In a previous posting (Determining the Control Valve Pressure Drop for Control Valve Sizing Purposes) I discussed the correct method for determining the control valve sizing pressure drop when the system already exists or has been designed. The ideal situation is where you have a say in determining what the control valve pressure drop will be, usually by specifying the pump that will be used. Many designers successfully use a rule of thumb that recommends calculating all of the dynamic losses in the system, excluding the control valve, and then adding a pressure drop of one half the other dynamic losses for the control valve and specifying the pump head appropriately. This is equivalent to saying that the control valve pressure drop should be 1/3 of the entire system pressure differential. My preference is, whenever possible, to use an actual analysis instead of rules of thumb.
In this posting, I will show how an analysis using Metso’s Nelprof™ control valve sizing and selection software’s unique ability to graph the installed gain of a control valve can be helpful in selecting the optimum pump for a balance of good controllability and minimum energy use. Figure 1 shows an example system for which I will use Nelprof™ to help choose the optimum control valve pressure drop. If you are not familiar with the subject of control valve installed gain, you might find this article of interest: http://www.valin.com/resources/whitepapers/installed-gain-control-valve-sizing-criterion