Let the DCS Fit the Process

Distributed Control Systems (DCSs) Are Gaining New Capabilities, Such as Integrating with Safety Systems, to Match the Unique Needs of Their Process Applications

By Jim Montague

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In all, the revamped complex incorporates nearly 50 process controllers, 180 logic solvers and approximately 5,000 process and safety I/O points. Following commissioning, INA Rijek reports its operators were able to bring the plant on stream quickly and smoothly because, prior to start-up, they were trained off-line using Emerson's OTS simulator. This enabled them to respond to simulated process upsets in an off-line, no-risk environment prior to actual plant operation.

More Product, Less Variability

Beyond optimizing operating conditions, another persistent problem must be solved as process applications produce ever higher product volumes and varieties—how to reduce variability.

For instance, Glatfelter's 120-year-old Chillicothe mill in Ohio manufactures 400,000 tons of specialty papers per year on four paper-making machines, which are supplied by a timber yard, bleached Kraft pulp mill, eight batch digesters and four boilers, and also work with one coater and a variety of paper converting machines. The largest of the four machines, nicknamed the Chief, was built in 1980, renovated in 2000, and produces a wide range of specialty products.

However, as product mixes diversified, converting equipment evolved and paper chemistry improved, Glatfelter reports that coating buildups increased on its caliper gauges, which caused more measurement errors and more ridging, roping, wrinkles and other quality problems on its paper. "Bad readings create false control actions, which result in low-quality products, internal rejects and ultimately in low-quality yields," says Chad Biddix, Glatfelter's stock preparation superintendent.

To improve its caliper measurements and quality control, Biddix and his colleagues at Glatfelter decided to adopt ABB's Extended Automation System 800xA and two Network Platform NP1200 scanners with optical caliper sensors—one at the size press and the other at the reel. These optical caliper sensors stabilize the sheet on one side, and then make a confocal, non-contact measurement on the other.

Following start-up, the benefits of the new control system and scanners were immediate. Glatfelter reports that internal rejects due to caliper errors or mechanical defects caused by poor caliper control were virtually eliminated, and the time needed to make paper-grade changes was drastically reduced.

"Our controls group and ABB developed an automated paper-grade change process, which allowed us to make paper-grade changes with no losses," explains Randy Dittman, the Chief's superintendent. "Even when making the most challenging of changes, we can now reach the right specification in seven minutes. This means that paper-grade changes have less than a 1% impact on our efficiency. Also, despite all the paper-grade changes the Chief makes, its winder remains one of the most productive in North America. The paper must be perfect before going to the winder, which runs at 8,500 feet per minute. If it runs well on our winder, then we can be confident it will run well on our customers' winders, too."

Likewise, Dittman adds that 800xA controls allow Glatfelter to study the Chief's frequency of disturbances, and apply them to the mill's other machines. "It's allowed us to link changes in performance to process upsets," he explains. "This means we're able to identify and eliminate sources of variation. We can test as much as we want to test. The online, spectral analysis has been a big help with all of this."

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