Kv and Cv
Kv is the water flow in m3/h at a 1 bar drop; Cv the flow in US gallons per minute at 1 psi. Cv = 1.156 Kv, and Kv = 0.865 Cv.
Reference tool — not a selection
Size a valve by its Kv before it goes in an RFQ.
Kv and Cv
Three values, one equation.
Fill two of Kv, flow and pressure drop, and the calculator finds the third. With all three it compares the flow asked for with what the valve passes.
Examples:
All 130 engineering tools
Valve datasheets give the flow capacity as Kv (m3/h of water at a 1 bar drop) or Cv (US gallons per minute at 1 psi). Enter the coefficient, the flow or the pressure drop, leave empty what you want found, and the calculator converts Kv and Cv, solves the liquid flow equation, draws the flow curve and writes an RFQ line.
For liquids only. Gas and steam are sized on absolute pressures, and choked flow needs the valve maker’s method; the old valve’s datasheet decides.
How the coefficient works
Water, one bar, one hour.
Flow grows with the square root
For a liquid, flow is Kv times the square root of the drop over the relative density: four times the drop gives twice the flow.
Lighter liquid, more flow
A liquid of relative density 0.81 passes 1/0.9 times as much as water at the same drop; a denser one passes less.
| Kv | Cv | m3/h at 0.5 bar | m3/h at 1 bar | m3/h at 2 bar |
|---|---|---|---|---|
| 0.5 | 0.58 | 0.35 | 0.5 | 0.71 |
| 1 | 1.16 | 0.71 | 1 | 1.41 |
| 2.5 | 2.89 | 1.77 | 2.5 | 3.54 |
| 4 | 4.62 | 2.83 | 4 | 5.66 |
| 6.3 | 7.28 | 4.45 | 6.3 | 8.91 |
| 10 | 11.56 | 7.07 | 10 | 14.14 |
| 16 | 18.5 | 11.31 | 16 | 22.63 |
| 25 | 28.9 | 17.68 | 25 | 35.36 |
| 40 | 46.24 | 28.28 | 40 | 56.57 |
| 63 | 72.83 | 44.55 | 63 | 89.1 |
| 100 | 115.6 | 70.71 | 100 | 141.42 |
Water, relative density 1. The Kv values are a range of examples, not a list of valve sizes.
Sources
Definitions, the factor 1.156 and the liquid equation: Wikipedia, "Flow coefficient" (with the VDI/VDE 2173 definition of Kv), Project Materials, "Cv vs Kv", and BW Valve, guide to Kv and Cv. Gas needs absolute pressures: Wikipedia.
Before you send the request
Questions about Kv and Cv in an RFQ
What does Kv 6.3 mean?
The fully open valve passes 6.3 m3/h of water with a 1 bar pressure drop across it. In US units that is Cv 7.3.
How do I convert Cv to Kv?
Multiply Cv by 0.865, or divide it by 1.156: Cv 10 is Kv 8.65.
What Kv do I need for 50 m3/h at 2 bar?
Kv is the flow times the square root of the relative density over the drop: for water, 50 times the square root of 1/2, about 35.4. A valve of that Kv or more is needed.
What should the request include?
The Kv or Cv and where it was read, the flow and the pressure drop, the liquid and its temperature, the valve type, size and connection, and the maker and model of the old valve with a photograph of its plate.
Kv found?
Send the line with the valve plate.
A calculation is a cross-check, not a selection. Attach a photograph of the old valve’s plate, and Vessel Core checks its data before an offer goes back.
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