How the calculation works
For liquids, the flow through a wide-open valve follows the standard sizing equation, where the flow coefficient already bundles up the valve's geometry:
Q = Kv × √( Δp / SG ) · Kv = 0.865 × Cv
In metric units Kv is the flow of water in m³/h through the valve at 1 bar pressure drop; Cv is the same idea in US units, in GPM at 1 psi. The two differ by a fixed factor, so once you know one you know the other. Divide the required Kv by the pressure drop you are prepared to accept and you get the size of valve to order.
Sizing tips that avoid most commissioning calls
- Valve authority matters more than nominal size. If the valve drops 100 kPa in a circuit whose total drop is 150 kPa, authority is 0.67 and control is stable. Below roughly 0.3 the valve is oversized for the duty and control becomes erratic.
- Never size from pipe diameter alone. Two DN25 valves with different trim can differ several times over in Kv.
- Check the minimum controllable Kv. A valve whose required Kv falls below about 10 % of its rated Kv will hunt; for wide load swings, consider a characterised trim or split the load over two valves.
- Water above 100 °C or a large drop across the seat can flash or cavitate — those cases need a proper check, not just the flow coefficient.
- Two-way on variable flow, three-way on constant flow. Most modern FCU installations use two-way valves with a variable-speed pump.