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OCR for page 43
1
Chapter 6
BASIS FOR REC:OMMENDAT TONS
Previous reports concerning the prevention of grain dust explosions in
grain~andling facilities have presented numerous recommendations. The panel
found no major fault with these recommendations except that they were not
ranked in term of priority, value, or economic feasibility. To be of value
to an industry composed of facilities as varied in size, construction, and
purpose as those handling grain, recommendations- must take into account the
fact that no single recommendation will suffice to solve the problem in every
facility and not every recommendation can be applied to all facilities.
Further, recommendations must address factors beyond technical ones.
Figure 5 illustrates the broader perspective taken by the panel to examine
subtle but consequential facets of the explosion problem. For example, the
personal cosmology of both grain elevator managers and workers (i.e., how they
perceive who they are and the meaning of life) influences their attitude in
taking action to prevent explosions.
If resources were unlimited, the panel believes that the dust explosion
hazard could be reduced to a negligible level in every type of facility.
Recognizing, however, that resources are not unlimited, the panel concentrated
its study on first determining what could be done and then on assessing each
preventive action's potential for hazard control. To accomplish the latter,
each preventive action was ranked as high (H), medium All, or low (L) in terms
of:
1. Efficacy - the degree to which the hazard would be eliminated or
.
controlled by the action;
2. Feasibility - the acceptability of implementing the action in light of
the economic, legal, cultural, political, social, and technical considerations
depicted in Fi gore 5;
3. Ef ficiencY - the cost-e ffectiveness of the action in terms of
potential dollar loss if no action i" taken versus the cost of the proposed
action.
The panel's recommendations fall into two main groups:
(1) recc~mendations to the grain-handling industry end its trade associations
concerning hazard reduction in existing facilities, needed research, and the
design of future facilities and (2) recommendations to the government
concerning more effective regulations. In the following discussion, the
recommendations on a specific subject are presented first and then the need
43
OCR for page 44
44
.1
CULTURAL
As'
\
~ TECHNICAL >\
/SOCIAL \tethod. ~ Rate of Grain Transport`/ LEGAL \
/ ~ Groin Typo, Density, ~ Age;
/ Previous Experience,\ ~ Prevailing ~
/ ~Weather, Elevator Design,, \
/ Training, Expectation., ~ ~ Litigious Attitude, \
/ Delegator Cteentine's,, \
/ Language Proficiency, Age, ~ ~ Reduredency, Conflict, an`/07 \
~Ign'ffon Sourcing \
Sensory Perception, Strophes, ~ Ab~enceof Regulation' ~ Laws; \
\ EXPLOSIONS / ~
Credibility of Explosion Potential, Inter- a Intra-60vernmental /
\Per~onal Cosmology, Family Life,/ ECONOMIC \ Agencies, lureaucratle Inertia,/
\` Follclore, Tradition', / Tariffs, \ Impact on Foreign Pollen l
\ Value Hierarchy, ~ ~ Conflict Among
\ ~ Grain Value, ~
\ Custom' ~ ~ Specie! Interim/
, _ . · \ /
Recial/Ethnic Sensitivity, ~ GRAIN ~ Enforcement Intent,
Edueation, Prejudices /
DUST
Criteria, ~ Capability
FOURCAL
/ Grain-Handling Schedulee,
Elevator ~ Grain Insurance; \ /
By
Salvage, locator Relelion.
-
-
FIGURE 5 Facets of a systems approach to grain elevator explosions.
1
OCR for page 45
1
45
for them is explained. The panel also realizes that sane recommendations
may already be in effect and that the cost of implementing these recommended
actions will be different for different facilities. The panel hoper that
its presentation of The recommendations in this manner will convince owners
and managers to examine the list, to determine where their operation is
deficient, and to take remedial action within the limits of their economic
and administrative capability. The same procedure should apply to those
recommendations aimed primarily at labor organizations, trade and
professional organizations, and federal, state and local governments.
EXIST ING FACILITTE;
1
The following recommendations address engineering matters and
administrative actions.
Dust Control
Re oononendations Ef ficacy Feasibility Ef f~v
. .
1
Btabl ish a housekeeping H M H
program involving a mechanical
dust collection system
supplemented by manual or other
means .
Apply state-of-the-art H L M
te chniques to reduce the
concentration of airborne
dust in and emanating from
e levator legs .
Control dust generation M M fir
and airborne dust at
a 11 gra in transf er and
discharge points.
E1 iminate all nonessential M M M
horizontal surfaces .
Coat all ncnhori zontal L L M
surfaces exposed to airborne
dust with materials that will
prevent the build-up of layered
dust.
Apply s tate-of-the-a rt
te chniques to reduce the
concentration of airborne dust
below the lower explosive
limit where possible in
enclosures other than legs.
1
L
L
OCR for page 46
46
There can be no doubt that overall dust control is the most important
action that can be taken to reduce the explosion hazard. It also is the
step that will be the most ccst-effective. The panel's own investigation of
explosions and studies of past explosions indicate that a preventable
accumulation and suspension of dust was the basic feature of every explosion.
There is'no question that dust explodes; the important point is that the
accumulation or suspension was preventable.
Of all of the locations in an elevator where duct can exist in '
suspension, the leg is by far the most hazardous. This has been recognized
time and again by groups studying elevator explosions. It is the one piece
of equipment with an environment of suspended dust that is continually
subject to "chokings (boot fills with grain and buckets will not turn),'
electrical faults, bearing failures, mechanical misalignments, ingestion of
foreign material, etc. ~
of a dust collection system in the leg sufficient to reduce the-concentration
of suspended dust. A means for 'preventing the accumulation of dust on the
walls of 'tine leg or a means for periodically removing any accumulated dust
also must be provided. A particularly difficult problem is presented by -
those legs in which the middle portion of the enclosure consists of only the
concrete walls of a headhouse. Manual cleaning or enclosing of the belt and
buckets in a close fitting, dust-tight metal casing appear to be the only
solutions.
Suspensions of dust in enclosed volumes other than legs present a
danger second only to that of legs. Silos, bins, garners, enclosed
conveyors, etc., contain little if any equipment that can serve as an
ignition source and are therefore only rarely the point of an initial
explosion. However, ignition of dust concentrations in these enclosure.,
whose volume can be much larger than that of legs, can result in an explosion
of much greater magnitude than that in a leg. These enclosures present two
hazard conditions: first, the air-suspended dust that is present when they
are being filled and, second, the dust clinging to the walls and ceilings
that Q n be loosened by the chock of an initial explosion. The removal of
the airborne dust can be accomplished using the same methods as in the leg.
The dust problem in silos and the generation of additional dust can be
lessened somewhat if dead boxes, grain ladders, and filling spouts that
entrain a large portion of the dust in the grain stream are used.
The movement of grain from one point to another results in the
creation of dust and in the suspension of scene airborne dust at transfer
points. A dust collection system should be used at every point where grain
falls through the air (e.g., when it is transferred from one belt to another
or from a spout onto a belt). There is little danger of an initial explosion
occurring at these points, but without use of a dust collection-system, most
of the dust will settle on the floors, walls, ledges, ducts, etc., in the
work space. This dust then "n became the fuel for secondary explosions in
tunnels, galleries, headhouses, and other work spaces. Thick layered dust
around working equipment presents the ideal conditions for initiation and
concealment - of smoldering dust f ires that can serve as the ignition source
for an explos ion .
OCR for page 47
1
47
Even if dust control is applied as indicated above, there will be a
need for manual housekeeping. The manual vacuuming of all exposed surfaces
i. recamended over blow down or sweeping, which tends to raise dust clouds
and usually does nothing more than redistribute most of the dust. (See, for
example, National Fire Protection Association's NETA No. 618, Code for the
Prevention of Dust Explosions in Terminal Grain Elevators.) As mentioned
earlier in this report, the danger of layered dust in the work space was
vividly exhibited to the panel in one of its explosion investigations. An
initial explosion in a headhouse was followed by an explosion in a silo.
This secondary explosion was initiated by a flame that propagated near the
gallery floor due to a layer of dust until it came to an open empty silo. In
numerous other explosions, headhauses, galleries, and tunnels have contained
enough layered dust on exposed surfaces to fuel secondary explosions within
these structures.
As a complement to manual housekeeping, the panel recc~'wnends that all
unnecessary horizontal surfaces be eliminated and all nonhorizontal surfaces,
both those in enclosures and those in the working areas, be coated with a
material that will inhibit the layering of dust. Rough concrete and wood
surfaces are particularly susceptible to a buildup of layered dust. Surface
coatings over metal should be somewhat conductive.
In summary, dust control is most important in reducing the dust
explosion hazard in grain-handling facilities. Same aspects of the dust
control recommendations are relatively more expensive than others and sane
may already be in effect. The value of each of the recamended actions
depends, of course, on how well they are applied or performed (e.g., dust
control systems that do not keep the dust concentrations below the rawer
explosive limit or manual housekeeping poorly performed are dangerous since
they instill a false sense of security). The total dust control efforts
should be based on a performance standard and not merely on the application
of the recommended actions.
Every elevator having interior legs should utilize an adequate dust
collection system in the leg because of the extremely hazardous condition
resulting from suspended dust in proximity to potential ignition sources.
The other dust control recommendations will contribute to a reduction of the
hazard and are, to some degree, interdependent. For example, the application
of surface coatings reduces clinging to vertical surfaces but does not
eliminate the need for dust collection from enclosures other than legs;
without dust collection at transfer points, the need for manual housekeeping
increases greatly. Adequate manual housekeeping is possible only when there
is easy access to hidden spaces and all surfaces that can support layered
dust. Her~e, special attention needs to be paid to providing access ports
to dust-containing enclosures to facilitate cleaning. Easy access to large
expanses of walls and ceilings, such as occur in many headhouses, must be
provided . me panel, of course, advises that all of these recommendations
be implemented.
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48
Innition Control
Re commendations
Use a preestablished and
enforced permit procedure
whenever welding, cutting, or
other open flame work is to be
done .
Incorporate a system to indicate
belt slippage and misalignment.
Incorporate a method to check
frequently the temperature and
vibration of cri tical bearings.
Use devices to extract foreign
materials from theincoming grain
stream.
Ground all conveying and
electrical equipment.
Use only equipment and installation L
stands rds meeting National Electrical
Code requirements.
Ef f icacy By Ef f iciency
H H H
H ~ H
H ~ H
M H H
L H - H
Next to the control of dust, the control of ignition sources is the most
effective means for reducing the explosion hazard. Since the data on ignition
in actual explosions are poor, it is not possible to give a meaningful ranking
to ignition sources; therefore, the panel arbitrarily divided the sources into
the eight categories shown in Table 2 and then assessed the probability of
their occur rence and the ease of their elimination.
The major deterrent to spontaneous ignition of stored grain as a likely
source of ignition is the necessity of preserving the commercial value of the
grain. In modern operating practice, if the grain is to be in residence for
more than a few weeks, the temperature i. closely monitored. A rise of a few
degrees in grain temperature indicates-insect infestation and fungus, which
reduce the grain's value and mandates countermeasures, cooling of the grain by
pulling air through it or by turning it. Thus, spontaneous ignition is not
considered a probable source.
me probability that electrical apparatus and wiring selected and installed
in accordance with the provisions of the National Electrical Code (NEC) will
be a source of ignition is extremely law under either normal or fault
conditions. The code provisions and apparatus standards place limits on
temperature of exposed surfaces and mandate enclosures that exclude dust that
could contact energized parts.
OCR for page 49
1
TABLE 2 Ignition Sou roes
49
Probability Ease of
Source of Occurrence Elimination
Spontaneous ignition Low Easy
Arcing from electrical apparatus
1
· Normal operation - Low Easy
· fault Me ration Low . Easy
Sparks from foreign materials
Elevators, ferrous metals Low Easy
Elevators, nonferrous metal Low Easy
Elevators, other Low Easy
Mills High Moderate
Static electricity
· Moving belts High Moderate
· Moving grain/dust Low Difficult
Hot Surfaces
o Lad Low Easy
· Bearings High Moderate
· Radiators/Pipes Low Moderate
Friction
· Rubbing head pulleys Moderate Easy
· Slipping. belts High Easy
· Scraping buckets Moderate Moderate
(misaligned belts)
Open f lame Hi gh Moderate
Welding and cutting High Easy
1
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50
If the installation is not in accordance with the NEC, the probability
of the electrical apparatus or wiring becoming a source of ignition is
higher, but in itself such an installation is not likely to be a source of
ignition unless open contact switches or other arcing parts are exposed
-directly to accumulated dust. However, if NEC guidelines are not followed
there is a much higher probability that there is also little control of the
use of portable equipment such as drills, hand lamps, and grinders that may
be ignition sources. Failure to observe NEC requirements often can be an
indication of more serious problems rather than an imminent danger itself.
Even in an installation that does not strictly reflect NEC requirements,
the immediate hazard may be only moderate if the equipment is nonsparking,
enclosures are dust-tight, and a good standard of housekeeping is observed.
This does not obviate the need, however, to follow NEC rules because a
conforming installation is forgiving of other problems whereas a nonconforming
installation in combination with other bad practice may become a hazard.
Most investigations of the production of sparks between combinations of
ferrous, nonferrous, and rocky materials have been concerned with the ignition
of methane. Although there have been many investigations, no simple picture
of the conditions required for dust ignition has emerged. It generally is
agreed that the the rmite reaction between aluminum and rusty-steel under some
conditions can ignite methane. Investigators differ on whether impact or
friction is the important parameter and whether steel-steel or rock-metal
impacts are ignition capable.
The range of energy reported for methane ignition by sparks generated
by impact is wide, ranging from 10 to 400 ft-lb, but in most reports above
200 ft-lb. It seems unlikely that a piece of tramp metal or a rock small
enough to pass through a 1-1/2 in. grate would result in enough impact energy
to ignite grain dust. The weight of a piece of foreign material that could
pass a l-1/2 in. grate could be as high as 1 lb for the case of a steel object.
A 1 lb. object weld have to fall 200 ft to have an impact energy of 200 ft-lb.
Although it is not impossible, it does not seem likely that a piece would fall
from a bucket and drop that distance without first striking another bucket or
the leg enclosure. The above numbers are for methane. Spark ignition energies
for grain dust are at least twenty times higher than that required for methane.
Even if one takes into account that slower release of energy accompanies a
friction spark (a hot particle) and that dust is more easily ignitable by
long-duration electrical sparks (Eckhoff 1975), one concludes that sparks from
tramp material falling in an elevator leg are not likely to be a prime ignition
source.
The potential for tramp metal to~cause a primary explosion does exist in
a hammer mill. The energy released when a small piece of metal is struck by
hammers in a mill is more than sufficient to ignite grain dust. Pieces of
metal as well as other hard objects also can damage hammers in mills and metal
buckets in an elevator. Although the objects themselves may not produce sparks
sufficient to ignite dust, the damage they cause may lead to an explosion
through friction heating or spark generation by the damaged hammers or buckets.
OCR for page 51
51
It has been suggested that the use of plastic buckets instead of metal buckets
would eliminate ache possibility'of sparking and reduce friction heating
resulting from damaged buckets striking or rubbing against the leg enclosure.
More study of this za needed since it is not certain that plastic buckets
will not introduce new and equally dangerous conditions.
The literature on ignition by static electricity under conditions fauna
in a grain elevator is sparse. Palmer {1973) offers qualitative guidance only
and advises the grounding of all ducts and metal. The Canadian Grain Handling
Association {1979) concluded, primarily based on the work of Morse of the
National Research Council of Canada, that static electricity generated in
maying grain is not'likely to be a source of ignition. A more recent study
(Safety Consulting Engineers, Inc. 1980) of electrostatic properties of grain
cites the need for further investigation of the properties in conjunction
with grain-handling facilities. Static discharges from belting have, in
conventional wisdom, been presumed to be ignition capable (University of
Sa~thhampton 1980~. Industry practice has been to bond the metal framework
of conveyors to ground to eliminate build-up of static charges, and this
should be considered standard practice. Although the presence of large
charges has not been'conclusively shown to be an ignition source, it seems
likely that arcing between parts of metal framework not bonded together could
release sufficient energy to ignite a dust cloud or layer. Additional
experimental data on ignition by discharges from betting, the presence of
static charges in the leg, and the likelihood of' static induced ignition in
dust collection system are needed.
me ignition temperature of grain dust layers exceeds 200°C. Hot
lump surfaces can serve as ignition sources if they do not meet NEC
requirements for use in dusty locations. If lighting fixtures are selected
and installed in accordance with the NEC, they should not be considered an
ignition'source except if installed in a position that permits dust to
accumulate on' the hot,- glass surface in a way that impedes beat transfer.
Fires due to hot bearings have been reported, and one must conclude
that bearings are a likely ignition source if only because they are so
numerous in an installation using conveyors. Boot and head pulley bearing
failure is especially hazardous. Two methods for reducing the hazard due to
hot bearings have been proposed: locating the bearings outside of conveyor
and leg enclosures so that overheating will not cause ignition of surrounding
dust and monitoring the temperature of the bearings. The application of
these methods will be inexpensive in new construction but only the second is
applicable to an existing facility. Both still require a relatively high
standard of housekeeping to keep dust layers from accumulating on external
bee ring su rfaces .
Slipping belts, especially at the head pulley of a leg, have been
blamed for grain elevator explosions, in many cases because friction ignited
the belt, which then parted and dropped dawn the leg. The universal
application of underspend devices that prevent operation of the elevator
under this condition can eliminate this source of ignition. A somewhat more
OCR for page 52
52
remote hazard, though it has occurred, is friction heating of the belt due to
a slipping or locked boot pulley; therefore, the underspend device should
monitor the speed of the belt as well as that of the head pulley. (In one
explosion (Finn 1976), the pulley speed was maintained but the belt slipped
because the lagging was worn off and caught fire and dropped down the leg.)
This remedy is applicable to existing as well as new facilities. Interlocks
to shut dawn moving systems when part of the system fails' or when dangerous
choke conditions occur should be standard features.
Ve nting
Pe commendation
.
Follow, to the extent practical
The Na tional Fi re Protection
Ass'ociation's Standard on explosion
venting (No. 6 8) for all enclosures.
Concrete structure. should be vented
by windows or other openings of the
size dictated by this standard.
l
Bf ~cY Feasibility Efficiency
L ~ '' H
Venting can be considered to be the third most important area (following
dust and ignition source control), but its effectiveness is limited since it
is effective only during the occurrence of an explosion. Many also have
reservations about the effectiveness of venting. The American Insurance
Association (1978),-for example, states: When the rate of pressure rise
exceeds 3,300 psi/e, it apparently becomes impossible to design an effective
explosion relieving system." Nevertheless, since the greatest amount of
damage and human injury usually is; caused by secondary explosions, venting
should be considered if it can reduce the connection between primary and
secondary explosions.
Many locations within a grain-handling facility can be regarded
literally as Loaded cannons" when they contain sufficient dust (either in
layers, in a cloud, or both) to support an explosion. Examples are legs,
empty bins, tunnels, duct work, headhouses, enclosed conveyors, and galleries.
If there is sufficient fuel, an explosion in any of these enclosures will
propagate through it until sufficient pressure is built up to rupture the
walls or until the pressure is relieved at the end of the enclosure'. (If the
end of the enclosure is strong-enough to withstand the pressure, the
reflection of the pressure wave back dawn the enclosure adds to the magnitude
of the already existing pressure.) She bursting strength of existing
structures is small compared to the maximum pressures generated by most
well-fueled grain dust explosions.
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53
Technical information on the merits of venting grain-handling
structures is practically nonexistent. Particularly hazardous is the
formulation of size of vent openings based only on the total volume enclosed,
without giving consideration to the effect due to the length/diameter (L/D)
ratio of the enclosure, or roughness. When the ratio, L/D, equals or exceeds
10 the equivalent diameter of the vent opening exceeds D. Numerous elevator
legs have a vent panel only at the top. A well-fueled explosion, initiated
at the bottan of the leg, will generate enough pressure to rupture- the leg
casing as it progresses up the leg long before the pressure wave reaches the
vent. Indeed, the panel has investigated explosions in which the leg casing
ruptured and the vent, in operable condition' was still closed.
For those cases where L/D is less than 10 (e.g., garners, scales, and
bag houses}, venting can provide she protection against rupture of the
enclosure but a flame will propagate through the normal or vent openings of
the enclosure. In addition, the vents must release to the outside of the
facility if they are to protect employees in the working areas from exposure
to flames.
Examples of incidental venting have been noted in a number of
explosions. Headhouses having numerous windows or steel sheathing walls
suffer much less damage than those constructed of concrete with only a few
windows or none at all. Believing explosive pressure by blowing out windows
or frangible sheathing is preferable to spraying the landscape with large
pieces of concrete from a concrete headhouse that has contained the explosion
until the pressure tee cones sufficient to rupture its walls.
The modification of existing structures to provide venting is often
impossible and always very expensive. For example, nothing can be done about
venting tunnels that are already completely below grade {e.g. those below
silos). Thus, venting should be applied to exterior structures (e.g., bag
houses, exterior legs- or other conveyors, and exterior ductwork).
Sur'are As ion
Be Commendation
Impractical for the workplace.
Possibly feasible for the interior of equipment.
Devices for the suppression of explosions can be installed in legs,
ductwork, and other narrow enclosures. These devices are containers-of
pressurized dry pander or inert gases, usually Halons, which release the gas
when triggered by actuators sensitive to flame (infrared) or pressure rise.
They are very effective in suppressing explosions in enclosures, especially
legs and dust collection systems, but they have two drawbacks: they are
relatively expensive for small facilities and they are not 100 percent safe
against false actuation, which adds to their operating cost because recharging
is expensive. Research-and development being conducted by the manufacturers
of these devices should be followed closely to determine if they are becoming
more cost-e ffective for small facilities.
OCR for page 54
S4
Passive and active barrier systems that have been extensively employed
by the coal mining industry for explosion suppression in tunnels (Cybulski
1975, Liebman et al. 1976) must be examined separately. These devices spread
an extinguishing agent-water, Purple R. rock dust--across the tunnel ahead
of the advancing flame front. The passive devices consist of frames
supporting water containers near the tunnel roof. The airflow behind the
pressure wave created by the explosion dumps the water. With the active
devices, a sensor detects the explosion and actuates the dispersion devices.
Extensive testing has led to optimum designs for these barriers and they may
be applicable in elevator galleries and tunnels. Specifically, the water
barrier. are relatively inexpensive to construct, require little maintenance,
and are reliably triggered.
Fire suppression by use of automatic sprinkler systems teas only
marginal value in the prevention of explosions. {me protection of the
physical plant from damage due to fire alone is not the subject of this
report. ~ The initiation of a number of explosions can be traced back to
smoldering dust that could never have triggered an automatic sprinkler
system.
Inert iw
Pe oonanendation
Do not use inerting because it is too expensive
and is dangerous to personnel.
Operating a-grain-handling facility in an inert atmosphere to prevent
explosions has been considered in the past. It has been judged to be
completely impracticable from both a mechanical and economic point of view.
In addition, the inerting of large volumes is dangerous because workers- can
be asphyxiated.
Education
Recommendations Ef ficacy
Establish an information
center to distribute actively
all available information
on elevator and mill dust
explosions and the ir causes
and prevention.
Investigate and report on
explosions in a manner that
re fleets the recommendations
made by the panel in its report,
Report NMAB 367-1.
Feasibility Efficiency
.
M L H
L ~ L
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1
55
As noted many times in this report, ignorance of the explosion hazard is
very prevalent and reflects both poor dissemination of what is known and lack
of knowledge concerning hazardous factors or situations. The degree of
ignorance is inversely proportional to ~
the unit Blue In the 1naustry (l.e.,
the management or large grain-nanoling corporations is much better informed
than the management of "mall mills and country elevators); therefore, given
the preponderance of small facilities, lack of information is an important
factor. The degree of ignorance also increases in going from overall
management to the lowest paid employee.
Ignorance of the dust explosion hazard can be alleviated by collecting
available information in a central repository, a relatively simple but
laboricus task and by distributing the available information. Numerous
organizations now are engaged in disseminating information {e.g., the
National Grain and Feed Association, the Grain Elevator and Processing
Society, the Department of Agriculture, NIOSH, OSHA, the trade and union
press, and various university and private research organizations), but it
does not seem to reach the Grass roots. of the industry. Regardless of the
reason for this, it is a problem that should be overcame. The panel suggests
that information-be channeled through the Department of Agriculture to the
state Directors of Agriculture down to the county agents of the Cooperative
Extension services. me elm OSLO ensure that each graln-processlng zaclllty
is informed without having to request information. Organizations such as
the U.S. Fire Administration, OSHA, NIOSH, trade and professional
associations, unions, insurance groups, and trade publications also should
receive all available information.
it.
Recommendations Efficacy ~ Efficiency
,
Conduct rigorous Preventive H H H
maintenance, especially on
all parts of bucket elevators.
Notify all facility managers
that safety is a non delegable
re sponsibility. If author) ty is
delegated it must be to an employee
who reports directly to the plant
manager .
L
H
H
Es tablish a f ire and explos ion M L H
prevention training program in
each facility.
If dust is returned to the grain M L L
stream do it in the least hazardous
manner.
OCR for page 56
S6
The manager of each facility is ultimately responsibile for the safety
of his plant. Either the manager or a designated employee who reports
directly to the manager should be responsible for the day-to-day safety.
This person should be completely familiar with all of the plant operations
and should perform a system safety analysis of the whole plant. It is
important that he understand that all explosions are preceded by a sequence
of events that may have begun quite sometime before actual ignition of a
dust cloud. '
'Operating'procedures affecting safety are numerous and varied but can
be classified into a few categories. The diversity of sizes and types of
elevators and mills dictates that the details of these procedures be left to
each plant or facility; numerous examples t for each overall subject are '
readily available.
The first and most important action is to insure that every employee,
visitor, contract employee, local firefighter, and any others who may be in
the facility are aware of the hazard of dust explosions and the means for
their prevention. Numerous examples of explosions resulting from welding
operations appear in the literature and the panel is aware of a number of
instances when ignorance of the proper method of fighting a dust fire led'
directly to an explosion.
Housekeeping, including continuous surveillance for dust emissions and
deposits, must be treated as a first priority activity in plant operations.
The panel has seen two instances where several feet of dust had collected in
boot pits and ultimately led to explosions. It was said that the pits were
cleaned out regularly every few months' The importance and degree of
housekeeping should be directly proportional to the degree of activity of
the facility, not inversely proportional.- '
'Maintenance is related to ignition sources in the same fashion as
housekeeping is related to dust control. It is assumed that any normal plant
operation should include preventive maintenance; however, in facilities
where flammable dust is a problem, maintenance to prevent ignition sources
assumes greater importance. Problem sites are bearings, belts, buckets,
augers, pulleys (including lagging), trippers, motors, dryers, and dust
collection systems. Because of poor design, however, it frequently is
'difficult to conduct effective maintenance. Head pulley gear boxes may have
. .
no work platform around them, tail pulley bearings may be in an unlighted
boot pit next to a wall, and legs and enclosed conveyors may not have
inspection ports at critical locations. Recognized problem sites and
maintenance areas must be made accessible through the use of platforms,
removable sections, and hinged ports that can be used by a mechanic (and
possibly a helper) who may have tools in both hands. Dryers fueled by
propane or butane, which are heavier-than-air gases at room temperature, can
be particularly hazardous. Leaks in fuel lines can spread a layer of highly
flammable, transparent gas throughout the lowest points in a plant.
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1
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Recommendations for the disposal of collected dust are certain to
create vigorous responses from industry, labor, and government. As has been
noted before, the question of returning the dust to the grain stream is the
point at which safety and economics can come directly into conflict. Several
aspects of the economic problem were considered by the panel, ranging from
total cleaning of the grain to re-introduction of the collected dust into the
grain stream. A properly designed and operated dust collection system
collects only those dust granules that become airborne. The amount of dust
of this size per bushel of grain, although a small percentage, can change
radically from crop to crop, season to season, and grain type to grain type.
It therefore is difficult to predict the economic impact of discarding the
collected dust other than that there is some lost involved. After
considering and weighing all factors, the panel ha's concluded that much more
attention must be given to the method by which dust is re-introduced into
the grain stream and its effect on downstream elevators. (The panel has
-.een'one particularly bed 'example in which dust collected on the upside was
delivered directly into the downside of an elevator leg')
This conflict between safety and economics can be resolved if the
industry will assume the responsibility for developing and demonstrating a
method for re-introducing collected dust that will not increase the explosion
hazard above that resulting from disposal of the dust. Methods of
re-introducing dust and possible alternative uses of dust that would lessen
the incentive to return it to the grain stream are discussed below.
RESEARCH
Dust Control
Recommendations Efficacy Feasibility Efficiency
Continue research on methods M H H
for reducing the dust concen-
tration in legs to a level
below the lower explosive limit.
Continue research on methods L M M
of reducing dust concentrations
below the lower explosive limit
in enclosures other than legs.
Conduct research to develop L M H
economic uses for collected
grain dust.
In conformance with some of its other recommendations, the panel
believes that research aimed at reducing the dust concentration in enclosed
areas will produce the greatest decrease in the explosion hazard. Thus, it
believes that research should be directed at developing thorough understanding
1
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of the movement of grain from the boot to the discharge spout and of the
airflow needed in the enclosure for an effective dust control system. Other
topics such as'bucket material and design, belt material and design, and
placement of'input and output openings also should be examined. The aim of
the research should-be twofold: to identify reasonable modifications that
can be made in existing elevator legs and to develop the optimum design for
legs in new facilities. Inclined belt conveyors, because of their length, are
not always suitable replacements for legs. Their capital and operating costs
are greater than those of legs and they require a horizontal space that may
not be available.
,
The problem with other enclosures is different from that with legs since
it involves the release of dust from a falling grain stream. This was amply
demonstrated to panel members who observed barge loading. When the end of
the loading spot was kept level with the surface of the grain no dust
appeared; when the spout was a few feet above the grain considerable dust was
released into the air. Research in this area should focus on identifying
methods for preventing dust from becoming airborne when filling an enclosure
and collecting the duct that may become airborne. The latter task is
complicated by the fact that most dust collection systems can serve as "sneak
paths. for transmitting explosions from one enclosure to another. Hence,
several smaller dust control systems may be preferable to a large system.
me use of grain dust for pelletized animal 'feed does not require
additional research, only ecoriomic development. The few pellet mills already
in existence can sell all they produce and portable mills can service small
elevators having only relatively small amounts of dust. If the economic
value of dust can be increased, the costs of dust collection will become much
more acceptable and the tendency to return dust to the grain will decrease.
Information on the present disposition of collected dust, the amounts
collected by elevators of various sizes and locations, and the cost of
transporting dust is needed.
Research also should be conducted to answer such questions as: How
clean is clean? Is it dangerous if the dust layer on the floor is deep enough
to show footprints? Will a primary explosion disperse enough dust to cause a
secondary explosion if the floor layer is 1/8 in. thick or 1/16 in. thick or
even smaller? How much dust will adhere to concrete walls? How well will
various thicknesses of dust propagate a flame? No single answer to these
questions will be applicable to all enclosures in elevators and mills.
However, experiments conducted under rigorous, well defined conditions can
establish meaningful reasonable upper or lower bounds.
' AS noted previously, the burden of research on safe ways to
re-introduce dust into the grain stream and the proof of their efficacy
should fall upon the industry. Using the values previously mentioned in this
report of 13,000 tons of dust collected from a large elevator in a year's
time, with the value of grain about $150/ton and the value of dust (for
pellets) of about S50/ton, it is easy to see that there is a difference of
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about $1,000,000 involved. This should provide some incentive for the
research. On the other hand, it does seem inconsistent to place a large
elevator in jeopardy to recover two thirds the grain value of airborne dust,
which may amount to only a fraction of a percent of the grain handled. (A
recommendation applying to this topic has already been made. See page 55.)
Research on the application of a substance to grain to inhibit the
formation of dust is now in progress. The results are too preliminary for
the panel to make any recommendation on the subject.
Ignition Sources
Re commendation
Investigate the e ffect of
electrostatics and humidity on
the explosion hazard, including
an examination of conveyor belt
conductivity and the 'charging of
ungrounded conductive structures.
Efficacy ~ Efficiency
L ~ L
Host of the ignition sources in elevators and mills are self-evident
and the reasons for their occurrence are not mysterious. 'The one exception
is electrostatic discharge. Two research topics are involved: the build-up
of electrostatic charges on conveyor belts and other poorly conducting
materials and the accumulation of static charges on grain and grain dust.
'This work should encompass a number of different topics. Those readily
apparent are: (1) the static charging and release characteristics of
conveying belts of various materials and various conductivities; (2) the
potential for build-up of charges in dust clouds in silos, bins, garners' or''
other enclosures ; (3) electrostatic phenomena occurring in pneumatic svs_~ms
conveying dust and grain; {4) electrostatic conditions in legs using metal
buckets and using plastic buckets of various conductivitie's; and (5) the
effect of atmospheric conditions on the buildup of charges and on the energy
needed to ignite grain dust.
Among the aims of'work done concerning topic 5 should be to establish
the facts concerning the danger of low relative humidity and low absolute
humidity. (The effect on the explosion hazard due to agglutination of
layered dust resulting from high humidity has never been examined.) Other
factors to be considered are differences in electrostatic properties for
different types of dust and the basic electrostatic characteristics of
grain-handling machinery (i.e., when the machinery is operating but no grain
is being handled). Considerable thought and care should be given to the
design of experiments in this area since electrostatic phenomena in
industrial locations are so elusive and ephemeral. The results of this
research obviously should be accompanied by recommendations for the
elimination of any electrostatic hazards uncovered.
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FUTURE FACILITT E S
Re commendations
Treat'the avoidance of dust
explosion hazards as an initial
design criteria in the cons traction
of new mills and elevators and the
modification of existing structures.
Examine the overall functions of
mills and elevators to develop a
totally new system less subject to
the hazards of dust explosions.
EffiCDCV Feasibility Bffici~
L
M
~ H H
- The first recommendation on the design of future facilities requires
little discussion. New facilities should be designed to incorporate adequate
dust control, 'to avoid dust generating operations, to facilitate housekeeping,
and to be well vented. Design criteria should reflect these concern. so they
are not considered only after the final design i. completed when any changes
become expens ive .
One problem to be considered in the design of elevators and mills is
the "response. of the facility if, for- sane reason, there is a primary
explosion. Thus, design criteria should consider' the isolation of sites
where primary explosions may occur from those that may produce secondary
explosions-. The use of outside legs, pressurized electrical vaults, and
isolated dryers are e'xamples that readily cane to mind.
Conservative des ign has been the rule in the grain-handling industry
and although most new elevators and mills incorporate advances in technology
(e.g., television surveillance, electrical interlocks, and dust collection
systems), they still handle and process grain in fundamentally the same manner
as has been used for the past 100 years. A study of the functions of
elevators and mills (e.g., grinding, blending, and storing of grain) is
needed to serve as the teas 'is for totally new elevator and mill designs that
will reduce the explosion hazard without decreasing efficiency or increasing
cots, both capital and operating. This study should be limited only by the
fact that grain must be transported from the farm to the ultimate consumer
with various processing operations occurring along the way. The rapid
increase in grain production in the past 20 to 25 years, in a broad sense,
changed the function of elevators from storage facilities to surge tanks in a
pipeline. The gradual evolution of elevators to accommodate this change
unfortunately carried along the hazardous features and, in sane cases,
intensified them. Considering that ache grain-handling industry accounts for
more explosion" than any other single industry, it would seem worthwhile to
re-examine the entire grain-handling process.
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GOVERNMENT REGULATIONS
Recommendations for actions by governments at any level--federal, state,
or local--in matters of hazard reduction cannot be ignored, especially in
cases pertaining to employee safety. Government regulations applying to
grain-handling facilities are no different in purpose than those applying to
other industries having safety problems: they are intended to point out
state-of-the-art practices that will provide a safe working environment and
they serve as ~laws. whose willful violation will result in punishment that,
in turn, will convince other possible violators not to follow the same path.
Grain industry and government understanding of safe operating practices
in elevators and mills' is minimal. Although large grain corporations will
disagree vigorously with this point, there are literally thousands of
elevators and mills operating as independent entities whose understanding of
the hazard is at best limited to a knowledge that elevator explosions are
fueled by grain dust. The federal, state, and local occupational safety
enforcement agencies are in no better position to decrease the dust explosion
hazard for a 'number of reasons. First, there are no regulations that apply
specifically to elevators and mills. Second, the protection of elevators and
mills from explosions i" only a small portion of their responsibility and!
consequently, they have allocated only a small portion of their staff's time
and effort to the problem. {The majority of contacts between industry and
safety enforcement agencies occur either after the fact--following an
explosion--or during infrequent safety inspections. The panel was privately
advised that in one state the available manpower was such that only about
2 percent of the elevators and mills could be inspected each year.) Gird,
animosity exists between industry and regulatory agencies and, whether for
real or imagined causes, 'it is a hindrance to safety.
Fortunately, some progress is being made in alleviating ' the contentious
situation between industry and government (OSHA). This may result in progress'
in reducing the explosion hazard. Since this panel's formation, OSHA has
taken two positive steps. First, it has developed a training program for its
explosion investigators to enable 'them to determine belter' the "causes of
explosions. If these investigators consider their primary task to be a
determination of cause, the mystery attached to explosions should be reduced
and better relations with industry may result. Second, it conducted a series
of meetings {New Orleans, Superior, and Kansas City) in 1980 on hazards in
grain-handling facilities that demonstrated its willingness to accept
industry's input. Industry, itself, cited the need for performance standards
at these meetings. It is, however, too early to assess the results of this
effort.
Appendix D consists of a report on recommended standards prepared by
this panel's & bpanel on Recommended Standards and Regulations. That report
should be considered as a beginning step in formulating standards by a
cooperative action between industry and government.
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Changing the grain grading system so as to penalize for the amount of
dust, with the objective of decreasing the explosion hazard, has often been
mentioned. The panel is of the opinion that, besides meeting considerable
· objection from sellers, buyers, and handlers of grain, the definition of a
standard solely for.this purpose is impractical. The only effect would be to
pass part of the hazard cost back to the deliverer of the grain since the
grain would be accepted, clean or dusty. It is recognized that the return of
dust to the grain stream places a heavier burden on the downstream handlers
of grain and their dust control systems. A standard, based on the assumption
that the degree of cleanliness of the grain (as now delivered to elevators
and mills).is directly proportional to the safety of the facility, ignores
the hazard due to dust generation in the facility.
REFERENCES
.
American Insurance Association, Special Lies Control Bulletin,.New York
.City, N.Y., 1978.
Canadian Grain Handling Association! Fire and Explosion Task Force Report
No. 1, Winnepeg, 1979.
. Cybulski, W., Coal Dust Explosions and Their Suppressions, NTIS TT
73-57001, National Technical Information Service, Springfield,
Virginia, 1975.
Eckhoff, R.K., Towards absolute minimum ignition energies for dust
clouds, Combustion and Flame 24 (1975~:53-64. .
Finn, W.D., Report of the Commission on Health and Safety in Grain
Fn evators--Burrard Terminal Fire and Explosion of October 3, 197 5,
University of British Columbia, Vancouver, 1976.
Liebman, I., Carry, J., and Richmond, R., H2O Barriers for Suppressing
Coal Dust Explosions, U.S. Bureau of Mines, Washington, D.C., 1976.
Safety Consulting Engineers, Inc., Rosemont, Illinois, Electrostatic
Characterization of-Grain Products, prepared for the National Grain and
feed Association, Washington, D.C., October 1, 1980.
Palmer, R.N., Dust Explosions and Fires, Chapman and Hall Ltd., London,
19 73.
University of Southhampton, England, Study of Static Electricity on Grain
Conveying Belts, prepared for the National Grain and feed Association,
Washington, D.C., November, 1980.
Representative terms from entire chapter:
explosion hazard
