I
IRVING HARTMANN and JOHN NAGY Central Experiment Station, U. S. Bureau of Mines, U. S. Department of the Interior, Pittsburgh, Pa.
Venting Dust Explosions Adequate explosion vents in plants are highly effective in reducing structural damage D u s r EXPLOSIONS are rapid uncontrolled combustion of dust in air, characterized by evolution of heat, high flame speeds, and increased pressure. Pressure increase results chiefly from heating and expansion of air in the explosion space; but with some dusts it is caused partly by generation of new gases during combustion. .4t the start of an explosion the pressure rises slowly. Time is required to disperse the dust, mix it in proper proportion with air, and preheat part or all of the mixture to ignition temperature. Furthermore, during the initial stage of most dust explosions there is an induction or ignition lag period sometimes attributed to the initiation of a chain reaction. After this stage, rates of chemical reaction and pressure rise in most explosions are greatly accelerated. It is important, therefore, that alleviating measures against explosions are taken during the initial phase. T o prevent dust explosions, all potential sources of ignition should be eliminated from dusty areas of plants, and good housekeeping should be practiced so that combustible dust is not disseminated and allowed to collect on exposed surfaces. particularly on elevated surfaces. Other helpful measures under some conditions are limiting production of fine dust; keeping concentrations of dispersed dust below- the lower explosive limit; using inert gas for processing highly combustible dusts; diluting combustible dust with noncombustible dust; pulverizing or processing the powder or dust under a liquid; and/or treating the surface of the particles with a protective film to prevent rapid -oxidation. In addition to preventive steps, precautionary measures to limit structural damage from potential dust expiosions are essential in many industrial operations. These include segregating hazardous processes from the main plant area; batch handling of combustible powders to limit the fuel available in an explosion; making the equipment and other affected structures strong enough to withstand maximum explosion pressures; providing chokes, diverting gates, or quick-acting shut-off valves to limit
1734
spread of explosions; cooling or otherwise quenching the flames; and installing explosion vents in equipment and rooms to be protected. Function of Explosion Vents
Adequate explosion vents in plants are highly effective in reducing structural damage from dust explosions. The purpose of venting is to promote rapid release of heated gases. flame, and burned and unburned dust from an enclosure during an explosion. By thus removing heat energy and fuel from the explosion space, rate of combustion is reduced and development of destructive pressures can be prevented. Free or unrestricted openings generally provide the most effective vents. However, because of protection needed against weather changes. or escape of dust and vapor from equipment during normal operation, or for other reasons, it is rarely feasible in practice to use free vents. Therefore, it is necesary to cover the vent openings with diaphragms or bloivout disks, poppet-type closures, louvers, hinged panels, hinged windows with proper latches, scored glass panes, light sections in wall or roof surfaces, or other rapidly opening devices. For the best effect. the equipment to be vented should be located as near as possible to outside walls or to the roo€ in a plant. \Vhere this cannot be arranged, ducts must be connected between the vents and the outer atmosphere. The proper areas of vents and suitability of vent closures depend on several factors-strength of equipment or structure being vented; intensity of an expected explosion, particularly maximum explosion pressure and rates of pressure rise produced by the dust being processed; position of the vent relative to the origin of an explosion; bursting strength of the vent closure or, if movable as a whole, minimum pressure required to open closure; inertia of the vent closure; and length of ducts attached to the vents. Intensity of explosions and venting requirements are affected by chemical and physical characteristics of the dust
INDUSTRIAL AND ENGINEERING CHEMISTRY
The Bureau of Mines used these galleries, shown in their open shelter, to study dust explosion venting. The gallery in the foreground has a volume of 64 cubic feet and that in the background, 2 16 cubic feet
which in turn determines rate of combustion and heat generation. Such characteristics are composition of the dust, its affinity for oxygen, particle size and shape, ease of dispersion, volatile content, and moisture content; concentration of the dust cloud; composition of the surrounding atmosphere, especially its oxygen content; nature of the ignition source; size, shape, and surface characteristics of the explosion space, and the distribution of dust therein. Importance of these factors has been discussed (7, 9. 77). Until now little progress has been made toivard a sound, theoretical solution of venting requirements in either dust or gas explosions. A number of proposals have been advanced for computing vent areas to limit pressures, but the computations are based on over simplified and unrealistic assumptionse.g., that no unburned fuel escapes through the vent and no heat is lost from the explosion space; that the fuel burns at a uniform rate, and that no gases are generated or consumed during the explosion. Knowledge of the mechanism of dust explosions is limited, and does not provide a sound basis for solving the complicated problem of venting which
This atomized aluminum powder explosion was released f r o m the 2 1 6-cubic-foot gallery through vents in two sides
involves a thermochemical process that results in heat generation, gas development or consumption in some instances, and simultaneous flow of heat, gas, and dust from the combustion space. For the time being, therefore, aside from empirical methods (74) useful for specific situations, design of explosion vents must be based on experience and on experimental evidence such as is presented in this report. Scope
and Design of Experiments
Systematic experiments on venting dust explosions have been limited. Studies of venting grain dust and cornstarch explosions were performed by the U. S. Department of Agriculture more than 20 years ago (I). Venting tests and other explosibility studies on various dusts have been performed a t laboratories of industrial insurance associations (3, 4,5). Venting cork dust and aluminum powder explosions in cylindrical chambers and ducts have been studied in England (2). Explosion venting studies were begun in the Bureau of Mines’ laboratories about 13 years ago. Initially, the experiments were conducted in a cubical chamber or gallery having a volume of 64 cubic feet and equipped with vents in three vertical sides and the roof (8, 72). Dust clouds were formed by dispersion of weighed quantities of dust from hemispherical cups with jets of compressed air; the resulting explosions were mild. In a later study (73),the relative effectiveness of unrestricted vents and of paper, cloth, metal foil and other diaphragms, hinged doors or panels, and several kinds of scored and unscored glass panes were investigated. Recently, venting requirements of dust explosions were studied in cubical galleries having volumes of l , 64, and 216 cubic feet, with and without ducts at-
tached to the vents. Parameters investigated include particle-size distribution of dust, concentration of dust clouds, and admixture of inert with combustible dust; manner of dispersing dust into a cloud; nature of igniting sources and their timing with formation of clouds; size, number, shape, and location of vents; length, shape, crosssectional area and orientation of ducts, bends and diaphragms in ducts, presence of dust on the duct floor, secondary vents in ducts, and weatherhoods in ducts. Also, venting finite volumes to large adjoining chambers (70), prevention of secondary explosions, and simultaneous venting and flame quenching of dust explosions were investigated (6). In most experiments, only a single vent, generally in a vertical wall, was used in all three galleries to limit the explosion pressure. Vents of circular, square, and rectangular shapes were studied. For comparing and plotting the test data, it was useful to divide the areas of vents by the volume of the explosion chamber and to express the resulting “vent ratio” in square feet per 100 cubic feet. In this investigation, the vent ratios generally ranged from less than 1 to about 10 to 15 square feet per 100 cubic feet. I n a few experiments with magnesium and aluminum powders, vent ratios as high as 25 to 30 square feet per 100 cubic feet were used. Most tests with diaphragm- and hinged-paneltype vent closures were made in the 64-cubic-foot gallery. Effects of square ducts (18 X 18 inches, up to 32 feet long) and rectangular ducts (18 X 30 inches, up to 12 feet long) were studied for explosions in the 64-cubic-foot gallery. Similar studies were made in the 1-cubicfoot gallery with square (37,’s inches), rectangular (2l/4 X 6I/2 inches), and circular (4I/q- and 53/4-inch diameter) ducts, ranging u p to 17 feet in length.
Vent characteristics of explosions of 15 different dusts and powders were studied; for a number of materials several samples were used. They were commercial products and by-products formed in processing the bulk solids and included atomized aluminum, flaked or stamped aluminum, milled magnesium, cocoa, cornstarch, sugar, soybean protein, bituminous coal dust, cork, wood flour, sodium lignosulfonate, soap powder, cellulose acetate, phenolic resin, and polystyrene copolymer. For the aluminum powder explosion illustrated, all particles in the samples passed through a No. 20 U. S. Standard sieve (840 microns). I n most samples 95 to 100% by weight of the particles passed through a No. 200 sieve (74 microns); and in a few samples, 85 to 95y0 passed through a No. 200 sieve. One coarse, milled magnesium powder tested in the early investigation, contained only 51.6% of particles finer than 74 microns. A limited number of tests were made to study the effect of particle size on explosion venting. When tested, all dusts contained less than 7Yo moisture and most less than
5%. Dust clouds were formed in the 1-cubicfoot gallery by directing a timed jet of compressed air downward against the dust in a hemispherical cup in the gallery floor. For this small enclosure this was a fairly efficient procedure. I n the larger galleries the weighed dust was placed in one to eight paper bags suspended in several positions at one or two levels. Within the dust in each bag was placed an electric detonator, the firing of which dispersed but did not ignite the dust. For some tests in the 1-cubic-foot gallery, a high-voltage, electrical induction spark ignited the dust cloud. I n others the flame of the small tuft of guncotton carefully timed with the dust cloud VOL. 49, NO. 10
OCTOBER 1957
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was used. I n the larger galleries ignition in all except early experiments was accomplished by guncotton flame. T o evaluate the relative effectiveness of various venting devices and arrangements, pressure-time records of the explosions were obtained a t one or two positions in the gallery, and in some tests also a t various positions in ducts. I n many special experiments, flame speeds of the explosions propagating through the duct were determined (6). Pressure manometers used were of the diaphragm type with optical magnification of the diaphragm deflection ( I I). Timing of the guncotton ignition and starting and stopping of the recording pressure manometers and other instruments were accomplished by electronic and mercuryswitch timers.
Test Parameters The experiments were planned to advance knowledge of the general principles of venting, rather than to furnish definitive design data for particular plant conditions. The recorded pressures would be different if, for example, two samples of a given material with different particle shapes. fineness, and surface characteristics were tested. Furthermore, in performing repetitive tests it is difficult to produce two dust clouds of the same uniformity and to ignite, explode, or vent them exactly alike. Certain parameters that have been studied intensively are important for the careful conduct of meaningful experiments, but in general, plant designers and operators can exercise little, if any, control over them. These parameters include : 1. Method of Dispersion. This factor is important because it affects uniformity of the dust cloud and therefore intensity of the explosion. I n the 64-cubicfoot gallery, dispersion by compressed air gave the least uniform clouds; dispersion with a detonator from one bag or container gave a better cloud. Simultaneous dispersion from eight bags produced the most uniform dust clouds and the strongest explosions. 2. Ignition Source. Size, duration, and intensity of the ignition source affect the lower explosive limit of the dust cloud and have an important role in initiating. developing, and venting an explosion. For most dusts studied, a flame was more potent than a high-voltage electrical induction spark. I n two otherwise identical series of cellulose acetate tests in the 64-cubic-foot gallery, with vent ratios between 2.3 and 5.3 square feet per 100 cubic feet, the dust clouds were ignited by flame from 5 - and 15-gram tufts of guncotton. Maximum pressures produced by the larger flames were from 20 to 100% higher than in the corresponding weaker explosions. 3. Concentration of Dust Cloud.
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Dust clouds like gas-air mixtures have well defined lower explosive limits, but their upper limits are indefinite. Between these two limits is an optimum concentration at which the strongest explosions are produced. This is generally higher than the stoichiometric concentration of the dust. In this study, the optimum concentration was determined experimentally for each dust, and most tests were made at these concentrations. Exceptions were a number of tests with aluminum and magnesium powders, which, because of strength limitations of the galleries, were performed at low concentrations. 4. Timing. Dense, explosive clouds of relatively coarse dust particles studied cannot be kept in uniform suspension for longer than a fraction of a second. Therefore, to achieve ignition a t the optimum concentration, it is important that ignition and dust dispersion be carefully coordinated. Proper timing for each dust and each gallery was determined in preliminary experiments. 5. Size of Explosion Gallery. Several dusts were tested in all three galleries, partly to determine what scaling factors could be used and partly to obtain venting information for much larger chambers in commercial plants. Theoretically, similar explosions in large enclosures should produce higher pressures and higher rates of pressure rise because as the volume increases, heat lost to the surroundings, which is a function of the surface-to-volume ratio of the enclosure, decreases. However, with increased volume, it becomes more difficult to produce uni-
Vent Ratio, Sq.
form dust clouds that fill the chamber. I n the experiments, this second factor was more important than the former, and with several dusts, stronger explosions were produced in the 1-cubic-foot gallery than in the 64-cubic-foot gallery. These were, in turn, somewhat stronger than explosions in the 21 6-cubic-foot gallery. With some dusts, these differences were slight, and in one complete series of cornstarch explosions, no measurable differences were observed.
Venting of Mild and Strong Explosions By varying the foregoing parameters, particularly method of dispersion and gallery size, data were obtained on venting requirements for relatively mild, strong, and some intermediate explosions of several dusts through unrestricted vents directly to the outside atmosphere. Mild explosions were produced in the 64cubic-foot gallery by dust clouds formed by compressed air jets, and the strong explosions were for the most part produced in the 1-cubic-foot gallery. Figure 1 is a semilogarithmic plot of the pressure-vent ratio relations for mild explosions of nine dusts. Figure 2 shows the relationship for strong explosions of 12 dusts. The relationships follow straight lines within an appreciable range of the test data, but the lines cannot be extrapolated to zero vent ratio nor to high vent ratios where zero pressure is approached. The equation of the lines can be log P = log A - Kr (1) where P = maximum pressure, 7 =
Ft. per 100 Cu. Ft.
Figure 1. Effect of unrestricted vents on pressures produced by explosions of various dusts
INDUSTRIAL A N D ENGINEERING CHEMISTRY
mild
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Vent Ratio, Figure 2. Effect o f unrestricted vents on pressures produced by strong explosions of various dusts
vent ratio, A and K are empirical constants, A is the intercept on P axis, and K is the slope of the line = (log P1 log P2)/(rz - T I ) . The pressure-vent ratio relations can also be expressed by the equation p = Ae-kr
(2)
where e = base of natural logarithms (2.71828), and k . logloe = 0.4343k = K in Equation 1. Test data and equations show that maximum explosion pressure decreases exponentially with increased vent ratio. Therefore, a small increase in the vent area at low vent ratios results in a much greater reduction in the explosion pressure than at high vent ratios. To limit the maximum pressure to 2 pounds per square inch (288 pounds per square foot), for mild explosions of coal dust or wood flour, the vent ratio should be approximately 1.5 square feet per 100 cubic feet and for strong explosions it should be 5.5 to 6.0 (Figures 1 and 2). Similarly, for mild explosions of soybean protein or cornstarch, the vent ratio corresponding to a maximum pressure of 2 pounds per square inch, is about 1.75. However, for strong explosions of soybean protein, the required vent ratio is 8.5, whereas for cornstarch it is 13.0.
Sq. Ft.
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20
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per 100 Cu. Ft.
Figure 3. Relative effectiveness o f circular, square, and rectangular vents for relieving cellulose acetate dust explosions from the 1-cubic-foot gallery
were equally effective in venting explosions. When the openings were sealed by strong paper, empire cloth (varnished cambric), or other diaphragms, rectangular vents were slightly more effective than square vents, and both were more effective than circular vents of like areas (Figure 3). This is probably because at equal pressures, stresses are higher at midpoints of the sides in rectangular and square diaphrams than in circular diaphragms. In a few tests with cornstarch explosions in the I-cubic-foot gallery, effectiveness of circular vents was increased (maximum pressure reduction of 10 to 30%) when the internal edge of the vents in l/d-inch thick brass wall plates was rounded to promote streamline flow of the combustion products. In the relatively small, cubical explosion chambers used, position of vents, whether in vertical walls or in the top, was immaterial. Two or more openings were as effective as a single vent of the same total area. However, in much larger or elongated enclosures, location of vents relative to the source of ignition is important. In long chambers several vents in various positions are more effective than a single large vent.
Shape and Location of Vents
Fineness of Dust and Admixture of Inert Matter
Unrestricted circular, square, and rectangular openings of equal areas
Explosion hazards of combustible dusts increase with decrease in particle
size for a number of reasons. Therefore, venting requirements should be more severe as the dust becomes finer. T o verify this, a number of experiments were performed with two samples of cellulose acetate dust which contained 85 to 9470 of particles which passed through No. 200 U. S. Standard sieve. Explosions of finer dust produced between 30 and 50Y0 higher pressures than the coarser dust (Figure 4A). I n another set of experiments, explosions of cork dust having particle sizes with sieve numbers ranging from 100 to 140 (149 to 105 microns), 140 to 200 (105 to 74 microns), and through No. 200, were produced in the l-cubicfoot gallery to which an 11-foot duct was attached. The maximum pressure decreased considerably with increase in the average diameter of the dust particles (Figure 4B). In certain industrial processes, noncombustible and combustible powders are blended. Sometimes it may be possible to distribute an inert dust, such as pulverized limestone, near an operation where highly combustible dust is processed. Then when ignited, the combustible and noncombustible dusts become mixed by the initial disturbance. When the noncombustible dust constitutes an appreciable proportion of the total mixture, it exerts an important effect in reducing the explosion hazard. Figure 5 shows the effect of increasing proportions of finely divided, calcined VOL. 49, NO. 10
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U. S. STANDARD SIEVE SCALE
0
A, Vent Ratio, Sq. Ft. per 100 Cu. Ft.
50 100 150 Average Particle Diameter, Microns
6.
Effect of particle size on explosions in vented 1-cubic-foot gallery
Figure 4.
A, ce!lulose acetate relieved through unrestricted vents; B, cork dust vented through a duct 4I/4 inches in diameter and 1 1 feet long
fuller's earth in mixtures with cork and cellulose acetate dust on maximum pressures developed by explosions in the 64-cubic-foot gallery equipped with vents of three sizes. Effect of Diaphragms and Hinged Panels
Diaphragms, hinged panels, and other closures on explosion vents restrict efflux rate of combustion products from an enclosure and reduce effectiveness of vents (Figure 3). Various diaphragms (73) had pressure-relieving capacities about inversely proportional to the static bursting strengths of the diaphragm materials. Effectiveness decreased as
the diaphragm thickness increased and, as mentioned before, shape of the diaphragm-covered vent also had an effect. Nature of the vent closure was less important in rapid aluminum powder explosions than in slower coal dust explosions. To facilitate rupture of strong diaphragms at low pressures, cutters can be placed near their outer surfaces and arranged so that when an explosion starts the diaphragm is pierced or weakened. Comparatively slow explosions of coal dust and other dusts were vented nearly as effectively through lightweight (l/~einch thick sheet metal) swinging panels or doors as through unrestricted openings. Heavy doors, however, resulted in higher
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MIXTURE COMPOSITION, PERCENT
Figure 5. Effect of inert dust on explosions of cork and cellulose acetate dusts in vented 64-cubic-foot gallery
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
pressures. Hinged steel doors, ' / 4 inch thick, on vents in the 64-cubic-foot gallery increased maximum pressure in coal dust explosions about two to three times and in soap powder explosions to nearly twice the pressures developed with unrestricted vents of similar dimensions. In rapid explosions such as atomized aluminum powder, even light-weight swinging doors result in increased pressure. When using swinging doors or panels. unless the equipment is quickly flooded with carbon dioxide or other gas for fire fighting, provision should be made to prevent development of destructive negative pressure (partial vacuum). This may occur soon after the outflow of combustion products when the door closes and cooling takes place. It can be prevented by quickly equalizing the internal and external pressure or by providing stops or louvers to avoid complete closure of the door. However, after the initial explosion, closing the vent opening is sometimes desirable to prevent inrush of fresh air that might cause a second explosion. In Great Britain and Germany, a combined bursting diaphragm and hinged door is sometimes used. The door is normally held open by a weighted bell crank. If an explosion occurs, the diaphragm breaks and the door, lifted slightly off its support, then drops into a closed position ovex the vent opening. Venting Explosions through Ducts
In many plants ducts through which explosion products must flow are needed between equipment to be vented and the roof or outside wall of the building. This reduces effectiveness of the vent because of the time required to move the column of air within the duct and because of frictional resistance of the duct surfaces to the flow. For unrestricted ducts attached to the explosion galleries, maximum pressure increases appreciably and directly, though not quite linearly, with increase in length of duct (lower curve. Figure 6A). Effects of ducts with circular. square, and rectangular cross sections of equal areas were virtually identical. Empire-cloth diaphragms between the gallery and duct had a greater effect than moderate increase in the length of the duct (Figure 6 4 . For cornstarch explosions, a diaphragm set in a duct 11 feet long at various distances from the gallery caused pressure increases up to 20% with increase in distance. I n some industrial processes, vent diaphragms are exposed to corrosive fumes or high temperatures that tend to weaken them. In such instances, two closely spaced diaphragms have been considered. I n experiments with cornstarch explosions in the 1-cubic-foot gallery with
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5 10 15 20 Length o f Duct, Ft. 6. Distance of Vent from Gallery, Ft. Figure 6. Cellulose acetate dust explosions vented from the 1 -cubic-foot gallery through 3'/*-inch square ducts
A.
Dust concentration in aallery. 0.50 oz./cu. foot. unrestricted vent location in i;-foot d u l t
A. effect of duct length;
an 1 1-foot duct attached, two diaphragms 1 foot apart at various positions along the duct resulted in pressures virtually identical to rhose from a single diaphragm. Limited experiments with vertical and horizontal ducts ranging from 2 to 16 feet long and attached to an explosion gallery. showed that they were equally effective in releasing explosions. In a number of tests pressures were measured in the ducts as well as in the explosion gallery. This was done, for example, in cellulose acetate explosions in the 1-cubic-foot gallery to which an unrestricted duct 17 feet long and 4
E , effect o f 4-inch square
inches square was attached. The maximum gallery pressure was approximately 3000 pounds per square foot, and the corresponding pressures in the duct were 2700 at 4 feet from the gallery, 2600 a t 8 feet, 2250 at 12 feet, 1600 a t 14 feet, and 800 at 16 feet. T o limit pressure in the gallery it is occasionally advisable to provide one or more secondary vents in the duct. The effectiveness of such vents decreases with increased distance from the source of the explosion (lower curve, Figure 6B). For these tests, area of the secondary vent, placed in one side of the duct, was equal to the duct area.
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Length of Duct, Feet Figure 7. Venting cornstarch dust explosions from 1-cubic-foot gallery through circular ducts A, unrestricted vents; 6, vents with k r a f t p a p e r diaphragms
---4 I / d n c h vent plus 4l/d-inch duct -.- 4l/4-inch vent plus 6-inch truncated cone plus 53/a-inch duct --
5'/4-inch vent plus 5a/4-inch duct
The upper curve in Figure 6B shows pressures developed in explosions when cellulose acetate was distributed uniformly along the duct floor. These pressures are higher than those without dust in the duct, but such increase (if any) depends to an important degree on the dust concentration in the explosion space, along the duct, and on availability of sufficient oxygen for the combustion. Increase in the cross-sectional area of ducts attached to vents of like areas in the explosion galleries was accompanied by reduction in maximum pressure. This was true for unrestricted ducts and also those with a kraft paper diaphragm a t the vent. Because in many types of industrial equipment it is impossible to use vents of recommended size, a study was made of the effect of connecting a small vent through a truncated-cone shaped funnel with a larger duct through which the explosion products would flow (Figure 7). With unrestricted vents and ducts, a 6-inch long truncated cone between a 41/4-inch diameter vent in the I-cubic-foot gallery and a 53/4-inch diameter duct was nearly as effective in limiting the explosion pressure in the gallery as a 53/4-inch diameter duct on a vent of the same size (Figure 7 A ) . When a diaphragm was placed at the wide (outer) end of the truncated cone, the combination of small vent, cone, and large duct was nearly as effective as a large duct and large vent (Figure 7B). When the vent diaphragm was placed a t the narrow end of the cone near the gallery, the combination was less effective but still better than a narrow duct. In venting explosions through ducts, attempts are generally made to avoid turns, particularly sharp changes in direction. This cannot always be accomplished. T o evaluate the effect of bends, experiments were made with 4-inch square ducts of 3- to 7-foot lengths connected to the 1-cubic-foot gallery, the ends of which had 45-, 90-, 1 3 5 , and 180-degree turns with average radii of 7 inches. In some tests. additional straight lengths of duct were connected beyond the bends. In explosions of cellulose acetate 45-degree bends at the ends of 3- and 6-foot ducts raised the gallery pressures in pounds per square foot from 1000 (without a bend) to 1350 (with a bend), and from 1600 to about 2000, respectively. Further increase in the bend to 90, 135, and 180 degrees caused little change in pressure. With bends from 0 to 180 degrees at the end of 10-foot long ducts, a small gradual increase occurred in gallery pressure-from 2400 to 2700 pounds per square foot. With 17-foot ducts, the effect of bends was barely measurable; the pressures ranged from 2900 to 3000 pounds per square foot. I n general, the effect of a 90-degree benp VOL. 49, NO. 10
OCTOBER 1957
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Sodium lignosulfonate -Cocoa
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Max. Pressure in Vented Gallery, Lb./Sq. Inch (Vent Ratio, 5 Sq. Feet/l 00 Cu. Feet) Figure 8. Explosion pressures in vented 1 -cubic-foot gallery compared with average rates of pressure rise in 1.3-liter laboratory bomb
about equaled that of an additional straight duct whose length is two to three times that of the bend. Bends in the ducts had a greater effect on pressure near the outer end of the bend and in the duct beyond it. This pressure increased nearly linearly with the degree of bend from 0 to 180 degrees. I n several tests, pressure beJ'ond a 90 degree bend was nearly three times as great; and beyond a 180-degree bend it Ivas five to six times as great as at the corresponding duct position without any bend. In some plants long ducts contain S-shaped offsets, and secondary vents are provided a t the bends in line with each straight section of duct. Correlation of Test Data from Vented and Unvented Explosions
One effect of venting is to change the pressure-time characteristics of explosions. Comparison of tests in vented explosion chambers with tests of corresponding dusts in the sealed 1.3-liter. cylindrical. laboratory test bomb shows that the time required for an explosion to reach maximum pressure (which for most dusts studied ranges from less than 10 to about 100 milliseconds) does not differ appreciably with and without vents. However, in sealed chambers, the rates of pressure rise (a few hundred to several thousand pounds per square inch per second) are a hundred times greater than those in the vented explosion galleries. A fairly good correlation exists between pressures developed in vented chambers and pressures, and particularly average rates of pressure rise produced in sealed test bombs (Figure 8). Venting tests are relatively time-consuming and expensive; therefore, they cannot be performed on a large number
1740
of industrial dusts, whereas tests in small, sealed test bombs can be performed more readily on a wide variety of samples. Because such test data can be correlated, venting requirements of dusts on which only laboratory tests had been performed can be estimated with some degree of certainty. The relationship of small-scale laboratory tests with cornstarch to results obtained in a 4000-cubic foot vented explosion chamber has been shown ( 3 ) .
20
Venting requirements for explosions of a number of inorganic and organic dusts were studied, and pressure-vent ratio relationships for relatively mild and strong explosions of several dusts were determined. Of the dusts investigated, aluminum and magnesium powders were the most difficult to vent. The nature of diaphragms, hinged panels, and other closures on vent openings is highly important. The frequently necessary interposition of ducts between vents and the outside atmosphere may greatly increase maximum pressures. Secondary vents in such ducts increase their effectiveness, especially when located near the potential source of an explosion. Products of combustion should be directed through vents in such a way that neither personnel nor property will be adversely affected and damage to the vent closures by wind, snow. ice, rust, or friction should be avoided. These problems and general principles of venting dust, gas, and vapor explosions have been discussed ( 7 5 ) . Although these experiments were done to obtain knowledge of venting principles rather than information for plant design, results from relatively small explosion chambers can be useful for protecting equipment and also for larger commercial structures. Venting requirements based on these experimental data, however, are generally too severe for large rooms or structures; in the latter, explosions of maximum violence occur rarely-the entire volume is not filled with a dust cloud of explosive concentration and ignitions do not occur at the most hazardous moment. It has been estimated (3) that in rooms of manufacturing plants of ordinar) size, it is improbable that more than one sixth of the volume is filled with an explosive dust cloud. By selecting vent ratios corresponding to maximum pressures to which the equipment can be safely subjected, these test data can be used to obtain the required vent sizes for equipment of known strength. Brick walls in most industrial buildings may be destroyed by sustained internal pressures of less than 1 pound per square inch. The National Fire Protection Association recommends vents ranging in size from 1 square foot for
INDUSTRIAL A N D ENGINEERING CHEMISTRY
each 10 to 30 cubic feet for small enclosures of light construction, to 1 square foot for each 80 cubic feet of volume for large rooms lrith heav!reinforced concrete walls. The British Factory Inspectorate suggests 5 square feet of relief area for each 100 cubic feet in systems subject to carbonaceous dust explosions, and 10 square feet when the risk is from magnesium or aluminum dust. I n most industrial equipment, it is impossible to provide vents of the desired or recommended areas because of design or other limitations. I n such instances, the largest possible vent should be provided, even though it may not be fully adequate. Even small vents give great reduction in the maximum explosion pressures. Other remedial steps are increasing strength of the equipment; reducing internal volume; where ducts are needed, increasing diameter of the duct by using an expanding funnel-shaped section between the vent and the duct; moving equipment out-of-doors; using inert gas in processing highly explosive dusts; blending incombustible dust with the combustible; if possible, arranging the operation so that the dust concentration is below the lower explosive limit; and most important of all, practicing good housekeeping and taking special care to prevent ignitions. Literature Cited (1) Brown: H. R., Hanson, R. L.: .\'at/. Fire Protect. Assoc. Quart. (April
1933).
( 2 ) Brown, K. C., British Safety in Mines Research Rept. 22, 1951. (3) Cotton, P. E., N a t l . Fire Protect. Assoc. Quart. 45, 1957-64 (October
1951). ( 4 ) Factory Insurance Assoc., Special Hazards Studv No. 4. 1940. Ibid.. KO. 5. Harfmann, Irving, Chem. Eng. Progr. 53, 107-11h4 (1957). Hartmann, Irving IND.EXG. CHz:M. 40, 752-33 (1948). Hartmann, Irving, Nutl. Fzre Protect. Assoc. Ouart. 40. 47-53 iJulv 1946). ( 9 ) Hartmann, Irving, Sci. .Vlonthly 79, 95-108 (1954). (10) Hartmann, Irving, Jacobson, M., Williams. R. P , U. S. Bureau Mines,.~Rept. Investigations 5052, 1954. (11) Hartmann, Irving, Nagy, John, U . S. Bureau of hfines. ReDt. Investigations 3751, 1344. (12) Zbid., 3924, 1946. (13) Nagy, John, Zeilinger; 3. E., Hartmann, Irving, Did., 4636, 1950. (14) Natl. Fire Protect. Assoc., Code for Prevention of Dust Explosions in the Plastics Industry, NFPA 6541946, pp. 17-20. (15) Natl. Fire Protect. Assoc. Guide for Explosion Venting, NFPA 68,1954. ~
A
RECEIVED for review March 21, 1957 ACCEPTED August 6 , 1957 Division of Industrial and Engineering Chemistry, Symposium on Safety, 131st Meeting, ACS, Miami, Fla., April 1937.
