Water hydraulics
Required liquid valve Kv
- where
- valve flow coefficient (m³/h at 1 bar)
- volume flow (m³/h; any unit you pick is converted)
- liquid specific gravity to water
- valve pressure difference (bar)
Explain this equation
Works out the valve flow coefficient Kv needed to pass a given flow with a given pressure drop. Use it to pick a control valve size.
Kv is the flow of water in m³/h that a valve passes with a 1 bar drop across it. q is the design flow in m³/h. SG is the liquid's specific gravity compared with water. Δp is the pressure drop allowed across the valve, in bar.
Flow through a restriction rises with the square root of the pressure drop. Heavier liquids flow less for the same drop, so SG appears under a square root.
Sense check: 12 L/min (0.72 m³/h) at 0.1 bar needs a Kv of about 2.3. Halve the drop and the Kv needed rises by about 41%.
Watch out: liquid only, not cavitating. Kv is not the same as the US Cv; Cv is about 1.16 × Kv.
Enter your values to see the result.
Good to know
Single-phase liquid, non-cavitating conditions. Valve authority, noise, control range and product limits need separate selection.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.