Q: Can you explain the difference between the control valve linear and equal percentage characteristics and when should one or the other be specified?
A: The linear characteristic is the easiest one to understand. There is simply a linear relationship between valve travel and Cv. For example, a valve with a perfect linear characteristic and with a fully open Cv of 100 would have a Cv of 10 at 10 percent travel, 20 at 20 percent of valve travel and so on. Interestingly, the linear flow characteristic is typically specified only about ten or twenty percent of the time. The reason is because of the installed characteristic. Most control valves are installed in systems that include other pressure-consuming elements, such as pipe, fittings, isolation valves, and process equipment such as filters and heat exchangers. Centrifugal pumps have the same impact on flow. The equal percentage valve has a flow characteristic where each time the valve travel is increased by ten percentage points, the Cv increases by, typically, 50% of what it was before the ten percentage point increase was introduced. The name “Equal percentage” comes from the fact that Cv increases for each ten percent increase in valve travel are approximately the same percentage or “equal percentage” increases. This results in a flow characteristic like the red “=% VALVE” line in Figure 2.
It is necessary to make the correct choice between a valve with an equal percentage or a linear inherent flow characteristic because to obtain good control throughout the specified flow range, the relationship between valve travel and flow must be a linear or nearly linear one. In a system with very little pipe or other pressure-consuming elements the valve in Figure 1 with a linear inherent characteristic would meet this criterion. In a system with considerable pipe and/or other pressure-consuming elements, the valve in Figure 2 with an equal percentage inherent characteristic would meet this criterion. Figures 3 and 4 summarize the reason that the choice between equal percentage and linear inherent flow characteristics must meet the above criteria.
About the author
Jon F. Monsen, PhD, PE, was a control specialist with over 45 years of experience in the control valve industry. He lectured nationally and internationally on the subjects of control valve application and sizing. Jon’s website, www.Control-Valve-Application-Tools.com freely shares articles, training and professional development materials, and Excel worksheets that might be of interest to those who use or specify control valves. Jon passed away in December 2023 and his series of columns is being published in accordance with his wishes.
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