Milling applications cover a broad range of equipment types. Typically, material passes down a inlet duct to the center of the mill from where it is thrown outwards to impact opposing sets of overlapping pins, or mechanically ground to finer particle sizes. Collisions between particles, or between particles and metal, create a very fine material which is discharged to a receiving vessel. This can be done mechanically through a conveyor, pneumatically, or through a gravity drop.
Milling systems present a special hazard. Not only do they produce fine materials, their moving parts and the energy created during their operation produces the potential for ignition. In addition to internally created ignition sources, foreign materials in the feed stream can cause impact sparks. Only some of these foreign materials, such as tramp metal, are removed by the magnetic separators commonly installed on mills. Although many mills are designed to withstand the dynamic pressures of an internal deflagration, adjacent equipment and duct work may be vulnerable to the violent effects of explosion propagation.
Protection System Description
Often, the close proximity of the mill and receiving vessel makes practical to include them in a single protected zone. Larger systems may be divided into several zones which initiate explosion suppression independently of each other. Explosion pressure detectors are mounted both on the mill and the receiver. They detect the pressure excursion from an impending explosion. Infrared detectors are also a common detection means for mills utilizing pneumatic conveying of the ground material. The detectors transmit a signal to a control panel, which triggers high rate discharge extinguishers while simultaneously shutting down the process. The extinguishers mounted on the mill hopper and collector body are designed to quench the fireball before maximum pressures are reached. The isolation extinguisher with hose assembly on the mill feed duct, and isolation extinguisher with spreader nozzle on the conveying duct disperse suppressant to mitigate the passage of flame.
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