The Explosibility of Starch Dust IRVING HARTMANN AND JOHN NAGY, Explosives Branch, U. S. Bureau of Mines, Pittsburgh, Pa.
Potential explosion hazards most be kept in mind by any com pany contemplating large scale use of starch and starch mixtures JL HE recent severe starch dust explosion in a large candy factory and newly planned applications of starches have aroused great interest in their fire and explosion hazards. Published records of the Na tional Fire Protection Association indi cate that, between 1901 and 1946, 38 dust explosions occurred in starch and cornproducts plants in this country. However, many manufacturers are still unaware of the fact that starch dust can be explosive under certain conditions, and other pro ducers believe that high atmospheric humidity or the normal moisture content of starch would prevent explosions. The ruling of the Food and Drug Ad ministration of the U. S. Department of Agriculture to prohibit the use in the near future of nitrogen trichloride or Agene for bleaching and maturing wheat flour has resulted in a search for other suitable agents. Among the compounds tried are potassium bromate and chlorine dioxide. One proposed procedure involves mixing minute quantities of the bromate powder into the flour. To assure proper mixing, concentrates or premixes of starch and po tassium bromate are being used in current experiments. If this practice should re place the Agene bleaching process, it would result in a new use of starches on a large scale, in some cases by companies which are entirely unfamiliar with the po tential explosion hazards.
electrical charges along the walls of the explosion space and thereby reduces the chances of ignition by static sparks. Highly hygroscopic dusts might absorb sufficient moisture from the surrounding humid air to cause agglomeration of the particles, thereby making the dust less readily dispersible. Some dusts, espe cially certain metal powders, oxidize rather rapidly in humid surroundings or when moisture is deposited on the surface of the particles. In some cases this re sults in the formation of an insensitive oxide or hydroxide layer, but under cer tain conditions the reaction causes heating of dusts, which might result in spontane ous ignitions. High content of free mois ture in dusts reduces the ease of dispersion, raises the ignition temperature and the minimum energy required for ignition of dust clouds, and reduces the rate of burn ing and flame propagation. However, numerous experiments made with coal dust and some work with starch and other dusts have shown that even moist dusts can produce violent explosions if thrown into suspension in air and ignited. These studies, as well as experience, indicate that, as a rule, neither high humidity in the air nor high natural moisture content of dusts can be relied upon to prevent dust explosions.
The object of this note is to point out some of the factors which govern dust ex plosions, to make available recent experi mental data on the explosibility of starch and starch mixtures, and to call attention to safeguards for preventing explosions or of reducing their violence. Explosibility Factors In general, dust explosions can be pro duced only when flammable dust is dis persed and ignited in air or in some other gaseous atmosphere which will support combustion. However, even undispersed dust on exposed surfaces in a plant can contribute to an explosion since a rela tively minor initial disturbance might throw it into suspension and cause a vio lent secondary explosion. The ignition of dust particles and the intensity of a dust explosion depend, among other things, on the chemical and physical properties of the dust, including the particle size, the concentration of the dust cloud, the turbu lence in the cloud, the nature and intensity of the ignition source, the initial tempera ture and pressure, the oxygen content and other properties of the gaseous atmosphere, and the size, shape, construction, and other characteristics of the explosion space. The effect of atmospheric humidity on the explosibility of dusts has not been studied thoroughly. High relative humid ity in the air promotes leakage of static
Test Data As a part of a research program designed to reduce explosion hazards in mines and in industry, the Bureau of Mines has investigated the explosive properties of hun-
Laboratory Explosibility Test Results UNDRIED
1 380
SAMPLE N U M B E R
DRIED
6 510
2 490
470 440 Ignition temperature of duet cloud, ° C 500 Relative flammability, %: e 90+ 90+ 90+ 90 + 90 + In furnace at 700° C. 90 + 85 90 85 85 80 In standard spark apparatus 80 Minimum energy required for ignition of dust cloud by 0.10 0.50 0.06 0.06 0.50 electric spark from condenser discharge, joule 0.04 Mini mum explosive concentration, ounces per cubic foot 0.055 0.050 0.045 0.075 0.120 0.100 Maximum pressure (p.s.i.) and rates of pressure rise (p.s.i. per second) developed by explosions of dust clouds in a closed bomb at concentrations of: 0.100 ounce per cubic foot: 35 35 Maximum pressure 19 500 400 Average rate 150 1100 800 Maximum rate 250 0.200 ounce per cubic foot: 53 48 41 45 52 Maximum pressure 28 950 300 250 500 650 Average rate 400 700 550 2350 1050 Maximum rate 700 1950 0.500 ounce per cubic foot: 68 54 72 69 Maximum pressure 51 1350 900 700 450 1050 Average rate 850 3350 1250 2000 850 2150 Maximum rate 1600 1.000 ounce per cubic foot: 70 55 Maximum pressure 58 650 900 Average rate 1100 2000 1850 Maximum rate 2150 Limiting percentage of oxygen in atmosphere to prevent ignition of dust clouds: 11 By electric spark 5 By hot surface at 850° C. α Percentage of inert dust needed in a mixture with the sample to prevent ignition of dust clouds.
VOLUME
2 7, N O .
29
.
JULY
18,
1949
6 480 90 + 85
7 400 90 + 85
8 460
90 + 85
5 460 90 + 85
0.04 0.045
0.10 0.090
0.12 0.065
0.045 0.050
470
9 460
10 480
0.025 0.050
90+ 80 0.035 0.075
90+ 70 0.26 0.130
36 650 1350
32 400 750
23 200 400
90 + 85
53 750 1350
54 950 2150
52 1100 3150
59 1250 2350
61 1150 3550
53 600 1350
17 100 15·
72 1050 2150
68 1250 1650
78 1050 1650
81 1750 4400
79 1150 3550
72 800 3000
68 750 2000
87 900 1750
101 800 2250
100 900 2250
90 600 1350
13 7
2071
8.000 6.000 4.000
% 2.000 CO
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4 8 12 VENT RATIO. SO. FT. PLS 2 0 0 CU. FT.
Effect of unrestricted relief vents on pressure developed by explosions of corn starch in three galleries dreds of dusts and powders. The test pro cedure and apparatus are described in Bureau of Mines Report of Investigations 3751. Samples recently studied included four ordinary corn starches (Xos. 1, 2, 3, and 4), two modified corn starches (Nos. 5 and 6), one mixture of 9 5 % corn starch and 5 % KBr0 3 (No. 7), one mixture of corn plus wheat starch and 5 % KBrO? (No. 8), one mixture of corn plus wheal starch, 5 % KBK) 3 , and 1 0 % inert in organic salts (No. 9), and one mixture of corn plus wheat starch, 5 % KBr0 3 , and 3 0 % inert inorganic salts (No. 10). A summary of the test data for these samples is given in the accompanying table. All tests were made on the dusts of 9 0 % or more through 200-mesh fineness after drying at 75° C. for 24 hours. Sam ples 4, 5, and 6 were, however, tested be fore as well as after drying; the moisture loss on these samples was 6.2, 7.2, and 6.0%, respectively. The tests showed that dust clouds of starch can be ignited readily by hot sur faces, by weak electrical sparks, and un doubtedly also by other common igniting sources. Undried dusts were more diffi cult to ignite and produced less severe explosions than similar dried samples, but the moisture definitely did not elimi nate the explosion hazard. Addition of 5 % potassium bromate to starch increased the explosibility somewhat, but not un duly. The addition of a greater proportion of bromate to starch might, however, intro duce more severe fire and explosion haz ards. Addition of small quantities of in ert inorganic constituents to the starchbromate mix reduced but did not elimi nate the danger of explosions. The data reported here cover only corn starch and corn starch mixtures. Tests made several years ago by the U. S. De partment of Agriculture had shown that pressures produced by explosions of dif
2072
ferent starches do not differ greatly; in those experiments pressures of decreasing intensity were developed by explosions of alkali starch, rice starch, corn starch, po tato starch, and wheat starch. Reduction Explosion
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of Hazard
Detailed recommendations for prevent ing dust explosions are contained in codes published by the National Fire Protection Association; one code deals with starch factories. Important safeguards that ap ply generally to all dusts and plants in clude the maintenance of good housekeep ing to prevent the dissemination of dust into the atmosphere and its collection on exposed surfaces and the elimination of all sources of ignition from areas where dust clouds are likely to be formed and where dust is stored or handled. In storing large quantities of starch attention should also be given to the po tential fire hazard due to external heating or spontaneous self heating. When a small layer of sample No. 2 was placed in a fur nace at 200° C. for 10 minutes, its color changed to a *pale brown; at 250° C. the starch turned deep brown in 10 minutes, and at 275° C. it charred within 10 min utes. When the test was repeated with fresh portions at higher temperatures, it was observed that flame issued from the starch after exposure at 410° C. for 3 min
Photography
in
utes, 30 seconds. A test report by a Neth erlands investigator indicates that fires in dextrin, a starch derivative, may start spontaneously if quantities of the powder are left in unventilated spaces for a long time at comparatively low temperatures. One sample reportedly ignited at 150° C. after 150 hours, and at 155° C. ignition occurred in 18 hours. T o reduce structural damage from dust explosions adequate relief vents should be provided in the wall surfaces of process ing equipment and rooms. The vents serve to release quickly the pressure de veloped during the incipient stages of a dust explosion. The proper sizes of the vents depend on the strength of the struc ture to be protected, the dust involved, and other factors. The attached figure shows the effect of increase in the vent area (expressed as a ratio of area to volume of the explosion space) on the pressures de veloped by experimental corn starch ex plosions in cubical test chambers or galler ies of three sizes. In these tests the vent opening was unrestricted. Experiments with other dusts have shown that when the opening is closed off by a diaphragm, by a hinged panel, by glass, or otherwise, higher maximum pressures are produced. Studies have also shown that when ducts are inter posed between the vent on the explosion chamber and the outside atmosphere, the effect is to increase the explosion pressures.
Science
Electron micrograph by Charles W. Hock of the Hercules Powder Co. Experimental Station showing a single cotton fiber shadowed with chromium and magnified 2 3 , 5 0 0 times. Photo was an entry in the Photography in Science competition spon sored by the Smithsonian Institution and Science Monthly·
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