Q: I recently specified a ten-inch segment ball valve for an application. Our purchasing agent suggested that a ten-inch high-performance butterfly valve would be less expensive. Can you please comment on this? I have attached the process conditions.
A: This is an application where an analysis of the installed flow and installed gain of the valve in the specified system can demonstrate how well a particular control valve will be able to control the process.
Using the control valve sizing Excel workbook application that is posted on my website (www.control-valve-application-tools.com), available at no cost, and examining the installed flow and installed gain graphs shown below, here are my comments.
Note that I have superimposed the graphs for the two valves for both installed flow and installed gain onto a single graph to make it easier to compare the results. This is not a feature included in the Excel workbook but was simply done in PowerPoint. The dotted red lines on both graphs represent the specified minimum and maximum flow rates.
My recommended criteria for installed flow is that, between the specified maximum and minimum flow, the flow vs. valve travel be as linear as possible. Also, it should be as symmetrically placed over the full range of valve travel.
My recommended criteria for installed gain that will ensure good and stable control over the specified flow range is that the gain should not be less than 0.5 or greater than 3. Also, the gain should not change by much more than a 2-to-one ratio over the specified flow range.
The installed flow graphs of both valves look good. The segment ball valve is very slightly more linear and will use just a bit more of its travel range. The installed gain graphs of both valves meet the suggested criteria. Based on my recommended installed flow and gain criteria, I believe that either valve would do a satisfactory job of controlling the process.
If you prefer to use a control valve sizing application that you are familiar with, there is also an Excel sheet that works in conjunction with your preferred sizing application to calculate and graph the installed flow and gain.
About the author
Jon F. Monsen, PhD, PE, is a control valve technology specialist with more than 45 years of experience in the control valve industry. He has lectured nationally and internationally on the subjects of control valve application and sizing. Jon currently hosts a website, www.Control-Valve-Application- Tools.com where he freely shares articles, training and professional development materials, and Excel worksheets that might be of interest to those who use or specify control valves. He can be reached at cvapptools@gmail.com.
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