The liquid sizing equation
Cv is defined as the US gallons per minute of 60 °F water that pass through the valve at 1 psi drop; Kv is the m³/h of water at 1 bar drop. Both are the same physical idea — a valve's flow capacity — in different unit systems, which is why the conversion is a fixed factor. The equation is the basic turbulent, non-choked liquid form of IEC 60534 / ISA-75.01.
Worked example
45 m³/h of water with 1.2 bar allocated across the valve:
- Kv = 45 × √(1.0 ÷ 1.2) = 41.1
- Cv = 1.156 × 41.1 = 47.5
- Select a valve with rated Cv around 60–65 so max flow lands near 75% travel — e.g. a typical 3-inch globe valve
Field notes
- Size for the pair (flow, ΔP) that occur together. Max flow usually coincides with minimum available ΔP — using max flow with max ΔP undersizes the problem.
- Check the turndown too: the valve must also control at minimum flow. If min flow needs less than ~10% travel, consider a smaller valve or a characterised trim.
- Oversizing is the classic sin: a valve running at 15% open hunts, seats wear, and control is poor. The next pipe size is not a sizing method.
- Vapour pressure check: if P₂ or the vena-contracta pressure approaches the liquid's vapour pressure, run the full choked/cavitation analysis before ordering.
Frequently asked questions
What is the Cv formula for liquids?
Cv = Q × √(SG ÷ ΔP), with Q in US GPM and ΔP in psi. In metric terms, Kv = Q(m³/h) × √(SG ÷ ΔP bar), and Cv = 1.156 × Kv.
What Cv should the selected valve have?
Common practice is choosing a valve whose rated Cv puts the maximum-flow operating point around 70–80% of travel, and normal flow near 50–70%. A valve exactly at 100% of its Cv has no control margin.
Which pressure drop do I use?
The drop across the valve itself at the sizing flow — not the total system drop. If the valve ΔP is not fixed by design, a common allocation is 25–33% of the total dynamic system loss at max flow.
Does this formula handle flashing or cavitation?
No — the basic equation assumes fully turbulent, non-vaporising liquid flow. If the pressure inside the valve approaches the liquid vapour pressure, choked flow, cavitation and flashing checks (per IEC 60534 / ISA-75.01) are required.
Basic turbulent-flow sizing for reference and education. Final valve selection must follow IEC 60534 / ISA-75.01 and vendor sizing software. See our disclaimer.