Designing and Planning Existing Control System Upgrades

Once the decision has been made to replace or retrofit your plant's or facility's control system, it's time to start a crucial part of the process: designing and planning. This stage must be completed diligently and methodically, as a poorly designed or planned control system will not last or perform well. Before designing and planning can start, it is also important to select the right control system. You can learn more about that process in a previous article.

 

How to Design and Plan a Control System

Once you have your budget and new control system choice, it's time to develop a plan and start designing your system. 

Designing a replacement control system can be more difficult than designing a control system from scratch. In designing a replacement to an existing control system, you may have to make decisions you normally would not have to make if it was a brand new installation. Your existing plant may need to be operational during the retrofit, or the replacement may need to occur swiftly during an outage. You may also need alternative designs or be flexible with your design in case something doesn't quite turn out as you expect.

The first key objective to planning your replacement is documenting the existing installation. This may involve gathering existing drawings, documents, databases, control system logic, HMI screen configurations, etc. Many times, existing documentation can be suspect or altogether incorrect. 

It is a good idea to go over the existing documentation and verify key parts of it with the existing installation. If existing documentation is not available, you must document the existing system status. Accomplish this with notes, sketches, photographs, video, plant personnel interviews, or other means. Even if the existing documentation seems accurate, photographs or video is an easy way to ensure that you have captured as much of the existing installation possible. Photographs and videos can also help if you are working on the design away from the control system site—it gives you an easy reference to installation without physically being there.

 

Figure 1. These pictures won't win any photo contests, but they can be a valuable reference during a control system retrofit. 

 

When documenting with photographs or video, make sure your pictures or video are clear and complete. Don't forget to capture every area of the existing control system. Organizing your pictures right after you take them is a good way to make sure they make sense days, weeks, or even months down the road as you work on the new system's design. Videos can help augment pictures. You can sweep an entire area or control cabinet slowly with the video and go back later to pause the video to see certain areas you may have missed with photographs.

Getting the electronic configuration of the existing system is just as important as the physical documentation. Suppose the existing configuration can be maintained electronically on the original hardware while implementing the configuration on the new hardware. In that case, this can be a handy way to duplicate and test the new configuration. You can refer back to the original configuration as needed to complete the new configuration as many times as needed. 

In addition to an electronic copy, a hard copy of the complete configuration and logic should also be created as a reference. This documentation may be used months, even years after the new control system has been integrated to verify a suspicious setting, range, or piece of logic that has been questioned once the system has been up and running.

 

Implementing New Equipment in Existing Control Systems

There are numerous ways to tackle your retrofit or replacement. Your plan may involve a phased approach if you need to keep things up and running or have very little downtime available.  You may also opt for the "rip and replace" method, where you completely remove the old system and replace it with a new system. Each of these methods can have additional options within themselves. You may be re-using the existing field terminations or utilizing a manufacturer's conversion system for field terminations. 

Some phased approaches can even use the existing I/O hardware along with the field terminations. This can be adventitious for numerous reasons but does not eliminate obsolete hardware if that is one of your concerns.

 

Figure 2. I/O wiring conversion system. Image used courtesy of Allen-Bradley

 

Logic Considerations for Control System Retrofits

When converting existing logic, there are several methods available to accomplish this part of the process. Many control systems have auto-conversion tools available to speed up the conversion process. Be careful with some of these tools; they get the job done quickly but may not get it done in the manner you would if you were programming manually. Many are not complete either. You have to "read the fine print," if you will, on the capabilities of some of these tools. 

Sometimes, the auto-conversion tools prevent you from taking advantage of newer or more advanced functions in the new control system. Formatting might be another area where auto-conversion tools fall short. The auto-conversion tool has been programmed to convert the logic in a set way. This may not be the most advantageous way, and the format may not fit your taste or plant guidelines.

Using the current program as a reference, manual programming may be slower than the auto-conversion process but may afford you the best results when comparing all the alternatives.  With manual programming, you can take advantage of new or more advanced features of the new control system while still having peace of mind that you are using trusted logic that hopefully was fully functional in the past. You have more control over the program's formatting and can review the program for errors, inconsistencies, or inefficient logic. Existing logic may have evolved over the years and been under the care of multiple individuals who may have added logic and not followed consistent programming practices. Manual programming offers the perfect opportunities to clean things like this up.

If the existing logic is unavailable or is a mess and has been prone to errors and caused the users nothing but headaches in the past, then neither of the above methods seem to be a good fit for the new control system logic. Sometimes, starting from scratch can help achieve the best results from your new control system. Careful planning and reviewing existing documentation will help get total re-programming of a system underway just a little faster than designing a brand-new installation.

 

Figure 3. Allen-Bradley control system equipment. Image used courtesy of Allen-Bradley

 

To summarize, below are some key recommendations for a successful control system retrofit or replacement:

• Documentation: As stated above, in the design and planning stage, document the existing installation thoroughly. Provide very clear and detailed drawings and documents for the new installation as well.
• Pre-assembly: Assemble as much of the new system as you can upfront while the old system is still operational.
• Simulation - If possible, simulate or test the new system offline before taking the old system down. If you can operate both the old and new systems side by side, you can eliminate many errors by pre-checking the new system implementation while referencing the old system.
• Flexibility: Have alternate solutions readily available for issues that you can predict. You will need to be prepared to provide alternate solutions on the fly for other issues that you didn't see coming.
• Education: Visit other facilities, read articles, or talk to others that have completed retrofits, especially of the same type, and get their feedback.
• Research: Research your proposed conversion and integration solutions thoroughly, as the advertised results from the manufacturer are not always as they seem. There are usually trade-offs when using a manufacturer's conversion system instead of completely replacing hardware or terminations.
• Manage closely: Do not rely solely on drawing and documents to get your design across to the installers. Make frequent visits, review the upcoming scope of work with the installers, and ensure all is clear.