In spite of my best efforts to use persistent RSS search feeds in order not to miss any news about Emerson experts in action, here’s one that got by me.
Mark Coughran, a consultant on Emerson’s Advanced Applied Technology team, shared this control challenge question he answered with me. You may recall Mark from earlier posts.
The question he addressed appeared on the ChemicalProcess.com’s Ask The Experts website. The question, Control pressure at discharge, was:
I have five pumps running parallel, transferring water. Due to pressure fluctuation at discharge, which depends on the flow requirements of the user, I am planning to install a pressure control valve at the pump discharge to keep the pump running at an optimum condition… What kind of valve is best for a 14-in discharge?
Mark notes that he’s seen problems with butterfly valves used on large water lines, but that things have improved with better valve, actuator, positioner, and application software. Common sources of problems include wrong valve size, shape of butterfly disk, backlash in disk-to-shaft and shaft-to-actuator connections, poor valve positioner performance, and insufficient torque.
Control valve suppliers have addressed these issues in a number of ways. Examples include better valve sizing software, improved butterfly valve disk shapes, zero-backlash connections, valve positioners responding to 0.1% signal changes, and sizing software that predicts installed torque.
Mark points out that globe valves are typically too expensive for this application. Butterfly or segmented ball valves may be better suited if the supplier’s test data for the valve + actuator + positioner shows suitability in similar applications.
Mark’s final guidance concerns the control strategy. He recommends a controller tuning method that does not oscillate, but responds at the application’s required speed, such as Lambda tuning. He advises:
If you need to control the five lines separately, there will be interaction and balancing concerns. The options range from individual PID controllers to a multivariable controller. All the options are easy to configure and tune in a modern DCS.
