Plan for PID Control Differences in Control System Migration Projects

Emerson's Laurie Ben

Emerson's Aaron Crews

Emerson's James Beall

The typical lives of industrial control systems (ICSs) are measured in decades. Even so, the time comes when reliability and securing replacement parts becomes nearly impossible given the rapid advancement of technology.

There is much to plan and do when the time comes to migrate and modernize the control system. In a Chemical Processing article, Avoid Startup Surprises in Control System Migration, Emerson’s Laurie Ben, Aaron Crews and James Beall share guidance and recommendations on how to avoid common pitfalls.

Chemical Processing: Avoid Startup Surprises in Control System MigrationThey open describing a hot cutover to a new distributed control system (DCS) gone awry:

The operator puts the control loops in the proper mode. Everything looks good initially. Thirty minutes later, though, the key loops on the column start to cycle. Soon, the top and bottom composition (temperature) loops have hit their respective alarm limits, indicating near out-of-spec products. The operator takes manual control trying to recover. It’s too late. The overhead and bottom streams have switched to recycle mode.

A common cause for this situation is the difference in proportional-integral-derivative (PID) control algorithms between the legacy and modern control system. Part of the planning effort is to not only understand and apply the additional power in modern PID control algorithms, but also understand and adjust for the differences.

Project teams must focus on forward engineering the settings to attain optimal control and process performance. That means concentrating on improvements that are attainable, such as implementing existing functionality with new features to ease use and maintenance. In addition, look at changes with new relatively simple features (e.g., an easily selectable PID structure to reduce process disturbances) that may not have existed in the legacy DCS; these can improve control and reduce variability.

For differences, understand the form and structure of the PID control algorithm.

The most common PID forms typically are series or standard, with the difference being in how the derivative action is applied mathematically. Less common is a third form called parallel. The use of different names for the form, e.g., interacting rather than series, can further complicate ascertaining the legacy PID form.

Laurie, Aaron and James recommend:

…that you standardize on one form (typically the standard) in the new system and properly convert the tuning from the legacy system based on the form and tuning parameter units of the legacy and new systems.

Other considerations of differences to plan for include:

…the structure (error handling), additive or multiplicative feedforward, nonlinear gain options, deadtime compensation and anti-reset windup settings, among many other possibilities.

They shared an example of the effects of a PID form mismatch upon cutover between old and new:

After the cutover of two fermenters, the pH cycled dramatically… unlike before the cutover.

What the project team discovered that fixed the problem:

…was that a different PID form was selected for the conversion but the units for the tuning parameters weren’t taken into account. To correct the problem, the loop was placed in manual, the correct tuning for the selected form was calculated so a download wasn’t needed, and the loop then was returned to auto.

Here are some signs of PID controller differences to watch out and correct for:

  1. Cycling controller outputs or PVs [process variables] when in auto;
  2. Loop control that either is super slow, super fast or “just different” compared to the legacy system; and
  3. Loop in control but stabilizing at an offset to the set point.

Read the article for ways to prevent other common mistakes so that your hot or cold control system cutover project goes smoothly.

Connect with Emerson modernization consultants to help you justify, plan and execute your control system migration project. You can also connect and interact with other migration and modernization experts in the Improve & Modernize and DeltaV groups in the Emerson Exchange 365 community.

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