You’re ready. Installation is complete. Your distributed control system (DCS) modernization project nears its end. The control loops of the distillation column have been in manual for the last four hours to allow a hot cutover to the new DCS. The project engineering team said the new controls should work just like the old ones.
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.
Now the team’s focus switches to troubleshooting. This, of course, delays the modernization project and cutover. Risks to both costs and schedule rise. Management looks at you and asks: “What just happened?”
After hours of investigation, the control engineer says that many aspects of the proportional-integral-derivative (PID) control conversion weren’t considered when moving from the legacy DCS to the modern automation system.
Appreciate The Differences
Nightmarish stories of modernizations gone wrong might make some people reach for spare-parts order sheets to keep their vintage DCS on life support. However, modernizations in chemical processing can and will have happy endings when well planned. Today’s DCSs bring benefits that old systems can’t match: productivity, process availability and enterprise profit increases to name a few. These are lasting differences that create successful operations.
Proper controller tuning paves the way for state-based control algorithms and other high-value functionality. Tools available now for systems provide help with auto tuning and advanced tuning techniques. In addition, knowledgeable control engineers can help with complex problems using the functionality correctly. In fact, controller tuning techniques can be used to minimize impact of any upsets or disturbances in hot cutovers as well as improve control performance after cold cutover startups.
