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Appendix B
DUST EXPLOSIONS
1
EXPLOSIBILITY OF GRAIN DUST
The dusts generated in grain processing are composed of moisture,-starab,
protein, fat, and ash. The chemical elements are typical of all natural
organic substances derived from plants (i.e., carbon, hydrogen, nitrogen, and
oxygen) plus traces of various minerals. The grain dusts collected in dust
control systems are 60 to 80 percent combustible (the balance is ash and
moisture) and have mean diameters of frown 17 to 120 ~m. The heat of caution
of such dusts ranges from 12,000 to 20,000 J/g (for comparison, a paraffin
hydrocarbon, CnH2n+2 · releases at least 4 S. ,000 J/g) .
When grain dust is suspended in air at a concentration of about lSO g/m3,
the mixture composition is correct for complete combustion of the dust
{i.e., the mixture is stoichiometric). However, in actual dust explosion
tests, it is found that the energy release and rate of energy release is a
maximum at concentrations well above the stoichiometric concentration
(usually at least 3 to 4 times that concentration). Additionally, dust
explosions, in contrast to gaseous explosions, show a rather flat response in
terms of maximum pressure and rate of pressure rise versus concentration.
Thus, for a dust-air mixture there is a large range of concentrations from
approximately stoichicmetric (150 g/m3) to approximately 10 times
stoichiametric (lSOO g/m3) over which the pressure rise in an enclosure is
approximately independent of dust concentration. Under these circumstances,
an explosion of a dust-air mixture in the enclosure if not relieved by
rupture of the enclosure will generate a pressure rise of approximately 6 to
10 a t:m, which is the same as that generated by a vapor or gaseous fuel
explosion (Bartknecht 1981) . Thus, once a system contains the minimum amount
of dust needed to sustain an explosion, the basic explosion hazard potential
has been established; additional amounts of dust increase the damage
potential by only a minor amount and even large excesses do not reduce it
significantly. The measurable properties pertinent to an understanding of
the ignitability and explosibility of a particular dust are the following:
~ 1 . = ~ ~ We Clou d--!rhe
apparatus typically used to measure this property is the Godbert-Greenwald
furnace (Godbert 1952, Godbert and Greenwald 1936~. A sample of dust in a
dust holder is blown downward through a vertical furnace of specified
dimensions and construction that has been heated to she initial temperature.
If a sheet of flame appears at the exit of the furnace, the furnace
temperature is said to be above the auto-ignition temperature of the dust
l
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cloud. If no flame appears, the furnace temperature is assumed to be below
the auto-ignition temperature. The experiment is performed over a range of
dust concentrations by varying the.amount of dust in the dust holder. The .
minimum value of the auto-ignition temperature and the concentration at which
it occurs.are then recorded.
2. Layer Ignition Temperature--A standard experiment has been devised
(National Materials Advisory Board 1982) to measure the surface temperature at
which a standard thickness of a dust will ignite within.a specified time
{usually 30 min). In this experiment, a layer of dust of a specified
thickness is placed on a thermostated heated metal surface. The ignition
temperature is determined by noting the surface temperature at which a glow or
flame appears or a thermocouple placed in the dust sample reads a.specified
higher temperature (usually 50°C) than a thermocouple imbedded in the hot
surface.
3. .
and Flammability Limits--In the test for maximum pressure rise and maximum
rate of pressure rise, a dust sample is dispersed into a vessel by a.burst of
high-pressure air. After some delay time the dust is ignited with a
high-energy electrical spark and the pressure-time curve of the explosion
process is recorded. Two types of vessel have been used for these
Measurements. One is the Hartmann test apparatus.that consists of a small
(75 inch vertical tube approximately 1 ft long. The other consists.of a
spherical vessel with central ignition. Bartknecht (1981) has used different
size spherical vessels and he has found that for a sufficiently large vessel
. .
(greater than 20 liter capacity) one can define a constant:
. kSt = (dP/dt~maX V1/3,
based on the maximum rate of pressure rise, (dp/dt~maX, and the vessel
volume, V. The constant, kSt, is unique for each dust type. It essentially
g Ives an indication of the combustion rate of that dust and there fore the
relative damage due to an explosion associated with that dust. Bartknecht
points out that f or vessel volumes less than approximately 20 1 iter, kSt
ceases to be a constant and becomes smaller as the vessel size decreases.
Thus, the rate of pressure rise observed in small vessels is less than the
potential rate of pressure rise that one would observe in a large vessel (it
is probable that radiative losses to the wall considerably lower the
combustion rate in the smaller vessel) . In the test for explosibility limits,
a bomb is used and the minimum dust concentration that can be.ignited by a
high~energy spark is determined. The LEI,s for coslunon grain dusts are shown in
Table B-1.
4. Ignition EnergY--The ignition energy of a combustible mixture
usually is measured by determining the amount of energy required in a
capacitance spark to just cause ignition of the combustible mixture. This
also is true in the case of dust ignition. Eckhoff (197 S) has concluded that
the ignit- on energies of dust are different from those of vapors and gases in
1
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1}7
TABLE B-1 Employ ire Properties of Colon Grain Dusts .
Maximum
Rate of Hinir~m Lower
Maximum Pressure Ignition Temperature Ignition Explosive
Pressure Rive Cloud Layer Energy Limit
Type of Dust (kPA) (M]?a/s ~ ~ °C) ~ ° ~ ~ J ~ (g/m3
Alfalfa meal 455 7.6 460 200 0.32 100
Cereal grass 360 3.5 550 220 0.80 200
Corn 655 41 400 250 0.04 5S
Corncob grit 760 21 450 240 0.045 45
Corn dextrin pure 725 48 400 370 0.04 40
Cor ns ta r ch cause rc ia l
product 745 48 380 330 0.04 45
Corns tarch
through 325 mesh 790 62 390 350 0.03 40
Flax strive 560 5.5 430 230 0.08 80
Grain dust, winter
wheat, corn, oats 790 38 430 230 0.03 55
Grass seed, blue 165 1.4 490 180 0.26 290
Rice 640 18 440 220 0.05 SO
Rice bran 4 20 9 .0 4 90 --- 0 .08 4 5
Safflower meal 580 20 460 210 0.025 55
Soy flour 540 5.5 540 190 0.10 60
Soy protein 660 65 520 260 0.05 35
Wheat, untreated 710 25 500 220 0.06 65
Wheat flour 655 26 380 360 0.05 50
Wheat starch, edible 690 45 420
0 .025- 45
Wheat straw 680 41 470 220 0.050 55
SOURCE: U.S. Bureau of Mines 1961.
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that the ignition process is dependent on details of the spark ~hape.and
discharge circuitry. The ignition energies of dusts also have been found to .
be considerably larger than those of typical vapors or. gases. However, the
panel during.its explosion investigations noted that typical ignition sources
in grain elevators exceed these values by several orders of magnitude.
The U.S. Bureau of Mines (1961) performed extensive experiments in the
1960s and measured many of these properties for a large number of dusts
including many common agricultural dusts (Table B-1~. It devised an '
explosibility index based on these measurements that is used to rank dusts in
terms of their explosion hazard. Unfortunately, this index wan based on
Hartmann bomb measurements and therefore tends to under-rank the most
dangerous dusts including certain agricultural dusts. Bartknecht (1981) has
taken a different approach. He measures kit and then segregates all dusts
into three categories. His approach, which is based only on the rate of
pressure rise in a spherical vessel, can be'used to determine proper vent
areas-for dust explosions (National Fire Protection Association 19787.
EXPLOSION DYNAMICS
Combustion explosions of the type that occur in grain elevators take
place because a combustible mixture of grain dust (fuel) and air. (oxidizer)
exists inside an enclosure (e.g., an elevator leg, a garner bin, or even the
building itself) (Strehlow 19801. As mentioned.above, a mixture of grain
dust and air can be combustible if the dust concentration exceeds some .
minimum value. Combustion of the explosive mixture in an enclosure will '
release sufficient heat to produce a pressure rise of approximately 6 to 10 atm
if the enclosure is strong enough to contain the.mixture during the entire
combustion process.
For combustion to occur, an ignition source is-necessary. Thin can take
many forms (Table B-23. It is important to note, however, that in grain
elevator explosions, the ignition source almost invariably is a ~soft. one
that has an energy considerably above the minimum ignition energy of the
dust-air. mixture. This means that the ignition source initially causes a
low-velocity flame to propagate away from the source region. The subsequent
behavior of the flame and the explosion process that occurs after such soft
ignition is very strongly.dependent on the geometry of the enclosure and the
location and size of the various pieces of equipment, etc., that are located
inside the enclosure. ' .
Two different flame and explosion process ' behaviors have.been observed
and most grain elevator explosions fall somewhere between the two extremes
(Bartknecht 1981)'. First, if the vessel has a very small length to diameter
{L/D) ratio (i.e., is almost spherical in shape), the flame will not
accelerate to high velocity and the pressure will rise rather slowly in the
enclosure (in large vessels the pressure rise may take as long as 10 s).
Under these circumstances,.all the walls of the vessel will be.pressurized
uniformly and, if of relatively equal strength, will fall away at about the
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TABLE B-2 Probable Ignition Sources
source No. of Facilities
USD a
Un known
Welding
E1 ectrical failure
Tramp metal
Fire other than welding
Other foreign objects
Friction, choked leg
Overheated bearings
identi f fed ape rk
Friction sparks
Li ghtning
Extension cords in legs
Faulty motors
Static electricity
Fire from slipping leg belt
Elaounable vapor
Smoldering grain
Smoking material
Fi re cracker
Voltatile chemical from soybean processing
External cob pile fire
Heating system
Gas in bin ignited
Extinguishing fire
Leak in gas pipe ignited
Electrical control panel explosion
S1 ipping conveyor belt
EMRI T Datab
Un known
Welding
Fr i ction i n e levator 1 eg
Fire other than welding
E1 ectri Cal
Lightning
M\ otors
Spontaneous combustion
Other fore ign objects
Static electricity
NOTE: Data probably are not mutually exclusive.
a U. S. Department of Agriculture 1980.
b Ve rkade and Chiotti 1976.
1
103
43
10
10
10
9
8
7
7
7
6
4
4
3
3
3
2
2
1
1
1
_
TOTAL
250
85
14
12
11
6
4
3
l
TOTAL 13 9
4
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same time. Certain headhouse explosions exhibit this behavior. On the other
hand, if the vessel has a large L/D ratio (erg., a Texas house or gallery) or
contains many obstacles (e.g., the usual tunnel of. a slipped formed facility),
an ordinary slow combustion wave can induce gas motion ahead of itself. This
causes turbulent eddy shedding and growth of turbulent 'wall boundary layers.
When the flame reaches these, it accelerates and generates a more rapid
pressure rise, which causes even more violent eddy.shedding and turbulent
boundary layer growth. This behavior results in extremely high propagation
rates (up to detonation velocities).
A detonation is a stable, supersonic combustion wave that travels at a
velocity determined by the amount of heat released in the fuel-air mixture.
These velocities can be as high as 2000 m/s. Furthermore, once such a wave
' is started. in a large L/D-ratio vessel, it usually propagates the length of '
the vessel or until it runs out of fuel. . In general, large L/D-ratio regions
or spaces with obstacles suffer much more devastating grain dust explosions
than law L/D. vessels because the supersonic.wave nature of the detonation
process produces a local pressure rise of about 20- atm with a rise time of
less than 0.1 s. Under these circumstances, the.walls cannot relieve in time
to stop the pressure from rising to its maximum value, the facility is
shattered with pieces thrown great distances, and a sizable external.blast
wave is produced. The missing bins in the Houston incident undoubtedly
experienced this type of a c~bust.ion explosion.
As was stated above, grain dust and air mixtures can support a ..
propagating flame. above a certain concentration limit. This limit is. well
above the level that is tolerable to man (e.g., grain dust in suspension' at
the LEL will not transmit light over a distance of about 3 ft) and is not
permitted in personnel areas. It is, however, tolerable in process equipment
and quite frequently is reached inside pieces of machinery. In general, the
internal explosion'of a piece of process equipment, particularly if it is. of
weak construction (e.g., sheet metal walls tack welded or clamped with weak
clamps), will not cause extensive . external damage because pressure relief
allows much of the combustion to occur externally in a much larger 'vessel
(the romp and the pressure never builds to a damaging level.
Given this s ituation, one might ask why damaging dust explosions occur
in the grain-handling industry. The answer is that even though high
concentrations of airborne dust are not tolerated in personnel areas, dust
accumulates on exposed surfaces (i.e., the layered dust) and constitutes a
fuel source for an explosion in the personnel area itself. Two common
sequences of events can be used to illustrate what can occur. In the'first
sequence, a spark is inadvertently generated in a piece of process equipment
and the dust concentration is high.enough to support combustion. The weak
piece of equipment ruptures, as it was designed to, at a very low
overpressure . The explosion produces ' a reasonably large fireball of Burning
dust near the piece of process equipment and, because of vibration and.air
motion ahead of the burning region, stirs up dust that was layered in the
personnel area and the entire atmosphere of the area becomes combustible.
This leads to a major secondary explosion in the facility. In 'the second
1
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sequence, a hot piece of equipment (e.g., a light bulb) in an explosionproof
housing or an explosion-proof motor is covered with sufficient layered dust
to lead to layer ignition and smoldering. A workman discovers the smoldering
pile and either he or a fireman attempts to extinguish it with a water.apray
or chemical extinguisher spray. This stirs up a considerable amount of dust,
some of which already is burning, and causes a primary explosion that can
then trigger a secondary explosion if the rest of the area is dirty.
REFERENCES
8artknecht, W., Explosions--Cause, Prevention, Protection,
Springer-Verlag, Berlin, 1981.
Eckhoff, R.R., Towards absolute minimum ignition energies for dust clouds,
Combustion and Flame 24 (1975~: 53-64. ~
Godbert, A.L., and Greenwald, H.P., Laboratory Studies of the Inflatability
of Dusts, U.S. Bureau of Mines Bulletin 389, U.S. Department of the
Interior, Washington, D.C., 1936.
Godbert, A.L., A Standard Apparatus for Determining the Inflammability of
Coal Dusts and Mine Dusts, Report 58, Safety in Mines Research
Establishment, 1952.
National Fire Protection Association, Guide for Explosion Venting, NFPA
No. 68, NFPA, Boston, Massachusetts, 1978. .
National Materials Advisory Board, Classification of Dusts Relative to
Electrical Equipment in Class II Hazardous Locations, Report NMAB
353-4, National Academy Press, Washington, D.C., 1982. .
Strehlow, R.A., Accidental explosions, American Scientist 68 (1980~.
U.S. Bureau of Mines, Explosibility of Agricultural Dusts, Investigation
· 5753, U.S. Department of the Interior, Washington, D.C., 1961.
U.S. Department of Agriculture, Prevention of Dust Explosions in Grain
Elevators--An Achievable Goal, USDA, Washington, D.C., 1980.
Verkade, M., and Chiotti, P., Literature Survey of Dust Explosions in
` Grain-Handling Facilities: Causes and Prevention, Report IS-EMRRI-2,
Energy and Mineral Research Institute, Ames, Iowa, 1976.
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Representative terms from entire chapter:
grain dust
