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Explosion Protection

Inadequate Treatment of Dust Explosions in Atex?

| Author / Editor: Rolf K. Eckhoff / Marcel Dröttboom

Fig. 1: The catastropic dust explosion at the plant of Imperial Sugar Co. at Port Wentworth, Georgia, USA, caused 14 deaths and injured 38 other people. (Picture: U.S. Chemical Safety Board)
Fig. 1: The catastropic dust explosion at the plant of Imperial Sugar Co. at Port Wentworth, Georgia, USA, caused 14 deaths and injured 38 other people. (Picture: U.S. Chemical Safety Board)

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The European 'Atex' Directives 94/9/EC and 999/92/EC exhibit a lack of differentiation between explosive gas mixtures and explosive dust clouds. Consequently, some very substantial differences between gases/vapours and dusts have either been neglected or addressed inadequately.

A new approach for standardization of design of electrical apparatuses for 'explosive atmospheres' has been adopted by the International Electrotechnical Commission (IEC) as well as in the European Union (CEN and Cenelec). A central concern is to combine and align standards for combustible dusts with the corresponding ones for combustible gases/vapours.

The European 'Atex' Directives 94/9/EC [1] and 1999/92/EC [2] appear to be a main source of inspiration in this effort, due to the lack of differentiation between explosive gas mixtures and explosive dust clouds in the directives. Consequently, in producing the new aligned/combined standards some very substantial differences between gases/vapours and dusts have either been neglected or addressed inadequately.

Basic Differences between Dust and Gases

Explosive gas mixtures and explosive dust clouds, once created, do exhibit very similar ignition and combustion properties. This may give rise to the misconception that they can be considered more or less identical in all respects. However, as discussed by Eckhoff [3], some very basic differences between gases and dusts need careful attention when designing electrical apparatuses for use in the presence of combustible dusts.

Firstly, the physics of generation, up-keeping and migration of dust clouds and premixed gas/vapour clouds are substantially different. Once a quiescent gas mixture has been created, it will for all practical purposes stay homogeneous because of the random molecular motion.

In dust clouds, however, the fuel particles are generally so much larger than the molecules of the air (in most cases between 1 and 100 mm), that their movement within the air is controlled by inertial forces, including gravity, rather than by random molecular motion. Therefore, in most situations where accidental explosive gas/vapour clouds may form and persist quite readily, the generation and persistence of explosive dust clouds would be highly unlikely.

Fig. 2 illustrates this, and the question is: "Is the explosion hazards created by the butane and the maize starch by opening the valve and the paper box respectively similar or different?" The answer is obvious. Opening the butane container creates a very significant gas explosion hazard, whereas the dust explosion hazard created by just opening the maize starch box is zero. Figs. 3 and 4 illustrate the difference.

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