Resource Magazine September/October 2013 : Page 15

Figure 8. Characteristic pressure vs. time curves for “dust X” at 73 g m -3 . Figure 6. Characteristic pressure vs. time curves for CGD at 1000 g m -3 . 80 g m -3 in the CAAQES chamber, and figure 8 shows the characteristic pressure vs. time curves for “dust X” at its MEC of 73 g m -3 . The results are in The ASTM criterion for determining whether or not a dust is explosible involves testing concentrations of the dust in an enclosed 20 L chamber and measuring the resulting pressure. If a deflagration occurs and the resulting pressure is 1 bar (14.5 psig) or more, then the dust is considered explosi-ble. SCE previously conducted CGD tests at a concentration of 1000 g m -3 with a flame from a 10 kJ source and measured a pressure of 5.6 bar (81 psig). The team at SCE therefore concluded that CGD is explosible. However, the ASTM protocol for explosible dust testing can result in incorrect indication of a deflagration because it uses pressure as the only indicator of a deflagration, with a flame as the igniter. For a true deflagration, the resulting pres-sure must be the result of a self-propagating flame. The ASTM protocol also does not mimic the conditions in a grain elevator, with a primary explosion ignited by a stationary source followed by one or more secondary dust explosions. The ASTM method does not require that a dust have an MEC to be explosible, nor does it acknowledge that an MEC must be present before an explosion can occur. In contrast, the CAAQES protocol involves conducting tests with a wide range of dust concentrations. If any concen-tration results in a deflagration in the CAAQES chamber, then the dust is explosible. The CAAQES method can also be used to determine the MEC of a dust. In fact, a dust must have an MEC to be classified as explosible. The CAAQES method of mimicking a grain dust explo-sion in a grain handling facility—using three criteria for deflagration as well as determining the MEC of the dust—is more accurate than the ASTM method. Using only a fraction of the chamber volume for the dust concentration, rather than the entire chamber volume, is also superior to the ASTM method. Finally, using a stationary coil rather than a flame as the igniter ensures that the pressure increase is the conse-quence of a flame self-propagating through the dust cloud. Based on the findings of the CAAQES method, and given the superiority of this method, we concluded that CGD is not an explosible dust. A S A B E m e mb e r C a l v i n B . P a r n e l l J r . , P. E . , Professor,, A S A B E m e m b e r R u s s e l l O . M c G e e , Assistant Research Scientist,, and A S A B E m e m b e r B a l a j i G a n e s a n , Graduate Student,, Department of Biological and Agricultural Engineering, Texas A&M University, College Station, USA. Figure 7. “Dust X” being tested in the CAAQES chamber at 80 g m -3 . RESOURCE September/October 2013 15

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