Column
Control valve Q & A
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 pressureconsuming 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 .
Figure 1 . In a system with very little pipe the pressure drop across the valve will remain constant throughout the flow range and the installed characteristic will be the same as the valve ’ s inherent characteristic .
Figure 3 . A nonlinear installed flow characteristic makes it difficult to tune a loop for stable and good control throughout the required flow range .
Figure 2 . In a system with a lot of pipe ( or other pressure consumig elements ) the pressure drop across the valve will decrease with increasing flow . Using a valve with an equal percentage inherent characteristic will result in an approximately linear installed flow characteristic .
Figure 4 . A linear installed flow characteristic makes it easy to tune a loop for stable and good control throughout the required flow range .
14 www . valve-world . net Valve World June 2024