Background:

Kennecott Utah Copper Mine (KUC) is fully integrated, from extraction and concentrating to smelting and refining. It is part of the Rio Tinto family of mines. At over 100 years old, the mine has produced more copper than any other mine in the world. It is currently more than 2.7 miles across and roughly 0.75 miles deep, making it the world's largest human excavation on earth. In fact, if you stacked two Sears Towers on top of each other at the bottom of the mine, they would still not make it to the top of this mine.

More than 150,000 tons of ore is moved via conveyor from the mine to the Copperton Concentrator facility every day for processing. The first stage of processing in the concentrator is the semi-autogenous grinding (SAG) mills. The SAG mills pulverize the ore into smaller particles. As the dual, 6000 horsepower motors rotate the drum, a series of lifting plates built into the walls of the drum repeatedly lift weighted balls and chunks of ore along the sides of the drum and drop them back onto the ore at the bottom of the drum. This grinding process is critical to extracting the copper from the ore in later steps.

The concentrator's grinding operation operates 24 x 7 x 365. So uptime is critical and any unplanned downtime reduces the facility's potential throughput to the smelter and refinery.

Between 2007 and 2011, Kennecott Utah Copper replaced three SAG mill motors. Each motor requires up to five days of downtime to replace.

In addition to the financial implications, potential failure of these assets also represents a safety risk to personnel involved in the process when the failure occurs, and to the maintenance personnel tasked with swapping the motors out.

Due to the critical nature of the SAG mill motors and because they are so labor-intensive and costly to replace, KUC's predictive maintenance team utilizes a suite of monitoring and maintenance techniques to keep the motors running at peak efficiency. The cost for the monitoring processes is viewed throughout the organization as a worthwhile investment that pays dividends by preventing losses.

Grind Floor at the Copperton Concentrator

Problem Faced:

Of the six SAG mill motors, three are over 20 years old. The combination of age and criticality of the equipment makes proactive condition monitoring imperative for site profitability.

Condition monitoring technologies utilized in SAG mill motor monitoring included:

  • Continuous monitoring of motor temperature (via RTDs), motor current, airflow, and water flow rates.
  • Vibration and ultrasonic are monitored periodically and trended over time to spot developing changes.

One aspect of motor health that had been a contributor for motor failures was brush wear. But the commutator block that houses the brush assembly is not transmissive to infrared radiation. Safety and practicality prevent the removal of the commutator housing during operation. Therefore, the brushes are not accessible for direct line of sight infrared imaging and wear monitoring. As a result, KUC was missing this critical equipment health data point and needed to develop a safe and efficient manner of inspecting the commutator and brushes.

Between June and October 2011, KUC installed two Exiscan XIR-S-4 series IR windows on the motor access doors of six SAG mill motors. Inspections are performed by KUC thermographers every nine weeks (or more frequently as requested). Within the first two months, the windows enabled KUC's predictive maintenance staff to gather accurate motor temperatures, which have helped the maintenance staff improve the operation and efficiency of the motors.

Results:

Case 1:

The PLC indicated that an RTD on motor #6 was running higher than normal. It was thought the RTD was out of calibration and the temperature reading was inaccurate. A thermographer was sent out to inspect the motor. An external scan of the motor did not reveal a significant difference in temperature. However, an infrared analysis of the inside of the commutator, via inspection through the IR window, confirmed the RTD's higher temperature measurement and confirmed that steps should be taken to bring the temperature of the motor back into specification.

Case 2:

Spring pressures and brush seating have been issues in the past on the SAG mill motors. Infrared inspections through the IR windows identified brush seating issues before much damage could be done. The results of these saves have been:

  • More reliable and predictable equipment operation.
  • Improvement in asset management.
  • Maintenance cost savings: rebuild of the motor after a failure of brush assemblies would typically run $300k-$500k in parts and labor, plus the potential seven to 10 days of unplanned outage resulting in millions in lost opportunity costs.

Economic and Safety Benefits of Inspection via IR Windows

The ability to inspect the SAG mill motors with infrared monitoring through IR windows is generating a short term return on investment (ROI). Early identification of commutator and brush anomalies has enabled KUC maintenance staff to fix minor issues more cost effectively and efficiently than if those issues had progressed to the point where they would have been picked up by other technologies. The long-term prevention of unplanned, catastrophic failure on these critical motors is expected to generate further ROI on the $20k investment in infrared windows.

Inspection via IR Windows

KUC also anticipates an improvement in safety as a result of using infrared inspection through IR windows by:

  • Eliminating or reducing equipment failures by reducing the amount of change-out, rebuild and repair work. Consequently, the reduction in these activities will reduce risks to personnel who would otherwise be charged with the various stages of repairing or replacing those assets (electrical, mechanical and transport tasks at every stage).
  • Reducing the risk of failures also reduces the related safety risks to personnel who might be in the proximity of the failure when it would have occurred.
  • Eliminating the high-risk tasks associated with opening the motor cabinet. In fact, the practice of inspection through an open panel is prohibited at KUC due to the obvious safety concerns.

Conclusion

The SAG mill motors are so critical to Kennecott Utah Copper Mine's 24x7 processes and are extremely labor-intensive and expensive to change out in the event of failure. Installing IR windows to monitor the commutator and brush assembly during operation has allowed thermographers to safely and efficiently gather a critical data point that will help protect SAG mill motors from unplanned and catastrophic failures. In fact, use of the IR windows has helped KUC's predictive maintenance staff identify some minor problems that were easily corrected within the first few months of use and are anticipated to help KUC maximize uptime of its Copperton concentrator processes.

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