A chemical reaction process began with feeding a combustible fine powder into a reactor through a charging hopper. The reactor and the charging hopper were fitted with a nitrogen inerting system to prevent dust explosions. Surprisingly, a dust explosion occurred when feeding the powder into the reactor from a FIBC. Onsite investigations found that the design of the inerting system allowed a sufficient amount of air to enter the charging hopper. There were no mechanical, electrical and thermal sources of ignition inside the closed type reactor-charging hopper system prior to the explosion. Electrostatic discharges appeared to be the ignition source; however, the ignition source could not be identified without further analysis through pertinent tests. Therefore, various dust explosion and electrostatic tests were conducted to determine the minimum amount of electrostatic discharge energy for igniting the dust cloud and the likelihood of various types of electrostatic discharges capable of igniting the dust cloud under the given material, equipment, and process conditions. The tests results show that the powder suspended in air at the optimal dust concentration requires less than 3 mJ of spark energy to ignite. A glass-line pipe at the bottom of the charging hopper was found to be the most likely ignition source. Propagating brush discharges on the glass lining of the pipe were produced in the laboratory.
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