The ignition and propagation of flame in a dust cloud depend on a number of factors such as dust concentration, the composition of the dust and moisture content, particle size and shape, and turbulence in the system. These factors not only determine the severity of the explosion but also influence the type and degree of precautions which it may be necessary and practicable to take.When a mass of solid flammable material is heated it burns away slowly, layer by layer, owing to the limited surface area exposed to the oxygen of the air. The energy produced is liberated gradually and harmlessly because it is dissipated as quickly as it is released. The result is quite different if the same mass of material is ground to a fine powder and intimately mixed with air in the form of a dust cloud. In these conditions the surface area exposed to the air is very great and if ignition now occurs the whole of the material will burn with great rapidity; the energy, which in the case of the mass was liberated gradually and harmlessly, is now released suddenly with the evolution of large quantities of heat and, as a rule, gaseous reaction products.
Composition of the Dust
The maximum pressures and rates of pressure rise which influence the severity of a dust explosion vary with the chemical constitution and certain physical properties of the dust. Some metal powders, for example, aluminium, magnesium and alloys of aluminium and magnesium, can generate maximum pressures of 12 bar and maximum rates of pressure rise in excess of 500 bar m/sec. Organics materials typically produce maximum explosion pressures up to 10 bar and the rates of pressure rise below 200 bar m/sec.Flammable materials which form dust clouds may be a mixture of two or more substances. Metal dusts may be contaminated with oxide; coal contains volatile components and variable quantities of incombustible ash; chemicals may contain solvents from previous processes and some dusts may be mixtures of a large variety of substances.Incombustible matter in the form of inert solid material, non-flammable volatile components or moisture tends to reduce the flammability of the dust both by chemical inhibition and by the cooling effect of the particles. High concentrations of moisture in the dust may also impede the formation of a dust cloud. Volatile combustible components increase the risk.
Composition of the Atmosphere
The flammability of a dust cloud is reduced by decreasing the oxygen content of the medium in which is is dispersed. As the oxygen content is reduced, the minimum hot-surface temperature necessary to ignite a dust cloud is progressively raised; and when a dust cloud is exposed to a source of ignition at a given temperature there is a critical percentage of oxygen below which the dust will not explode. This limit depends on the inerting properties of the other components of the atmosphere. Carbon dioxide and nitrogen are effective inerting agents for carbonaceous materials, but some metal powders may ignite and burn in carbon dioxide, and even in nitrogen under certain circumstances.
Particle Size and Shape
The particle size of the material exerts a considerable influence on the explosibility of a dust cloud. The finer the dust the more readily it is dispersed into a cloud and the longer it will remain in suspension. A reduction in the size of particles means an increase in surface area per unit weight and an increase in the ease of ignition. The specific surface area is also determined by the shape of the particle. A thin flat particle is more readily ignited than a spherical particle of the same substance and minimum ignition energy is strongly influenced by particle size. The flammability of a dust cloud is reduced by decreasing the oxygen content of the medium in which is is dispersed.
Useful information for assessing a dust explosion risk may be obtained from laboratory tests. Samples of dust that are indicative of ‘worst case’ should be tested; that is, the finest and driest material likely to be encountered on plant. Initially, dusts are tested, or screened, to discriminate between dangerous dusts and dusts which do not ignite in the tests. The materials are classified as follows:Group A -Dusts which have ignited and produced flame in the tests Group B -Dusts which do not produce flame in the testsNote that explosion protection design parameters, such as limiting oxygen concentration (LOC), must be based on tests undertaken on representative samples. Published generic data should never be used for safety critical data as part of a basis of safety.
Presence of flammable gas or vapour
The presence of low concentration flammable gas (or vapour from highly flammable liquid) can increase the explosion violence of a dust cloud significantly. Such combinations are known as ‘hybrid mixtures’ and require special consideration; professional engineering advice should be sought. Classification of flammable/highly flammable liquids is determined by flash point testing.