GRAIN-Dust Explosions Cause Big Farm Loss

We are coming more and more to realize the need for the adoption of precautionary measures against dust explosions. Explosions in large industrial plants are particularly spectacular because of the heavy loss of life and extensive property damage, but the smaller explosions, far greater in number, represent an enormous loss in life and property.

FIG 107.—Dust explosion in a grain-threshing machine in eastern Washington. These explosions and fires have caused extensive losses to grain and machinery

Investigations by the Bureau of Chemistry have shown that practically all of the grain dusts are explosive when scattered as a cloud in the air and that the explosion hazard is present from the time the grain is cut in the field until it leaves the export terminal elevators. Many explosions have occurred during threshing, both in fields and in barns, in the small country elevators to which farmers deliver their grain, in the large elevators -where grain is stored, and in the mills and industrial plants where grain is milled into flour, manufactured into starch, ground into feed, or made into any of the numerous cereal products now on the market. All that is necessary to produce an explosion is to have a cloud of the finely-pulverized product in suspension in air ignited by a spark, flame, or heated surface. A spark of static electricity is sufficient to ignite the dust cloud. A hot bearing on machinery can start a fire which may cause a dust explosion.

Dust explosions and fires in grain-threshing machines have been most frequent in the wheat-growing territory of eastern Washington (fig. 107), northern Idaho, and northeastern Oregon, although they have been reported from many other sections of the country.  As a rule these explosions occur in dry sections or in places where the humidity is low. In the Northwest the explosions usually begin with the opening of the threshing season in July and continue until the middle of September. Here the explosion hazard is increased by the presence of wheat smut. When the smut balls are broken during threshing, the light, fine dust produced floats in the air to form a highly explosive mixture. Before the development of methods of preventing such explosions the losses in the Northwest alone were very high, being estimated at $1,000,000 during 1914 and 1915. In many cases not only was the machine destroyed (fig. 108), but the fire spread to the straw stack, the sacked grain, and the grain standing in the field. Even since the development of equipment to prevent such explosions and fires it has been estimated that the preventable losses in this part of the country amount to from $15,000 to $75,000 each season.

FIG. 108.—All that remained of a threshing machine following a dust explosion and fire

Devices for Threshing Machines

To prevent dust explosions of any kind it is necessary either to eliminate the dust cloud or the source of ignition or to change the atmospheric conditions in such a way that combustion can not occur.  In the case of threshing machines the problem has been attacked in several ways.

Fans have been designed to collect the dust formed in the interior of threshing machines and thus prevent the formation of explosive mixtures. These fans are light and relatively inexpensive and require little power for their operation. They are usually installed on the deck of the machine.. (Fig.109.) Besides reducing the explosion hazard, they help to clean the grain, thus improving its grade and also prevent the dissemination of smut spores, which are likely to infect the ground and attack the next year’s crop.

Investigations in the field indicated that a large proportion of the explosions occurred when dust clouds in or about the machine were ignited by sparks of static electricity produced by the friction of moving parts of the machine. Tests showed that heavy charges of static electricity were present on many of the machines—in some cases the measurements showed more than 50,000 volts. To remove these charges a system of wiring was devised in which the various parts of the machine were connected by wires with a main wire or cable that was thoroughly grounded by being attached to an iron rod driven into the earth. So long as positive contacts were maintained between the ground wire and the various parts of the machine this method proved effective in reducing the hazard of explosions due to the ignition of dust by sparks of static electricity.

To provide protection against loss from fires starting in threshing machines a chemical fire extinguisher was designed. This equipment consists of a cylindrical steel tank connected with pipe lines leading to different parts of the machine. The extinguishing liquid is held in the tank until it is released through the pipe lines by the opening of a valve. The valve can be opened by hand or automatically operated by the heat from a fire in the machine. The extinguisher is mounted on the deck of the machine. After the harvest season it may be taken down and used for general fire protection about the arm.

FIG. 109.—A threshing machine equipped with a dust-collecting fan

Hazards in Elevators

The interest of the farmer is not confined to dust-explosion prevention on the farm. He is interested also in the work being done to reduce the explosion hazard in the elevators or warehouses where his grain is stored. Perhaps he has a share in a cooperatively operated elevator. Perhaps he has suffered a loss on grain which he has stored in an elevator without protection against damage from dust explosions. Again, the destruction by a dust explosion and fire of an elevator in his territory may mean a longer haul to get his grain to market or even make it impossible for him to market his grain when he wishes. For all these reasons the farmer, as well as the grain producer, the elevator operator, and the miller, or user of grain and cereal products, has a vital interest in the work of the Bureau of Chemistry on the cause of dust explosions and the development of methods of preventing them.