Calculating Control Valve Sizing


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Recently i am dealing with the calculation of control valve capacity Cv. In case of standard method of calculation, so far i know first i have to assume a Cv at the 100% opening condition of the valve according to my valve size for starting iteration. So for doing so when i explored into the FISHER Catalog, i found different Cv for 100% opening with respect to different trim size. Now i am confused which one should i select for calculation as Cv is different at the same opening for different trim size?
you calculate the Cv for the maximum flow condition, then you select a valve/trim combination to handle that flow based on your Cv.

When you have laminar, cavitation or two phase flow conditions, with liquids, the calculation is a bit more complex. High pressure gases and steams are another matter.

Basically work through the calculations until you understand them.