In an ISA Intech article, Know when to look outside the PID tuning, Emerson’s Mark Coughran provides 10 actual examples from his and his fellow Control Performance Consultants' experiences where the problems were outside the loop tuning parameters.
Mark opens noting that PID loop tuning skills are important and a good starting spot:
However, a proper loop tuning exercise—including measuring the process dynamics—should be the first step when approaching most loop performance problems. Many loop problems can be solved readily by tuning, and testing for the process response often reveals loop behavior that can help locate problems outside the controller.
The ten real-world examples Mark cites include: control valve response, valve positioner assembly, transmitter failure, signal communication, extra filter in logic, low cutoff in logic, master loop parameters for cascade, lessons learned.
I’ll share one example and invite you to read the article for the others.
Control valve response
After the heater was retrofitted with NOX controls, the plant operators could not control the draft pressure PC-2 and asked the engineers to tune the controller. This loop uses pressure in the heater as the PV [process variable] and sends an output to the final control element based on the difference between SP [setpoint] and PV.
The main symptom of the PC-2 loop was that the PV could not reach SP for any value of output. A consultant worked with the plant operator to put the controller in manual mode and make steps on the output, but still saw no recognizable pattern of the PV responding to the output.
The consultant then asked the instrument technician for an inspection of the damper. After a sweaty climb up three ladders, they arrived at the damper, which turned out to be a very large butterfly valve—found in its fully open position. The consultant asked the operator, via radio, to put the controller in manual and step the output. Large output steps drove the actuator pressure—displayed on the local gauges—from zero to full supply and back again, showing correct response from the positioner. But the valve did not move, because it was jammed in the fully open position.
Repairing the control valve was required so it would respond to the positioner. The consultant also recommended bringing the HART position signal into the control system, so that diagnosis of control valve problems would be faster in the future.
There are many great examples from many different industries, so if you are responsible for keeping the control loops performing properly, this article is well worth your time to read.
You can connect and interact with other control performance experts in the Improve & Modernize group in the Emerson Exchange 365 community. There is also a great educational course, Emerson EnTech Course 9030 – Process Dynamics & Tuning Fundamentals that will occur December 4-7 in Houston, Texas. It is highly recommended by one of the Control Performance Consultants, James Beall.