Flames dust explosions

Dust explosions usually occur in pairs. The first explosion involves dust already in suspension. This jars dust from beams, ledges, etc, creating a second cloud to which the explosion propagates, resulting in a secondary explosion. Dust clouds have been ignited by open flames, electric sparks, hot... 

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. 

Increasing the surface area of a combustible solid enhances the ease of ignition. Hence dust burns more rapidly than the corresponding bulk solid combustion of dust layers can result in rapid flame spread by train firing . Solid particles less than about 10 pm in diameter settle slowly in air and comprise float dust (see p. 51 for settling velocities). Such particles behave, in some ways, similarly to gas and, if the solid is combustible, a flammable dust-air mixture can form within certain limits. Larger particles also take part, since there is a distribution of particle sizes, and ignition can result in a dust explosion. 

Finely divided aluminium powder or dust forms highly explosive dispersions in air [1], and all aspects of prevention of aluminium dust explosions are covered in 2 recent US National Fire Codes [2], The effects on ignition properties of impurities introduced by recycled metal used to prepare dust were studied [3], Pyrophoricity is eliminated by surface coating aluminium powder with polystyrene [4], Explosion hazards involved in arc and flame spraying of the powder are analysed and discussed [5], and the effect of surface oxide layers on flammability was studied [6], The causes of a severe explosion in 1983 in a plant producing fine aluminium powder are analysed, and improvements in safety practices discussed... 

General References Crowl and Louvar, Chemical Process Safety Fundamentals with Applications, 2d ed., Prentice-Hall, Upper Saddle River, N.J., 2002, Chaps. 6 and 7. Crowl, Understanding Explosions, American Institute of Chemical Engineers, New York, 2003. Eckoff, Dust Explosions in the Process Industries, 2d ed., Butterworth-Heinemann, now Elsevier, Amsterdam, 1997. Kinney and Graham, Explosive Shocks in Air, 2d ed., Springer-Verlag, New York, 1985. Lewis and von Elbe, Combustion, Flames and Explosions of Gases, 3d ed., Academic Press, New York, 1987. Mannan, Lees Loss Prevention in the Process Industries, 3d ed., Elsevier, Amsterdam, 2005, Chap. 16 Fire, Chap. 17 Explosion. 

D. Rae, Safety in Mines Res Estab, Res Rept 253 (1967) CA 69, 1140 (1968). The main characteristics of slow coal-dust explosions and their relation to the testing of barriers. The main characteristics of slow coal-dust explosions in the new gallery at Buxton are a const initial flame front acceleration from... 

Combustion, Flames and Explosion in Gases , 2nd Editn, Academic Press, NY (1961), 322—23 (Dust quenching occurs at a critical value of the surface area of the dust per unit vol of the suspension, and depends on the nature of the salt. Better results are obtained with salts having a mp under 200°. Alkali halides are better than carbonates, potassium better than sodium, fluoride better than iodide and better than chlor-ide. If the dust concentration is high enough, even detonation waves can be extinguished)... 

The flame of explosion. Experiments by Hiscock [27] and T. Urbanski [28] showed that the flame of safety explosives in firedamp and coal-dust is very small and its intensity insignificant. These explosives, differed little in the dimensions and intensity of flame produced in these tests. 

Probe, in Get) (Medicion de la longitud y la duracion de la llama, in Span). The teat is based upon the belief that the greater the length of the flame an explosive emits and the longer the time during which that flame endures, the greater are the chances that such a flame when shot into the atmosphere of a coal mine will ig-nite inflammable or expl mixes of mine gas and air of coal dust and air or of mine gas, coal dust, and air... 

In dust explosions the combustion process is very rapid. The flame speed is high compared with that in gas deflagrations. Detonations normally do not occur in dust explosions in industrial plants. 

In dust explosions the combustion process is very rapid. The flame speed is high compared to that in gas deflagrations. However, detonations normally do not occur in dust explosions in industrial plants. In a serious industrial dust explosion, two steps often occur. First, a primary explosion occurs in part of a plant, causing an air disturbance. Second, the air disturbance disperses dust and causes a secondary explosion. The secondary explosion is often more destructive than the primary explosion. There is a great deal of literature on dust explosions, which is available to the process designer. See, for example, Lees (1980) for a bibliography on dust explosions. 

Unlike a gas-phase explosion or detonation in which the.flame speed is close to the speed of sound (340 m/s), the flame speed in a dust explosion is usually relatively slow (about 10 m/s). Here the explosion is described as a deflagration. This means that the shock wave in the explosion precedes the flame, resuspending settled dust and enhancing conditions for a second, larger dust explosion. Thus dust explosions tend to propagate as a series of increasingly severe explosions, utilizing resuspended previously settled dust as the explosive material. 

In addition to reducing the probability of a dust explosion, it is possible to take steps to mitigate the consequences of a dust explosion, should one occur. Use of automatic vents to prevent the buildup of destructive overpressures and constructing areas with blowout partitions are examples of such steps. Also, spaces which may contain these high dust concentrations can be kept fairly open to minimize the development of turbulence in a flame front, should an explosion occur. 

EARTHS. A strong reducing agent. Moderate fire hazard ignites spontaneously in air at 150-180°. Moderate explosion hazard in the form of dust when exposed to flame. The metal or its alloys spark with friction. Many alloys are pyrophoric in air. See also IRON DUST. Explosive reaction with zinc. Very exothermic reaction with antimony or bismuth. Ignites when heated in atmospheres of CO2 + N2, CI2, or Br2. Violent reaction when heated with phosphoms (400°Q, silicon (1400°C). 

Flammable in the form of dust when exposed to heat or flame. Moderately explosive in the form of dust when exposed to heat or flame. Mixmres of hydrogen peroxide + trioxane explode on contact with lead. Rubber gloves containing lead may ignite in nitric acid. Violent reaction on ignition with chlorine trifluoride, concentrated hydrogen peroxide, ammonium nitrate (below 200° with powdered lead), sodium acetylide (with powdered lead). Incompatible with NaNs,... 

The main characteristics of slow coal-dust explosions in the new gallery at Buxton are a const initial flame front acceleration from... 

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