While many chemical plants rely on a single—or a few—distributed control systems (DCS) for their primary process control tasks, programmable logic controllers (PLCs) often are abundant and diverse, and thus present a significant lifecycle management challenge for process manufacturers. Whether deployed as part of an independent safety instrumented systems (SIS), for machinery control and protection, or as part of a vendor-packaged unit, "our ability to manage and maintain these PLCs has a direct impact on plant reliability and safety," said Chris Wells, senior staff engineer, ExxonMobil Chemical Co.
Today's process plants are going eight or more years between maintenance intervals, Wells said in his presentation "PLC Lifecycle Management," at last week's gathering of users of Invensys Operations Management's Triconex safety systems in Galveston, Texas. "And long times between turnarounds means limited opportunities to interrupt the process."
Limited turnaround time means that PLC programming changes and version upgrades should be doable online, Wells said. "We need designed migration strategies for when upgrades are required—with no rewiring of I/O and no impact to operations." Standardization, too, can be difficult due to regional differences in support and availability, OEM controls on vendor-supplied units and specialized applications, Wells said. "This leads to challenges in spare parts management and training."
Integration with other plant systems needs to be reliable, flexible and secure with consistently updated protection from malware and hackers. Add to all this a desire to source and use components for 20-30 years, as well as backward compatibility and continued availability of support and expertise, and Wells' requirements constitute a significant challenge to any maker of programmable systems.
He did, however, offer recommendations for how manufacturers could help users better manage the lifecycle challenges of their PLC investments. He called for increased flexibility in design and more scalable systems using the same hardware in order to address standardization issues. Configurable I/O would further boost flexibility and reduce costs, Wells added.
He also noted that the cybersecurity of many current PLC designs is inadequate; suggesting more secure programming interfaces, more secure control systems interfaces and designs to the ISA99 EDSA (Embedded Device Security Assessment) certification standards would help ease users' lifecycle management tasks.
Industry also needs improved PLC diagnostics, Wells continued. Not just red blinking lights or cryptic error codes, but a "clear visual indication of what we're supposed to do, along with better diagnostics to indicate failures that result in loss of redundancy," Wells said.
"PLC reliability is critical," Wells summarized. "Longer component life and ability to make online changes are needed." Cybersecurity, too, is an important emerging issue, Wells added. "Design improvements and better tools are needed—all of which need to be maintainable in lockstep well into the future."