VOLUME II
GRAIN DUST LEVELS CAUSED BY
TENT CONTROL OF SHIP LOADING
EMISSIONS COMPARED TO MINIMUM
EXPLOSIVE LEVELS
Final Report
GCA/TECHNOLOGY DIVISION
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GCA-79-06-G(2)
GRAIN TERMINAL CONTROL STUDY
Contract No. 68-01-4143
Technical Service Area 1
Task Order Nos. 24, 47 and 69
EPA Project Officer
Mr. John R. Busik
Division of Stationary Source
Enforcement
401 M Street, S.W.
Washington, D.C. 20460
EPA Task Officer
Ms. Betty Swan
U.S. EPA, Region 10
1200 Sixth Avenue
Seattle, Washington 98101
VOLUME II
GRAIN DUST LEVELS CAUSED BY
TENT CONTROL OF SHIP LOADING
EMISSIONS COMPARED TO MINIMUM
EXPLOSIVE LEVELS
Final Report
by
William Battye
Robert R. Hall
Pedro Lilienfeld
GCA CORPORATION
GCA/TECHNOLOGY DIVISION
Bedford, Massachusetts
January 1980
U.S. ENVIRONMENTAL PROTECTION AGENCY
Division of Stationary Source Enforcement
Washington, D.C. 20460
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DISCLAIMER
This Final Report was prepared for the Environmental Protection Agency
by GCA Corporation, GCA/Technology Division, Burlington Road, Bedford,
Massachusetts 01730, in fulfillment of Contract No. 68-01-4143, Task Order
Nos. 24, 47 and 69. The opinions, findings, and conclusions expressed are
those of the authors and not necessarily those of the Environmental Protection
Agency. Mention of company or product names is not to be considered as an
endorsement by the Environmental Protection Agency.
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ABSTRACT
Explosions in grain handling facilities late in 1977 and early in 1978
generated considerable concern with respect to any operation where grain dust
is generated and contained. As a result of this concern, the use of tents
to control particulate emissions from ship loading at the Bunge and Louis
Dreyfus terminal grain elevators in Portland, Oregon was temporarily halted.
This has resulted in an increase in the suspendable particulate emission factor
for ship loading at these facilities from about 6-10 g/1000 kg (0.012 - 0.020
lb/ton) of grain loaded to about 40 g/1000 kg (0.08 lb/ton). The purpose of
this study was to determine whether the use of tents at the Bunge and Dreyfus
ship loading facilities would, in fact, pose an explosion hazard. Measurements
of dust concentration were made in the holds of ships at the United Grain
terminal at Tacoma, Washington, at Bunge, and at Dreyfus during tent con-
trolled loading and during uncontrolled loading. Both loading and weather
conditions during these tests were typical of the Bunge and Dreyfus facilities.
A total of 67 samples were taken over 5-20 min intervals. Average dust con-
centration was 0.40 g/m3 with a maximum value of 1.1 g/m3, well below the
minimum explosive limit of 40 g/m3.
iii
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CONTENTS
Abstract iii
Figures v
Tables vi
1. Summary and Conclusions 1
2. Background Information: Conditions Required for Dust Explosions . . 3
3. Measurements - Procedures and Results 6
Procedures and Equipment 6
Results 11
References 27
Appendix
A. Reprint of "Special Report on Dust Explosibility" 29
iv
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FIGURES
Number Page
1 Effect of particle size on the minimum explosive concentration
of cornstarch 5
2 Sampling device used to measure grain dust concentrations in ship
holds 7
3 Schematic of dust sampling system 8
4 Pressure drop across particulate filters 9
5 Particle size distribution measured with Andersen impactor at
United Grain 13
6 Measuring probe locations at United Grain 17
7 Sample positions at Bunge Corporation, March 26, 1979 20
8 Sample positions at Louis Dreyfus Corporation, March 27, 1979 ... 21
9 Sample positions at Bunge Corporation, March 28, 1979 22
10 Sample positions at Louis Dreyfus Corporation, March 29, 1979 ... 23
11 Log-probability plot of measured dust concentrations 25
12 Log-probability plot of estimated 1-min dust concentrations .... 26
v
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TABLES
Number Page
1 Minimum Mass Concentrations Required for Wheat Dust Explosions ... 3
2 Minimum Explosible Dust Concentration of Agricultural Grains .... 4
3 Particle Size Distribution Measured with Andersen Impactor at
United Grain 12
4 Relative Humidity Results at United Grain 15
5 Results of Dust Concentration Measurements at United Grain,
Tacoma, Washington 16
6 Summary of Grain Dust Concentrations Measured in Ship Holds at
Portland Terminals 19
vi
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SECTION 1
SUMMARY AND CONCLUSIONS
Explosions in grain elevators in late 1977 and early 1978 caused consid-
erable concern with respect to any operation where dust, particularly grain
dust, is generated and contained.
Ship loading at terminal grain elevators results in the aerosolization of
dust formed by abrasion of the grain in numerous transfer operations. The dust
becomes aerosolized when the grain is dropped down long chutes (15 to 25 m, or
50 to 80 ft) into the ship holds. Preventing the emission of this dust requires
proper control equipment design because of variations in ship and hold sizes,
and because deck height will vary not only with tide or river level but with
ship trim as well.
One method of controlling dust emissions during ship loading involves
covering of the hold opening with tents, and aspiration of dust laden air
under the tents to a fabric filtration system. At most terminals using tents,
the tents are used only during general filling of bulk carriers, since they
may interfere with loading operations if used during topping off or during
tween-decker loading and are not needed for tanker loadings. Bulk carriers
carry Borne 90 percent of the grain shipped in the United States, and 75 to
85 percent of the grain loaded to a bulk carrier is loaded in a general filling
mode. Use of tents to control emissions from bulk carrier loading reduces
the suspendable particulate emission factor (including topping off) from
40 g/metric ton (0.08 lb/ton) loaded to 6-10 g/t (0.012 - 0.02 lb/ton).1
Before early 1978, tents were used to control emissions from ship loading
at the Bunge Corporation and the Louis Dreyfus Corporation terminals in
Portland, Oregon. Tents were and are used at other grain loading facilities
in the U.S. as relatively inexpensive method to reduce particulate emissions.
In early 1978, the use of tents generated concern among Portland area steve-
dores, as they suggested that an explosion hazard could be caused by the
dust concentrations in tent controlled ship holds. • The tents are not pre-
sently used by the stevedores in Portland although they are still made avail-
able by the elevator operators and they are used in Tacoma, Washington.
This report describes tests by GCA/Technology Division for the U.S.
Environmental Protection Agency, Oregon Operations Office, to determine
whether or 'not the use of tents to control particulate emissions during ship
loading represents an explosion hazard. Initial tests were conducted at the
United Grain terminal in Tacoma, Washington during November 1978. In March
1979, additional tests were conducted at the Bunge and Dreyfus terminals.
1
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Background data in the technical literature2 indicates that the lower
explosive limit for grain dust is 40 g/m3. If this concentration is reached,
an explosion is still only possible if an ignition source with enough energy
Is present. Suggested ignition sources include static electricity or tramp
metal in the grain although documentation showing the presence of these
sources is lacking. Data on dust concentration in ship holds were collected
in this study and showed that dust levels are well below the minimum explosive
limit.
A total of 67 measurements were conducted at United Grain (Tacoma,
Washington) and Bunge and Dreyfus (Portland, Oregon). Average dust concen-
tration over the 5-20 min sampling intervals was 0.40 g/m3 with a maximum
value of 1.1 g/m3.
Based on the change in filter resistance as a function of time during
each sample run, estimates of 1 minute concentrations were made. As
expected, these indicated a slightly wider range of values. However, the
maximum estimated concentration was 2.3 g/m3 which occurred only once in
600 1-min intervals.
Graphical plots on log-probability paper of the distributions of mea-
sured and estimated dust concentrations indicated an insignificant (less
than 1 chance in 10,000) chance of exceeding about 5 g/m3 as either a 1-min
or 10-min average.
2
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SECTION 2
BACKGROUND INFORMATION: CONDITIONS REQUIRED FOR DUST EXPLOSIONS
A dust explosion is a reaction in a mixture of finely divided particles
and a gas (generally air) which is initiated by a local heat source. For an
explosion to occur, the simultaneous presence of an "explosible" dust cloud
and an ignition source of sufficient magnitude is required. The explosibility
of a dust cloud in air is determined by a number of factors, including type of
dust; its concentration; the moisture content of the air and the dust; the
flow dynamics and dimensions of the dust cloud; and the extent to which it is
contained (see Appendix A for a more detailed discussion).
Dust concentration has a profound effect on explosibility. In fact, for
a given dust, there is a minimum concentration below which an explosion cannot
occur. It should be understood that at dust concentrations below this level
it is essentially impossible to initiate an explosion even if an active flame
exists within the dust cloud. In this case, only those particles which come
in contact with the flame will burn, as there is insufficient heat transfer
to ignite neighboring particles.
The precise minimum concentration at which an explosion can be initiated
cannot be specified with complete certainty because of the many other factors
that influence explosibility. Based on laboratory investigations by several
researchers, however, it appears that one can infer a consensus on the minimum
explosible concentration for a given dust. Table 1 summarizes values obtained
from several sources for the minimum explosive concentration of wheat dust:
TABLE 1. MINIMUM MASS CONCENTRATIONS REQUIRED FOR
WHEAT DUST EXPLOSIONS2
Mass concentration (g/m3) 20 - 50 40 10.3 50 - 100 23 70
Literature source (reference) 3 4 5 6 7 8
The low value of 10.3 g/m3 from Reference 3 probably resulted from the experi-
mental method used for that determination which frequently yields underestimates
of the minimum explosive concentrations. It thus appears that the average
minimum concentration for the other sources is about 40 g/m3 with a low limit
of about 20 g/m3. Table 2 lists minimum explosive concentrations measured for
other agricultural dusts. This tends to confirm the 40 g/m3 level which we
have selected as the minimum explosive concentration for wheat dust.
3
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TABLE 2. MINIMUM EXPLOSIBLE DUST CONCENTRATION OF
AGRICULTURAL GRAINS9
Type of dust Minimum explosible concentration (g/m3)
Alfalfa 100
Coconut shell
35
Coffee
85
Coffee, instant
280
Corn cob
45
Cornstarch
40
Cottonseed meal
55
Malt barley
55
Rice
50
Soya flour
60
Soya protein
50
Sugar
45
Wheat flour
50
Wheat starch
45
Yeas t
50
4
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As would be expected, moisture tends to reduce the explosibility of dust,
and the effect is shown by increases in the minimum explosible mass concentra-
tion and minimum ignition energy. Typically, the minimum dust concentration
for explosibility increases by about a factor of 10 for admixed moisture con-
centrations of the order of 100 g/m3, with respect to explosible dust concen-
trations at zero moisture levels.9 Similarly, it has been determined that grain
dust explosions are inhibited for dust moisture content in excess of about 20
percent.3 It can thus be stated that small amounts of water adsorbed onto the
dust particles drastically reduce the danger of explosibility given that the
dust concentration is sufficiently high for such a danger to exist.
Particle size also has an effect on the explosibility of a dust cloud.
The minimum explosive concentration tends to increase with increasing particle
size for two reasons. First, for a given mass concentration, the number of
particles increases with decreasing average diameter causing a decrease in the
distance between the particles. This facilitates heat transfer between neigh-
boring particles. Second, for a given mass concentration, the total surface
area increases with decreasing size, and reaction rates between solids and gases
are generally strongly dependent on the contact surface. Figure 1 shows the
effect of average particle size on the minimum explosive limit of cornstarch.^
The limit is constant up to about 100 ym, and increases drastically between
100 ym and 150 pm. Particles smaller than 75 urn are generally used in Bureau
of Mines investigations of explosibility of dust clouds.8 It should be noted
that, in practice, the size distribution of dust generated by various means is
not extremely variable as particles larger than 100 ym tend to settle out
rapidly, leaving only smaller particles.
0 40 80 120 160 200
Avaroga Particle Diameter,
microns
Figure 1. Effect of particle size on the minimum
explosive concentration of cornstarch.
5
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SECTION 3
MEASUREMENTS - PROCEDURES AND RESULTS
In order to determine whether the use of tents to control ship-loading
emissions poses an explosion hazard, one must determine first whether the tents
cause an appreciable change in the character of the atmosphere inside the hold,
and second whether the resulting atmosphere is an explosible one. Background
material on the conditions necessary for dust explosions indicates that the
parameters which are most important in determining the explosibility of a dust
cloud are (1) dust concentration, (2) dust size distribution, and (3) moisture
content. Measurements of dust concentration and relative humidity were made
at various locations in the holds during both uncontrolled loading and con-
trolled loading with various aspirations rates. Based on background data on
dust explosions (see Figure 1) and on the fact that the Bureau of Mines mea-
sures lower explosive limits using particles less than about 75 pm, it was
decided that the concentration of only the dust smaller than about 100 pm
should be measured. The aspiration which was varied by removing suction hoses
from the hold was also measured. Particle size distributions were measured
for both tent controlled and uncontrolled loading.
PROCEDURES AND EQUIPMENT
Dust Concentration and Humidity Measurements
Dust concentrations (smaller than about 100 ym or 0.0039 in.) and humid-
ities were measured by a probe consisting of a remote humidity and temperature
sensor and a dust collector designed and built by GCA. The probe can be sus-
pended in a hold by an 18 m (60-foot) umbilical cord which contains shielded
cable for the humidity sensor and air and pressure tap lines for the dust
sampling device.
Dust Concentration—
The specially designed dust sampling probe is illustrated in Figure 2. Air
is drawn through the probe at a.predetermined rate by a remote pump. The rate
is maintained using a dry gas meter and an orifice flow meter. A gravitational
preseparator at the inlet of the probe allows only those particles with aero-
dynamic diameters smaller than about 100 ym to be drawn into the probe. Larger
particles have settling velocities greater than the velocity of air in the
preseparator, and are, thus, not collected. After passing through the pre-
separator, the air, with particles smaller than about 100 urn, passes through a
filter holder. In the filter holder preweighed glass fiber filters are used
to remove the remaining dust from the air. There are pressure taps to the air-
stream on either side of the filter holder which.allow the measurement of the
pressure drop across the filter at any instant. A schematic of the sampling
syBtem is presented in Figure 3.
6
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TO DECK
CONNECTINO FIN
(SOLDERED)
ORAVITATIONAL
PRESEPARATOR
UMBILICAL CORD
CONTAININO ' AIR LINE TO PUMP,
PRESSURE TAP LINES,
HUMIDITY SENSOR TRANSMISSION
CABLE.
AIR FLOW («*»20 cm/aac)
PRESSURE TAP
TO MANOMETER
i=r
j=rr 1—¦"
PRESSURE TAP
TO MANOMETER
SAMPLE AIR
TO PUMP VIA
UMBILICAL CORD
GRAVITY
I Inch
I 1
SC41S
Figure 2. Sampling device used to measure grain dust concentrations in
ship holds.
7
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Figure 3. Schematic of dust sampling system.
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The dust filter was changed every 5 to 20 min (typically 10 min) during
the tests, and each filter change constituted a run. During each run the
pressure drop across the filter was recorded once a minute for the duration
of the run. The total number of runs made under various conditions was 67.
From the total volume of air drawn through the filter during a given run,
and the dry weight of the filter before and after the run, one can determine
the average concentration of particulates less than about 100 ym diameter in
the sampled air.
From readings of the pressure drop across the filter at different times
during the run, it is possible to estimate dust concentrations for shorter
time intervals. It has been found that the relationship between the dust cake
thickness on a fabric filter control device and the pressure drop across the
fabric takes the general form of curve (a) in Figure 4.
Figure 4. Pressure drop across particulate filters.
For the thick glass fiber filbers used in these experiments, the relationship
between dust cake thickness and pressure drop across the filter approximates
the straight line (b). Thus, one can-expect a linear relationship between
the rate of increase of the pressure drop across the filter and the rate of
dust deposition on the filter. Since the flow rate of air through the filter
is maintained at a constant value, the rate of dust deposition is directly
proportional to the concentration of particulate matter in the air. To obtain
the average dust concentration for a given minute of a given run, then, one need •
only multiply the average dust concentration over the entire run by the ratio
of instantaneous pressure drop increase for a given minute to the average rate
of pressure drop increase over the entire run.
Relative Humidity—
The remote humidity sensor was obtained from Phys-Chemical Research. The
device contains two integrated circuits whose impedance is directly related to
9
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the temperature and humidity of the air around them. The circuits in the probe
are connected by shielded cable to an indicator box built by GCA, and from
which the temperature and humidity can be read. The temperature can be obtained
directly from the impedance of one of the circuits, while the impedance of the
other, when corrected for temperature, can be used to obtain the relative humid-
ity. In the range of 60 to 90 percent relative humidity, the sensor is accurate
to within 2 percent relative humidity.
Relative humidity measurements were made' inside and outside the hold for
16 of the dust concentration runs.
Aspiration Rates
For each loading spout at the United Grain terminal there is one aspiration
pipe attached to the spout and two flexible aspiration hoses which can be inserted
under the tents at the edge of the hold. During loading, the two hoses and the
pipe for the loading leg in use are always pulling air to the fabric filtration
system. Generally, hoses for other legs are pulling air as well. The aspiration
rate was adjusted during the course of the measurements by putting different num-
bers of hoses into the hold being loaded, or by turning off the suction system
entirely. Aspiration rates for individual tubes were measured by GCA using a pi-
tot tube. The tube was used to find air velocites at various distances from the
center of the tube, and the total flow rates were found using these velocities and
tube areas. Similar measurements of aspiration rates were made during the tests
at Bunge and Dreyfus.
Particle Size Measurements
An Andersen particle fractioning sampler ("Andersen impactor") was used to
obtain typical particle size distributions during tent controlled loading and
also during uncontrolled loading (topping off).
Air is drawn through an Anderson impactor at a predetermined rate by a
remote pump. The sampling rate is maintained constant by a dry gas meter and
an orifice flow meter. The impactor itself contains eight plates mounted in
series, each having a pattern of precision-drilled orifices. The orifices are
Bmaller for successive stages. Generally, a cyclonic preseparator and a backup
filter are mounted at the inlet and outlet of the impactor, respectively. Large
particles are removed from the sample air stream by the cyclonic precollector
Smaller particles are inertially impacted onto eight preweighed glass fiber
substrates which are mounted below the precision-drilled plates. The aero-
dynamic size ranges of the particles deposited in the precollector and on the
eight substrates can be calculated using the exact flow rate of air through the
impactor and constants which are determined by the precollector dimensions and
the diameters of the precision-drilled orifices. Particles which pass through
the precollector and the impactor are trapped on the backup filter. Thus, dust
entering the impactor-precollector-filter system is fractioned into ten size
ranges. There are effectively nine size ranges, however, since the cutoff
diameter for the precollector is lower than that for the first impaction plate.
From the dry weight of the dust trapped in the precollector, and the dry weights
of the eight substrates and the backup filter before and after a run, one can
10
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determine the total average dust concentration in the sampled air; and the size
distribution of the dust.
An Andersen impactor is generally used to sample gas flowing out of a
stack. When this is done, the inlet nozzle for the precollector is chosen so
that, when the impactor is run at the desired flow rate, the gas velocity
through the nozzle will be the same as the gas velocity in the stack. Since
there is no constant airflow velocity or direction in the hold of a ship, this
procedure could not be used. Instead, the nozzle size was chosen to minimize
the airflow velocity through the nozzle and, thus, keep the collection efficiency
for different size particles relatively constant. The impactor was held in
such a way that the nozzle was mounted horizontally. It is estimated that par-
ticles up to about 150 ym (0.0059 in.) are collected using this technique,
with collection efficiency becoming smaller as particle size increases.
RESULTS
Size Distribution
The results of Andersen impactor measurements of particle size distribu-
tions at the United Grain terminal are presented in Table 3 and Figure 5. Two
runs were made, one during tent-controlled loading with an aspiration rate of
about 225 m3/min (8000 acfm) (run Al), and one during uncontrolled loading
(run A2). In each case, the impactor was located in a moderately dusty sec-
tion of the hold, somewhat removed from the grain impaction site. Figure 5
shows that the size distributions found in the two runs are essentially the
same. The mass median diameter of the suspended particles was about 11 pm
(4.33 x 10"4 in.) while the geometric standard deviation was about 3.2.
Particles larger than 100 pm made up an insignificant fraction of the particles
collected by the impactor.
Aspiration Rates
Aspiration rates for open hoses and tubes were measured during the loading
of one hold at United Grain. There was one aspiration tube attached to the
loading spout, and two tubes which could reach the hold being loaded. These
three tubes must be on or off in unison. The tube attached to the spout was
found to pull air at a rate of 160 m3/min (5700 acfm), while each flexible hose
maintained a flow of 70 m3/min (2500 acfm). Three flexible tubes which would
not reach the hold being loaded were valved open, but were placed end to the
ground so there was no significant flow through them. The measured total
amount of air being drawn through the fabric filtration system was about 300
m3/min (10,700 acfm).
Aspiration rates were also measured during the tests at Bunge and Dreyfus.
In both cases, the flow rate is a function of the number of hoses inserted
into the hold, the number of hoses left open, and the location of the hoses
in the manifold system. During the first series of tests at Bunge, one hose
with an aspiration rate of 127 m3/min (4500 acfm) was in use. Tenting and
therefore aspiration was not used during the second series of tents at Bunge.
At Dreyfus during the first series of tests, two hoses with a combined aspira-
tion rate of 158 m3/min (5600 acfm) were used. During the second series of
11
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TABLE 3. PARTICLE SIZE DISTRIBUTION MEASURED WITH ANDERSEN
IMPACTOR AT UNITED GRAIN
Run Time
no. (min)
Air
sampled
(m3)
Total
particulate
concentration
(g/m3)
Weight percent less than stated size*
Cyclone Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Stage 7
A1
0.0254
0.29
70.5
(19.6)
57.2
(16.4)
39.0
(11.2)
34.5
(7.63)
24.5
(4.75)
17.0
(2.10)
8.70
(1.44)
1.74
(0.883)
A2
0.0286
0.18
68.3
(18.4)
62.6
(15.4)
44.7
(10.5)
28.2
(10-5)
19.1
(7.16)
9.72
(4.46)
5.64
(1.35)
(0.825)
* Numbers in parenthesis are sizes in micrometers. Top numbers are percentage less than the stated sizes.
-------
IOOjQ 1 1 1 1 1 1 r
900-
800-
roc-
600-
90jC-
40fl -
2QC"
A RUN Al (TENTS IN USE )
O RUN A2 (NO CONTROLS)
_u
3 10 20 30 40 50 60 7X3 80 90 95 98 99
WEIGHT PERCENT SMALLER THAN STATED SIZE
Figure 5. Particle size distribution measured with Andersen
impactor at United Grain.
13
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tests at Dreyfus, a single hose with an aspiration rate of 122 m /min (4300
acfm) was used. The higher flow rate through a single hose occurred because
the hose entered the manifold system near the fabric filter and fewer
hosea were left open.
Relative Humidity
The results of relative humidity measurements at United Grain are pre-
sented in Table 4. The table also indicates the general conditions under
which the measurements were taken. Aspiration rates were estimated by con-
sidering which tubes were in use and the aspiration rates measured for such
tubes. It is assumed that the aspiration rate for flexible hoses and the
tube above the loading spouts are the same regardless of which loading spout
is in use.
There does not appear to be a significant dependence of the humidity in
the hold with whether or not tents are in use. There is some dependence of
the difference between inside and outside humidity on outside humidity. For
outside humidities of 72 percent and less, the humidity inside the hold was,
on the average, only about 1 percent less than that outside. For outside
humidities of 80 percent and above, the humidity inside the hold was on the
average 7 percent below the outside humidity. This was true even when no
test was used. In any case, the humidity inside the hold is not appreciably
lower than the outside the hold.
Particulate Concentrations
As was previously mentioned, 67 measurements of. dust concentrations were
made during topping-off, during tent controlled loading with different aspira-
tion rates at various locations in the holds and during uncontrolled loading.
Because nearly all of the particulate matter collected in the two Andersen
impactor runs were smaller than 80 to 100 ym (see Figure 5), the total dust
concentrations measured in these runs can also be considered in any deter-
minations of average suspended dust concentrations.
Renults of the measurements at United Grain are presented in Table 5 and
the location of the measuring points is shown in Figure 6. The time weighted
average concentration was 0.34 g/m3 and the highest measured concentration was
a 12-min average of 1.1 g/m3.
Results of the measurements at Bunge and Dreyfus are presented in Table 6.
The location of the measuring points are shown in Figures 7, 8, 9 and 10. At
Bunge the time weighted average concentration was 0.51 g/m3 while the highest
measured concentration was 0.85 g/m3 as a 10-min average. Results at Dreyfus
show a time weighted average of 0.32 g/m3 and a highest value of 0.74 repre-
senting a 10-min average.
After a brief examination of the above data, one could reasonably conclude
that the chances of measuring dust concentration greater than 40 g/m3 is prac-
tically nil. Further information is provided by the distribution of the re-
sults on log-probability paper as shown in Figure 11. Data that have a log-
normal distribution, plot as a straight line in this type of graph paper.
14
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TABLE 4. RELATIVE HUMIDITY RESULTS AT UNITED GRAIN
Relative
humidity
Run (percent)
code
Conditions
Difference Tent in Aspiration
^ jj t jj use? rate
Outside Inside , 3 , . .
hold hold (X) (»3/»in)
1 60 63 +3 X 225
2 63 61 -2 X 225
3 63 59 -4 X 225
4 63 58 -5 X 225
5 63 58 -5 X 160
11 62 63 +1 X 0
12 62 62 0 X 0
13 62 59 -3 X 160
15 68 70 +2 X 160
16 68 70 +2 X 160
20 82 82 0
21 82 73 -9
22 82 73 -9
23 82 73 -9
24 80 70 -10
25 70 72 +2 X 0
15
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TABLE 5. RESULTS OF DUST CONCENTRATION MEASUREMENTS AT UNITED GRAIN, TACOMA, WASHINGTON
Run
Date
Time
Sampling
Duration
(min)
Visual
observation
of probe area*
Tents
in use
yes (X) or no (-)
Aspiration rate
(m3/min)
Hoses
in use1
Dust
concentration
(g/m3)
1
10/27
11:35
15
D
X
225
A, B
0.02
2
13:30
7
D
X
225
A, B
0.34
3
14 :45
15
D
X
225
A, B
0.11
4
14:26
15
D
X
225
A, B
0.07
5
14:48
20
T
X
160
A
0.08
7
15:55
12
T
X
160
A
0.45
8
16:15
15
T
X
160
A
0.18
9
10/28
08:50
6
T
-
-
-
0.18
10
09:30
10
T
X
0
-
0.83
11
09:45
10
T
X
0
-
0.82
12
10:00
10
T
X
0
-
0.75
13
10:15
17
T
X
160
A
0.66
14
10:35
13
T
X
160
A
0.30
15
11:10
15
D
X
160
A
0.38
16
11:40
10
T
X
160
A
0.18
17
13:24
2
T
X
160
A
0.61
A1
14:30
7
T
X
225
A, B
0.29
18
15:00
12
T
X
225
A, B
0.87
19
15:13
10
D
-
-
-
0.75
A2
10/30
08:15
7
D
-
-
-
0.18
20
08:44
10
T
-
-
-
0.04
21
09:05
15
T
-
-
-
0.17
22
09:20
15
T
-
-
-
0.01
23
09:40
6
T
-
-
-
0.02
24
09:52
1.83
T
-
-
-
0.02
25
10:30
7
D
X
0
-
1.1
* D = Dustier than the rest of the hold
T = Typical
t A = Pipe attached to grain spout
B = One flexible hose
-------
5T
Al
A5.7.8
o.
1,3,4
A18,25
AW,". 12. '3.14
® GRAIN SPOUT
@ ASPIRATION HOSE ATTACHED TO SPOUT
(b) FLEXIBLE HOSE
GRAIN IMPACT SITE
j| FALLING GRAIN
Figure 6. Measuring probe locations at United Grain.
-------
a
A9
KEY-
A PROBE LOCATION
® 6RAIN SPOUT
SjT, GRAIN IMPACT SITE
o
A 20 I9.A2A
A24
o
A2I
A22.23
Figure 6 (continued).
-------
TABLE 6. SUMMARY OF GRAIN DUST CONCENTRATIONS MEASURED IN SHIP HOLDS
AT PORTLAND TERMINALS
Run
No.
Location Data
Time
Sampling
Duration
(mln)
Grain
type*
Tent
In uae
yea (X) or no (-)
Aaplratlon
flow rate
(m3/mln)
Sample
concentration*
(g/m3)
1
bJ 3/26
10:30 a.m.
15
WW
X
127
0. 324
2
B
10:37 a.m.
10
WW
X
127
0.467
3
B
15
WW
X
127
0.536
4
B
1:21 p.m.
10
WW
X
-
0.2 36
i
B
1 :4b p.m.
10
WW
X
-
0.414
6
n
1 :59 p.m.
10
WW
X
-
0. ft 70
7
B
2:10 p.m.
10
WW
X
127
0. 609
8
B
2:27 p.m.
10
WW
X
127
0.417
9
B
3:00 p.m.
10
WW
X
127
0.84 7
10
D1
9:05 p.m.
15
WW
X
79
0.135
11
D
9:25 p.m.
15
WW
X
79
0. 167
12
0
11:10 p.m.
15
WW
X
79
0.695
13
D
11:30 p.m.
10
WW
X
79
0. 740
14
D
11:45 p.m.
10
WW
X
79
0. ft*)2
15
D 3/28
12:55 a.m.
15
WW
X
158
0. 149
16
D
1:25 a.m.
ft
HRW
X
158
0.121
17
D
1:36 a.m.
10
HRW
X
158
0. 143
18
D
2:00 a.m.
20
HRW
X
158
0. 127
19
H
1:59 p.m.
10
NS
-
-
0. 326
20
B
2:14 p.m.
10
NS
-
-
0.429
21
B
2:2b p.m.
10
NS
-
-
0. 751
22
B
2:40 p.m.
10
NS
-
0.527
23
B
2:38 p.m.
10
NS
-
-
0.647
24
B
3:09 p.m.
10
NS
-
-
0. 776
25
B
3:25 p.m.
10
NS
-
-
0.561
26
B
3:30 p.m.
10
NS
-
-
0.449
27
n
3:55 p.m.
5
NS
-
-
0.099
28
D 3/29
9:25 a.m.
10
DNS
X
-
0./13
29
D
9:42 a.m.
10
DNS
X
-
0.141
30
D
9:45 a.m.
10
DNS
X
-
0. 318
31
D
10:07 a.m.
10
DNS
X
122
0. 226
32
D
10:22 a.m.
10
DNS
X
122
0.468
33
D
10:40 a.m.
10
DNS
X
-
0.475
34
0
10:52 a.m.
10
DNS
X
122
0.399
35
D
11:0b a.m.
10
DNS
X
122
0.463
36
D
1:15 p.m.
10
DNS
X
122
0.201
37
D
1:25 p.m.
10
DNS
X
122
J
38
D
1:40 p.m.
10
DNS
X
122
J
39
D
1:51 p.m.
10
DNS
X
122
0. 3 J 7
40
D
2 :09 p.m.
10
DNS
X
122
J
4J
D
2 :20 p.m.
10
DNS
X
122
J
42
D
2:32 p.m.
10
DNS
X
122
0. 320
* WW lndlcaton Wcntorn White, HRW Indicates Hard Red Winter, NS indicates Northern S|.rlnR and
DNS Indicate*) Dark Northern Spring.
I Corrocted lo standard conditions.
t H Indicates Bungc CorporntLon.
• I) Indicates Lou If Droyfua Corporation,
f SampLa mishandled - duot was Lost.
19
-------
3
A3,<,3
£?
'.2
^
I ®
I
I
CD
KEY-
A PROBE LOCATION
® GRAIN SPOUT
Q CAIN IMPACT site
Figure 7. Sample positions at Bunge Corporation, March 26, 1979.
-------
1
1
1
^12.13,14,15
i
P
{ ®
1
KEY:
A PROBE LOCATION
® GRAIN SPOUT
& GRAIN impact site
Figure 8. Sample positions at Louis Dreyfus Corporation, March 27, 1979.
-------
NJ
t-O
HATCH
COVER
tW/V//
TENT
NOT IN
USE
TS//A \
GRAIN
SPOUT
V////
Al9.20.2l.22
»t^rrcTc» ¦*•'• •+':• - _
^vC^-V i'~, S~kif,i Ph^T:
zzzzzzzzza
i ¦' 1 " ' » ¦ " " *' • " %
.¦ . -i *
KEY:
A PROBE LOCATION
® GRAIN SPOUT
O GRAIN IMPACT SITE
Figure 9. Sample positions at Bunge Corporation, March 28, 1979.
-------
1
1
1
1
A",33,34 .sjs
1
1
1
\ !
V
1
> :
® !
&3
I
€,37
ro
U)
HATCH—7 TENT
COVER f J
7ZZZZZZ2ZZ\ - --
//; ;/ S/A
A36,
32,33,34,35
y;s//l-\rr777\
A« fa
A40'41
^ V • • • » » #\
yv ' . * \ • '• K\%»\
>> ... , ; • * - ,« « ' •• ¦ \
^ ^ ••
¦' •">¦•
i *v
KEY:
A PROBE LOCATION
® GRAIN SPOUT
& GRAIN IMPACT SITE
Figure 10. Sample positions at Louis Dreyfus Corporation, March 29, 1979.
-------
40
20
10
8.0
>o
E
fi.O
§ 4.0
)-
4
QC
»-
Z
u
s
o
o
NOTE1 THE HIGHEST MEASURED VALUE Llg/m3. NO OTHER
SAMPLES WERE GREATER THAN 0.90g/m3.
2.0
o
Ui
a:
i.O
0.8
0 6
0.4
80 70 60 50 40 30 20 10 3 2 1 a2 0.1
PERCENT OF MEASURED DUST CONCENTRATIONS GREATER THAN
OR EQUAL TO THE INDICATED VALUE
0.01
Figure 11. Log-probability plot of measured dust concentrations.
24
-------
The data in Figure 11 show that there is less than a 2 percent chance of
measuring a concentration, over the typical 10-min interval, of greater than
0.9 g/m3. The curve can be graphically extrapolated in either of two extreme
ways - ignoring the lower half or the upper half of the curve. Extrapolation
of the upper half indicates only one chance in 10,000 of exceeding about 1.5
g/m3 while extrapolation of the lower half indicates one chance in 10,000 of
exceeding about 5-6 g/m3. In either case, one must conclude that there is no
significant chance of exceeding 40 g/m3 as a 10-min average.
It should be further noted that the measurements were deliberately biased
to measure high concentrations by taking samples in the dustiest portions of
the holds. Also, the chances of explosion are further reduced by the fact
that two events must occur at the same time - an ignition source and a dust
concentration over 40 g/m3 must both be present.
As previously discussed, it is possible to estimate 1-min dust concentra-
tions from the rate of change in pressure drop across the filter. For example,
assume that an average concentration of 1 g/m3 was measured during a 10-min
sample and the pressure drop across the filter rose by 2.49 kPa (10 in H2O).
Then, if over a selected 1 min interval the pressure drop increased by 0.249
kPa (1 in H2O), the estimated concentration for that interval would be 1 g/m3.
Similarly, if the pressure drop increased by 0.5 kPa (2 in H2O) in 1 min the
estimated concentration would be 2 g/m3. These calculations were performed
for all sample runs yielding about 600 1-min concentration values. The dis-
tribution of these values is presented in Figure 12.
The results for the estimated 1-min concentrations are similar to the
longer term averages. This is not surprising because the rate of pressure
drop increase did not exhibit extremely large variations. The 1-min con-
centrations do, however, show a few values outside the range of the longer
term averages. The highest estimated 1-min concentration was 2.3 g/m3 and
the second highest was 1.7 g/m3. Extrapolation of the curve in Figure 12
indicates that there is one chance in 10,000 of finding a 1-min concentration
greater than 2-5 g/m3. Again the implication is that the chances of an ex-
plosion are insignificant.
The small differences in measured values between tent controlled loading
with and without aspiration and between tent controlled and uncontrolled
loading combined with the scarcity of measurements under some of these condi-
tions and other variables such as the type of wheat make the evaluation of
other data groupings rather tenuous.
25
-------
40
20
10
8.0
"L e .o
8
¦s.
a>
4.0
8
o
H 2.0
0
Ui
1 ,0
h
i/i
w 0.8
0.8
0.4
0.2
0.1
NOTE1 THE HIGHEST ESTIMATED I minute CONCENTRATION
WAS 2 3g/nr.3 NO OTHER ESTIMATES WERE GREATER THAN l.7g/m3
.A'
/
y
/
/
i i i i
90 80 70 60 90 40 3 0 20 10 8 2 1 0.2 0.1 0.01
PERCENT OF ESTIMATED l-minute DUST CONCENTRATIONS GREATER
THAN OR EQUAL TO THE INDICATED VALUE
Figure 12. Log-probability plot of estimated 1-rain dust concentrations.
26
-------
REFERENCES
1. Battye, W. , and R. R. Hall. Feasibility of Control of Particulate Emis-
sions from Grain Elevators (Draft Report in preparation). GCA/Technology
Division, EPA Contract No. 68-01-4143, Technical Service Area 1, TO No. 47.
January 1979.
2. Lilienfeld, Pedro. Special Report on Dust Explosibility. GCA/Technology
Division, GCA-TR-78-17-6. EPA Contract No. 68-01-4143, Technical Service
Area 1, Task Order No. 24. March 27, 1978.
3. GooLJer, H. de, et al. Literature Investigation into the Dust Explosion
Danger in Industries Storing and Processing Cereals and Flour, TNO
Report 8398 Rigswijk, Netherlands. October 1975.
4. Grain Elevator Industry Hazard Alert, OSHA, January 5, 1978.
5. Beyersdorfer, P. Staub-Explosionen, Verlag von Theodor Steinkopff,
Leipzig, 1925.
6. Schierwater, F. W., Die Konzeption der neuen Explosionsschutz-Richtlinien,
insbesondere im Hinsblick auf die Schutzmassnahmen gegen Staub expllsionen,
Staub. 36:43. 1976.
7. Weber, R., Verh, Ver. Beforderung Gewerbefleiss, p. 83. 1978.
8. Marks, L. S. (ed.). Mechanical Engineers Handbook, Sixth Edition.
McGraw-Hill. New York. 1958. p. 7-44.
9. Palmer, K. N., Dust Explosions and Fires, Chapman and Hall, London.
1973.
10. Hartmann, I., A. R. Cooper and M. Jacobson. U.S. Bureau of Mines,
R.I. 4725. 1950.
11. Dennis, R., R. W. Cass, D. W. Copper, R. R. Hall, V. Harapl, H. A. Klemm,
J. E. Langley, and R. W. Stern. Filtration Model for Coal Fly Ash with
Glass Fabrics. GCA/Technology Division, Bedford, Massachusetts.
EPA-600/7-77-084. August 1977.
12. Consultation with R. Dennis, GCA/Technology Division, Bedford, Massachusetts.
27
-------
APPENDIX A
REPRINT OF
"SPECIAL REPORT ON DUST EXPLOSIBILITY"
29
-------
GCA-TR-78-17-G
FEASIBILITY OF CONTROL OF PARTICULATE
EMISSIONS FROM GRAIN TERMINALS
Contract No. 68-01-4143
Technical Service Area 1
Task No. 24
EPA Project Officer
Mr. John R. Busik
Division of Stationary Source
Enforcement
401 M Street, S.W.
Washington, D.C. 20460
EPA Task Officer
Mr. Norman Edmisten
U.S. EPA,
Oregon Operations Office
522 S.W. Fifth St.
Yeon Building
Portland, Oregon 97204
SPECIAL REPORT ON
DUST EXPLOSIBILITY
by
Pedro Lilienfeld
CCA CORPORATION
CCA/TECHNOLOGY DIVISION
Bedford, Massachusetts
27 March 1978
U.S. ENVIRONMENTAL PROTECTION AGENCY
Division of Stationary Source Enforcement
Washington, D.C. 20460
-------
DISCLAIMER
This document was furnished to the U.S. Environmental Protection Agency
by GCA Corporation, GCA/Technology Division, Bedford, Massachusetts 01730.
The opinions, findings, and conclusions expressed are those of the authors and
not necessarily those of the Environmental Protection Agency or of cooperating
agencies. Mention of company or product .names Is not to be considered as an
endorsement by the Environmental Protection Agency.
i i
-------
SECTION 1
INTRODUCTION
The recent explosions in grain elevators and the resultant loss of lives
has generated considerable concern with respect to any operation where grain
dust is generated and, in particular, contained.
Grain loading of ships is accompanied by the aerosolization of the dust
mixed wLth the graLn. This aerosolization results from the free fall of the
grain from the chute opening and the impact of this stream on the deposited
grain within the ship hold.
In order to prevent and/or reduce the fugitive emission of this airborne
dust from the face of the hold during grain loading various measures have been
taken, among which is the method of covering the face of the hold by means of
a tent or tarpaulin. This method is usually combined with a high-flow rate
suction system whereby a hose is inserted into the hold and air Is pulled into
a fabric-filter baghouse. Dilution air thus flows from the outside, under the
tent into the hold in order to provide this aspiration flow. This system
usually prevents any visible emissions from being detected above the hold during
loading.
The containment of grain dust by means of tents has generated an acute
concern as to the possibility of explosions under such enclosed loading con-
ditions, and as a result OSHA has recommended that this method be discontinued
until the safety from dust explosions has been determined.
The purpose of this report is to provide information from which a prelim-
inary determination of the safety of tent-controlled ship grain loading opera-
tions can be made. Furthermore, this document will summarize additional informa-
tion required to arrive at a more definitive conclusion of this question.
1
-------
SECTION 2
HISTORICAL BACKGROUND
Agricultural dust explosions have been a problem for many years. They
arc not unique to our times although reliable records of such accidents are
restrie ted to the last 200 years. Probably the first recognized explosion
of this type occurred Ln Italy on December 14, 1785 involving flour dust.1
Increasing industrialization thereafter is correlated with an increasing num-
ber of such explosions both ln Europe and the U.S.2 From about 1860 on, grain
dust explosions become, considerably more frequent. In the U.S. about 100
violent conflagrations associated with flour and grain dust were reported
between L870 and 1922,? and 59 explosions in industries storing and processing
cereals and fLour, between 1949 and 1973.3 It is noteworthy to indicate that
none of the reviewed grain dust explosions occurred on ships as a result of
grain loading operations. As a matter of fact, no grain dust explosions on
ships are mentioned at all.
The primary causes for grain dust explosions had remained largely unknown
until recently, when Improved surveillance and reporting methodology have been
applied. Thus for 535 grain dust explosions between 1860 and 1973, 46 percent
of them were of unknown origLn, whereas for the period 1949 to 1973, of 128
such accidents, 26 percent were unexplained. This number drops to only 9 per-
cent during that latter interval for grain dust explosions in the Netherlands.3
For conflagration in grain elevators alone 62 percent of these had unknown
causes lor the perLod 1958 to 1975 in the U.S. Primary causatory factors
for grain dust explosions identified by various authors are: welding, hand-
lamps, open fires or flames, frictional heat, mechanical sparks, electrical
.sparks, sparks from foreign materials, etc. In addition, less frequent causes
are: lightning, spontaneous combustion, etc.
2
-------
SECTION 3
EXPLOSION MECHANISMS AND REQUIRED CONDITIONS
PROPERTIES AND CHARACTERISTICS
A dust explosion Is a reaction In a mixture of finely divided solid
substance and a gas, mostly air, which is initiated by local heat supply; the
reaction then proceeds quickly through the entire mixture. Initially, the
reaction is restricted to a fraction of the dust cloud. This reacting part
forms the flame. The heat released in the flame is transferred to nonreacting
particles at the flame front. As a result of this heat transport, the flame
front advances. The speed at which this flame front moves is called the
propagation velocity or flame velocity. The propagation velocity is the re-
sultant of the burning velocity and the expansion velocity of the hot gaseous
products. Under nonchanging conditions (temperature and pressure) the burning
velocity Is constant. This velocity is called linear burning velocity. In
case of a "point" ignition the flame spreads like a sphere.
if the expanding gas develops turbulence, the flame front area will ex-
pand considerably and the mass-conversion rate increases. The heat transfer
of the reacting dust particles to those that have not yet reacted plays an
Important part In the propagation of the explosion. The heat transport takes
place by conduction, convection and radiation. It is assumed that radiation
In particular makes an Important contribution.
Furthermore, the rate at which the substance decomposes under the in-
fluence of high temperature yielding combustible gases (pyrolysis), and the
rate at which the oxygen required for the reaction is supplied by diffusion,
are Important factors determining the development of the explosion. The
violence or intensity of the explosion depends on how finely the airborne
material la divided (i.e., particle size), since the reaction velocity between
the solid substance and the gas is strongly dependent on the dimensions of the
contact surface. Thus, rigorously it is not the mass concentration of the
dust particles, but the specific surface concentration what determines the
reaction velocity. For practical reasons, however, mostly the mass concentra-
tion Is used In this context and the particle size stated when known.
Since optical aerosol measurement methods (i.e., light scattering and
extinction) are more closely related to particle area than to particle volume,
tiii'se methods should he considered uniquely compatible with dust cloud charac-
terizations from the point of view of explosiveness.
3
-------
The energy released at the combustion of a dust particle will have to
bring an adjacent particle to such a temperature that this particle itself
starts reacting with the oxygen in the ambient air, and, as a result, starts
functioning as a new heat source. Thus, the distance between particles must
be below a certain value. This means that a minimum number concentration of
particles must exit for an explosive condition. This minimum concentration
Is the lower explosion limit. If the dust concentration is raised gradually,
the upper explosion limit will be reached above which no explosion occurs any
longer. This condition is reached as a result of the cooling action of the
excess of dust. Between the two limits there is the explosive region.
An Important parameter Is the energy required for the ignition of an ex-
ploHive dust/air mixture (ignition energy). This energy depends on the com-
position of the explosive mixture and the particle size (i.e., specific sur-
face). As the particle size decreases, the ignition energy decreases and the
combustion rate Increases.
The effect of a dust explosion on the environment is determined by the
velocLty at which the combustion reaction progresses (propagation velocity
or flame velocity), by the heat of reaction and by the gas volume formed by
the reaction. In Figure .1 are shown the values characteristic for the effect;
i.e., the maximum overpressure, the maximum rate of pressure rise and the
average rate of pressure rise.
Figure 1. Pressure versus time of a dust explosion in a closed vessel.
The effect of an explosion depends on:
1. the nature, shape and dimensions of the substance;
2. the amount of oxygen available;
shape and size of the volume in which the explosion
taken place;
t\. turbulence.
7 71
time
-------
The burning velocity In a dust cloud is many times greater than at an
ordinary combustion process (for example a massive solid substance).
The maximum overpressures that may occur at dust explosions are between
5 and 10 atmospheres. The average maximum overpressure amounts to 7 to 8
atmospheres, if the initial pressure is 1 atmosphere. Most customary building
constructions cannot resist a higher internal explosion overpressure than
about 0.3 to 0.5 atmosphere. The pressure-rise process takes place in a very
short time (0.01 to 1 second), the surrounding gas being heated to some
thousands of degrees centigrade.
EXPLOSIVITY CONDITIONS
Several dust cloud parameters determine the explosive potential. The most
important are: dust concentration, particle size, and moisture.
Dust Concent rat i.on
The precise minimum dust concentration level at which a grain dust ex-
plosion can occur cannot be specified with complete certainty because particle
size, particle size distribution, type of grain, moisture content, dimensions
of the contained volume, flow dynamics, etc., all influence the onset of an
explosion of this type. In practice, however, and based on laboratory inves-
tigations by several researchers, it appears that a general consensus can be
Inferred. Table 1 summarizes this values as obtained from several represen-
tative sources:
TABLE 1. MINIMUM MASS CONCENTRATIONS REQUIRED FOR GRAIN
DUST EXPLOSIONS
Mass concentration (g/m3) 20 to 50
40
10.3 50 - 100 23
Literature source (reference) 3
4
2 5 6
.
The low value of 10.3 g/m3 from Reference 2 probably resulted from the
experimental method used for that determination which frequently yields under-
estimates of the minimum explosive concentrations.3 It thus appears that the
average minimum concentration for the other sources is about 40 g/m3 with a
low Limit of about 20 g/m3.
Concentrations of airborne dust of these values are extremely high. Dust
clouds with concentrations of the order of 0.5 g/m3 usually produce nearly
100 percent opacity over path lenghts of the order of 10 meters, and may be
easily misjudged as potentially explosive. It should be understood that for
dust concentrations below the explosive minimum it is essentially impossible
to initiate an explosion even it an active flame exists within the cloud, in
which case only those particles that come in direct contact with this flame
undergo combustion but without propagation to neighboring particles. Table 2
lists specific types of agricultural grains and their minimum explosive
concentrations./
5
-------
PABLE 2. MINIMUM EXPLOSIBLE DUST CONCENTRATION OF
AGRICULTURAL GRAINS7
Type of dust
Minimum explosible concentration (g/m3)
Alfalfa
100
Coconut shell
35
Coffee
85
Coffee, instant
280
Corn cob
45
Cornstarch
40
Corn flock.
50
Cottonseed meal
55
Malt barley
55
Rice
50
Soya flour
60
Soya protein
50
Sugar
45
Wheat, fiour
50
Wheat starch
45
Yeast
50
Table 2 again confirms tlie general range of minimum explosivity concen-
trations above 40 g/m3 for these types of dust. The explosibility of agri-
cultural. grain dusts may be somewhat enhanced by the concomitant presence of
i.nsect ic:ides as recently reported. The contribution of these chemicals,
however, will probably be restricted to reducing the minimum ignition energy
nt the ml.nl.mum mass concentration of dust.
Mo I.Mturc:
/
As would be expected, moisture tends to reduce the explosibility of dust,
:md the effect Is shown by increases in the minimum explosible mass concen-
tration and minimum Ignition energy. Typically, the minimum dust concentra-
tion Tor explosibility Increases by about a factor of 10 for admixed moisture
concentrations of the order of 100 g/m3, with respect to explosible dust con-
centrations at zero moisture levels.7 Similarly, it has been determined that
grain dual explosions are inhibited for dust moisture content in excess of
about 20 percent.3 It can thus be stated that small amounts of water adsorbed
ont.o the dust particles drastically reduces the danger of explosibility given
that tlie (Ju.st concentration is sufficiently high for such a danger to exist.
6
-------
I'artlcle Size
In general, as mentioned previously, for a given mass concentration of
airborne dust the explosibility increases with decreasing particle size because
the ratio of particle area to volume is inversely proportional to size, and
at Che same time the average distance between particles is proportional to
size. It must be considered, however, that in practice, the size distribution
of grain dust generated by various means is probably not too variable since
the large particles tend to settle out rapidly leaving only a population of
those smaller than about 100 ym in diameter which are also the more important /,
contributors to a potential explosion.
PREVENTIVE MEASURES
Grain dust explosions can be prevented or their effect drastically re-
duced by several means which will now be discussed in the context of loading
ship holds.
The most obvious preventive method is centered around the reduction of
the concentration of airborne dust well below the explosibility levels men-
tioned previously. The containment technique using a tent or tarpaulin over
the hold in itself would tend to increase this concentration, but in combi-
nation with aspiration the concentration can be reduced below the level that
would exist in the hold without tenting. The equation for the average mass
concentration as a function of time within the hold can be calculated from:
C(t) = ^— (l-e~tQa/vh)
where. C(t) is the concentration as a function of time (g/m3)
m
Is the rate of dust generation (g/sec)
Qa Is the aspiration flow rate (m3/sec)
is the hold volume (m3).
ThLs equation assumes that the aspiration flow rate exceeds the airflow
entrained through the chute by the inflowing grain; i.e., that the pressure
Inside the tcnt-covered hold is negative with respect to the surrounding atmo-
sphere. The gradual decrease of the actual hold volume as the grain fills
it has no practical effect on the above analysis since it occurs slowly with
respect to other time-dependent variables.
The in-flow of dilution air from the outside of the hold has another
Important effect in the case of grain loading operations into slip holds.
For obvious reasons, the air surrounding a ship usually has a high moisture
content and If this air is drawn into the hold by the aspiration suction,
the relative humidity of the air under the tent is raised considerably. As
mentioned before, this will tend to drastically reduce any degree of explosi-
bility as the aLrborne dust will adsorb water vapor.
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further measures that should be considered in order to prevent any danger
of explosion are: proper attention to grounding all metallic elements that
come Into contact with the grain flow and the airborne dust, such as chute
ductw, aspiration hoses, machinery, etc. Proper electrical grounding is re-
quired to prevent build-up of static charge, and should be checked on a regular
basis since salt spray-produced oxidation or corrosion contamination of inter-
faces by dust and oils, etc., will tend to rapidly degrade electrical ground-
ing contacts.
The tent itself represents an effective means of pressure relief in the
extremely unlikely case of a conflagration within the hold. Since the over-
pressures thus generated are of the order of 5 to 10 atmospheres, the upward
force on a tent of 10 x 10 meters (1000 sq. feet) would be of the order of
10® newtons (~ 107 lbs) and nearly instantaneous relief would thus result.
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SECTION 4
PRELIMINARY CONCLUSIONS BASED ON RECENT
MEASUREMENTS BY GCA
The dust measurement program recently performed by GCA/Technology Division
personnel in Portland, Oregon® was aimed at assessing the amount of fugitive
particulate emissions resulting from the grain loading operations within the
port of that cLty. Although that program was not directed at obtaining infor-
mation concerning explosibility problems within ship holds during grain load-
ing procedures, however, some of the determinations and measurements provide
pre)Imlnary indications of the concentration levels to be expected within
ship holds when using tents as a means of dust control in combination with as-
piration fJow. Two ship loading operations with tents were monitored within
that program, Bunge and Dreyfus.8 Fugitive emissions directly adjacent to
the bitter were measured at 200 mg/m3; it should be noted that at Dreyfus no
aspiration flow from the hold was used. At Bunge no visible emissions were
detected from the tent-covered hold with a reduced aspiration flow of only
5000 cfm. During this same period, however, measurements taken with the GCA
mode] KDM-101 resplrable mass monitor with its inlet placed within a 1-foot high
and b-foot long opening on the side of the tent indicated resplrable (i.e.,
less than about 3.5 um diameter unit density spherical particles) mass con-
centrations ranging from 1.7 to 5.5 mg/m3 with an average of about 3.6 mg/m3.
These values, in combination with the particle size distribution of the dust
without the tent cover (see Figure 1, curve (1), Reference 8) during topping
off indicate that the concentration within the tent-covered and aspirated
hold did not exceed, but probably was less than, the total concentration of
89 mg/m3 observed without the tent. This conclusion is qualitatively supported
by the Dreyfus measurements without aspiration, mentioned above.
If indeed the mass concentration of airborne grain dust within aspirated
tent-covered holds is of the order of 100 to 200 mg/m3 (0.1 to 0.2 g/m3) the
ronceritrut ion safety margin is of the order of 100 to 200 within respect to a
most conservative (or pessimistic) explosibility level of 20 g/m3. It should
be considered that even if highly localized (e.g., .near the stream of grain
falling from the chute) areas may exhibit higher concentrations, any significant
propagation of an explosion requires a distributed high concentration which
appears extremely unlikely based on these preliminary determinations. The
in-flow of moist dilution air into the hold should further contribute to the
.safety of the operation.
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SECTION 5
PROPOSED ADDITIONAL EFFORT
[11 order to develop further substantiating and supporting evidence to the
preliminary assessment presented within this report, additional measurements
should be performed aimed specifically at determining the mass concentration
and Its spatial distribution in the interior of tent-covered holds during
grain loading operations. Since most of; the measurements cited in this report
were performed above or near Lhe edge of the holds, the proposed additional
determinations should be performed preferentially well within the hold volume,
JiiHt above the grain level and towards the central regions of the hold. In
addition It would be desirabie to determine the relative humidity of the air
at those same .locations. A monitoring program of this type, whose objective
is to solidify the Initial safety contention of these operations, will be
proposed In more detail in a separate document.
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REFERENCES
1. Amer. Miller, 17:20. 1889.
2. Beyersdorfer, P., Staub-Explosionen, Verlag von Theodor Steinkopff,
Leipzig, 1925.
3. Gooijer, H. de, et al. Literature Investigation into the Dust Explosion
Danger in Industries Storing and Processing Cereals and Flour, TNO
Report 8398, Rigswijk, Netherlands, October 1975.
4. Grain Elevator Industry Hazard Alert, OSHA, January 5, 1978.
5. Schierwater, F.W., Die Konzeption der neuen Explosionsschutz-Richtlinien,
lnsbesondere im Hinblick auf die Schutzmassnahmen gegen Staub explosionen,
Staub. 36:43. 1976.
6. Weber, R., Verh, Ver. Beforderung Gewerbefleiss, p. 83, 1878.
7. Palmer, K.N., Dust Explosions and Fires, Chapman and Hall, London, 1973.
8. llal.l, R.R. and W. Battye. Feasibility of Control of Particulate Emissions
From Grain Terminals. GCA Monthly Progress Report No. 4, EPA Contract
No. 68-01-4143, Task Order No. 24. 6 February 1978.
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