Valve Cv Calculator
Required valve flow coefficient per ISA-75.01.01 / IEC 60534-2-1 for liquid, gas, and steam service, with choked-flow, cavitation, and piping-geometry corrections.
When to use this calculator
Use when sizing a control valve or checking a vendor selection against your process conditions. It sizes on the full ISA-75.01.01 method rather than the textbook square-root formula: liquid sizing applies the choked-flow limit through F_L and the liquid critical pressure ratio F_F, gas and steam sizing applies the expansion factor Y and the terminal pressure drop ratio x_T, and both correct for the piping geometry factor F_P when the valve is smaller than the line. Enter minimum, normal, and maximum flow cases together and it sizes for all three, which is how rangeability and controllability problems show up before the valve is bought.
Required inputs
- Service — liquid, gas, or steam
- Flow cases — minimum, normal, and maximum, each with flow, P1, P2, and temperature
- Liquid properties: specific gravity or density, viscosity, vapour pressure, critical pressure
- Gas properties: molecular weight or SG, compressibility Z, specific heat ratio k
- Valve type and size, line size, and optional F_L / x_T overrides
Expected outputs
- Required Cv (and Kv) for each flow case
- Choked-flow limit and whether the case is choked
- Expansion factor Y and pressure drop ratio x for compressible service
- Piping geometry factor F_P when valve and line sizes differ
- Fully substituted equation per case for the datasheet
Formula overview
Flow accepts m³/h, gpm, lpm, Nm³/h, Sm³/h, SCFH, kg/h, and lb/hr; pressures in bar g/a, kPa g/a, psig or psia; temperature in °C, K, °F, or °R. Inputs are converted to a metric basis internally, so the equation constants N are always the metric set.
ISA-75.01.01 / IEC 60534-2-1.
Liquid:
Cv = q / (N1 · F_P · √(ΔP_sizing / G_f))
ΔP_max = (F_LP / F_P)² · (P1 − F_F · P_v)
F_F = 0.96 − 0.28 · √(P_v / P_c)
Gas and steam:
Cv = q / (N · F_P · P1 · Y · √(…))
Y = 1 − x / (3 · F_k · x_T) with Y ≥ 2/3
F_k = k / 1.4
x_limit = F_k · x_TP
ΔP_sizing is the lesser of the actual ΔP and ΔP_max — beyond ΔP_max the
flow is choked and extra pressure drop buys no extra flow.Worked example
Water at 20 °C, globe valve (F_L = 0.9), normal case:
q = 100 m³/h, P1 = 6 bar a, P2 = 5 bar a, ΔP = 1 bar,
G_f = 1.0, P_v = 0.023 bar a, P_c = 221 bar a, F_P = 1.0
Choked-flow check:
F_F = 0.96 − 0.28 × √(0.023/221) = 0.957
ΔP_max = 0.9² × (6 − 0.957 × 0.023) = 4.84 bar
ΔP = 1 bar < 4.84 bar → not choked, size on the actual ΔP
Sizing:
Kv = 100 × √(1.0 / 1) = 100
Cv = 1.156 × 100 = 116
Select a valve whose rated Cv puts the normal case near 60–70% travel.Common mistakes
- Applying the liquid square-root formula to gas or steam. Compressible flow needs the expansion factor Y and the x_T limit; ignoring them under-sizes badly once the pressure drop ratio passes about 0.5.
- Sizing on the maximum case alone. A valve chosen for maximum flow often sits below 10% travel at minimum flow, where gain is poor and the loop hunts — size all three cases and check the travel span.
- Ignoring the choked-flow limit. Beyond ΔP_max additional pressure drop produces no additional flow, so sizing on the full available ΔP overstates capacity, and the same condition brings cavitation damage in liquid service.
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