Fire on the Move
Some mining operations install large-scale conveyors, called overland conveyors, as a cost-effective alternative to trucks for the movement of aggregates over long distances, with lengths measured in miles rather than feet. The typical overland conveyor travels at speeds around 1,000 feet per minute. These types of conveyors take significant time to start and stop due to their large system inertia.
Just like all other conveyors, overland conveyors have a tail end, where material enters, and a head end, where material exits. A take-up carriage at the head end controls the belt tension and a training pulley aligns the belt for proper operation. Belt alignment requires periodic manual adjustments.
Fires can and do take place on conveyors, including overland conveyors. The scale of overland conveyors and the fact that most operate with limited supervision can complicate fire investigations. In one EDT case, a fire on an overland conveyor belt traveled over five miles prior to discovery. A review of the records and operating parameters were critical for narrowing the search for the fire origin.
Operational records are used to create a timeline of events, a key tool for forensic investigations. Important records include start and stop times. In the case of the fire discovered five miles away from its origin, the conveyor had been running for several hours earlier in the day but was shut down for 30 minutes before restarting. The fire burned on the moving conveyor belt for about 30 minutes after the restart.
Except for a 250-foot section, fire damage to the five-mile conveyor was minimal. Yet, evidence of minor, sporadic fire damage existed along the ground underneath the entire length of the conveyor, indicating that a section of the belt was on fire along the path of travel. Knowing the startup time, steady-state speed, and the shutdown time of the conveyor, the belt travel distance was calculated. The calculations indicated that the fire-damaged portion of the belt was near the head end when the conveyor was restarted.
Examination of the head end found a major belt misalignment at the head pulley. Further, the paint on the frame next to the head pulley exhibited thermal damage. A polished inner frame surface and piles of rubber fragments confirmed rubbing between the belt and the frame for an extended period of time. After reviewing the available data, the damage to the paint indicated temperature excursions adequate to ignite the rubber belt. However, the belt was parked for 30 minutes at the rubbing location with indications of high temperatures, but no indications of fire. Why was the fire discovered later, five miles away?
To explain this phenomenon, we must revisit the basics of fire. For a fire to take place, it needs fuel, heat, and oxygen. For this case, friction between the steel frame and the moving rubber belt created the heat source. The fuel was the rubber belt. During operation, the belt moved quickly past the steel frame, limiting the amount of time for heat to transfer into any one area of the belt.
The heat that transferred to the belt material dissipated after it left the rubbing area. When the belt stopped, heat transferred to a small portion of the belt in contact with the frame, raising its local temperature. The limited amount of oxygen between the frame and the belt prevented ignition. When the conveyor restarted, the hot section of the belt moved away from the frame, exposing it to ample oxygen and consequent ignition. The burning section of the belt was on the bottom of the conveyor, so as it burned, flaming debris dropped along its path creating minor fire damage underneath the conveyor. The belt continued to burn as it traveled the five-mile journey to the tail end. The fire consumed the rubber, but the belt’s steel reinforcements held it together.
In the end, belt misalignment resulting from poor maintenance practices caused the fire.
About the Author
Timothy M. Himes, M.S.E., P.E., CFEI, CVFI is a consulting engineer with our Birmingham, AL office. Mr. Himes provides consultation services and analysis of mechanical systems, investigation, and assessment of incidents involving machinery and equipment, interpretation of codes and standards, and evaluation of fire and explosion origin and cause. His services also include the failure analysis of fire suppression systems. You may contact Tim for your forensic engineering needs at email@example.com or (205) 838-1040.
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