ISA / IEC 60534-2-1 · Engineering Tool
Control Valve Cv (Liquid)
Cv = Q · √(SG / ΔP)
Liquid control-valve sizing per ISA/IEC 60534 — required Cv, choked-flow check, and a cavitation index.
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Cv — required flow coefficient
15.8
- ΔP = P₁ − P₂
- 40.0psi
- FF (critical ratio factor)
- 0.957
- ΔPchoked = FL²(P₁ − FF·Pv)
- 80.6psi
- Sizing ΔP
- 40.0psi
- Cavitation index σ
- 2.49
How it works
Control-valve sizing fixes the required flow coefficient Cv from the design flow, pressure drop, and fluid specific gravity. For non-choked liquid service the sizing equation is Cv = Q·√(SG/ΔP), with Q in US gpm and ΔP in psi.
When the pressure drop approaches the choked limit, vaporisation at the vena contracta caps the effective ΔP. The choked pressure drop is ΔPchoked = FL²·(P1 − FF·Pv), where FL is the valve liquid pressure-recovery factor and FF is the liquid critical-pressure-ratio factor. Once ΔP ≥ ΔPchoked the sizing ΔP is held at ΔPchoked and the flow is choked.
A low cavitation index σ = (P1 − Pv)/(P1 − P2) flags cavitation risk; valve- and trim-specific incipient cavitation data should be confirmed against the manufacturer before a final selection.
Assumptions & limitations
- Liquid, non-flashing single-phase service. Gas, two-phase, and flashing applications need a different sizing path.
- FL is the valve pressure-recovery factor — use the manufacturer value where available (default 0.9 is a generic globe-valve estimate).
- Viscosity correction (Kv) and piping-geometry factors are taken as unity — viscous or close-coupled installations need the full IEC 60534 treatment.
- FF is computed from the liquid vapor and critical pressures; for water at ambient conditions FF ≈ 0.96.
Related Reading
Companion Articles
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