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The handling of grain in elevators produces large quantities of dust at all handling and conveying locations within the elevator. This dust must be collected by efficient dust collection systems and removed by regular housekeeping to minimize the accumulation of dust throughout the elevator. Large concentrations of dust in an elevator increase the possibility of a severe fire or explosion.
Mr. Palmer points out in hls book, "Dust Explosions and Fires," that, "generally, an explosion occurs only where the dust is dispersed in the air, or can be dispersed by some means whilst the source of ignition is present, whereas dust fires develop in heaps, storage bins, and other deposits. Transition from fire to explosion and vice versa can readily occur. If burning dust is suddenly disturbed and a suspension is formed in the air, a 'flash' is easily produced which in turn can disturb more dust and cause a more violent effect. In contrast, if a suspension already formed is ignited, and causes an explosion, burning dust particles are likely to fall out of suspension onto nearby surfaces and spread fire. In either event, the damage can be considerable ..
It is not generally known that, in the United States, grain dust explosions are ranked No. 1 as the cause of industrial dust explosions. The following table shows figures for explosion incidences in grain elevators and feed mills from 1948 to 1977:
TABLE 1.—EXPLOSION INCIDENTS IN GRAIN ELEVATORS AND FEED MILLS, 1958-77
Source: U.S. Department of Agriculture, "Prevention of Dust Explosions in Grain Elevators-An Achievable Goal," draft report, Washington, April 1979; taken from table 2-1.
4 Palmer, K. N. "Dust Explosions and Fires." London, Chapman and Hall Ltd., 1973, p. 3.
In the Department of Agriculture report, tabular data show that grain production and export has markedly increased since 1962, i.e., total grain production in the United States increased from 7,537.4 million bushels in 1962 to 11,614.8 million bushels in 1976, and total exports increased from 1,564.6 million bushels in 1962 to 36,666.5 million bushels in 1976 (see Table II). In addition, it is pointed out that these "massive increases in U.S. grain exports have created a stress situation in the export grain handling system. The volumes of grain actually handled are approaching the design handling limits of the system." 5
Wheat, corn, and soybeans accounted for about 92 percent of the total inspected grain exports. Most of the U.S. grain exports are handled by Gulf Coast terminals; e.g., in 1975, Gulf Coast ports in Federal Grain Inspection Service (FGIS) Regions I and III (see Figure 1) handled 57 percent of inspected wheat, corn and soybean exports and 59 percent of the exports of inspected wheat, corn and soybeans in 1976 and 1977. In addition to the increased volume of grain exports, the volume of corn production and exports also increased. In 1962, wheat (649 million bushels) was the major export grain, corn (416 million bushels) was second, and soybeans (180.5 million bushels) was third. However, in 1975, corn was first with 1.71 billion bushels exported, followed by wheat at 1.17 billion bushels exported, and soybeans at 555 million bushels exported.
This increase in the volume of corn exported also produces more dust than usual because handling and processing corn generates substantially more dust than wheat. It is noted that this increase in corn volume as well as other grains handled results in increased amounts of dust available for igniton. "This constitutes a significant hazard when the volumes of grain and dust actually handled are near the design handling capacity of the grain handling system or any of its component parts."
Present dust control methods are not specifically applicable to the unique characteristics of corn or the large volumes of corn present in the U.S. handling systems. The Department of Agriculture report points out that an analysis of state totals for corn produced for grain in 1978 showed that "11 states grew 85 percent of all corn produced for grain *** Eight of these 11 states are also among the top 11 states in explosions. The production of corn and its handling and storage play a significant role in grain facility explosions.'
U.S. Department of Agriculture, "Preventlon of Dust Explosions In Grain Elevators-An Achievable Goal." Draft Report, Washington, April, 1979, p. 7-34.
Op. cit., p. 7-35.
Op. cit., p. 3-45.
TABLE II.-U.S. GRAIN PRODUCTION AND EXPORTS BY MARKETING YEAR 1
1 Year beginning June 1 for barley, flaxseed, oats, rye, and wheat; August 1 for rice; September 1 for soybeans; and October 1 for corn and sorghum.
2 Includes rough rice and milled rice converted to rough rice equivalents.
* Includes wheat, flour, and other products expressed in wheat equivalents.
Note.-Production and export figures have been rounded to the nearest 100,000 bushels where possible.
Source: U.S. Department of Agriculture. "Prevention of Dust Explosions in Grain Elevators-An Achievable Goal," draft report, Washington, April 1979, pp. 7-15. Compiled from "Agricultural Statistics 1978," USDA, Government Printing Office, Washington, D.C. 1978.
SOURCE.-U.S. Department of Agriculture. "Prevention of Dust Explosions in Grain Elevators-An Achievable Goal." Draft report. Washington, April 1979.
III. DESCRIPTION OF A GRAIN ELEVATOR
A grain elevator is a building or complex of buildings which elevate, store, discharge, and sometimes process grain. A grain elevator provides both storage facilities and merchandising functions and contains equipment for mechanically loading, unloading, weighing, transferring, turning (moving grain from one storage bin to another), blending, cleaning, fumigating, drying, or aerating the grain (wheat, oats, barley, soybeans, rye, corn, alfalfa, sorghum (milo), rice, triticale (wheat and rye hybrid), and oil seeds (linseed, sunflower, cotton seed, and rape seeds)).
Grain elevators are constructed of monolithic reinforced concrete or are steel-framed and sheathed with steel. Some elevators may be wood frame structures which may have sheet metal cladding applied to the exterior walls and roof of the building. "Factors which influence the choice of materials include availability and cost of materials and labor, insurance rates, and personal preference. Insurance rates tend to decrease in the following order: wood, wood frame with sheet metal cladding, steel frame sheathed with steel, monolithic reinforced concrete.' ." The elevators are usually built in the form of cylindrical towers or silos, and frequently, several silos are connected. There are about 15,000 grain storage and handling facilities in the United States that employ 225,000 workers. (Approximately 10,000 of these facilities are grain elevators and the rest are feed mills, flour mills, and other grain processing plants.) Out of the 10,000 grain elevators, approximately 80 are export terminal elevators with over half of these elevators operating on a year-round basis. The other grain elevators,10 i.e., country elevators, operate seasonally, with a workforce that fluctuates with many part-time employees being used during the harvest season which is the peak employment period. During these peak employment periods, many grain elevators operate 24 hours per day.
When the elevators are operating and the grain passes through each step of the process, i.e., it is moved, stored, weighed, etc., clouds of highly explosive dust are raised that only need a spark to be detonated. Only three components are necessary for a grain dust explosion. They are (1) the presence of oxygen above the minimum concentration permissible to cause ignition; (2) grain dust in suspension between the lower and upper explosive limits of the explosive range; and (3) a source of ignition (spark, heat, or flame) having adequate energy to ignite the dust cloud.
8 Verkade, M. and Chiotti, P. "Literature Survey of Dust Explosions in Grain Handling Facilities: Causes and Prevention." Energy and Mineral Resources Research Institute, Iowa State University, Ames, Iowa. March 25, 1976.
"Export terminal elevators" purchase grain from country, sub-terminal, and inland terminal elevators. Export terminals are located in major ports. Grain is received by truck, rail, barge, or ship.
10 "Country elevators" are the first receiving point for grain from the farm. They usually are equipped for only truck receiving, but may have rail and truck load-out facilities. Country elevators may also merchandise fuels, fertilizer, feed supplies, hardware, and other farm supplies.
"Sub-terminal elevators" purchase grain from farmers and country elevators. Sub-terminal facilities are located in rural grain-producing areas and receive grain by rail or truck.
"Inland terminal elevators" purchase grain from country and sub-terminal elevators, Inland terminals are usually located in principal grain marketing centers. Grain is received by rail, truck, and barge (if located on a river).
Note: Elevator definitions were taken from: Steve Jones and John Steelnack, "Safety and Health Hazards in Grain Elevators v.s. Occupational Safety and Health Administration," Washington, March 1978.