BIZOMAT MAKEOS · Hydraulics
Valve Flow Coefficient (Kv and Cv)
What Kv does this valve need for the flow and pressure drop, or what flow will a valve pass?
Valve flow coefficient Kv (and Cv) for a liquid from flow, pressure drop and specific gravity, or the flow through a valve of known Kv.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Calculate | choice | Find the Kv needed, or the flow through a valve. |
| Flow | m³/h | Liquid flow through the valve. |
| Pressure drop across the valve | bar | Pressure difference across the valve. |
| Valve Kv (flow mode) | m³/h | Kv from the valve data sheet. |
| Specific gravity of the liquid | Specific gravity (water = 1). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Calculate
- What it is
- Whether to find the Kv a valve must have for a given flow and pressure drop, or the flow through a valve whose Kv you know.
- Units and format
- Choose one of 2 options from the list: Kv needed for a flow and pressure drop; Flow through a known Kv.
- Example
- The form opens with “Kv needed for a flow and pressure drop” selected as a worked example. Choose the option that matches your case.
- Get it right
- Only the input for the chosen direction is used: flow in the first case, valve Kv in the second.
What this means: Flow
- What it is
- The liquid flow that must pass through the valve; used when finding the Kv needed.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0.000001 m³/h to 1000000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The Kv needed rises in direct proportion to flow.
- Get it right
- Enter cubic metres per hour; 1 m³/h is 16.67 L/min.
What this means: Pressure drop across the valve
- What it is
- The pressure difference between the valve inlet and outlet at the flow considered.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Kv needed falls with the square root of the pressure drop; flow rises with it.
- Get it right
- Use the drop across the valve only, not the whole system’s pressure.
What this means: Valve Kv (flow mode)
- What it is
- The valve’s flow coefficient: the flow of water in cubic metres per hour it passes at one bar pressure drop, from the valve data sheet; used when finding the flow.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0.000001 m³/h to 1000000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Flow rises in direct proportion to Kv.
- Get it right
- Do not confuse Kv with Cv, the US gallon-per-minute coefficient; they differ.
What this means: Specific gravity of the liquid
- What it is
- The specific gravity of the liquid: its density divided by the density of water, which is one for water.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A heavier liquid needs a larger Kv for the same flow.
- Get it right
- It is a ratio, not a density in kg/m³.
Method and formulas
- Q (m³/h) = Kv √(Δp (bar) ÷ SG).
- Kv = Q √(SG ÷ Δp).
- Cv (US gal/min at 1 psi) = 1.156 Kv.
- Example: 10 m³/h of water at 0.5 bar drop needs Kv 14.142 (Cv 16.348).
Limitations
- Turbulent, non-flashing, non-cavitating liquid flow; gases and choked flow need other formulas.
- Control valves are usually selected to run at 20–80% of their rated Kv.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a control or isolation valve
- Find the pressure drop through a valve
- Convert between Kv and Cv
Who it is for
- Process engineers
- Maintenance engineers
- Design engineers
What you get
- Kv
- Cv
- Flow
Interpreting the result
- Choose a valve whose Kv gives the required flow near the middle of its travel.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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