Safety shutdowns in process control applications have always been more complex than just turning off a switch, but they've become a lot easier at Georgia Pacific Chemical's nine U.S. plants thanks to coordinated efforts between their Experion PKS and Safety Manager systems.
"Five years ago, we began implementing a new safety system across our plants," said Jarmo Salminen, manager of process control engineering at GP Chemicals, based in Atlanta, Georgia. "In the past, shutdowns had been handled by the basic process control system [BPCS] or single-loop controllers, so we really needed to enhance our safety system. We began to research safety control solutions, and eventually selected Honeywell Safety Manager. This was a pretty natural move for us because we'd already standardized on Experion."
Salminen presented "Safety Shutdown System Integration with Experion: Benefits of Standardizing on Distributed Control System (DCS) and Safety Instrumented System (SIS)" this week at Honeywell Users Group Americas 2016 in San Antonio.
Quench systems need testing
Though its plants aren't especially big—the largest has eight reactors and the smallest has two—Georgia Pacific Chemicals produces a wide variety of products. They consist mainly of resins, especially thermo-set resins for plywood and other products, as well as specialty chemicals and controlled-release nitrogen fertilizers. In general, the nine U.S. plants maintain rigorous separation between their controls and safety functions, and perform root-cause analyses of all safety shutdown trips.
Because of the unique characteristics of its applications, Salminen explained that, "We can't have normal shutdown systems. We need to stop chemical reactions, and this means adding materials to stop or slow them. We call this process 'quenching,' and it involves keeping two types of quenching materials—for either acid-catalyzed reactions or base-catalyzed reactions—in tanks at about 100 psi. We use pressurized air as the motive force because it's safer than nitrogen, which poses a possible asphyxiation risk. One quench system can handle up to five reactors."
To maintain the quenching capability required for its safety shutdowns, GP Chemicals maintains and tests both automated and manual valves. They're designed to be SIL 2-rated, undergo standard testing every 15 and 30 days, complete full tests annually and logic-proof testing every five years. "The 15- and 30-day tests are performed with timers, which means new batches can't be started until the test is successfully completed," said Salminen. "We also have a manual maintenance mode for opening, moving and closing valves, and for doing other safety tasks."
Integration without compromise
While most traditional safety shutdown systems have used completely separate human machine interfaces (HMIs) and other equipment, GP Chemicals takes advantage of the fact that Experion and Safety Manager can combine several useful DCS and SIS functions. While their actual safety equipment and controls remain separate, Safety Manager's indicators, functions and data for GP Chemical's quenching recipes are displayed on existing Experion HMIs at its plants, which helps maintain operator awareness and performance.
"Whether they're doing regular production or carrying out a quenching recipe, our operators see the same types of graphics, and that's a big help for them," added Salminen, who reported that other key benefits of combining Experion and Safety Manager include:
- Standardization across sites. This is critical for enabling a centrally provided support model; allowed programmatic rollout of a common DCS/SIS approach; and allows standardization down to software revisions.
- Maintenance of BPCS and SIS separation. This means SIS modification can only be made by central engineering, and not site staff. Also, different programming environments ensure that no one can unintentionally work on a safety function. But, everyone can still benefit from using a common operator interface.
- Sequence of events (SOE). Integrating safety data with Experion simplifies root-cause analysis of trips.
"Experion does the process control, and Safety Manager does the safety shutdown functions, and we saved by not having to invest in a separate HMI for each," added Salminen.