Ignition of Dust Layers by Mechanical Sparks

Dust explosions represent a considerable safety risk in process industry. Statistical records of dust explosions show that 32.7 % of these explosions are initiated by mechanical sparks or hot surfaces due to mechanical friction [1]. Laboratory investigations however show that direct ignition of dust clouds by mechanical sparks and hot surfaces caused by friction processes is not that straightforward [2]. A possible explanation of this anomaly may be that the mechanical sparks and hot surfaces cause an indirect ignition of a dust cloud through ignition of a dust layer. This paper addresses the ignition of dust layers by mechanical sparks.

An experimental study has been performed investigating ignition of dust layers by sparks generated from metal rods pushed against a grinding wheel. A large number of parameters were varied including: ? Contact pressure and contact time between metal rod and grinding wheel ? Distance between grinding wheel and dust layer ? Type of metal used

? Ventilation flow rate across dust layer

? Dust type (in total 17 different organic and inorganic dusts were involved in the investigation), ? Moisture content of dust ? Particle size distribution of the dust ? Density of dust layer ? Dust layer thickness

Of the dusts investigated standard dust combustion properties (minimum ignition temperature of dust layer and burning velocity of dust layer) and particle size distribution and moisture content were determined.

The main experimental findings are: The experiments showed that a relatively high spark density is needed to establish smoldering combustion in dust layers for the dusts investigated. Ignition of a dust layer by a single impact spark therefore seems to have a very low probability. A relatively large median particle size dust is easier ignited by mechanical sparks than fine dust. Dust layers with a median particle size diameter of < 70 µm could not be ignited. The results showed no clear correlation with regard to the ignition probability of a dust layer by mechanical sparks and especially the dust layer combustion properties. Ignition probability was promoted by higher flow rates across the dust layer

The paper will give more detailed conclusions. [1] Beck, H. and Jeske, A. Berichte über Staubexplosionen ? Einzelereignisse und Dokumentation, VDI- Berichte, 1272, 365-387, 1996 [2] Bartknecht, W., Explosionsschutz. Grundlagen und Anwendung, Springer-Verlag, 1993