Input to modification block
100 90 80 70 60 50 40 30 20 10
Column
0
0 10 20 30 40 50 60 70 80 90 100 Output from modification block
Figure 3 . Simply relabel the axes of the relative Cv vs . relative travel graph ( Figure 2A ) to obtain a modification graph to program into a controller output block that will linearize controller output vs . flow .
1.0
Installed Relative Flow vs . Relative Travel
Relative Flow
0.8 0.6 0.4 0.2 0.0 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0
Relative Travel
Figure 4 . Installed flow graph from Enhanced version of graphing control valve installed flow and gain . The worksheet places the dotted red lines at the user ’ s specified minimum and maximum flow .
flow range , making it more difficult to come up with a set of controller tuning parameters that will result in fast and stable control . One well-known valve manufacturer ( Neles ) includes installed flow graphs in its valve sizing application , and at least one other can furnish installed flow graphs on request . There are also three sets of Excel worksheets on my website that graph control valve installed flow . The simplest of them is listed on the website as “ Control valve sizing worksheets .” The download consists of three Excel worksheets , one for liquid , one for gas flow in volumetric units , and one for gas flow in mass flow units . On the Installed flow tab of each is a space to enter a table of Cv and of x T or F L of the valve you want to analyze . There is a more comprehensive set of sheets listed on the website as “ Control Valve Sizing application workbooks ”. These Excel Workbooks are intended as full function control valve sizing applications and have built-in Cv tables for a variety of generic control valve styles . The worksheets in both these sets include valve noise prediction calculations in accordance with the current IEC control valve noise prediction standards . The most popular worksheet , which is labelled “ Enhanced version of graphing control valve installed flow and gain ”, makes it possible to create installed flow graphs in conjunction with the user ’ s preferred control valve application . Figure 4 is an installed flow graph made using this Excel sheet . The installed flow characteristic is pretty good as is , but since it is so easy to do , it could be helped a bit by applying the same method used in linearizing an equal percentage inherent flow characteristic .
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