΔP, ΔPchoked) Viscous Cv = Cv(turbulent) / FR, FR from the valve Reynolds number Rev Gas Cv = W / (N6·Fp·Y·√(x·P1·ρ1)) N6 = 2.73 (kg/h, kPa, kg/m³) x = ΔP/P1, Fγ = k/1.4, Y = 1 − x/(3·Fγ·xTP), x ≤ Fγ·xTP (choked) Fittings Fp = [1 + ΣK/N2·(Cv/d²)²]^−½, ΣK = 1.5·(1 − d²/D²)² (inlet + outlet reducers) FLP = FL/√(1 + FL²·ΣKi/N2·(Cv/d²)²), xTP = (xT/Fp²)/(1 + xT·ΣKi/N5·(Cv/d²)²) Kv = 0.865·Cv

Valve factors and rated Cv values are typical for each style, not a specific product. Confirm final selection with the manufacturer's data and sizing software, and check noise (IEC 60534-8) separately.