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.

This thread seems to have gone all over the place.  
You asked how to get the 100% Cv of a valve, knowing the Cv at 80%.  

If that's all the information you have, you can only estimate.  


MOST control valves have equal percentage characteristic.  These vary, but 80% of TRAVEL with =% is about 45% of CAPACITY.  So if you have (for example) a Cv of 20.7 at 80% of stroke, the full 100% travel Cv is 46. If it's linear characteristic, you can estimate pretty closely by multiplying the 80% Cv by 1.25.  The characteristic also should be on the serial plate.  Cv46P means that the Cv is 46, percentage.  

You mentioned the Masoneilan software.  I haven't used it recently but I did have a relationship with Masoneilan for 18 years, and their software, (and their sliderule), were all keyed to the 100% Cv which has always been posted in their literature. And if it's a Masoneilan valve, the serial number plate was stamped with the rated Cv, and the Characteristic.    BTW: The numbers in the previous paragraph were from the Masoneilan 21000 series valve bulletin.  

Psafety gives you an effective "Plan B" by calling the Manufacturer with the serial number and asking for a data sheet.  

Many valve manufacturers engrave the part number of the plug in the flats at the top of the stem.  If the serial number plate is gone, you might be able to read the plug part number, then check the parts supplement or the manufacturer's customer service rep, and get the Cv of the plug with that part number.

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