Conveyor systems aren’t just subject to considerable wear. Even in normal use, they can face the risk of fire due to equipment failure or ignition of the materials being transported. The Harrington plant, owned and operated by Xcel Energy, gets its coal primarily from the Powder River Basin (PRB) in Wyoming. A special property of the PRB coal is its propensity to self-ignite. Another characteristic this coal is its friability, creating combustible dust that can penetrate into bearings and other parts of the conveyor system.

For the Savage maintenance department, handling PRB coal has introduced extraordinary hazards. Careful management of these hazards is a must. A mechanical fault in the bearing of a roller, for instance, could cause ignition of the belt or coal. Friction between a seized roller and the belt could also lead to fire. The site’s online condition monitoring program plays an essential role in dealing with the safety issues that come with the handling of PRB coal.

Savage implemented its condition monitoring program to monitor plant machinery and detect potential equipment failure at an early stage. In late 2009 and early 2010, the Intellinova® online system from SPM Instrument was installed to monitor 40 conveyor and crusher bearings. The condition of these bearings is measured with SPM HD® measurement technique. Particularly well suited for low-rpm applications, this technology can be utilized on rolling element bearings throughout the range of 1-20,000 RPM. At the Savage Harrington location, the conveyor system runs at approximately 120 RPM.

Initial readings in June, 2010 indicated deteriorating condition on one of the pulley bearings. With the online monitoring system, Savage personnel were able to keep a watchful eye on the development of the damage for a full 15 months before the bearing needed replacement.

June 2010 – October 2011

As can be seen in the trend graph, taken from Condmaster® software, the HDm readings (black) were already in the yellow warning zone, when measurements began. Shortly thereafter, they started to move into the red, but the condition degradation was relatively slow. In September, 2011, a significant increase was seen, leading to a decision to replace the bearing in a planned maintenance action. Immediately after replacement, the readings dropped into the green zone.

Examination of the replaced bearing showed severe damage on both the inner and outer rings of the bearing as well as the rolling elements; yet no secondary damage to the shaft or bearing housing was caused, and since the bearing was replaced during normal downtime, no loss of production was incurred.