Flow caracteristic

[Destroyer]

hello,
I have a guestion. Different producents of valves give the flow caracteristics for Kv values. I need to kwow if these caracteristics stay the same if i use air as medium.
Example: if i have a linear flow caracteristic for %opening of valve versus %Kv, does the caracteristic stays linear for air? 50%opening, does this give me then 50% of max airflow???

Thanks a lot

 

[marklobo]

Kv (Cv in U.S. units) is the same for a valve regardless of fluid. The characteristic is simply the variation of Cv with travel.

The answer to your second question is yes, provided that the differential pressure is unchanged and the flow remains turbulent.

 

[4carats]

The answer to your question is 'no'.
The Installed Characteristic (i.e. after the valve is installed in the process) can be quite different from the Inherent Characteristic. Depending on how much the valve is oversized, the fraction of the dynamic pressure taken by the valve, and the rangeability of the valve, a linear characteristic (inherent) can turn out to be quick opening (installed). This has been vigorously calculated in some chemical engineering text books and in agreement with experimental values.