Some key considerations when evaluating explosion severity of nanopowders

Abstract

Protection from explosion events requires the determination of key safety parameters like lower explosion limit, maximum explosion over-pressure, and maximum rate of pressure rise. These parameters are routinely obtained through standard tests performed typically either in a 20 L sphere or a 1 m 3 container. But several aspects are worth a closer investigation. Firstly, the test apparatus must be able to disperse a fairly uniform dust cloud. However, previous investigations showed that actually the current dispersion system can be improved. Indeed, as a function of the sample and the dispersion conditions, the dust concentration in the ignition zone can be significantly different from the nominal dust concentration. Moreover, due to agglomeration/fragmentation phenomena, the particle size distribution can greatly evolve during dust injection. Secondly, the influence of humidity on the explosivity is not considered in current standards. It is just stated that the relative humidity should be checked and noted down, though some provisions exist in American standards. Even when performing the powder injection with synthetic air, the water contained in the residual air of the sphere can impact the chemical reactions occurring during the explosion. Thirdly, the ignition delay time is sometimes modified to study the impact of the dust cloud turbulence on flame propagation but is often misunderstood. For instance, by decreasing the ignition delay time, the injection time of the powder becomes shorter for the same pressure difference. This modifies the dispersion kinetics and can change the particle size distribution. Finally, for very sensitive powders, pre-ignition can occur or the ignition energy sets at 10kJ can lead to an overdriving phenomenon. Maybe these aspects have not been thoroughly considered for micron powders. However, in the case of nanopowders, the importance of these influencing factors was shown in order to duly evaluate explosion parameters. Experimental evidences confirm these aspects and alternative solutions will be presented.