Liquid Cv Calculator

This calculator sizes a control valve for non-compressible (liquid) service using the equations of IEC 60534-2-1. It computes the required flow coefficient (Cv or Kv), checks whether the flow chokes at the vena contracta (the onset of heavy cavitation and flashing), and reports the limiting pressure drop used for sizing.

Turbulent flow, no choking:
Cv = Q × √( SG / ΔP ) Kv = Q × √( SG / ΔP )
Q in gpm and ΔP in psi → Cv | Q in m³/h and ΔP in bar → Kv | SG = liquid relative density (water = 1)

Choked-flow check:
ΔP_choked = F_L² × ( P₁ − F_F × P_v ) with F_F = 0.96 − 0.28 × √( P_v / P_c )
F_L = liquid pressure-recovery coefficient (globe trim ≈ 0.85–0.95; high-recovery rotary ≈ 0.6–0.8). P_v = vapour pressure, P_c = critical pressure, both absolute.

Input Parameters

All pressures are absolute. Switch units first, then enter your operating data.

Unit system

Pressure unit follows the selection (psi / bar).

Flow rate Q

Specific gravity SG (at flowing T)

Water = 1.0 at 60°F / 15°C.

Upstream pressure P₁ (abs)

Downstream pressure P₂ (abs)

Vapour pressure Pv (abs)

At flowing temperature. Water at 20°C ≈ 0.023 bar.

Recovery coefficient Fₖ

From the valve datasheet. 0.9 is a typical globe value.

Critical pressure Pc (abs)

Water ≈ 221.2 bar (3208 psi). Change for other liquids.

ΔP = P₁ − P₂—

Liquid critical-pressure ratio factor Fₛ—

Choked pressure drop ΔP_choked = Fₖ²(P₁−FₛPv)—

Sizing pressure drop used ( lesser of ΔP and ΔP_choked )—

Required Cv—

Equivalent Kv ( = 0.865 × Cv )—

Suggested next size up ( ≈ 1.3 × required Cv )—

Flow regime—

How the calculation works

Standard method. The sizing equation is the incompressible-flow equation of IEC 60534-2-1 (identical form in ISA-75.01.01). US units give Cv; metric units give Kv. The two are related by Cv = 1.156 Kv.
Choked flow. As the downstream pressure falls, flow stops increasing once the vena contracta pressure approaches the liquid vapour pressure. Beyond ΔP_choked, extra pressure drop buys no extra flow — it only buys cavitation damage and noise. The calculator therefore sizes with the lesser of the actual and the choked ΔP, exactly as the standard requires.
Regime verdict. If ΔP ≤ ΔP_choked the flow is sub-critical (turbulent, non-choking). If ΔP > ΔP_choked, flow is choked: expect flashing or cavitation depending on the P₂/Pv relationship, and consider a multistage or cavitation-resistant trim.

Assumptions and limits

No piping geometry factor (Fₙ = 1). The installed reducer/expanders are neglected. With unusual line sizes, apply Fₙ per IEC 60534-2-1 Part 2.
Fully turbulent flow. Low-Reynolds-number correction (Fₛ) is not applied — valid for most water-like services in control valves; check IEC 60534-2-1 for viscous duties.
Single-phase liquid. Mixed-phase (cavitating/flashing two-phase) flow needs the standard's two-phase method; this tool flags the condition rather than sizing for it.
Educational sizing. Results are for education and preliminary selection. Final sizing must use the full standard method with the manufacturer's certified Fₖ, Fₙ and FL data.

Next tools in the series: gas & steam Cv (expansion-factor method), cavitation margin check and Cv/Kv conversion are under engineering review — see Engineering Tools.

Tool disclaimer. This calculator implements published IEC 60534-2-1 equations with stated assumptions. It does not replace the full standard calculation, manufacturer sizing software, or engineering judgment for the actual installation.