Protective Measures Against Dust Explosions

When speaking about explosions, preventive and constructive measures are discriminated. This is also done in the case of protection against dust explosions. When discussing the preventive measures which have the task to eliminate the chance of a possible event actually occurring, the question of substance substitution is the first to be asked. If a dust explosible substance can be replaced by one which does not have this property, this substitution is the most effective measure. The probability that this exchange is possible in practice is, however, comparatively low. 

In most situations it remains necessary to rely on other alternatives. The most rec-ommendable method to prevent the occurrence of dust explosions is inertization. Some details to be observed in its application will be presented in Section 7.4. At this point, some general remarks shall just be made. The highest allowable oxygen concentration is substance specific. Experimental methods for the determination of this limit concentration are well known. In a first approximation it may be assumed, however, that the oxygen limit concentration for organic powders and dusts will not be lower than 8%. For metal dusts this value has to be halved. 

It must further be observed that inertization is acknowledged as a protective measure only if it is continuously supervised by measurements. A vacuum purge inertization of an apparatus repeated once or twice without any subsequent supervision cannot be accepted as a preventive protecting measure. 

A further decisive factor determining the efficiency of this measure is to safeguard the absence of a substance which decomposes without the presence of oxygen, as such a process would render an inertization absolutely useless. 

Another measure is vividly discussed regarding its efficiency the elimination of ignition sources. Today s view of experts supports any attempt pursued to eliminate ignition sources but denies it the quality to he sufficient as a single measure on which to base the safety of a process or a plant. This very critical assessment of the measure is the result of the negative experience that incidents have become known which took place after supposedly fiill elimination of ignition sources. It is more responsible to act 

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Bartknecht, W., Prevention and design measures for protection against dust explosions, in Industrial Dust Explosions (K. L. Cashdollar and M. Hertzberg, eds.) ASTM tech. pub. 958, pp. 158-190 (1987)... 

The minimum dust concentration for explosion is measured in the vertical tube apparatus and is used to give an indication of the quantities of air to be used in extraction systems for combustible dusts. Since dust concentrations can vary widely with time and location in a plant it is not considered wise to use concentration control as the sole method of protection against dust explosion. 

The investigation of a sugar dust explosion in a Belgian factory is discussed and preventive and protective measures are described in detail [1], Sugar dust explosions and safety measures against them are reviewed [2]. 

Taking measures to protect against the consequences of potential dust cloud explosions. Explosion protection measures include ... 

The air cleaning devices (air filters, dust collectors, cyclones, etc.) should be protected against the consequences of dust cloud explosions. This includes measures to prevent propagation of the explosion back through the exhaust ducting to other areas of plant and equipment. 

In some powder handling processes it is not possible to avoid the simultaneous presence of an explosible dust cloud and a hazardous buildup of charge. In those situations, measures should be taken to protect against or prevent explosions. These include inerting, use of explosion-resistant equipment, explosion venting, or explosion suppression. 

The European directives for explosion protection, Directives 99/92/EC [2] and 94/9/EC [1], define measures for the protection against explosions of explosive atmospheres under atmospheric conditions. The two directives define an explosive atmosphere, as previously mentioned, as a mixture of air and combustible gases, vapours, mists or dusts under atmospheric conditions in which, after an ignition, combustion processes propagate to the complete unburned mixture. However, the two directives do not define atmospheric conditions . 

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