How can i obtain the the CV value of a control valve at 100% open, knowing the CV value at 80% open for example?"We don't believe things because they are true, things are true because we believe them.""Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."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.Standard curves exist for control valves. You need to determine which trim exists. Some manufacturers provide tables showing the percentage travel and the valve coefficient Cv at each 10%, or each 10 degrees for rotary valves.A common characteristic selection for throttling valves is described as equal percentage. With an equal percentage characteristic curve, equal change in the travel provide equal percentage changes in the flow coefficient. Cage style globe valves can be machined for several common characteristic curves. Equal percentage is likely the most common, followed by linear and fewest control valves being quick opening. A similar curve for old style plug and seat valves was known as modified parabolic.

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