It’s a rite of passage that every long-running plant must pass through at some point: when to pull the plug on control technology that is still functional but no longer supported by its maker. That necessary investment often pays for itself in improved reliability and system performance down the road, but there are often a number of paths from which to choose.
Such is the crossroads recently faced at Chevron’s refinery in southern California, where now obsolete Rockwell Automation PLC-5 controllers still perform many essential functions in coordination with Honeywell Process Knowledge System (PKS) distributed control systems.
“The PLC-5s were discontinued in 2017, but we still have a lot of them running,” said Maggie Sun, Chevron senior control systems engineer, in her presentation at this week’s 2023 Honeywell Users Group meeting in Orlando, Fla. Given the potential difficulties that the obsolete controllers represent in terms of spare-parts availability and potential unit shutdown, the organization set out to replace them, Sun said. The PLC-5s also did not provide for controller redundancy, so that was a contributing factor in the decision to replace them.
Three modernization options made the first cut for evaluation: switching out the PLC-5s for Rockwell Automation’s current ControlLogix controllers, switching them out for Honeywell C300 controllers or switching them out for Honeywell’s ControlEdge UOC controllers. The PLC-5s had also been tightly integrated with the Honeywell PKS and controlled through the PKS console, and a comparable level of integration was expected post-migration. Other key decision-making criteria included upfront costs, necessary I/O modifications, Experion integration effort, Experion HMI modifications and current engineering skillset and project timeline—given an upcoming shutdown window for the project.
The move to ControlLogix was appealing because of the controller’s powerful CPU, readily available spare parts and lower upfront costs for a standalone system, Sun said. But ControlLogix also entailed extra engineering effort and costs to integrate it with Experion. It also had an independent database to maintain, and it communicated limited data for troubleshooting back to Experion.
The second path forward was to replace the PLC-5s with Honeywell’s C300 controllers. On the positive side, they integrated natively with the Experion system, they leveraged a pre-existing skillset among the staff, and spare parts were readily available. On the opposite side of the ledger were higher upfront costs and the need to redo all of the I/O. “The C300s were more expensive and were already in use, too, but reworking the I/O was too labor-intensive to meet our project deadlines,” Sun explained.
The third and final candidate was the ControlEdge UOC. Relative to the C300, a key advantage was that it offered straight one-to-one wiring relative to the PLC-5. It was also native to the Experion platform, and project execution fit within the planned timeline. “In the end, we decided the ControlEdge UOC was our best option,” Sun said.
Implementation challenges
The project team did run into a few challenges when it came time for the implementation. First, the team had trouble switching the ControlEdge processor from PLC mode to UOC. After consultation with Honeywell, it was found that the latest firmware manager “had a bug,” and a downgrade from R143.1 to R142.1 fixed the problem, Sun said. The team also had to migrate the associated Experion system from R511.4 to R620.1 TCU3 or newer to mitigate an anomaly in the now redundant processor configuration that could result in a loss of control, Sun said. “Loss of control is never a good thing when you’re running a refinery,” she said.
But the biggest implementation hurdle arose when the team went to install the new I/O cards, only to find they had been shipped non-isolated versions. “The I/O modules did not match the drawing and isolated versions were not available. We explored external isolation and that was a ‘no’—but the clock was ticking,” Sun said. At every morning meeting during the shutdown, the team was asked, “Have you found the missing part yet?”
In the end, a member of the Honeywell Project Team located the correct I/O cards at a warehouse in Mexico, flew down to retrieve them and brought them back in his carry-on luggage. “The cards were quickly installed, and we eventually completed our cutover on time, and we restarted on schedule,” Sun said.
Longer term, the team expects to see overall system reliability improved through redundancy. Further, cutover costs were reduced by 50%. “And now we have extensive access to system data that we did not have with the PLC-5,” Sun added. Other advantages included shared skills across the UOC and PKS platforms, easier licensing management and consistent local support. “Working directly with Honeywell we were able to achieve our objectives and continue meeting the energy needs of the world,” she said.
