Investigation of marginally explosible dusts

“Marginally explosible dusts (MEDs)” are a group of combustible dusts that are distinguished by relatively low volume-normalized maximum rate of pressure rise (KSt) and maximum explosion pressure (Pmax) values. Earlier studies have suggested that dusts with KSt values less than 45 bar m/s in the laboratory-scale 20-L chamber would not explode in the 1-m3 chamber and therefore not on an industrial scale. Conversely, for some metallic dusts, significantly higher KSt values are generated in the 1-m3 chamber. Industries that handle MEDs continue to search for answers to the questions “are they explosible or not?” and “should we protect or not protect against potential explosions of these dusts?“. To answer these questions, four well-characterized materials namely, carbon black, oat grain flour, urea, and zinc were tested in the current study. These materials were selected because they exhibit different combustion behaviors and also cover a range of industries. Five ignition energies (i.e., 0.5, 1, 2.5, 5, and 10 kJ) were used in the 20-L chamber tests. The results show a clear case of overdriving (as a result of the high ignition energy density in the 20-L chamber) with respect to urea and carbon black dusts. Nevertheless, the urea dust is non-explosible. The data also suggests that the choice of test chamber to generate suitable explosion data is strongly dictated by the combustion path of MEDs. However, there are exceptions. The study also establishes the importance of both chemico-physical and thermal analyses to understanding the explosion behavior of MEDs. With reference to urea dust, a new definition for MEDs has been suggested as dusts having Pmax < 3.0 bar(g), KSt < 20 bar m/s (in the 20-L chamber), MEC >1000 g/m3, MIE >1000 mJ, and MIT >600 °C. This work provides guidelines to industries that handle MEDs on the explosibility classification of these dusts, thus addressing the existing difficulty, and informing industry on the safety strategies required when handling this group of dusts.