Using Control Valve Installed Gain Calculations

With the 17-hp pump, besides requiring a more expensive 6-inch valve, the gain graph looks terrible. The installed gain is the highest of the three (meaning a larger flow error for the same valve position error), it drops to 0.4 as it approaches the maximum design flow (the red vertical line at 1.0 on the Q/Qmax scale), and the variation in gain over the flow range is almost 7:1, much greater than the recommendation of 2:1. This is large enough that it would be difficult to come up with proportional-integral-derivative (PID) tuning parameters that would provide good and stable control over the entire required flow range. The gain graphs of the 23-hp and 29-hp pumps fall within the recommended gain criteria, but the 23-hp pump is the winner, because its gain is closer to 1.0, and it also is the more economical of the two to operate.

References

1. Monsen, Jon, Rules of Thumb, Flow Control, November 2012 pp. 24–26
2. Monsen, Jon, An Insider’s Guide to Installed Gain as a Control Valve Sizing Criterion, Flow Control, May 2015, pp. 22-25.
3. Monsen, Jon, Modern Tools for Sizing Control Valves & Actuators, Processing, January 2018 pp. 12-14
4. Monsen, Jon, Calculating the Installed Flow and Gain of a Control Valve, Process Instrumentation, March 2021, pp. 26–30. (The worksheet described in the reference and an enhanced version are available.)
5. Jessee, Peter, Determining Pressure Drop for Control Valve Sizing, Flow Control, August 2000, pp. 12-14.
6. Coggan, D. A, ed., “Fundamentals of Industrial Control,” Second Edition, Research Triangle Park, NC: Instrumentation, Systems, and Automation Society (ISA) (now the International Society of Automation—ISA), 2004. pp. 278–280.

This article was originally published in the December 2021 issue of InTech magazine.