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Control valve Q & A
Q . I need to specify a control valve for a liquid system involving very short runs of pipe both upstream and downstream of the valve . The source pressure will remain constant at all specified flow rates . The back pressure from the receiving equipment will also remain constant at all specified flow rates . Normally I would specify a globe control valve with a linear inherent flow characteristic . The problem in this case is that the fiber content of the process media is too high for a globe valve . In similar processes in my plant , but with longer pipe runs , we satisfactorily use segment ball valves , which are only available with an equal percentage inherent flow characteristic . I have attached a Cv vs . relative valve travel graph for my preferred manufacturer ’ s segment ball valve . ( Figure 1 .) Can you suggest a solution for this application ?
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 .
A . What I have done in similar situations is to add a flow modification block in the output of the controller , something that is usually easy to do with modern digital controllers . The first step is to normalize the vertical scale on the manufacturer ’ s Cv chart so that instead of reading in units of Cv it reads in relative Cv , that is from zero to 100 percent of the fully open Cv ( Figure 2 , A ). Then rotate your relative Cv vs . relative valve travel graph 90 degrees counterclockwise ( Figure 2 , B ). Finally , flip the graph horizontally ( Figure 2 , C ). You now have a modification graph to enter into your controller ’ s modification block . Although the text and numbers look a bit strange , upon relabeling the horizontal and vertical axes , as shown in Figure 2 , C you have a graph in a conventional format that you can use to program your modification block . Once you are satisfied that creating the modification block graph is based on a logical sequence , you can save a lot of time by simply relabeling the Figure 2 , A graph as shown in Figure 3 . Granted , it is not in conventional form ( input , horizontal axis , output vertical axis ), but no one will see it except you .
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Relative Travel , % Figure 1 . Manufacturers published Cv vs . relative travel
If you have a graph of a valve ’ s installed flow characteristic , you can apply the same process to it to remove any non-linearities in its installed flow characteristic . Any non-linearity in the control valve ’ s installed flow characteristic will result in installed gain changes throughout the specified
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A . Redraw Cv axis as 0 to 100 % relative Cv |
B . Rotata 90 degrees CCW . |
C . Flip Horizontal |
Figure 2 . Manipulate valve ’ s flow characteristic to obtain a modification that will linearize Cv vs . controller output . |
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