Valve Sizing (Cv) — Liquid
Size a control valve for liquid service: get required Cv from flow, pressure drop, and specific gravity.
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The engineering
Cv is the flow coefficient of a valve: the number of US gallons per minute of 60 °F water that passes at 1 psi drop with the valve wide open. Rearranging the ISA liquid-sizing relation gives the Cv you must specify for a duty point — pick a valve whose rated Cv comfortably exceeds it. This card covers non-choked, non-flashing liquid service; for gas, steam, or choked flow the full S75.01 correction factors (FL, FF, Y, xT) apply.
Rule of thumb: size so the normal duty lands between roughly 20% and 80% of rated Cv. A valve that runs nearly wide open has no room to control on upset, and one that barely cracks open throttles poorly and wears the trim. If ΔP is a large fraction of inlet-to-vapor-pressure headroom, check for choked/flashing flow before trusting this straight-line result.
Where this math comes from
The flow coefficient Cv was introduced by the Masoneilan valve company in 1944, an engineer's shortcut: rather than publish dimensioned orifice data, rate every valve against a single benchmark — water at 1 psi drop. The number folded valve geometry, port shape, and losses into one figure a plant engineer could read off a catalog and plug into a square-root formula.
The Instrument Society of America codified the practice so vendors could be compared apples-to-apples. ISA S75.01 (first issued 1977, now ISA/IEC 60534-2-1) added the physics Masoneilan's simple form omitted — piping geometry factors, liquid pressure-recovery FL, and the choked-flow limits — while keeping Cv as the headline number every technician still quotes.
- 1944MasoneilanIntroduces Cv, the flow coefficient rating valves against water at 1 psi drop.
- 1953Valve industryMetric Kv (m³/h at 1 bar) adopted alongside Cv in Europe.
- 1977Instrument Society of AmericaPublishes ISA S75.01 standardizing control-valve sizing equations.
- 2011IEC / ISAHarmonized as IEC 60534-2-1 / ISA-75.01.01 with full correction factors.
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