Control valve Cv for gas and steam
Required Cv and Kv for a gas, vapour or steam control valve by IEC 60534-2-1 — with the expansion factor and the choked-flow check.
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How this calculator works
Required flow coefficient for a control valve on gas, vapour or steam, by IEC 60534-2-1 (ISA-75.01.01) for compressible fluids in turbulent flow: Kv = W / (N6·Y·√(x·P1·ρ1)), with x = ΔP/P1, the expansion factor Y = 1 − x/(3·Fγ·xT) and Fγ = γ/1.40. Flow chokes when x reaches Fγ·xT; beyond that x is held at the limit and Y = 2/3. Cv = 1.156·Kv. Steam density and its isentropic exponent come from the IAPWS-IF97 steam tables.
Example: 20,000 kg/h of natural gas (M = 17.4, Z = 0.92, γ = 1.30) at 30 °C from 40 to 30 bar a, in a globe valve with xT = 0.72: x = 0.25, not choked (limit 0.669), Y = 0.875, Cv = 48.2 (Kv = 41.7). Saturated steam: 5,000 kg/h from 10 to 5 bar a needs Cv = 48.1. The calculation was checked against the IEC equations as coded in the open-source fluids library for six cases, including a choked one: agreement within 0.08 %.
Limits: line-size valve (piping geometry factor FP = 1) — with reducers the required Cv rises; turbulent flow (true for nearly all gas service). Use the valve maker's xT and check noise (IEC 60534-8-3) on high-drop valves. For liquids use the liquid control valve calculator. References: IEC 60534-2-1:2011, Industrial-process control valves — Flow capacity — Sizing equations for fluid flow under installed conditions; Emerson, Control Valve Handbook, 5th ed.