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Plcs Help Minimize Downtime and Training Costs at Norske Canada’s Crofton Division Paper Mill

A single supervisory Programmable Logic Controller (PLC) controls the entire TMP plant process — chip washing, screening, primary and secondary refining, cleaning and storage. The PLC controls 2,500…

A single supervisory Programmable Logic Controller (PLC) controls the entire TMP plant process — chip washing, screening, primary and secondary refining, cleaning and storage. The PLC controls 2,500 physical I/O points and 4,000 internal I/O points for communications to other devices. Machinery is powered by motors with a combined horsepower of over 80,000 HP, including six motors each larger than 10,000 HP.

The TMP plant’s existing I/O were recently upgraded to new controllers — without changing the existing I/O. “The fact that we could upgrade to new PLCs without having to change our existing I/O minimized equipment upgrade, downtime and documentation costs — a savings of $2 million”, observed Stephen Hinde, electrical/instrument maintenance manager, Norske Canada Crofton Division. Hinde plans to expand the TMP operations to include a third stage refiner.

The wood chips are ground to a fine pulp by three primary and secondary refiners, each of which consists of a large fixed metal disc located between two 6-foot diameter (1.83 m) metal discs. A shaft attaches all three discs to the motor. To ensure the chips are ground as finely as possible, the three discs are located only 1/10 of an inch (2.5 mm) apart. Should the plates touch, however, the grooves could lock, potentially costing half a million dollars of equipment damage and downtime, as well as the hazard of flying metal. A high level of vibration also creates the potential for serious equipment damage. “This is why we rely on a high speed, reliable vibration monitoring system to control the refiners, which considerably minimizes potential equipment damage,” Hinde explained The vibration monitoring equipment communicates to the PLC. This safety supervisory system ensures the gaps between the discs are maintained at an optimal width, thereby maximizing pulp quality while minimizing safety hazards, equipment damage and downtime.

The controllers’ high speed and reliability, and increased internal addressing space allow for more complex communication via the Modbus Plus bridge mux to the distributed control system and operator console and for programming logic to be configured to facilitate troubleshooting and improve internal program diagnostics.

Once refined, the stock (pulp) from the TMP plant and other sources is conditioned and blended. This process — the single link between the stock source (TMP) and the paper machines — has no surge capacity. If the machines go down, the entire plant shuts down, costing $30,000 per hour. “Because of this situation, we must avoid downtime in our stock preparation PLCs. They must be reliable and allow us to perform on-line programming changes”, Hinde confirmed. “That’s why we installed a hot standby PLC to control the stock preparation process.”

The blended and conditioned stock is processed through three high speed paper machines (with a combined motor horsepower of 20,000 HP) running at 1200 metres per minute. Each machine is controlled by a PLC. “Our choice of PLC was made for two key reasons: its ability to communicate via Modbus Plus to third party devices; and the ability to allow us to shut down a single machine for maintenance while keeping the other two running — essential to minimizing downtime,” Hinde explained.

When it exits the paper machines, the paper is slit into rolls. Each roll is barcoded, wrapped and labelled for shipment to the customer. A single PLC controls the finishing line. The inventory tracking/barcoding system communicates the packaging specifications for each customer shipment via Modbus to the PLC, which ensures the paper machines cut each roll to the required weight, length and diameter. The finished rolls are then packaged for shipment according to precise customer specifications — including the type of spool to insert in the core (that will fit on their specific printer) and the type of protective outer paper wrapping to use. For consistency of label positioning, a robot applies the barcoded shipping labels to the packaged rolls. A final machine adjusts the position of each roll to ensure that the shipping label, when palletized, can be easily read by the warehouse forklift operators. “Our packaging operations are very complex — we handle over 200 sets of different packaging specifications required by customers,” said Hinde. “So we installed a PLC to facilitate execution of the higher level functions necessary to communicate to the I/O devices.”

Hinde has standardized on Schneider Electric Telemecanique Quantum PLCs, not only because these highly reliable PLCs minimize downtime, but also because training and documentation costs are minimized. “Training is so important for us at Norske Canada Crofton Division that we’ve set up a dedicated PLC equipment training program at the British Columbia Institute of Technology (BCIT).”

Steven Hinde is the electrical/instrument maintenance manager at Norske Canada Crofton Division. Paul Galbraith is a senior technical representative at Schneider Electric.