Ignition sources, dust explosions

Dust Explosions Dispersion dryers can be more hazardous than layer-type dryers if we are drying a solid combustible material which is then dispersed in air, particularly if the product is a fine particle size. If this finely dispersed product is then exposed to an ignition source, an explosion can result. The following conditions (van t Land, Industrial Drying Equipment, Marcel Dekker, New York, 1991) will be conducive to fire and explosion hazard ... 

EXPLOSION and FIRE CONCERNS flammable in powder form NFPA rating Health 3, Flammability 2, Reactivity 0 reacts violently when heated with ammonium nitrate vigorous reaction when heated with nitryl fluoride explodes on contact with hydrazoic acid incompatible with ignition sources, dust generation, moisture, and excess heat cadmium and its salts are highly toxic when heated to decomposition, emits toxic fumes use dry chemical and carbon dioxide for firefighting purposes. 

Definition of Dust E losion A dust explosion is the rapid combustion of a dust cloud. In a confined or nearly confined space, the explosion is characterized by relatively rapid development of pressure with a flame propagation and the evolution of large quantities of heat and reaction products. The required oxygen for this combustion is mostly supphed oy the combustion air. The condition necessaiy for a dust explosion is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and a suitable ignition source. 

The first and third incidents are examples of hazards introduced by simple modifications (see Chapter 2). Many dust explosions caused by other sources of ignition are reviewed in Reference 10. 

Figures 7-65A through H are the venting area requirements for dust explosions. These are based on high energy ignition sources.

LEL (lower explosive, OR FLAMMABLE, LIMIT) The minimum concentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an efficient ignition source. Generally expressed as % by volume for gases and vapours, and as mg/m for mists or dusts. 

Table 5. Representative ignition temperatures, minimum explosion concentration, and minimum ignition energy for selected dusts, from NFPA Fire Protection Handbook (National Fire Protection Association, 1986) and other sources.

Electrostatics enters into the problem of secondary explosions in several ways. First, an electrostatic spark may be the ignition source of the primary explosion. Second, an electrostatic spark can serve to ignite a dust cloud if particulate spews out at high velocity as a result of a sudden piping or vessel containment failure. Similar containment failures have occurred with insulating liquids and can not be ruled out for powders. For example,... 

The subscript in vessel is for the reactor or building. The subscript experimental applies to data determined in the laboratory using either the vapor or dust explosion apparatus. Equation 6-20 allows the experimental results from the dust and vapor explosion apparatus to be applied to determining the explosive behavior of materials in buildings and process vessels. This is discussed in more detail in chapter 9. The constants KG and KSt are not physical properties of the material because they are dependent on (1) the composition of the mixture, (2) the mixing within the vessel, (3) the shape of the reaction vessel, and (4) the energy of the ignition source. It is therefore necessary to run the experiments as close as possible to the actual conditions under consideration. 

The explosion of a dust or gas (either as a deflagration or a detonation) results in a reaction front moving outward from the ignition source preceded by a shock wave or pressure front. After the combustible material is consumed, the reaction front terminates, but the pressure wave continues its outward movement. A blast wave is composed of the pressure wave and subsequent wind. It is the blast wave that causes most of the damage. 

All electrical devices are inherent ignition sources. Special design features are required to prevent the ignition of flammable vapors and dusts. The fire and explosion hazard is directly proportional to the number and type of electrically powered devices in a process area. 

See DUST EXPLOSION INCIDENTS, ELECTROSTATIC HAZARDS, IGNITION SOURCES... 

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