When selecting the correct valve type and size, various operands can be decisive. For example, the Kv factor, the flow rate and the characteristic value for the pressure drop help you to choose the right valve for the desired requirements and applications. Calculate these values easily online with our free Fluidic Calculator.
Bürkert Fluidic Calculator - free online calculation of the Kv factor
Do you want to calculate the flow factor, flow rate or pressure drop of a valve? Our free Fluidic Calculator provides you with the necessary support for this task. Choose from a variety of media or create your own.
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Flow factor
What does the flow factor Kv indicate?
The Kv factor is a standardised key figure for the achievable flow rate of a fluid through a valve and has been in use since the 1950s. The Kv factor is calculated according to standard DIN EN 60 534, wherein the value is defined according to VDE/VDI 2173 guidelines by the measurement of water at a pressure drop of approx. 1 bar and a temperature of 5-30°C. The unit of the result is given in m3/h.
In addition, this characteristic value of the valve only represents a certain valve stroke, i.e. a specific degree of opening. A valve has as many Kv factors as it has setting levels. An on/off valve therefore only has one Kv factor while control valves have Kv factors for each position. The key figure for the maximum 100% stroke is the Kvs factor.
Difference between Cv value & Kv factor
The often equated Cv value is the US measurement unit, which is given in USG/min (US-Gallon per minute), and can therefore not be compared with the Kv factor. Conversion formulas exist for these values:
Kv = 0.857 * Cv
Cv = 1.165 * Kv
Formulas for calculating the flow factor for various aggregate states
Kv calculation of liquids
To calculate the Kv factor for liquids, the flow rate in l/min or m3/h, the density of the medium upstream of the valve and the pressure drop across the valve must be known, i.e. the difference between input pressure and back pressure.
Q = volume flow in m3/h
Δp = pressure drop in bar
ρ = density of the liquid in kg/m3
Kv calculation of gases
When calculating gases, it is distinguished between a subcritical and a supercritical flow condition. Subcritical means that the input pressure and the back pressure of the valve determine the flow rate. The greater the back pressure, i.e. the pressure downstream of the valve (p2), the smaller the volume flow.
Supercritical, on the other hand, means that the flow rate depends only on the input pressure, thus resulting in a "choked" flow effect. At large pressure differences (Δp > p1/2), sound velocity theoretically occurs in the narrowest cross-section of the valve. The medium accelerated by the pressure drop cannot flow faster than the sound velocity (Mach 1), even if the back pressure is further reduced. With gases, the standardised calculation takes place at 1013 hPa and 0°C with QN as the standard flow rate and ρN as the standard density. The temperature influence must also be taken into account here.
Calculation with subcritical flow (subsonic velocity)
Calculation with supercritical flow (sound velocity)
p1 = input pressure in bar
p2 = back pressure in bar
Δp = pressure drop in bar
QN = flow rate, standardised, in m3/h
ρN = density, standardised, in kg/m3
T = absolute temperature upstream of the valve in Kelvin
Measurement setup for calculating the Kv factor of valves
The figure below shows a measurement setup for determining the Kv factor at a given pressure drop. In this case, 1 is the test specimen, i.e. the valve to be tested, and 2 is the flow meter. The test setup also includes measuring points for the input pressure (3) and the back pressure (4) and a flow control valve (5). Finally, a temperature measurement device (6) is connected for the measurement of gaseous media.
1 Test specimen
2 Flow meter
3 Pressure gauge: Pressure upstream of the valve (input pressure)
4 Pressure gauge: Pressure downstream of the valve (back pressure)
5 Flow control valve
6 Temperature measurement device
Flow rate
What does the flow rate Q indicate?
A further key figure in fluid technology is the flow, which is also known as the flow rate or volume flow. It indicates how much volume of a fluid flows through a valve at a given time.
To calculate the flow rate volume of a liquid, the Kv factor, the density of the medium and the pressure difference between input pressure and back pressure must be known. Media specified by Bürkert include oxygen, carbon monoxide or ethane. In this case, the respective density is already stored and the pressure difference is calculated automatically, so that only the fields of the Kv factor, the input pressure and the back pressure have to be completed.
Formulas for calculating the volume flow for various aggregate states
Flow rate calculation for liquids
Calculate the flow rate volume using the following formula:
Q = flow rate volume
Kv = flow factor in m3/h
Δp = pressure drop in bar
ρ = density in kg/m3
Flow rate calculation for gases
The standardised flow rate of a gas, on the other hand, also requires the Kv factor, as well as the standard density, the input pressure, the back pressure and the temperature of the medium. Furthermore, it is again distinguished between a subcritical and a supercritical flow.
Calculation for a subcritical flow
Calculation for a supercritical flow
p1 = input pressure in bar
p2 = back pressure in bar
Δp = pressure drop in bar
Kv = flow factor in m3/h
ρN = density in kg/m3
T = temperature in Kelvin
Pressure drop across the valve
How is the pressure drop across a valve calculated?
The pressure drop is the difference between the input pressure of the medium upstream of the valve and the back pressure downstream of the valve. This measured value refers to the energy loss of a fluid when flowing through a valve and is given in bar.
The Kv factor, the density of the liquid and the flow rate are required to calculate the pressure drop in relation to a liquid. The formula on which the calculation is based can be seen below.
Formulas for calculating the pressure drop for various aggregate states
Pressure drop calculation for liquids
ρ = density in kg/m3
Q = volume flow in m3/h
Kv = flow factor in m3/h
Pressure drop calculation for gases
In the calculation for a gaseous medium, a distinction is made between a subcritical and a supercritical flow and the following values are required: The Kv factor, the standard flow rate at 1013 hPa and 0°C, as well as the standard density, the back pressure and the medium temperature.
Calculation for a subcritical flow
Calculation for a supercritical flow
p1 = input pressure in bar
p2 = back pressure in bar
ρN = density in kg/m3
T = temperature in Kelvin
QN = flow rate, standardised, in m3/h
Kv = flow factor in m3/h
Choose from a range of existing media, such as bromine or neon, which are stored along with their density, or simply create another medium. All you have to do is specify the density and the aggregate state of the fluid. While you enter the necessary data for your desired value, the Fluidic Calculator begins working in the background and displays the final result as well as the intermediate results automatically in the upper right-hand box.
Start the fluid factor calculation now!
Do you want to calculate other substances, such as water vapour, or special flow conditions triggered by a very low flow rate or higher viscosities? Or are you looking for a process valve that best suits your requirements? If so, use our special valve design tool to select process valves. Design the valve now!
