Many Objectives, Many Worlds of Process Control

In many publications on process control, the common metric you see is integrated absolute error for a step disturbance on the process output. In many tests for tuning, setpoint changes are made and the most important criteria becomes overshoot of setpoint. Increasingly, oscillations of any type are looked at as inherently bad. What is really important varies because of the different loops and types of processes. Here we seek to open minds and develop a better understanding of what is important.

Many Objectives

  • Minimum PV peak error in load response to prevent:

–        Compressor surge, SIS activation, relief activation, undesirable reactions, poor cell health

  • Minimum PV integrated error in load or setpoint response to minimize:

–        total amount of off-spec product to enable closer operation to optimum setpoint

  • Minimum PV overshoot of SP in setpoint response to prevent:

–        Compressor surge, SIS activation, relief activation, undesirable reactions, poor cell health

  • Minimum Out overshoot of FRV* in setpoint response to prevent:

–        Interaction with heat integration and recycle loops in hydrocarbon gas unit operations

  • Minimum PV time to reach SP in setpoint response to minimize:

–        Batch cycle time, startup time, transition time to new products and operating rates

  • Minimum split range point crossings to prevent:

–        Wasted energy-reactants-reagents, poor cell health (high osmotic pressure)

  • Maximum absorption of variability in level control (e.g. surge tank) to prevent:

–        Passing of changes in input flows to output flows upsetting downstream unit ops

  • Optimum transfer of variability from controlled variable to manipulated variable to prevent:

–        Resonance, interaction and propagation of disturbances to other loops

* FRV is the Final Resting Value of PID output. Overshoot of FRV is necessary for setpoint and load response for integrating and runaway processes. However for self-regulating processes not involving highly mixed vessels (e.g., heat exchangers and plug flow reactors),  aggressive action in terms of PID output can upset other loops and unit operations that are affected by the flow manipulated by the PID. Not recognized in the literature is that external-reset feedback of the manipulated flow enables setpoint rate limits to smooth out changes in manipulated flows without affecting the PID tuning.

Many Worlds

  • Hydrocarbon processes and other gas unit operations with plug flow, heat integration & recycle streams (e.g. crackers, furnaces, reformers)

–        Fast self-regulating responses, interactions and complex secondary responses with sensitivity to SP and FRV overshoot, split range crossings and utility interactions.

  • Chemical batch and continuous processes with vessels and columns

–        Important loops tend to have slow near or true integrating and runaway responses with minimizing peak and integrated errors and rise time as key objectives.

  • Utility systems (e.g., boilers, steam headers, chillers, compressors)

–        Important loops tend to have fast near or true integrating responses with minimizing peak and integrated errors and interactions as key objectives.

  • Pulp, paper, food and polymer inline, extrusion and sheet processes

–        Fast self-regulating responses and interactions with propagation of variability into product (little to no attenuation of oscillations by back mixed volumes) with extreme sensitive to variability and resonance. Loops (particularly for sheets) can be dead time dominant due to transportation delays unless there are heat transfer lags.

  • Biological vessels (e.g., fermenters and bioreactors)

–        Most important loops tend have slow near or true integrating responses with extreme sensitivity to SP and FRV overshoot, split range crossings and utility interactions. Load disturbances originating from cells are incredibly slow and therefore not an issue.

A critical insight is that most disturbances are on the process input not the process output and are not step changes. The fastest disturbances are generally flow or liquid pressure but even these have an 86% response time of at least several seconds because of the 86% response time of valves and the tuning of PID controllers. The fastest and most disruptive disturbances are often manual actions by an operator or setpoint changes by a batch sequence. Setpoint rate limits and a 2 Degrees of Freedom (2DOF) PID structure with Beta and Gamma approaching zero can eliminate much of the disruption from setpoint changes by slowing down changes in the PID output from proportional and derivative action. A disturbance to a loop can be considered to be fast if it has a 86% response time less than the loop deadtime.

If you would like to hear more on this, checkout the ISA Mentor Program Webinar Recording: PID Options and Solutions Part1

If you want to be able to explain this to young engineers, check out the dictionary for translation of slang terms in the Control Talk Column “Hands-on Labs build real skills.”