In a plant that operates 24 hours a day seven days a week, control and safety are of paramount importance. According to Ian Craw, automation engineer at Nelson Pine Industries, “The plant is aging. To upgrade the chip mill we decided to start at the whole backbone of control to take advantage of advancing technologies and meet current safety standards.”
The chip mill is a large part of the site where logs are unloaded from trucks for processing. Two pivot cranes and a drum debarker handle 300 tonnes of logs per hour. Control and safety are critically important in the chip mill so the first stage of the upgrade involved replacing the existing PLC-5® hardware platform with a GuardLogix® Integrated Safety System.
According to Sean Doherty, account manager at Rockwell Automation, “The GuardLogix provides the benefits of the standard ControlLogix® systems but also includes safety features that support Category 4/PLe safety applications. The GuardLogix also offers integrated safety, discrete motion, drive and process control.”
“Nelson Pine has been particularly innovative in their approach. We often see safety systems bolted on to the control systems that may meet safety requirements but impact other business objectives, such as production rates and downtime. This type of solution was discounted early in discussions with Nelson Pine."
To allow for zone control, the chip mill building was split into two geographical safety zones, using some of the latest safety guard locking switches with RFID technology for controlling and monitoring zones.
The first safety zone incorporates a large drum debarker, which rotates the logs, removing bark before entering the chipper. Out-dated variable speed drives were replaced with eight, 90kW PowerFlex® 753 drives in a master/slave configuration. They receive their speed/torque reference via the DLR and achieve a Stop Category 0, (via safe torque off) to Cat3/PLd.
“The integrated safety provided by zone control allows the plant to shut down one zone while the other is still operating as usual, delivering improved production rates. The goal is zero harm but we also wanted to minimise impact to production schedules so we suggested a solution that helps achieve this,” said Doherty.
The second safety zone incorporates safe speed monitoring of the main, 1.8MW chipper motor and safe position monitoring of the 11kV motor breaker, to confirm lockout/tagout (LOTO) has been performed, before access is granted into the hazard zone.
“When upgrading equipment it was a priority to meet current safety standards. We are well on the way to complying with the Machine Safety Standard, EN ISO 13849, with the goal to achieve PLd across most of the site in the coming years,” said Craw.