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A new safety characteristic the 'dustiness' according to VDI 2263 part 9 (Verein Deutscher Ingenieure, 2008) is investigated. Dustiness means the tendency of a dust to form clouds. The paper deals with the influence of the dustiness on vented dust explosions. In order to look into the effects of the dustiness on dust cloud formation and explosion properties experiments and simulations in a vertical dust dispersion glass tube apparatus were carried out.
Preliminary explosion experiments showed that the dustiness has an influence on the reduced explosion pressure in a vented 75 L test apparatus. Dusts with comparable pmax and KSt values and different dustiness were tested. Dusts with higher dustiness produced higher overpressures, despite comparable safety characteristics. In order to verify the results for applications in the process industries further tests with different settings are planned as well as industrial scale experiments. Characteristics of the dust such as particle size, density, specific surface area and particle shape, which influence the dispersibility, have been determined experimentally.
The Euler/Lagrange and the Euler/Euler approaches are compared for simulating an exemplary dust/air mixture. Especially sedimentation and the ability of the approaches to simulate the tendency of dust to stay airborne were investigated. The Euler/Lagrange approach is better suited for simulating local dust concentrations, particle size distributions and particle forces. It could be used to point out regions of high dust concentrations in a vessel. With the Euler/Euler method it is possible to achieve fast solutions for one specified diameter, but the simulated dust/air mixtures are always more homogenous than in reality. ANSYS CFX version 13 was used in all simulations.
Measurement and numerical simulation of local dust concentrations over time in a vessel can only be done with some uncertainty due to the complexity of the behavior of dust/air mixtures. Dust concentration was measured in a 50 m³ vessel and compared to simulations with a commercial CFD code. A 50 m³ silo was used with two different filling methods. In one setup dust was conveyed with pneumatic filling at the top. The other filling was done with pressurized air and a homogenous injection via eight nozzles. Experiments were repeated three to four times with two kinds of dusts and the results were used to evaluate reproducibility of dust concentration measurements over time in a vessel depending on the filling method. Dust concentrations over time varied up to 30% from the average for homogenous injection and even more for pneumatic filling.
Numerical investigations were done with maize starch. Measured concentrations were compared to simulated ones with the commercial CFD code ANSYS CFX R14 using an Euler/Lagrange approach. Drag force, turbulent dispersion force, particle size distribution, particle surface area and particle/particle interaction were modeled. A general agreement of measurement and simulation was achieved. Numerical simulations of filling processes were used to predict parts of the vessel where the lower explosion limit is reached and exceeded. This could help to improve dust explosion protection, if it is used to find configurations where the dust concentration exceeds the lower explosion limit only in small parts of the vessel during filling, e.g. using different injection points or injection angles. The volume where LEL is reached or exceeded in a 50 m³ silo is shown for pneumatic and homogenous filling. Volume of combustible atmosphere in the vessel over time is compared for two pneumatic filling configurations and one worst-case homogenous injection configuration.