• Updating the HMI and operator screen. You must decide whether it's more important to implement the latest advances in alarm management and operator interface or to maintain the look and feel of the existing system.
• Complying with new codes and standards. Many regulations don't apply to systems in place before they went into effect . Migration may mean that a part of your process in compliance because of such "grandfathering" now requires installation of new equipment including, for example, a safety instrumented system with a safety-integrity-level (SIL) rating on your burner management applications.
Phased migration does have its drawbacks in terms of cost and time but is a lower risk approach with less downtime. You may reduce risk and downtime further by simulating the new system prior to installation and using the model to train your staff, especially panel operators, on how to use and interact with the new system.
Four factors frequently lead to migration project failure:
1. Lack of detailed upfront planning to identify all interactions among control system components, their impact on the process, and the skill sets of the individuals involved. As with any control project, the devil always is in the details.
2. Not considering ancillaries. In addition to the control system itself, the automation master plan must cover ancillaries including space allocation, HVAC, uninterruptible power supplies, grounding and power distribution systems to ensure removing the weakest link.
3. Third-party device communication and interface difficulties, especially for legacy protocols and any necessary buffering or gateways that may be necessary between the existing field equipment and the new controllers. Third-party communications also frequently require accurate mapping of a wide range of parameters — a task that's often labor intensive and prone to error.
4. Extended cutover delays and downtime. Control system migration, like most projects, is susceptible to "scope creep" — this may involve, e.g., calibrating field devices, tuning loops, and minor repairs to device ancillaries such as air supplies, tubing, impulse lines, mounting brackets, etc. This extra work can take a toll on the schedule.
One of the first tasks once the migration project gets the go-ahead is to build — and gain agreement on — a list of selection criteria for the new system. Develop evaluation criteria at the same time. Having evaluation criteria in place before sending out the request for quotation will minimize bias in the weightings for the decision analysis process and ensure the weightings reflect deliberations during list development.
As already noted, communication and interfaces with third-party devices can pose a major stumbling block to automation system upgrades. One way to get a generic or universal I/O gateway is to choose one of the many fieldbus interfaces as an output. The selection process then comes down to picking a bus that's natively supported by the control system.
A new installation likely will require an interface with the safety system. Many DCS suppliers now have integrated safety functionality into the control system via separate specialized I/O cards. The majority of safety systems continue to be stand-alone, though, and thus need a third-party interface with all that implies. However, because DCS companies have been integrating stand-alone safety logic solvers for years, most interfaces between these systems are well defined.
Control system migration will be part of our future. So, we must face its challenges of retaining institutional knowledge, timing the change for maximum return with minimum production impact, and integrating the many different signal types into a coherent whole. Fortunately, as with any project, good planning is key to success, which is something, of course, all good engineers already know.
IAN VERHAPPEN is director and principal consultant of Industrial Automation Networks Inc., Wainwright, AB. E-mail him at firstname.lastname@example.org.
1. Woll, D., "Optimizing the Control System Migration Value Proposition," ARC Insights, ARC Advisory Group, Dedham, Mass. (Dec. 2010).
2. Schnipke, E., "Hot Cutover Boosts Control System Migration," p. 39, Chemical Processing (May 2008).
3. Summers, A., "Can You Safely Grandfather Your SIS?," p. 42, Chemical Processing (Aug. 2005).