The Kv value and its relevance for valve technology
13.07.2023 The following article answers the basic questions about the Kv value and its significance in valve technology.
What is the Kv value?
The Kv value is essential for choosing the right valve size. It indicates the flow rate going through a valve under a given pressure differential. The measuring unit is m3/h of water.
We deliberately use water as the test medium here, as it is still seems to be used in practice.
Why is the Kv value relevant?
For any application that requires valves being installed you need to know the corresponding flow capacity of these valves to be able to professionally design the sytem. The Kv value indicates it accurately. The required Kv value differs depending on the project. For example, if a hydrogen refueling station is taken into operation, the key question is what type of vehicles are to be fueled up. Since trucks and buses have much larger tanks than passenger cars, a valve with larger Kv value is used for their fueling. On a general note, it should be mentioned that the Kv value for gases should be used for applications involving gas. For hydrogen in the high-pressure range and all other pressure ranges of gases, however, we recommend the C,b-mass flow model about which we will report at a later point.
How is the Kv value calculated?
There is a binding predefined rule for how the Kv value must be measured and calculated. In industrial practice, there are two standards and thus two different approaches that are widely used:
- IEC 60534-2-3 edition 2016-4 Industrial-process control valves
- VDI VDE 2173 edition August 2022
These two approaches have in common that the Kv calculation is measured or can be simulated with water as media. The simulation with water instead of gas is for historical reasons and therefore expressly not recommended anymore for gas valves.
An identical test system is used for both standards measurements and for the simulations. Two pressure measurement ports are arranged before and after the test specimen. At the inlet of the test line, water is pushed in the proper flow direction through the test specimen and collected outlet. In the process, the pressure differentials at the respective flow rate are measured.
The flow coefficient C is calculated with three different pressure differentials taken with turbulent flow. These pressures are measured at the two pressure measurement points on the test line. The valve open position for the control valves is predefined. The three results are averaged and the Kv value determined according to IEC 60534.
The Kv value is simulated according to VDI using the same test system as in the IEC method described above. Now, however, only one measurement is taken with water under differential pressure of 1 bar. Thus, the method is a little simpler.
The Kv value is calculated by a simple formula.
The two approaches differ in their testing procedure. The discrepancies in results are minor according to our knowledge, though the IEC method is preferable, as the reliability of the results is greater due to the multiple measuring points.
VDI formula for calculating the Kv value
Design of the measuring device
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