Focus on Harsh Environments: rock solid

The job of the mill operator in the Miller’s Creek gypsum quarry is to process rock as fast and at as high an average as possible. On July 17 this year though, the feeder that runs ore into the 100-ton primary crusher kept tripping out, even though the mill operator wasn’t running the equipment any faster than usual. But by reviewing the trending graphs in the mill’s computerized monitoring system, the mill personnel were able to quickly find the cause of the problem. The monitoring system is based on Rockwell Automation’s RSView32 integrated component-based software for Microsoft Windows environments, which is designed for monitoring and controlling machines and processes.

The Miller’s Creek quarry in Windsor, N.S., has been in operation since 1957 and produces 1.7 millions tons of dark gypsum a year. It is owned by the Fundy Gypsum Company, a division of USG Canadian Mining Ltd. Its parent, United States Gypsum Company, is the world’s largest manufacturer and marketer of gypsum-based products.

In October 1999, new crushing and screening facilities worth C$18 million came on line, complete with the computerized monitoring system, which monitors everything through an Allen-Bradley PLC-5/80E processor. It monitors and records the performance of every component, right down to bearing temperatures, lubrication flow and the centring of the mill’s thousands of feet of conveyor belts.

On July 17, the RSView trend graph showed that the average temperature in the feeder was unusually high. The maintenance people traced the cause to finer, heavier material that was being run through the mill, putting an extra load on the feeder. Linking cause and effect like this enabled the mill operator to lower the speed of the drive feeding the crusher when running heavier rock. It also saved the six maintenance workers in the mill department a lot of troubleshooting time.

From his room right over the primary crusher, mill operator John Zink demonstrated how he can call up system schematics and graphics on RSView as fast as he can point and click with his computer mouse. The screens change with no discernible delay, allowing him to, for example, zoom in from a screen showing the primary and secondary crushers and several conveyors (Fig. 1) to take a real-time look at bearing and motor data in the primary crusher. This graphic (Fig. 2) shows the frame and jaw bearing temperatures, lube flow, amperage and motor winding temperatures for both of the 200-hp primary crusher motors.

All of the equipment’s performance variables have preset thresholds below or above which alarms are triggered. If, for example, the thermal capacity of a motor exceeds a preset level, say 80 per cent, an alarm will come on and the operator can slow down the equipment — or the whole mill for that matter — in mere seconds, and thus bring down the operating temperature of the motors.

Responding to alarms

In the old mill, the mill operator might not be certain if he was running a piece of equipment too hard. This new ability to respond to alarms and intervene in real time means that the mill operator can, for example, prevent bearing wear due to high temperatures caused by an overloaded system or insufficient grease.

“The whole system is designed to allow us to have planned maintenance,” explains Bonnie Miles-Dunn, the Miller’s Creek quarry engineer. For example, if a hot bearing triggers an alarm, the mill operator can cut back the speed of the equipment and maintenance can be planned for a convenient time. Miles-Dunn adds, “[RSView] should eliminate a lot of surprises for us.” And, adds Cliff Power, a mechanic-welder in the maintenance department, “There are so many pieces of moving equipment here now, we can’t afford to have many surprises.”

Fifty- and 100-ton haul trucks bring the gypsum from the quarry to the primary crusher. There the ore is reduced to chunks about 12-in. across. A 12-ft-wide conveyor takes this ore to the secondary crusher, which further crushes the ore down to pieces roughly 6-in. across. If there is mud in the ore it is transported to an outside screen and removed. Otherwise, a 1,800-ft long, 60-in. wide conveyor transports the ore to a 276-ft long rock storage shed with a 25,000-ton capacity.

From the storage shed, railway cars transport most of the ore 10 miles to a 150,000 ton storage shed in Hantsport, alongside the tidal Avon River. There it is loaded into ships two to three times a week; the 20,000-ton capacity ships are loaded in three hours flat and sail at high tide.

Conveyors monitored

RSView monitors a lot of information about the mill’s 14 conveyors, including how many tons an hour of ore each is carrying. The conveyors are fitted with belt switches that shut down a conveyor if it wanders off centre. When a belt switch was activated in the old mill, maintenance would have to go hunting for it. If a belt had no switches, it simply got torn.

In the new mill, an off-centre belt will trigger a belt switch, which in turn triggers an alarm which is automatically posted in the RSView alarm log; for example, “Conveyor #12 Belt Alignment Switch.” The screen graphics show exactly where on the conveyor the switch was activated.

Alarms posted in the alarm log are acknowledged and cleared by maintenance.

Although placid-looking from the outside, the inside of the mill is a harsh environment in which to keep equipment operating properly. Drifts of fine gypsum dust coat everything in the crushers, plugging motor vent holes and causing them to overheat. But because RSView monitors all system variables, impending problems can be dealt with before they get out of hand.

If the maintenance department needs help interpreting any data, which RSView collects in huge quantities and presents as graphs showing historical trends, it is a simple matter to capture any screen image and e-mail it to the equipment or component manufacturer for a second opinion on a diagnosis.

“With the old mill, a lot of our troubleshooting time was spent finding the problem,” says Henry Smolenaars, an electrician at Miller’s Creek. “With the new mill, the operator knows almost immediately what the problem is, so it is just a matter of getting the right department to fix the problem.”

Montreal-based contributing editor Carroll McCormick travelled to Nova Scotia in the summer for this report.

The Magic Mineral

Gypsum is a non-metallic mineral, found as a rock composed of 79.1% calcium sulphate and 20.9% water by weight. Gypsum was formed about 300 million years ago. In absolutely pure form, gypsum rock is white. However, it contains impurities whose presence makes the rock appear gray, brown, pink or almost black.

The Ancient Assyrians called it Alabaster and used it for sculpturing. Five thousand years ago the Egyptians had learned to make plaster from it. The ancient Greeks named this mineral Gypsos, from which the name gypsum was created.

Today gypsum is used in the manufacture of many products including, but not restricted to, drywall board, plaster, sheathing, toothpaste, blackboard chalk, and filler in paints. It is also used for surgical casts and moulds for false teeth.