Upgrading control systems: Phase migration vs. complete replacement
After receiving the approval for a control system replacement project, how do you decide between phased migration and a complete replacement? Do you keep some of the old and integrate the new, or just go with a fresh start?
Now it’s time to setup the plant for the highest possible return on a project that was hard to justify in the first place. There are so many questions that should drive the selection of the control system. What is its expandability? How well does it play with my “other” systems? What is my support learning curve? When you have answered these questions and have selected a control system, the hard part is done right? Not even close; it is time to execute what model you choose. Let’s discuss two typical migration models, phased migration and complete replacement. So now you must consider what should stay and what should go.
Phased migration allows the full system modernization to be completed in gradual steps. The old and new control systems work simultaneously to help eliminate downtime and startup risk. This is my personal favorite. With this approach depending on the change, it can cause some difficulty for operations with two systems. The payoff of the new system can be validated in some cases while the old system is doing the work for a while. If done right, it can be switched back from new to old so production does not suffer the startup heartache.
This style of execution requires extra planning and time to roll out systems one at a time. I favor this approach because it allows everyone to get their feet wet without having to jump into the deep end. Training, drawings, and maintenance get the upper hand without having to change just for the sake of changing. The old should stay and the new should be as close to parity as possible. The control system can keep the best of what it used to do and leverage the best of new practices, such as alarm management, code standards, and standards for screens. This approach also allows plant personnel to become familiar with the new control system one process area at a time. The final product is different than in the past, making sure everything is once again organized.
Complete replacement allows the project to be completed all at once during a planned outage. This may be the best way to turn the page if there are a lot of legacy issues. Some system changes are so great that this will be the only option. However, this has the highest startup risk since everything old is no longer accessible or comparable. This puts everyone on the same page from the beginning, so if the system is vastly different the past can be a blessing.
In this case the old has to go. Move on with your plant, don’t look back, and build on what was there, but keep everything new. That includes new drawings and training for all levels of staff. New approaches to leveraging the new tools available make it possible to get a deeper fresh look at your operations. One must remember that no matter how new, the process did not change. There may be a new button sequence, queue, recipe manager to get it started, but what it does will be what it did before.
With a new control system you get the advantage of new data storage, trending possibilities, and other plant systems integration. If your plant is lucky enough to already have a historian this is where the old will save the new. Everyone will have a different perceived notion on how the system worked previously to the new control system. Actual run history removes the perceived notion and the thought of “I think it used to operate different” goes away. So that history, if available, is invaluable.
Do you need to import the old data into your nice new system? Probably not, but you need accessible data to help ensure that the new system is working properly. Remember that the physical capabilities and restraints of the system did not change with the new control system. Keeping the names of the devices close to what they were before will minimize the problems dealing with some kind of cross-reference list, not to mention any existing documentation or forms that already exist can be lost with name changes. It’s the control system changing and not the control points for the process; however, a clean start to operations can find process improvements.
So embrace the old with the new! No matter how you chose to replace your facility’s controls system, always leverage what is running now to keep from having to invent the wheel again. What model should you choose? If the answer to this question is unknown, then partner with someone that does this on a daily basis. There is a fit to your needs and if you are lost in the woods it helps to have a compass.
What would be the one thing that is a must in either of these cases? There is never a better opportunity now to build in that highly coveted simulation network, but that is a topic for another day.
This post was written by Dave Cortivo. Dave is a senior engineer at MAVERICK Technologies, which is a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.