**The flow coefficient (Cv)** is also additionally referred to as the **valve flow coefficient**.** ** It is an imperial measurement system of unit and is defined as the flow of water through a valve at 60°F in US gallons per minute (GPM) at a pressure drop of 1 pound/square inch (lb/in²).

**Cv = Q * √ (SG/ΔP) (units in US GPM, psi)**

Q = Flow in US GPM,

SG = Specific Gravity,

ΔP **= **Differential Pressure (psi).

**Flow factor (Kv)** is the metric system equivalent of the **flow coefficient (Cv)**. Also referred to as International System of units (abbreviated as SI system).

It is defined as the flow of water in cubic meters per hour (m³/hr) at a pressure drop of one bar with temperature ranging from 5 to 30 degree C.

**Kv = Q * √ (SG/ΔP) (units in m3/hr, bar)**.

Q = Flow in m3/ hr,

SG = Specific Gravity of water ( for water=1). ** **

**ΔP** = Differential Pressure (bar).

The **Flow Factor (Kv)** should not be misunderstood with the **Discharge Coefficient (k). **Discharge Coefficient (k) is a characteristic non-dimensional factor of a valve, used to calculate flow that discharges from a tank to the environment.

**The** **Relationship between Cv and Kv is**

Cv = 1.156 Kv **OR ** Kv = 0.864 Cv.

Unit conversions from Imperial to Metric (vice versa):

1 US GPM = 0.227125 m3/hr. 1 m3/hr = 4.402868 US GPM

1 psi = 0.06894757 bar. 1 bar = 14.503773773 psi.

60 deg F = 15.5 deg C.

**Conversion of units of Kv to Cv**:

Kv = 1US GPM * **√** (1/1psi)

Kv = 0.227125 m3/hr. * **√** (1/0.06894757 bar)

Kv = 0.2271*3.818* m3/ hr. * bar

Kv = 0.864 m3/ hr. * bar

**Kv = 0.864 Cv**

**Conversion of units of Cv to Kv**:

Cv = 1 m3/hr * **√** (1/1bar)

Cv = 4.4028812454 US GPM * 0.2625786985 *1/psi

Cv = 4.4028812454 US GPM * **√** (1/14.503773773 psi)

Cv = 1.1561 US GPM / psi

**Cv = 1.156 Kv**

**What is its importance?**

- Flow coefficient Cv and Flow Factor Kv are values used to describe the hydraulic capacity of a control valve.

- Quickly compare different brands of valves and their flow capacities.

- Help to select a valve that is the right size (and not oversized) for the application.

- One can quickly understand how the valve you select will have an effect on the pressure in your system.