Dust Explosions JOSEPH B. FICKLEN' The Travelers Insurance Company, Har~ford,Connecticut
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OME years ago practically everyone who attempted photography a t night used magnesium powder for illumination. The brilliant light produced by burning this powder was the simplest light source of sufficient intensity to take good night photographs. However, the terrific heat of some of these magnesium fires produced bums on careless photographers and often resulted in severe property damage. In recent years the use of magnesium powder by the general public for illumination in photography has been largely superseded by the use of photoflash or photoflood bulbs. Hence, many persons today are not aware of the disastrous effects that can be produced by the quick fire or explosion resulting from the ignition of magnesium powder. In fact, one type of incendiary bomb used in the present war depends on the burning of magnesium for its destructiveness. Recently the hazards of magnesium dust and even of large bits of magnesium, such as shavings, turnings, and borings, have been brought forcibly to the attention of industry. This is due to the ever-increasing use of magnesium metal in the construction of airplanes and of various machine parts where light weight is of primary importance, which has been accompanied by a number of serious fires and explosions. Here are the details of a magnesium dust explosion: In a certain plant, dust from the grinding of magnesium alloy castings was exhausted by a fan through a duct about 40 feet long and then was wet by a spray of water. The water spray should have been close to the grinding wheel. A casting had been made which had a wire nail in i t to hold the core upright. When a portion of the casting had been ground off, the grinding wheel struck the wire nail and sparks were produced which ignited the dust. An explosion followed which demolished the exhaust duct, air washer, and fan housing, and broke about two hundred panes of glass in the windows of the plant. The manufacture of another metal dust-aluminum dust or "powder" as i t is usually called-also may involve a considerable explosion hazard. The sheet aluminum is powdered in stamping machines and then is screened to the desired size. Not long ago an explosion occurred in a stamping and screening department of an aluminum "powder" plant. The force of the explosion blew off the roof and shattered the side walls. The two employees inside the building a t the time of the accident were injured so severely that they died later in the hospital. Water for fighting the fire was available but was of no use, because i t was known that the application of water to burning aluminum dust is positively dangerous.
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The same danger is present when using water to extinguish magnesium fires. Magnesium and aluminum react with water to produce hydrogen and the corresponding hydroxides. This reaction is very slow a t ordinary temperatures but the rate of reaction increases rapidly as the temperature rises. This explains the extreme violence of the combustion of wet aluminum or magnesium. The hydrogen produced introduces an additional hazard on account of its flammability. Much safer extinguishing materials have been suggested, such as powdered graphite or talc, fine castiron boriugs, and similar inert material. Sand is not particularly effective, especially in the case of magnesium; certain tars are more effective Practically everyone knows that the majority of dusts arising from the handling or processing of various grains, seeds, and agricultural products can form explosive mixtures in air. Certain finely divided metal dusts, as discussed above, as well as the dusts from some of the synthetic resins also can form explosive dust clouds in air. Many of us have read of grain elevator explosions, but few of us realize how disastrous they can be. For example, a few years ago a dust explosion and fire occurred in a grain elevator in Chicago. Nine persons were killed and about thirty others were seriously injured. The property loss was estimated a t $3,500,000. Not only was the grain elevator in which the explosion took place seriously damaged, but in addition three other grain elevators nearby caught fire and were
almost completely destroyed. Only a small amount of the pain stored in any of the four elevators was salvaged. Various devices hive been built for show in^ aualitativelv that a dust will exdode and for evaluatin~the
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relative amount of damage to be expected from explosions of different dusts. All this information is of great value in devising suitable precautionary measures. An apparatus has been developed which will demonstrate whether a certain dust in air is explosive. It consists of an electric motor and a metal box-or "explosion chamber" provided with a hinged cover. Inside the box there is a piece of leather fixed into the motor shaft which extends through the wall of the box. A sample of dnst is placed on the leather, and when the motor is started the dust is thrown into the air inside the box. An ordinary spark plug placed in the side of the box provides a source of ignition for the dust cloud. The explosion pressure, flame, and gas are vented by the hinged top which flies up when the explosion occurs. In order to get data on the intensity of the explosion, certain factors must be determined more accurately than is possible merely by the qualitative test described above. For example, the fineness of the dust and the concentration of the dust in the air have a bearing upon the maximum pressure developed and also on the rapidity with which this pressure is built up. Suitable apparatus has been devised for carrying on such experiments on a laboratory scale. A setup which approaches nearly actual plant operations is the one used by the United States Department of Agriculture a t the Arlington, Virginia, Experimental Station. This was built primarily to study the ways by which dust explosions may be rendered harmless. It appears that pressure built up at the time of an explosion may produce an undue spread of flames and also secondary explosions. These secondary explosions may be caused by an ignition of dust thrown into
Figures 2 and 3-consists of a room, a gallery, and a tower erected and connected as shown. Bowls for holding dnst are located throughout the structure as desired, and the dnst is thrown into the air by means
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the air (from beams or other parts of a building) by the first explosion, and may play a major part in the loss of life and damage to property. This setup-
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