An Alternate Approach to Determine the Explosibility of Dusts
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A dust is classified as explosible based on the laboratory tests specified by the American Society for Testing and Materials (ASTM) E 1226. This standard requires that a dust be uniformly dispersed into an enclosed 20-Liter (L) chamber forming a dust cloud; a 10,000 Joule (10 kJ) flame is subsequently forced through the dust cloud and the resulting pressure rise is measured. If the pressure rise exceeds one bar (14.5 psig), it is assumed that a deflagration occurred, and the dust is classified as class ?A? explosible dust. We have reported several flaws in the current ASTM testing protocols. The only indicator used by the ASTM method for assuming a deflagration had occurred in the test chamber is pressure rise. CAAQES has developed an alternative protocol that more accurately characterizes the explosibility of dusts. The CAAQES protocol for determining MEC is to test a wide range of concentrations of a dust in a 28.3-L (1 ft^(3)) Plexiglas chamber with a diaphragm and a stationary ignition source. If a self-propagating flame results as indicated by the diaphragm bursting, the Pressure vs. Time curve, and the flame leaving the chamber, a deflagration occurred during the test and the dust is explosible. Several dusts were tested along with cotton gin dust (CGD) for explosibility. The CGD does not have a MEC and hence it is non-explosible, contrary to the results reported by the Safety Consulting Engineers Inc. (SEC Inc.). The SCE Inc. tested CGD for explosibility based on the ASTM E1226 standard and reported CGD as class ?A? explosible dust. The difference in test results triggered a research on characterizing a dust for explosibility by the CAAQES. The MECs of dusts were reported and compared with the MECs determined by Palmer in 1973 and the U.S. Bureau of Mines in1964. Further studies were also conducted to determine the dust properties affecting the explosibility of a dust. The CGD consisted of 87% inerts with a low energy content of 1400 J g^(-1). The properties of CGD rendered it non-explosible. Dusts were mixed with Fuller?s earth and tested in the CAAQES chamber in order to study the effect of inert mass fraction of a dust on MECs. It was hypothesized that, at a specific concentration, the distance between the combustible particles must be at a certain distance to enable a flame to propagate from one particle to another. The distance between the combustible particles should be 450 to 700 m to propagate the flame from one particle to another in the CAAQES chamber. Approximately, 50% of inerts (Fuller?s earth) prevented a deflagration for all dusts. The energy content of agricultural dusts should be above 7000 J g^(-1) to result in a deflagration. It was also concluded that the dust properties influences the explosibility of a dust.
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