This guest post is authored by Greg McMillan.
In the ISA Automation Week Mentor Program I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The eighth question in the ISA mentor program is from Danaca Jordan (USA):
"What is the effect of the pump on control valves at lower rates?"
If the pump is only supplying the subject valves, at the lower rates the pump head developed increases causing the control valve to ride closer to its seat where the friction is greatest. Continual cycling at low rates is most likely caused by increased stick-slip most noticeable for valve positions less than 20%. At low rates (low system frictional losses), you are also subject to a greater effect of changes in source and destination suction heads as seen in the intersection of system curve with the pump curve. Adaptive control can compensate for changes in process gain from installed characteristics or the different effect of split range valves. For a big-small valve, valve position control (VPC) is preferable to split range control if the VPC is fast enough by the use of feedforward. See the entries “How to Succeed – Part 5” and “How to Succeed – Part 6” for how to get the control valve that will have the least stick-slip and the greatest linearity and rangeability.