Valve manufacturers state the Kv value, the plant delivers flow rate and pressures – the link between them is the sizing calculation per VDI/VDE 2173. The Kv Value Calculator for Gases (Valve Sizing) handles it, including the case distinction that is most often overlooked.
Two flow regimes, two formulas
With gases, the pressure ratio decides: if the outlet pressure p2 is greater than half the inlet pressure (p2 > p1/2), flow is subcritical – the flow rate depends on both pressures. If p2 falls below p1/2, the flow becomes choked: sonic velocity prevails in the narrowest cross-section, and no more than this flow will pass, no matter how far the outlet pressure drops. Anyone who keeps calculating with the subcritical formula sizes too small.
Step by step
- Select the gas: the calculator knows density and standard density of 8 common gases (N₂, O₂, Ar, He, H₂, CO₂, CH₄, air)
- Enter operating data: flow rate in Nm³/h, inlet pressure p1 and outlet pressure p2 (absolute!), temperature
- Read the result: required Kv value, flow regime and the recommendation with sizing margin
Worked example
10 Nm³/h nitrogen, p1 = 7 bar(a), p2 = 6 bar(a), 20 °C: subcritical flow, required Kv ≈ 0.152. With the usual margin you choose a valve with Kv 0.2–0.25 – but not more: a heavily oversized control valve operates only in the lowest stroke range and controls poorly.
Practical tips
Always calculate with absolute pressures – the classic mistake is inserting gauge pressure straight from the manometer. For valve selection, what counts is the largest required flow at the smallest available differential pressure. And for control valves: the rangeability must also cleanly cover the minimum flow. Manufacturer data (Kv at defined stroke, XT values) govern the fine sizing.
