Choose valve coefficient, flow rate, pressure drop or liquid specific gravity and enter the required inputs. Flow uses US GPM, L/min or m³/h; differential pressure uses PSI, bar or kPa.
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CV Flow Formula
The calculator uses the standard liquid valve flow coefficient relationship. Flow is converted to US GPM and pressure drop is converted to PSI before the formula is applied. One m³/h equals approximately 4.402868 US GPM.
- Q = flow rate, in US gallons per minute after unit conversion
- Cv = valve flow coefficient
- DeltaP = pressure drop across the valve, in PSI after unit conversion
- SG = specific gravity of the fluid, where water is about 1.0
To use the calculator, choose the value to solve for, enter the three displayed inputs, and select Calculate. For flow or pressure results, choose the answer unit. The simplified liquid equation excludes choking, cavitation and viscosity corrections.
Common Specific Gravity Values for CV Flow Calculations
Specific gravity affects the flow through a valve because heavier fluids require more pressure drop for the same flow and CV. The values below are examples, not fluid specifications; temperature, composition and water-reference conditions matter. NIOSH examples reference water at 4°C and are not precision valve-sizing inputs. Glycol mixtures differ from neat ethylene glycol. Use verified data for the actual liquid at its operating temperature.
| Fluid | Approximate specific gravity |
|---|---|
| Water | 1.00 |
| Gasoline | 0.72 to 0.76 at 60°F (NIOSH) |
| Diesel fuel | Use the actual product’s operating-temperature relative density |
| Kerosene | About 0.81 at 68°F (NIOSH) |
| Ethylene glycol | About 1.11 at 20°C for industrial ethylene glycol (BASF) |
| Seawater | About 1.025 as a rough illustration; temperature and salinity vary |
How to Read CV Flow Results
| Result type | What it means |
|---|---|
| Higher CV | The valve can pass more flow for the same pressure drop and fluid specific gravity. |
| Lower CV | The valve is more restrictive and needs more pressure drop to pass the same flow. |
| Higher specific gravity | A denser fluid reduces flow for the same CV and pressure drop. |
| Higher pressure drop | Flow increases with the square root of pressure drop, not in direct proportion. |
Example CV Flow Calculations
Example 1: Calculate CV
You have a flow rate of 50 GPM, a pressure drop of 10 PSI, and a fluid specific gravity of 1.00.
The required flow coefficient is 15.8114 CV.
Example 2: Calculate Pressure Drop
You have a flow rate of 20 GPM, a CV of 12, and a fluid specific gravity of 0.85.
The pressure drop is 2.3611 PSI.
FAQ
What does CV mean in flow calculations?
CV is the valve flow coefficient. For liquids, it represents the amount of water in gallons per minute that can pass through a valve with a 1 PSI pressure drop at about 60°F. A larger CV means the valve can pass more flow with less restriction.
Can this formula be used for gases or steam?
This calculator is for liquid flow using the standard incompressible flow CV equation. Gas, air, and steam sizing require different equations because compressibility, temperature, pressure ratio, and choking can affect the result.
Why does specific gravity change the CV result?
Specific gravity compares a fluid’s density to water. A fluid with a higher specific gravity is denser, so it needs more pressure drop to move through the same valve at the same flow rate. If all other values stay the same, a higher specific gravity increases the required CV.
