Valve World Magazine February 2026 | Page 21

Technical Article

These values indicate how much fluid the a valve can pass at a given pressure drop.

a. Kv( Flow Coefficient) is a unit of measurement used in the metric system. Its definition is as follows: It is the volumetric flow rate of water at 20 ° C passing through a fully open valve with a pressure drop of 1 bar, in cubic meters per hour( m 3 / h). Formula( For Liquids):

Kv Q

SG P

Q: Volumetric flow rate( m 3 / hour) ΔP: Pressure drop across the valve( bar) SG: Specific gravity of the fluid. Specific gravity( SG) is the ratio of the density of a substance to the density of a specific reference substance. This ratio is generally a dimensionless number, meaning it has no units. For liquids: Water at 4 ° C( the temperature at which it has maximum density) is generally used as the reference substance. The density of water at this temperature is assumed to be 1000 kg / m 3( or 1 g / cm 3).

SG � liquid

reference, water ρ liquid

: Density of the relevant liquid

ρ reference _ water

: Density of reference water( usually water at 4 ° C. For gases, air is generally used as the reference substance. If the fluid is water( ρ = 1000 kg / m 3, SG = 1), the Kv formula is often simplified to:

Kv �

Q

� P

Kvs Value: The maximum Kv value of a valve at 100 % open is called Kvs. It is important in determining the characteristics of control valves. b. Cv( Valve Coefficient) is a unit of measurement used in the imperial system of units( generally used in the United States and Canada). Its definition is: 60 ° F( approximately 15.5 ° C) through a fully open valve at a pressure drop of 1 psi, in US gallons per minute( US GPM). Formula( for liquids):

Cv Q

SG P

Fig. 3. Directional flow valves( check valves, multi-port ball valves etc.).

Q: Flow rate( US GPM) ΔP: Pressure drop across the valve( psi) SG: Specific gravity of the fluid( SG = 1 for water)

c. Conversion between Kv and Cv Kv and Cv can be converted to each other:

d. Why calculate Kv or Cv?

Choosing the right valve: Choosing the most appropriate valve for a system, ensuring it has the correct flow capacity. System performance: Predicting flow rate and optimising system energy efficiency and pressure drop. Design and maintenance: It is a critical parameter for engineers who design and maintain fluid control systems. When selecting a valve, it’ s important to consider all parameters, including the fluid type( liquid, gas, steam), temperature, flow rate and maximum allowable pressure drop across the valve. These values are typically specified in valve manufacturers’ catalogues and technical data.

e. Kv value for gases Kv( flow coefficient) value for gases is slightly different than for liquids because

Fig. 2. Flow control valves( needle).

the density of gases varies significantly with pressure and temperature. The Kv value expresses a valve’ s gas flow capacity in metric units. In gas flow, if the outlet pressure drops below a critical ratio of the inlet pressure( typically around P2 / P1 < 0.5), the flow velocity reaches the speed of sound, and the flow rate through the valve does not increase with further pressure drop. This condition is called critical flow or choked flow, and the Kv formula cannot be directly applied in this case. Manufacturer data or more complex calculations are required. Therefore, for gas flows, the flow condition must be determined and the relevant equations used.

Valve functions

Industrial valves are designed for different functions, and these are usually represented by symbols on the P & ID( Piping & Instrumentation Diagram). The main types of valves, classified by their primary function, are as follows:

• On / off Valves: These valves are used to start or stop fluid flow. This is the most basic valve function, and there are many different types, including ball valves, butterfly valves, diaphragm valves, plug valves and gate valves. These valves generally operate in either the fully open or fully closed position.( Fig. 1)

• Flow control valves: These valves gradually adjust the flow rate and are used especially in sensitive processes. Needle and proportional control valves perform this function.