How can i obtain the the CV value of a control valve at 100% open, knowing the CV value at 80% open for example?

I believe you can't. As the cv is the dimensioned measurement of flow (kv = m3/h of water at 20 deg C with a pressure drop of 1 bar, cv = kv x 1,2 (approximate definition)) there is no direct automatic mathematical connection between the flow at 100% and any lesser opening degrere.

The flow (cv) at any opening will be depending on the internal geometrical lay-out of the valve at the different opening degrees. Measured flow will give you the flow-curve (cv at different openings).

The flow curve may approximate a straight line, or a curved (normally convex) line.

The manufacturers of valves do not usually test each single valve for the flow curve, but select certain sizes and  certain opening degrees on the curve, and give the rest as approximations based on the known measured data.

The best you can hope for with your valve to guess the flow is the same. If you cant obtain the real flow curve for your valve you may compare and compute according to values from a similar (very similar!) known valve.

Normally the output from sizing software pertains to the valve coefficient at that specific operationg condition.  Often the process data includes minimum, normal and maximum cases.  The Cv calculated is the requirement for the specified condition - not the rated valve Cv.  If a specific valve was selected, the software may identify that specific valve opening that corresponds to that calculated Cv.

MORE NEWS