R. Todd Swinderman, Martin Engineering, explains how preventing fugitive material around conveyors can improve safety and reduce the costs of operations.
Most conveyors have some amount of material loss from spillage, leakage, dust, and carryback emissions, collectively called fugitive materials. This loss can range from 3% of the cargo in poorly operated and maintained systems to less than 0.1% for world class operations. The result is degraded component life, workers exposed to hazards, and reduced product quality. While fugitive materials cannot be totally eliminated from bulk material handling conveyors, the problem can be managed.
The root causes are often obvious but rarely addressed. Rather, the standard approach is to treat the symptoms. The symptoms of a failure to control fugitive materials include unplanned downtime, excessive cleaning costs, regulatory actions, poor public relations, and safety incidents. Addressing these symptoms with workable long-term root cause solutions will improve availability, housekeeping, safety, and improve company cash flow.
Types of fugitive material
The nature of fugitive material problems from any conveyor is indicated by the location and particle sizes of the accumulations. Fugitive materials are generally categorised into spillage, leakage, dust, and carryback.
Spillage refers to cargo that escapes the belt, accumulating on either side of the conveyor and is characterised by a range of particle sizes similar to that of cargo. Piles of spillage can accumulate quite rapidly and can occur intermittently from plugged chutes, overloaded belts, and mistracking, or continuously from poorly designed or inadequately maintained transfer points. Inclined loading zones can also cause material to roll backwards, resulting in spillage out the back of the transfer point.
Dust consists of fine particles, usually less than the diameter of a human hair, created during processing and through the degrading action as bulk material moves through a conveyor system. Fine airborne dust is emitted at locations where the cargo becomes airborne and is emitted from open inspection doors, missing covers, worn skirtboard seals, and the exit of transfers. Because dust particles are very small, they are easily dispersed by wind when dry, blanketing wide areas; however, the sources of dust are often visually obvious. In processing involving hot materials, the fumes released often saturate dust particles and cause them to build up inside chutes and on equipment.
Carryback is fine material that sticks to the belt surface or becomes lodged in the cracks and crevices of the carrying side of the belt. Carryback material collects on the components that the carrying side of the belt touches and eventually dries out, dropping beneath the system along the return path. Carryback accumulations tend to range from pyramid shaped piles of dry dust particles to puddles of muck. In severe cases, large chips of material form thick layers of buildup on return rollers and are thrown off return rollers as they dry and break up.
Issues created by design
Bulk material handling investments are often capital projects due to their significant costs and the expected lifespan of the equipment. Controlling fugitive material requires careful attention to detail during the design stage, which is often not considered because of the additional design time (capital cost) required. Furthermore, the perception that fugitive material control costs are hard to quantify makes it challenging to justify design improvements. Belt Conveyors for Bulk Materials and FOUNDATIONS are excellent resources for design.
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