Pulverizing, along with milling and grinding, are heavy duty operations in which both organic and inorganic materials are reduced in size. Typically, they consist of one or more grinding mills and a dust collection system connected by a pneumatic conveyor. By-products of the pulverizing process are high levels of dust concentrations near the mill outlet. These concentrations of finely divided particles can produce violent explosions when suspended in air in a confined volume.
Most often, the pulverizing mill provides the source of ignition as sparks are produced when metal or stone fragments come in contact with mill components. In some cases, material binding on the conveying system causes friction which results in the material catching fire.
Explosions occurring in pulverizers tend to spread rapidly to other areas. Therefore, proper explosion protection focuses on carefully analyzing the configuration of the process so as to fit critical components (mill, dust collector and conveyor) with an integral system capable of detecting and suppressing explosions in milliseconds...no matter where the explosion originates.
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
The example shown, the close proximity of the mill and collector make it 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 dust collector. They detect the pressure excursion from an impending explosion. 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. An explosion-proof rotary gate valve mounted on the collector hopper reduces the likelihood of burning materials passing downstream.