INDUSTRIAL AND ENGINEERING CHEMISTRY
752
2. The effect of the ratio of tube length to tube diameter on the heat transfer coefficient was discussed, and the results showed that, for a length to diameter ratio greater than 10, h was not dependent on this quantity. 3. The validity of the equations when applied to gases other than air was discussed. I t was shown experimentally that the expressions are adequate to predict coefficients for carbon dioxide, which has considerably different physical properties from those of air. I n this connection the incorporation of the Prandtl group in the equations was considered. 4. Finally, the application of the equations to tube sizes larger than 2 inches was discussed.
k
= thermal
Vol. 40, No. 4
conductivit,y of gases, B.t.u./(hr.) (sq. ft.)
( F.jft.1
p
= heat transferred through pipe wall, B.t.u./hr.
tl
= inlet gas temperature,
tz tw
= exit gas temperature, O F. = water temperature, F.
a
F.
w D, DL G
= mass velocity, based on open cross section, lb./(hr.)
R
= ratio of -, dimensionless
= Jyeight rate of gas flow, lb./hr.
= particle diameter, ft. = tube diameter, ft,.
(sq. ft.) NU Reo.'
hD Xu = Nusselt number 2, dimensionless k
Re = modified Reynolds number DPG -, dimensionless P
ACKNO W LEDGMEh-T
The authors wish to thank J. T'idosh and C. R. Siple for the sketches and graphs.
At = logarithmic mean temperature difference, p = gas viscosity, lb./(ft.) (hr.) 01 = proportionality constant,
'F.
LITERATURE CITED SOMENC LATURE
e
f
(1) Leva, Max, IND. ENG.CHEM., 39,857 ( 1 9 4 7 ) : 40, 415 (1948). (2) M o n r a d , C. C., a n d P e l t o n , J. F., T r a n s . Am. Inst. C h e w Engrs., 38, 593-611 ( 1 9 4 2 ) .
base of natural logarithms, dimensionless
=
= function of, dimensionless
h = gas film coefficient, B.t.u./(hr.) ( O F . ) (sq. ft.) h, = water film coefficient, B.t.u./(hr.) (OF.) (sq. ft.)
RECEIVED AMaroh1, 1947. Published by uermission of the Director, Bureau of Mines, U. S.Dept. of t h e Interior.
Recent Research on the Explosibility of Dust Dispersions IRVING H-4RTRIANN Central Experiment Station, U . S. Bureau of Mines, Pittsburgh, P a . T h e Bureau of >lines started to study coal dust and gas explosions in mines over 30 years ago a t its Experimental Coal Mine near Pittsburgh, Pa. This work, coupled with laboratory research on flames and combustion, has greatly advanced knowledge of the causes and characteristics of explosion phenomena and resulted in the development of effective preventive measures against mine explosions. In recent years a n intensive investigation of explosions of industrial dusts and powders has been undertaken. This paper reviews briefly the nature of the dust hazard and the importance of dust explosions, discusses the various chemical and physical factors that affect dust explosibility, and describes recent research by the Bureau of Mines in this field,
H
IGH concentrations of dust in air constitute several hazards.
Most dusts are harmfyl to breathe, reduce visibility, and may cause explosions., Physiologists generally agree that only dust particles of 1-micron size and less are a menace to the respiratory organs. Visibility is affected chiefly by dust particles that remain in suspension in still or in slowly moving airthat is, those of 10 microns and smaller diameter. On the other hand, explosions can be caused by dusts ranging from the finest particles to those 700 microns (about 20-mesh) in diameter. I n the normal air of a residential room there are less than 1,000,000 dust particles per cubic foot, whereas in the general
atmosphere of a dusty industrial plant the count may be as high as 1,000,000,000 particles per cubic foot. The U. S. Public Health Service recommends that where workers are exposed during a n entire daily shift to air containing free or uncombined silica dust, the count be limited t o 5,000,000 particles per cubic foot. For continuous exposure to ordinary nuisance dusts, including coal dust in mines, the health recommendations call for less than 50,000,000 particles per cubic foot. The latter figure corresponds approximately to 1.4 mg. of coal dust per cubic foot or 0.05 mg. per liter. In comparison, the lower explosive limit of coal dust in air is of the order of 35 mg. per liter, or about 0.035 ounce per cubic foot. Since a dust explosion can be produced only by dust that is dispersed in the atmosphere, the dust in the air of a n industrial plant, or in a mine that is kept safe from the standpoint of health, cannot start or of itself propagate a dust explosion. I n fact most such explosions originate either in the vicinity of a n extremely dusty operation or as a result of an unusual disturbance that suddenly disperses into the air a large quantity of dust from exposed surfaces. NATURE AND IMPORTANCE OF DUST EXPLOSIOXS
Practically every combustible solid material in finely divided form can produce a dust explosion if it is dispersed into air and ignited. The conditions necessary for an explosion are a sufficiently dense dust cloud in a n atmosphere that will support its combustion and, simultaneously, a source of ignition that will heat a portion of the cloud t o the ignition temperature. Under
April 1948
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
favorable circumstances the initial ignition will produce heat more rapidly than it is dissipated to the surroundings, successively adjacent portions of the dust cloud are ignited, and flame propagates throughout the mixture. The explosion is characterized by the rapid development of pressure, which frequently causes destruction of plant and equipment. The pressure is caused by the thermal expansion of nitrogen and other gases in the explosion space due t o the heat of combustion and in some dust explosions also by the generation of gaseous reaction products. The phenomenon is analogous to the explosion of gas-air mixtures, but owing to the much smaller area of contact surface of the reacting components-Le., dust and air-the rate of propagation is generally slower than in gas explosions. However, in spite of this and the attendant greater heat losses, the pressurq produced by many dust explosions are of the same order of magnitude as those produced by gas explosions. This results from the fact that the heat of combustion of the dust contained in a given enclosure in combining proportions with the air is greater than the heat of combustion of a stoichiometric mixture of most gases with air in a similar volume. I n the United States a potential dust explosion hazard exists in more than 28,000 industrial plants, which emplojr over 1,000,000 people and have a n annual production of over $10,000,000,000. The industries include grain elevators, flour mills, wood-working plants, feed and cereal mills, starch and corn product plants, dehydrated-food plants, fertilizer plants, cotton mills, paper mills, rubber factories, sugar refineries, pitch and rosin plants, spice and coffee plants, drug and chemical plants, pulverized coal installations, malt houses, plastics plants, and factories producing many miscellaneous organic and inorganic powders and dusts. Statistics of the National Fire Protection Association show that up to April 1, 1946, 888 dust explosions occurred; 575 persons were killed and $80,000,000 property damage resulted. These do not include dust explosions in mines and in military arsenals, nor many unreported small explosions. DUST-EXPLOSIBILITY FACTORS
The factors that affect the ease of ignition of dust clouds and the violence of the resulting explosions are many and involved. As very little is known at present of the basic mechanism of ignition of dust particles and of the mode of propagation of the explosion waves and the flame, the true significance of all the influencing factors is not fully understood. Studies of this fundamental problem are being undertaken. For many years investigators of coal mine explosions believed that mixtures of coal dust and air could be exploded only in the presence of methane or other inflammable mine gases. This view was proved t o be erroneous. Furthermore, many people believe t h a t the first stage in the ignition of dust particles is the ignition of vapors evolved from the solid. This might be the case in the ignition of some organic dusts t h a t decompose by spark or heat and liberate inflammable gases or vapors, and possibly for a few inorganic dusts, but it certainly is less true for many metal powders that have very low vapor pressures a t the ignition temperatures. The explosibility of coal dust is closely related to its volatile combustible content and it is probable t h a t initially these gases are ignited. The ease of ignition of dust particles in suspension depends among other things on the chemical composition of the dust, the shape, size, and surface structure of the particles, the ignition temperature, the energy required for ignition, and the initial temperature and pressure in the explosion space. Some of these factors are interrelated. The destructive effects of a dust explosion are largely determined by the maximum pressure i t produces, the average and maximum rates of pressure rise, the speed and extent of flame travel, the speed of the shock wave preceding the flame, the
753
duration of increasedopressure, and in some' cases the negative pressure resulting following the explosion. These are in turn influenced principally by the following factors:
-
Chemical and Physical Properties of the Dust. These include the composition, heat of combustion, rate of oxidation, oxygen required for complete burning, heat, fineness, and shape - specific . and structure of particles. Concentration and Uniformitv of Distribution of Dust Cloud. For an explosion to be develope& by a dust-air mixture, the dust concentration must be above a certain lower explosive limit and below a n ill-defined upper explosive limit. Properties of the Atmosphere in the Explosion Space. Important properties are the oxygen content, inflammable vapor and gas content, humidity, temperature, pressure, specific heat, and heat conductivity. For most dusts there exists a critical oxygen limit below which they will not propagate an explosion. A few exce tions are referred to below. Ignition Source. fndustrial dust explosions have been initiated by electric sparks and arcs in equipment and wiring, by static electric discharges, by frictional or metallic sparks, by open lights, by overheated machinery and other hot surfaces, by glowing particles, by spontaneous ignitions of dust, by chemical reactions between constituents of dust mixtures, by disturbance of burning dust layers, by application of streams of water, and by other causes. As a rule a n extensive flame or hot surface is more hazardous than a point source similar to a short rapid electric spark. Characteristics of Explosion Space. The size, shape, and construction of the enclosure in which a dust explosion occurs have a n important bearing on the damage that may be caused. The presence of pressure release vents in the structure is of special importance. Other Characteristics T h a t Are Known or Appear to Influence the Explosibility of Dusts. These include the ease of dispersion, which is in turn partly determined by the particle size and shape, the specific gravity, and the apparent density; the moistureabsorbing tendency of the dust; the accumulation of electrical charge on the dust particles; and the adsorption of oxygen and/or other gases by the dust. EVALUATION OF EXPLOSIBILITY OF DUSTS
The experimental work on explosibility of industrial dusts is being performed in special laboratory-scale apparatus and in somewhat larger explosion galleries. It consists of evaluation of the explosibility of many types of dusts and of various means for preventing ignitions and for reducing the severity of explosions. The laboratory tests normally performed include determinations of the following: 1. Particle size distribution, usually by sieving. 2. Moisture content. 3. Ignition temperature of dust cloud and of undispersed dust layer. 4. Relative inflammability of dust cloud-Le., percentage of a n inert dust (generally fuller's earth) required in admixture with the combustible dust to prevent ignition by a n electric spark and by a hot surface a t 700" C. 5. Minimum energy required for ignition of dust cloud (and of dust layer) by electric spark from condenser discharge. 6. Lower explosive limit. 7. Maximum pressures and rates of pressure rise developed by explosions of dust clouds of various concentrations. 8. Limiting percentage of oxygen in atmosphere containing dust cloud, below which dust cannot be ignited by electric sparks and by hot surfaces.
I n addition t o the above tests, for some samples submitted by industry studies are made t o determine sensitivity t o direct impact and frictional impact, ignition by frictional sparks, effects of humidity on explosibility, etc. The experimental data for many metal powders and for resins, molding compounds, primary ingredients, and fillers used in the plastics industry were published in two papers (1,5), which contain also a description of the apparatus and procedure. Since that time information has been obtained for a great number of other dust samples. Data on 17 recently tested dusts found to be highly explosive are given in Table I.
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Jaeckel (4) for thermal ignition of dust clouds in a Rates of space of constant volume. Pressure Rise, Limiting Oxygen Lb.i'Sq. In./ Percentage t o On one side of this space Second Prevent Ignition is assumed a plane hot surAver- Maxiof Dust Cloud b y age mum Electric Sparks face or similar initiating b 1450 5000 source of great area; heat b 4400 4750 2150 5700 flow occurs only in a di3 b 750 1650 rection at right angle to this 1250 3000 14 1350 3150 14 surface. The explosion of 400 1260 15 the dust cloud starts by 1760 3850 .. heating and ignition of the 1350 2350 .. dust particles in the air 2180 3150 .. layer parallel with and near850 3350 15 est to the hot surface. The 350 800 16 heat produced by this com700 1950 11 600 1450 16 bustion in turn heats and ignites the next dust-air 1050 2130 .. 660 1300 .. layer, and in this manner 750 2100 the explosion propagates throughout the dust cloud. In a lower limit mixture there are just enough particles in each parallel layer to produce the heat (of combustion) required to bring the adjacent layer to the ignition temperature and to take care of heat lost by radiation and , conduction to the surroundings. Neglecting heat losses, the heat of combustion generated in a unit volume of enclosure must a t least equal the heat required to raise the temperature of the dust in a like volume t o the ignition point and to heat the air to the same temperature or:
TABLE I. DUSTEXPLOSIOF CIWRACTERISTICS OF VARIOCS POWDERS
T y p e of Powder Zirconium Magnesium Aluminum Titanium Rosin Phenolic resin Polyethylene Allyl alcohol resin Cellulose propionate p-Oxybenaaldehyde Hard rubber, crude Coal Sulfur Phenothiazine Cornstarch, modified Soap Aluminum stearate
Ignition Temperature of Dust Cloud, 520 645 480 390 500 450
10 10 80
Minimum Explosive Concentration, Ounce/Cu. Ft. 0.040 0.020 0.025 0,046 0.015 0.025 0,025
500
20
0.035
68
c.
Minimum Spark Energy Required t o Ignite Dust Cloud, Miiiijouies 15 EO 50
..
Maximum Explosion Pressure, Lb./Sq. In. 50 72 89 52 56 61
83
460
60
0.025
66
430
15
0.020
58
350 610 190 540
50 40 15
0.025 0.035 0,035 0.015
57 46 41 43
40
60
0,048 0.045
72 60
15
0.015
62
470 430 400
'
I n examining the data in Table I it should be remembered that, 'because of variations in chemical and physical properties, probably no two samples from different lots or sources have exactly the same explosive characteristics. The data listed apply to the most hazardous sample tested for each type of powder. Of all the dusts investigated so far, zirconium powder was found the most easily ignitible, not only in suspension in air, but also in settled layers. Undispersed layers of this material could be ignited by an electric spark of less than 0.001 millijoule (0.001 milliwatt-second) energy. This powder is never stored or shipped in the dry state and only small amounts are dried shortly bcfore use. Dust explosions of magnesium and aluminum powders are generally very severe because they produce high pressures a t very high rates. Undispersed layers of one magnesium powder mere ignited by a spark of 0.24-millijoule energy. All other dusts listed (except zirconium) require stronger igniting sparks. Electrostatic spark energies of the order of 10 millijoules might be built up and discharged from a human body under some conditions. This indicates the necessity for preventing the accumulation of electric charge on workmen, as well as on equipment, in plants where zirconium, magnesium, and other highly inflammable powders are processed. RESEARCH ON EXPLOSIBILITY FACTORS
Much theoretical and experimental work remains to be done to evaluate the relative importance of the many factors that pertain to the explosion hazard of dust clouds. RIany experiments have already been made in this direction and some of the results are described briefly below. LOWEREXPLOSIVE LIMIT. The type of ignitioii source that initiates a dust explosion affects the value of the lower explosive limit. For example, it LYas determined in recent tests that whereas no explosions could be initiated in dispersions of an atomized aluminum powder in air by a continuous electric spark (weak arc) below a concentration of 0.05 ounce per cubic foot, explosions could be started a t 0.025 ounce per cubic foot by the rapid timed flame of a small quantity of guncotton. Similarly, in tests with the above igniting sources the lower explosive limits for dispersions of finely pulverized coal dust were determined to be 0.035 and 0.005 ounce per cubic foot, respectively. It is of interest to compare the above experimental lower limits with values computcd from a theoretical expression derived by
py = p ( T -
t)Ci
+ (T - t k c ,
( T - t)sc1
p = y - (2' - t)ca
rvhere p = lower explosive limit of dust concentration q = heat of combustion of unit weight of dust T = ignition temperature of dust t = initial temperature of dust cloud s = density of air a t initial temperature and pressure = specific heat of air at constant volume c2 = specific heat of dust The expression can be regarded only as an approximation for an idealized condition. I t oversimplifies the mechanism of heat transfer, does not consider possible dissociation, and takes no account of particle-size effects. The lower explosive limits for aluminum powder and for coal dust computed from the above expression are 0.019 and 0.017
LEGEND
Spark ignition oGuncotton ignition
5? 600 E5
I
I
i u '
400
f2 e
5
8
200
I
0
2 RAT
4 6 C 0 Ci RE-,EF AREA TO V O L d V E SQ FT PE9 100 C.' ,,hRLSTR
C-,
CTED VEIITS)
Figure 1. Pressures Developed by Dust Explosions Initiated by Guncotton and Spark Ignition
INDUSTRIAL AND ENGINEERING CHEMISTRY
il 1948
155
ce per cubic foot, compared wi’th the experimental minima of !5 and 0.035.
aeckel showed further that, when the ignition source apaches a point source within the dust cloud, the lower explosive it is very much higher than that given by the foregoing exssion. ;UNCOTTON FLAME.Under some conditions explosions inited by guncotton flame produced higher pressures than those {eloped by explosions of identical dust dispersions ignited by electric spark. This is illustrated in Figure 1 for two series tests with coal dust and cornstarch. The tests were made in a -cubic-foot gallery provided with relief vents of adjustable es through which the explosion pressure could be relieved iickly. One reason for these effects appears to be the fact that the ncotton flame is in contact with and initially heats to the nition temperature a larger volume of the dust-air mixture an is ignited by the spark, and therefore the explosion propaites more rapidly and with smaller proportional heat loss to the irroundings. It was also found that the ignition source has an important :wing on the limiting percentage of oxygen in the atmosphere mtaining a dust cloud at which ignition can be prevented. or ex ,mple, coal-dust clouds in air-carbon dioxide mixtures snnot be exploded by electric sparks if the oxygen content is less han about IS’%, whereas with a hot surface (lOOOo C.) igniting ource the oxygen content must be reduced below 10% to prevent ropagation. RELATIVE INFLAMMABILITIES OF DUSTS. Another illustration )f the effect of the initiating source on the severity of a dust bxplosion is given in Table 11,which records the relative inflammaiilities for several dusts a t various temperatures of a given initiatng source. The term “relative inflammability” is used to designate the necessary percentage of inert dust admixed with the inflammable dust in order t o arrest the flame. This value is important in coal mining practice, because it gives the amount of rock dust, generally limestone dust, that should be applied on the floor, roof, and ribs of mine passageways to prevent propagation of a coal-dust explosion. I n the experiments in question the dust mixtures were dispersed downward through a cylindrical electrically heated furnace with wall temperatures indicated in the table. The values of relative inflammability denote the percentages of inert dust (in the experiments fuller’s earth wab used) in the mixtures when no flame emerged from the bottom mouth of the furnace. DUSTCONCENTRATION. The effect of dust concentration on the pressures and rates of pressure rise produced by dust explosions was studied for many samples. Theoretically it might be expected that the strongest explosions would be produced a t a concentration when there is just sufficient dust in suspension in an enclosure to consume all the available oxygen, if the combustion involves only oxidation. This stoichiometric concentration can be computed if the chemical composition of the duet
80
z
s: 5 m,
60
w 3
D Y
40
I
a
9 I
20 I
I
l
l
I
1
1
I
1
a 04
08 12 16 DUST CONCENTRATION, OZ. PER CUBIC FOOT
0
2.0
Figure 2. Maximum Pressure and Rates of Pressure Rise Developed by Explosions of Fine Magnesium Powder
and of the atmosphere is known and if complete combustion is assumed. Actually in most experiments the maximum explosion pressures and rates of pressure rise were produced by dust clouds of somewhat higher concentrations than those corresponding t o the stoichiometric values. This is illustrated for a magnesium powder in Figure 2. The reasons for the variation might be imperfect dust distribution in the cloud, incomplete combustion, and some magnesium nitride formation. Figure 2 also indicates that a t high dust concentrations the curves approach the zeropressure line slowly, and therefore it is most difficult to determine experimentally the values of upper explosive limits for dust clouds. With increase in dust concentration beyond an optimum value, the combustion becomes more incomplete and the explosions decrease in force, but there is no definite limit at which propagation ceases. COMPOSITION OF ATMOSPHERE.The composition of the atmosphere, particularly its oxygen. content, has profodhd effects on the initiation and development of dust explosions. A
.
TABLE 11. EFFECTOF INITIATING SOURCD Furnace Temperature,
c.
315 350 400 500 570 600 610 650 700 750
Relative Inflammability, Per Cent Iron, Magnesium, Zinc Pittsburgh Hz-reduced milled powder coal dust
no
a Zero values correspond t o ignition temperatures of dust clouds of pure substances.
0
20
40
60
80
100
OXYGEN CONTENT OF ATMOSPHERE, PERCENT
Figure 3. Effect of Oxygen Content of Atmosphere on Ignition Temperature of Dust Clouds a€ -200-Mesh Pittsburgh Coal
756
Vol. 40, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY 0.5
Y
.4
.A
4 >E 3 Y z
3 I
.2
I 1
0
20
40
OXYGEN CONTENT
OXYGEN CONTENT OF GAS, PERCENT
Figure 4. Inflammability of Powders i n Mixtures of Air and Carbon Dioxide
complete explanation of these effects cannot be given until a better knowledge of the mechanism of dust explosions is acquired. Experiments have shobyn that as the oxygen content of the atmosphere is reduced the ignition temperature of dust clouds is raised. This is illustrated for fine Pittsburgh coal dust in Figure 3. The experiments were performed a t atmospheric pressure in oxygen, in air, and in mixtures of aii and carbon dioxide. Other experiments have shown that reduction of the oxygen content by dilution of air n i t h carbon dioxide or nitFogen reduces the relative inflammability of dust clouds, as illustrated in Figure 4. Figure 5 illustrates the increase in the electrical spark energy required for ignition of atomized aluminum dust clouds in atmospheres of decreasing oxygen content. Figure 6 shows the relation of the minimum explosive concentration of the same aluminum powder to the oxygen content of the atmosphere. The difference in the lower explosion limits of the dust in air (20.9y0 oxygen) and in pure oxygen is small. This has also been found true for explosive gas mixtures. One reason is that both gases contain far more oxygen than is needed for the complete oxidation of the lower limi@mixture and the rate of reaction in the two gases is apparently not very different. It was also found that the violence of explosions produced by dust clouds above the lower limit concentration is reduced with decreasing oxygen content. This is shown for ethylcellulose explosions in Figure 7 . An important cause of this variation is probably the reduction in the rate of diffusion of oxygen toward the surface of the burning dust particles with decreasing partial pressure of oxygen. I n atmospheres containing less oxygen than corresponds t o the stoichiometric mixture (approximately 15% oxygen for this dust cloud), reduction in the oxygen content results in the combustion of a smaller amount of dust and therefore in a reduction of heat generated by the explosion. FREOK.Dust clouds of magnesium, zirconium, titanium, and some magnesium-aluminum alloy powders can be ignited by electric sparks in a n atmosphere of carbon dioxide. Layers of these and some other metal powders burn or react with carbon dioxide and with nitrogen when heated a t elevated temperatures for several minutes. Recently the question arose whether Freon gas, normally considered inert, which is a good extinguisher for gasoline fires, might also be useful for abating magnesium fires. To answer the question, dust clouds of magnesium powder were dispersed in an atmosphere of Freon-12 in the presence of an electric spa1k. The trials resulted in violent explosions. The same type of reaction, but not so strong, was obtained with dust clouds of stamped aluminum powder in Freon. The dust clouds could also be exploded by a heated surface. When layers of the
OF
60 80 100 ATMOSPHERE, PERCENT
Figure 5 . Effect of Oxygen Content of Atmosphere on Minimum Energy Required t o Ignite Atomized Aluminum Dust Clouds by Electric Sparks
poTyders mere heated in Freon, the magnesium powder ignited at 410" C. and the aluminum a t 580" C. X-ray diffraction analysis of the residues showed that the reaction products were fluorides and chlorides of the metals, and for aluminum also a trace of carbide. I n comparison, ignition temperatures of layers of the same magnesium powder in air and in carbon dioxide were determined to be 490' and 570" C., respectively. These data indicate the risks involved in a przori generalizations for new powders and new situations at this stage of our knowledge of the mechanism of these reactions. SHAPE,SURFACESTRUCTURE, and SCRFACEAREA. The effects of shape, surface structure, and surface area of dust particles on their explosibility were studied for iron, aluminum, and magnesium powders. The surface area is important because combustion occurs a t the surface and the total exposed area of the particles affects the rate of the reaction. In Table I11 data are given on minimum energy requirements for ignition of dust layers and of dust clouds, and pressures and rates of pressure rise developed by explosions of a n atomized magnesium powder of approximately spherical shape, a milled powder of angular shape, and a stamped powder of thin flat shape. The sieve
0 28 u. I-
2
24
DI LL
0 Y
z 20 3 0 2
0 c c
16
I-
z 0 W
8
12
W
E: k m
.08
W
2 04 I
0
20
40
OXYGEN CONTENT
60
80
100
OF ATMOSPHERE, PERCENT
Figure 6. Effect of Oxygen Content of Atmosphere on Minimum Explosive Concentration of Atomized Aluminum Dust Clouds
~
I N D u s T R I A L A N D,' E N G I N E E R I N G
April 1948
,
I
c H E M I sTR Y
757
I 1,800
REQUIREMENTS FOR IGNITION TABLEf 11. MINIMUMENERGY Spark Energy
I
I
!
, I\ I , \
Magnesium Powder Atomized Milled Stamped
Dust cloud Concentration 0.100 01. pcr cu fl.
I
I
W\l I
!
1:. 3
600
2e b. 0
w
400
200
0 2
Figure plosion Rise of Powder
20
18 16 14 12 OXYGEN CONTENT OF GAS, PERCENT
0
7. Variation of Maximum ExPressure and Rate of Pressure -200-Mesh Ethylcellulose Molding (M-C-3) with Oxygen Content of Atmosphere
analyses of the three samples were approximately the same. As can be seen, the stamped powder, which has the largest surface area per particle and per unit weight, required the least energy for ignition and produced the highest pressures and highest rates of pressure rise, and the atomized powder with the least surface area required the strongest igniting sparks and produced the weakest explosions. PARTICLE SIZE. The cxplosion hazard of dust clouds increases with decrcase in particle size. There may be several rcasons for this. Smaller particles arc dispersed more readily, remain in suspension longer, and burn mote rapidly. Experiments havc shown that as the particle size decreases, generally the energy required for ignition is reduced, as illustrated in Figure 8; that the ignition temperature, as determined by short conTYLER SCREEN SCALE, MESH NUMBERS m 3 I 2 588 tact with a hot surface, decreases, as can be seen in Figure 9; that the lower explosive limit decreases, as shown in Figure 10; and that the maximum pressures and rates of pressure rise increase, as indicated in Figure 11. The chief reason for the latter effects is the increase in the rate of the reaction with decreasing size of particles. However, as the AVERAGE PARTICLE DIAMETER, INCH particle size beFigure 8. Effect o€ Fineness on comes very small, Minimum Energy Required for u ltimately the lgnition of Dust Clouds of Cellumaximum pressure lose Acetate Molding Powder (M-C-1) by Electric Sparks is determined alLD
dust layer
0.05 0.008 0.00024
dust cloud 0.24 0.08
0.08
Lb./Sq. Inch
dverage
57 62 72
750 1300 1450
1450 2600 4800
most solely by t,he quantity of dust burned, and a t a given dust concentration further decrease in particle size will not cause a rise in pressure; this is apparent in Figure 11. The data plotted in the foregoing figures are based on tests of definite separated size fractions of given dusts. Tests made with total aggregates of chemically similar atomized aluminum and with magnesium powders of different particle size distribution have also shown t h a t the explosibility inoreases with increase in the proportion of fines. For example, in tests of two identical aluminum powders, one of which contained 5% of -325-mesh particles, whereas the second did not have this fraction, the maximuin explosion pressures produced were, respectively, 54 and 49 pounds per square inch and the maximum rates of pressure rise were 2700 and 1900 pounds per square inch per second. TYLER SCREEN SCALE, MESH NUMBERS
700
u w'
% 600 c < w
i
5 500 + z -
0
400
300 0020
0016 0012 0008 0004 AVERAGE PARTICLE DIAMETER, INCH
0
Figure 9. Effect of Fineness on Ignitioh Temperature of Dust Clouds
An exception to this general behavior was noted many years ago by investigators at the Bureau of Mines, who studied the burning characteristics of closely sized pulverized-coal dusts. They observed that particles 0 to 10 and 10 to 15 microns in diameter produced lower explosion pressures than slightly larger particles. Three possible reasons were given for this: (1) I t may be the property of coal that when pulverized the very fine dust has EL somewhat different chemical composition than the coarser dust, because of a greater friability of some constituents than of others; (2) during the size separation by elutriation in air the finest particles may have oxidized more; (3) the very fine particles agglomerate or form groups of particles that resist dispersion into a cloud. REDUCTION OF EXPLOSION HAZARD
Based on practical experiences of safety engineers and on experimental studies, a number of codes for the prevention of dust explosions in various industries have been prepared by committees of the National Fire Protection Association of Boston, Mass. (5). The recommendations cover safety requirements in
758
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 4
TYLER SCREEN SCALE, MESH NUMBERS
R a t i o of Kelirf Vent Area t o Volunie, Sq. Ft./100 c u . Kt.
1
I
'0
I 0
I
0016
0012 0008 0004 AVERAGE PARilCLE DIAMETER, INCH
F i g u r e 10. Effect of Fineness on Minim u m Explosive C o n c e n t r a t i o n of D u s t Clouds construction of the plants and equipment, and in manufacturing proceduros. I m p o r t h t safeguards include elimination of all ignit,ion sourccs near dusty processes, reducing the product.ion of very fine dust as much as possible, use of inert gas atmospheres whcrc practicablc, and good houselicrping t,o prevent disscmination of dust outside of equipment. The latter poiiit is vital, bocausc in many disasters the greatest daniage resulted from the secondary explosion of the sett,lcd dust t,hat was thrown into suspwsion by an initial minor explosion. A preventive 'measure of greatest imgortancc against structural damage by dust explosions is the provision of adequate rapidly operat,ing relief vents through which the pressure of an incipient explosion can be relieved quickly. The vents can be simply unrestricted or free openings; hinged or pivoted sash, which swing outward a t a low internal pressure; fixed sash with light wall anchorages; scored glass panes; light wall pancls; monitors or skylights; poppet-type vent closures; paper, metal foil, or othcr diaphragms t,hat burst at lorn pressures; pull-out diaphragms, or other similar arrangements. An invest,igat,ion of this subject, is currently in progress a t the I3ureau of hIinos. l'rcliminary data have been published ( 2 ) on the effectiveness TYLER SCREEN SCALE, MESH NUMBERS
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cluuti cvncentralion 0 500 oz per cu. 11.
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3laximuin Explosion Pressure, Lb./Sq. Inoli
Average R a t e of
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Tot.al I ~ I I I J L I ~ ~ C ,
(Lb./Sq. 111o11) X Scconds
of unrestricted vcnts lor pressure .rcl(:as.c from a 64-cubic-foot gall(~r,vin explosions of coal dust, wood flour, soybean prot,c:in, curiistixcli, phenolic resin, inagncsiurn, and atoniizcd and stainpcd aluminum powders. Since that time 7 00 the study has bcen continued in the 600 ssinc gallory arid in two othcrs of c 0 500 1- arid 2lG-cul~icw foot volun1cs. The w relations between 5 SCQ u ' increase in t,hc vent 3 Ln area, or more 2a 3CO properly in thc 3 ratio of the vcnt z area to the volume 5 2c0 a of the enclosure in whicli an ex100 plosion occurs and t,hcresultant roductiori of tlic maxi0 niuni pressure, tho SQ F i . PER 100 CU. F i . rat,c of flamo propaF i g u r e 12. Relative Effectiveness gation, as irldicatctl o f Unrestricted Openings a n d IPcavy Paper D i a p h r a g m s in Heby the rate of lieving Pressures f r o m Coal-Dust pressure rise, and the total impulse Explosions I n i t i a t e d b y Electric Spark (pre s s u r c' t i m e s lime) of t,hc cxp'losioiis h a m bcen established. This is, shom-11 for a series of magnesium powder explosions in Table IV. It,was determined t.hat tlie vent area needed for pressure relcaso of rapid explosions, as of magnesium and aluminum p o ~ d e r s is , considerably grcater than is required for slower reactions, as coal-dust, or wood-flour explosions. The type of closurc on the venis n'as found to have an important effect on the area necessary to provide a desired pressure release. This is illustrated in Figure 12 for two series of coal-dust explosion?. As can be seen, for a given vent ratio a lower maximum pressure is produced by tlie explosion in the gallery when the vent is free or unrestricted than wheii it is sealed with a thick pkper diaphragm. CL
T
LITERATURE CITED
1 2
(1) Eartmann, Irving, and Nagy, J o h n , U. S. Bur. Mines, Eept. Investiuations 3751 (1944). (2) I b i d . . 3924 (1946). (3) Hartrnann, Irving, Nagy, John, and Brown, H. R., Ibid., 3722 (1943). (4) Jaeckel, George, 2 . tech. P h y s i k , 5, 67-78 (1924). (5) Natl. Fire Protectiou Assoc., "Natl. Fire Codes for Proventioil of Dust, Explosions," 1646.
F i g u r e 11. Variation of M a x i m u m Explosion F'rese u r c a n d H a t e of P r e s s u r e Rise w i t h Fineness o€ Cellulose Acetate Molding Powder
Rii,csrvsn Rlarch 27, 1947. Presented hPfore t h e Diviqion of Industrial and Engineering Clicmistry a t t h e 111th Rlceting of the h M E n 1 c A N ( ~ f r l a \ l I c A r . A o c I ~ r Y ,Atlantic City, N . J. Publi3hed by permission of the Direotor, U. S.Bureau of Mines.
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012 008 004 AVERAGE PARTICLE DIAMETER, INCH
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