Calculate Kv and Cv values for gas flow per VDI/VDE 2173 - with automatic detection of subcritical vs. critical (choked) flow and 8 gases.
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Kv value for gases: sizing valves correctly
The Kv value describes a valve's flow capacity – defined as water flow in m³/h at 1 bar pressure drop. For gases, compressibility creates two operating regimes which this calculator distinguishes automatically per VDI/VDE 2173.
Subcritical and critical flow
As long as downstream pressure exceeds half the upstream pressure (p₂ > p₁/2): Qn = 514 × Kv × √(Δp × p₂ / (ρn × T₁)). Once p₂ falls below p₁/2 the gas reaches sonic velocity in the vena contracta – flow cannot increase further no matter how low p₂ drops: Qn = 257 × Kv × p₁ × √(1/(ρn × T₁)). Using the subcritical formula in the choked regime sizes the valve wrongly.
Worked example
10 Nm³/h nitrogen, upstream 7 bar(a), downstream 6 bar(a), 20 °C. Pressure ratio 0.86 → subcritical. Kv = 10 / (514 × √(1 × 6 / (1.25 × 293.15))) = 0.152 m³/h. With 30 % margin the valve should have Kvs ≥ 0.2. The same flow vented to 2 bar(a) is choked – then only upstream pressure matters.
Kv and Cv
The US Cv (US gal/min water at 1 psi) equals Kv × 1.156. Catalogues state Kvs – the Kv at rated travel. The valve should operate at 60–80 % travel at the design point, hence the recommended margin.
