REDUCING HAZARDS OF EXPLOSION FROM DUST IN MANUFACTURING PLANTS
Safety Codes for Dust Explosion Prevention Have Been Adopted in Many Industries Where the Air is Unavoidably Laden with Dust Clouds
THE Chemical Engineering Division of the Bureau of Chemistry and Soils of the United States Department of Agriculture has record of 573 dust explosions in plants handling or milling products principally of agricultural origin. In 189 of these explosions, 440 persons were killed and 891 were injured, making a total of 1,331 persons involved. In 461 cases the property damage amounted to $46,779,910.
The largest number of dust explosions have occurred in grain elevators and in connection with grain-handling operations. There have been 101 explosions in grain elevators. In 30 of these explosions 74 persons were killed, and 160 were injured. In 91 cases the total property damage reported was $15,879,150.
There have been 81 dust explosions in flour mills. In 22 of the explosions in flour mills, 27 persons were killed, and 40 were injured. In 55 of these cases the property damage amounted to $4,387,900.
Feed and Cereal Mills
In feed and cereal mills there have been 75 dust explosions. In 30 of these explosions 53 persons have been killed and 183 injured. In 67 cases the property damage was $5,612,050.
Starch and Corn Products Plants
In starch and corn products plants 32 dust explosions have occurred. In 16 cases 128 people were killed and 123 were injured. In 27 cases the property damage was $5,095,850.
Wood Working Plants
There have been 70 dust explosions of wood dust in woodworking plants. In 30 of these explosions 26 persons were killed and 96 were injured. In 52 cases the property damage was $1,948,860.
Other Industries Affected
Dust explosions and resulting fires also have occurred in the following lines of industry: Cork handling plants, sugar refineries, malt houses, fertilizer plants, pulverized coal installations, bark grinding operations, sulphur grinding and handling, coffee and spice milling plants, metallic grinding and pulverizing operations, cotton and textile mills, paper manufactories, hard rubber recovery plants, phonograph record plants, resin handling and grinding, powdered milk plants, cocoa and chocolate plants, and rice mills.
Two hundred and fourteen explosions have occurred in these additional industries. In 61 cases 132 people were killed and 289 were injured. In 169 cases the property loss reported amounted to $13,856,100.
Economic Importance of Dusf Explosion Prevention
There are approximately 28,000 industrial plants in the United States that are subject to the hazard of dust explosions. These plants normally employ 1,325,000 persons and manufacture products of an annual value of more than $10,000,000,000 (ten billion dollars). The extent of this industrial hazard indicates the economic importance of research on this problem.
Reduction in Dust Explosion Losses
A careful study of the reports of dust explosion investigations will show that:
- In the grain-handling industries the losses of both life and property as the result of dust explosions have decreased in recent years.
- Although a larger number of dust explosions are occurring each year, largely on account of the increase in number of industries affected, the effect of explosions is localized and limited in extent as the result of the application of methods of control and prevention.
In the period from 1921 to 1931 the average loss gradually decreased from approximately $520,000 in 1921 to about $28,000 in 1931, a net average reduction of approximately $500,000. The yearly losses have dropped from $4,160,000 in 1921 to $1,100,700 in 1931.
This marked reduction in dust explosion losses in the grain-handling and associated industries has been due in a large degree to the splendid cooperation which the Department of Agriculture has enjoyed with all the industries, organizations. and agencies engaged in explosion and fire prevention work. This cooperation has resulted in a definite saving of both life and property.
Safety Codes for Dust Explosion Prevention
One of the principal accomplishments in dust explosion control and prevention has been the development of safety codes by the Dust Explosion Hazards Committee of the National Fire Protection Association. This committee, composed of representatives from the industries directly concerned, and from insurance and safety organizations. State and Federal officials, and construction and equipment engineers, works under the leadership of the Bureau of Chemistry and Soils, United States Department of Agriculture.
The committee has prepared safety codes for the following:
- Flour and feed mills
- Sugar and cocoa pulverizing systems
- Pulverized fuel installations
- Terminal grain elevators
- Starch factories
- Coal pneumatic cleaning plants
- Wood flour manufacturing establishments
- Spice grinding plants
- Inert gas systems for fire and explosion prevention.
These safety codes, which have been adopted by the National Fire Protection Association and by the National Board of Fire Underwriters and which have been approved as “American Standard” by the American Standards Association, have been very helpful in bringing about the application of precautionary measures that have been instrumental in reducing losses from dust explosions and fire.
Effect of New Manufacturing Processes
Although our attention was called to the dust explosion hazard in a definite way more than fifty years ago, we are still faced with this important, well-recognized industrial problem. The expansion of the various industries, the utilization of by-products and waste materials, the introduction of new methods and processes with correspondingly increased production, and new types and sources of ignition have produced additional hazards in some of our newer industries. The increased use of spray-drying equipment in some of the industries, for instance, has introduced new ignition sources, requiring that care be exercised in the installation of drier equipment of all types in which products are handled in powdered form.
Inert Gas Systems
In grinding and pulverizing operations it is practically impossible to prevent the formation of dust clouds or to eliminate entirely all sources of ignition. Extensive work has been done to develop inert gas systems for the prevention of dust explosions in grinding equipment. Describing the work of the Bureau of Chemistry and Soils on this subject, Hylton R. Brown, Engineer of the Chemical Engineering Division, in Technical Bulletin No. 74, entitled “The Value of Inert Gas as a Preventive of Dust Explosions in Grinding Equipment,” states:
“As a result of the experimental work and the observations made during actual operation of inert gas systems in industrial planls, the value of inert gas as a means of preventing dust explosions has been so well demonstrated that its use should be seriously considered wherever an explosion hazard exists which cannot be controlled through the elimination of the dust cloud or the source of ignition. The use of inert gas is particularly recommended in grinding, bolting, or any phase of a manufacturing process where an explosive dust Is produced or handled within an enclosed piece of equipment.”
Nature and Action of Dust Explosions
As the result of the experimental work on the subject, considerable knowledge as to the action and behavior of dust explosions has been acquired. It is now generally recognized that in their action and behavior dust explosions arc somewhat similar to gas explosions. It has been found that the different dusts have definite ignition temperatures, just as the gases do, and that they also have what might be termed the upper and lower limits of explosive concentration.
The ignition sources of explosive dusts arc practically identical with those of explosive gases. It must therefore be recognized that any source that will ignite explosive gases also will ignite explosive dusts.
One of the principal factors in the explosibility of a dust appears to be the degree of fineness. The explosion appears to be nothing more than flame propagation through a very finely and uniformly divided dust cloud. The flame travels at a very rapid rate, building up considerable pressure, thereby producing what we ordinarily call the explosion. It has therefore been extremely difficult in certain instances to determine definitely when the fire ended and the explosion started. It would appear that any fire might result in an explosion if combustible dust clouds are present.
Dust explosions usually occur in two stages—the “primary” and “secondary.” The original ignition of the explosive dust-air mixture is termed the primary stage. As a rule the damage resulting from this primary ignition is limited and the explosion is not very violent. This primary ignition, however, is accompanied by sufficient concussion to shake any settled or “static” dust into suspension, and this newly formed dust cloud is ignited by the flame from the first or primary ignition and the secondary explosion follows. Secondary explosions are more serious than the primary. Therefore, if we can limit or weaken the primary puff there will not be any extensive damage.
Explosion Venting Methods
Progress has been made in the development of methods for venting dust explosions in the primary stage. In order to obtain accurate data for various explosive dusts with the different types of venting, special equipment has been installed at the Arlington, Va., experiment station of the United States Department of Agriculture. This experimental structure, in which the venting of dust explosions can be studied, consists of a room, a gallery, and a tower, with a total volume of 333 cubic feet.
The fixed glass windows in the structure provide a factor of safety by breaking when the venting area is not sufficient to prevent the development of excessive pressures. Dust from hemispherical cups may be sprayed into suspension in the air within the structure by jets of compressed air. Ignition is produced by a glowing electric heater coil. Pressure produced by the explosions are recorded by rubber diaphragm manometers, installed at appropriate points in the structure, to record pressures up to 800 pounds per square foot.
On the principle that the breaking of glass indicates the pressure at which structural damage would occur, tests were made to determine:
Venting area per unit volume necessary to prevent structural damage.
- Effect of location and distribution of vents.
- Effect of differences in size and type of ignition.
- Reduction of pressure due to increase in venting area.
- Effect of outside glazing.
- Effect of scoring glass to reduce breaking strength.
In these tests sixty-six explosions of grain dust and fiftyeight explosions of starch dust were produced. To provide satisfactory venting for an explosion of grain dust in a cube-shaped structure it is recommended that 1J4 square foot of vent be provided for each 100 cubic feet and for starch dust 3 square feet of vent for each 100 cubic feet.
The results of tests with sugar, wood flour, sulphur, cork, powdered milk, soap powder, and similar products indicate that the venting necessary for these dusts ranges between the requirements for starch and those for grain dust.
These venting tests, although confined principally to starch and grain dusts, indicate that:
- It is possible to vent dust explosions without structural damage.
- Fixed glass offers too much resistance to permit dependence upon it alone for the release of explosion pressures without structural damage.
- Many types of venting equipment, hinged doors, windows and panels may be satisfactorily used to release explosion pressures provided sufficient venting area is used.
- Vents located near the source of ignition are more effective than those located some distance away.
- The venting area required varies for different dusts.
- Secondary explosions are more difficult to vent than primary explosions.
- A definite reduction in pressure results as the venting area is increased.
- Pressures may be released by lowering the resistance of fixed glass by means of outside glazing or by scoring.
Practical Application of Venting Tests
It must be recognized that some dust explosions are certain to occur in grain elevators and grain-handling plants. The extent and seriousness of these explosions can be minimized by making practical application of the results of these venting tests. The proper use of light construction, hinged panels, or large glass and sash areas that give quick venting at low pressures should be embodied in specifications for construction of grain elevators and plants producing or handling combustible dusts, and all plants that are subject to the dust explosion hazard. Built up pressures produce undue spread of flame and result in secondary explosions, which are responsible for most of the life loss and damage to structure and equipment. This extensive loss can be prevented by the application of proper dust explosion venting methods.
(Excerpts from a paper read before the fifth annual convention of the Society of Grain Elevator Superintendents of North America.)