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APPENDIX
INVESTIGATIONS
During the period November 1978 to July 1981, the sites of a number of
grain elevator explosions were visited by members of the Explosion
Investigation Subpanel of the Panel on Causes and Prevention of Grain
Elevator Explosions.- The elevators ranged in size from a small country
elevator with a capacity of approximately 20,000 bushels to a large export
elevator with a capacity of approximately 6,000,000 bushels. In all cases,
excellent technical information on grain dust explosions was obtained. If
one includes two recent European investigations--the Roland Flour Mill in
Bremen and Euro silo in Ghent--this information can probably be considered
the most reliable contemporary data in existence. The six incidents
described here are typical.
Information from each accident is presented separately in the form of an
incident report. In the first section, Observations, factual information
collected at the site of the accident is given. In the second section,
Scenario, a plausible chain of events leading up to the accident is
presented. Some of this material is speculative. In the-third section,
Conclusions, information relating to grain elevator safety and the
successful conduct of investigations is discussed.
Incident No. 1
An explosion occurred at a concrete inland terminal elevator of medium
size. There were truck dump and loading facilities and rail dump and rail
loading facilities. A large headhouse was located between, but separated
from two silo complexes. There were three bucket elevators within the
headhouse having combination steel and concrete double legwells. One truck
dump was adjacent to the headhouse and not enclosed, another was located a
short distance away and enclosed. The rail deep and loading areas were
alongside the headhouse on two lines; the inside line served the dump pits.
There was a large drier between the headhouse and one silo complex with two
bucket elevators. A machine shop was located on the opposite side of the
headhouse. The office was located a moderate distance from the main
elevator accident. There was a primitive dust control system. The level of
housekeeping, as judged qualitatively by the subpanel, was not good.
Observations
The explosion occurred in mid-afternoon.
1
17
Severe structural damage was
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done to almost all of the bins in the headhouse and moderate damage was done
to most of the headhouse structure. The tops of almost all of the headhouse
bins had blown off, essentially destroying the top of the bin floor and the
top of the cleaner floor. In addition, some of these bins around the edge
of the headhouse caused the failure of the outside wall. In the headhouse
structure above the bin floor there were a large number of casement type
windows. All of these windows were destroyed but there was little damage to
the walls themselves. Fireballs had propagated through the basement, work
floor, bin floor, and scale floors. Severe damage occurred to all elevator
legs. Where the legs passed through the house bins in concrete legwells in
some areas the concrete had completely shattered. This was especially true
on the gallery floor where one of the legs vented. On other floors above
and below the bins many of the the metal leg casings were split wide open.
The one elevator leg showed severe fire damage with much of the belt and
many plastic buckets burned. The exterior wee and dry drier legs also
showed moderate explosion damage. Adjacent to the work floor the rail
loading and dumping area showed fire damage but only alight blast damage.
The two truck dumps showed evidence of fire and blast damage. The dust
collection systems adjacent to the headhouse showed slight explosion
damage. In the one silo complex severe explosion damage occurred to the
catwalk connecting it to the headhouse, the gallery at the far end of the
tunnel, and a small group of silos centered around an airshaft approximately
one-third of the way along the gallery. This happened to be the location
where the belt tripper was parted. Between the headhouse and this location,
gallery sidewalls and the roof had been displaced and the windows were
destroyed. At the location of this airshaft the gallery walls and roof had
been completely destroyed. Beyond this point the explosion damage to the
gallery was still significant but not as severe. At the location of the
airshaft the grain in the surrounding tanks was on fire. The interior
concrete bin walls had been extensively shattered leaving in many places
only the reinforcing rod. The tripper rails above this area had been bent
straight up. Concrete fragments from this area of the elevator had been
thrown several hundred feet into the adjoining rail yard. In the tunnel
under this silo complex only the portion from the airshaft to the far end
showed any fire or explosion damage. An extremely strong blast wave had
propagated down this tunnel shearing of f the grain Spouts at the level of
the bin bottoms and completely destroying the end of the tunnel. The
vertical slab had been moved several inches back into the earth. The other
silo complex showed practically no explosion damage with the exception of an
empty bin near the far end, which had exploded and caused moderate damage at
its bottom. Damage to the tunnel was slight and a fireball had propagated
down the gallery from the headhouse. Some damage from flying debris was
done to the office building and cars parked near the elevator. At the time
of the explosion milo had been unloaded from rail cars and corn was being
loaded into rail cars. The leg that had been carrying the milo was running
empty and another leg was carrying the corn. Both of the basement belts
were running and neither gallery belt was being used. The first evidence of
an explosion came from an employee in the basement near the north tunnel.
He stated that he heard a "pop" and saw a f ireball coming out of the boot of
the leg that had been running empty. An employee on the scale f loor f elt
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the explosion and sought shelter under a desk in the scale shanty. An
employee standing outside the office and looking at the basement of the
headhouse saw dust coming out of the basement windows and heard five
explosions. An employee on top of the railroad car saw a fireball come out
of the loading spout and was knocked to the ground. He thought that he
heard three explosions . Concre te f ragment s came through the roof of the
of f ice building but did not ~ trike any of the occupant s . An individual here
described the explosion as sounding like a train wreck or the bumping of
cars in a railroad yard. At this point the plant manager attempted to call
the fire department but the telephone was inoperative. Personnel from
surrounding industry called the fire department, who extinguished the fires
in the headhouse but not in the grain tanks near the airshaft. After the
explosion, regulatory agency personnel found extensive tramp metal in two of
the elevator boot pits. Elevator employees unloaded the hopper care that
had been filled near -the time of the explosion and found a bindicator (bin
indicator) and a portion of electrical conduit. The bindicator had been
located in the loadout bin. The fuse associated with this bindicator was
blown and the wiring and conduit remaining in the bin showed evidence of
arcing.
Scenario
A low level bindicator was installed in the loadout surge bin, which was
between the scale and the car. Near the bottom of the bin a bindicator was
installed inside the bin wall cantilevered on an electrical conduit elbow.
The bindicator weighed perhaps 10 pounds, was approximately 10 inches in
diameter, and had a depth of approximately 6 inches. The-face was a rubber
metal diaphragm, which made mechanical contact with a microswitch.
Appropriate installation of this bindicator would have made the diaphragm
flush with the bin wall. A load of grain that was dumped from the-scale
impacted upon the bindicator causing it to break loose from the conduit. -
This left exposed at the conduit the bindicator circuit wires with a live
potential of 110 volts. After several additional grain dumps from the scale
into the surge bin one of the dumps impacted upon the broken conduit and
exposed wires. This caused the wires to contact each other and the conduit,
producing an electrical arc. As the scale was continuing to dump the surge
that was ignited by the electrical arc, an explosion occurred in the loadout
bin and propagated into the adjoining house bins and the bin floor area
through the destruction of the bin walls and tops. The explosion was able
to go out the loadout spout to the car. The explosion also traveled up the
spout into the scale and then through the common dust collection system into
the bucket elevators. The legs blew out at various levels in the
headhouse. The leg blowout on the gallery floor allowed the explosion to
propagate in one direction toward the set of tanks where the airshaft was
located. Progress of the advancing explosion down the gallery was impeded
by the tripper, and the advancing airflow went down the ventilation shaft.
This airflow in a vertical shaft would be ideal for producing a well-mixed
dust-air mixture. The flame front then arrived as it too was deflected by
the tripper down this shaft. A very rapid rate of combustion occurred. The
combustion wave then came back up the shaft and returned toward the
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headhouse and went down the shaft into the tunnel where it went out the far
end. In the gallery a flame front from the exploded leg casing traveled
into the other silo complex. Conditions there were not as favorable for ,
propagation and only a low velocity flame front traveled near the floor.
However, at the end of the closed gallery it was reflected into the top of
an empty tank. The dust inside the tank was disturbed and an explosion
occurred.
Conclusions
.
Regulatory agencies have listed the tramp metal in the boot pit as the
cause of this explosion. All reasonable efforts should be made to keep this
foreign material out of the elevator equipment by the use of properly sized
gratings on all dumps and the use of magnets on bucket elevator feeds.
Regular cleaning of the boot pit will catch any foreign material that
escaped the other collection system. However, there seems to be little
information relating to the detailed mechanism of the heating and ignition
of grain dust by tramp metal. '
The ignition source considered by investigators from the subpanel to be
more likely is the failed Vindicator. Bindicators should, of course, be
installed in the specified fashion. Moreover, only' low voltage electrical
circuits should be used ' in Vindicator circuits to lessen the hazard caused
by arcing. Guidelines for design of circuits that are incapable of igniting
grain dust are contained in NFPA 493, "Intrinsically Safe Apparatus for Use
in Division 1 Hazardous Locations."
Other possible ignition sources that were rejected as causes because
they were not at the apparent point-of-origin were rubbing of the gallery
belt on the frame and the dragging of rail cars through the car dump using a
winch. The rubbing had begun to cut the frame and there is little doubt
that the metal was red hot. This could have led to a smoldering fire.
Friction sparks that could be causes by the dragging of the cars could have
fallen into the dump pit.
It may have been possible to have detected the loss of the Vindicator.
If there had been a grating on the hopper car fill the broken Vindicator
would have been caught at this point. If a truck had been filling,
presumably the driver would have seen it. Also, if the Vindicator signal
had been continually monitored, the loss of signal would have been
immediately noticed. A minicomputer is more reliable than a human in this
respect .
Once the explosion was initiated, it could propagate in several
directions. A Mayo spout was used on the bin floor. Once the explosion
reached thi 8 location it could enter every headhouse bin. Also, the dust
collection systems on the scale and the legs were interconnected, allowing
the explosion to propagate into the legs. European design practice is to
use several smaller dust control systems. The one leg was running empty and
there was probably a heavy suspended dust concentration. The legs
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effectively propagated the explosion downward. The headhouse had an
unusually large window area for its volume; it was quite well vented and
there was little structural damage. Most structural damage was done from
the explosion of headhouse bins (garner bins). Concrete fragments were
dispersed to rather distant places. Some of them impacted on the office
building. The commonly mentioned rule of thumb that a distance from the
elevator equal to the height of the headhouse should be a safe distance
would appear to be inapplicable. A plate glass window in a shanty structure
on the bin floor shattered and the fragments caused injuries. Within
elevators only shatterproof, transparent materials should be used.
In a large elevator where there are many employees there must be an
explosion-resistant emergency warning system to call for an evacuation if
there is time. In addition, provisions must be made for locating personnel
after a disaster. An assembly area should be designated in order to
identify the missing. Time cards can be used for employees but gate records
must be used for farmers, truckers, salespersons, etc. Some idea should
exist as to the previous locations of missing personnel. Power, water
hydrant, and telephone lines to the elevator should be blast resistant.
This probably implies burial at some distance from the elevator foundation.
Employees should wear fire-resistant or fire-retardant clothing over all
portions of the body, including the head and hands, to minimize the severity
of burns. This is perhaps most easily done by using coveralls or Jump suits
in conjunction with caps and gloves. Under no condition should employees go
shiftless, wear only undershirts' or wear synthetic fiber (edge, polyesters)
clothing, which considerably exacerbates burn injuries.
Complete cooperation by management is most helpful in conducting an
investigation. They can furnish blueprints, witness statements, flashlight
batteries, and hospitality. One witness who was badly injured had been in a
location where he could have clearly seen the course of the explosion but
declined to cooperate on the advice of legal counsel. He was concerned that
he would jeopardize any chance of financial compensation for his injuries.
It is not known whether this difficulty is real or perceived-, but whatever
the reason it can be a significant impediment to investigations. Two visits
were made to the site of the explosion. One was 2 days after the explosion
and the other after 2 months. Both resulted in valuable information. A
third meeting with members of the elevator management was made some 6 months
after the explosion. Because of the size and complexity of the explosion it
was not until after the third meeting that a reasonable scenario could be
assembled.
Incident No. 2
An explosion occurred at a large wooden country elevator. There were
rail loading and truck dump and truck loading facilities. There was a
separate headhouse with two metal-encased bucket elevators. Two storage
annexes were located on either nice of the headhouse and two metal grain
storage tanks were at the end of one annex. The level-of housekeeping, as
Judged qualitatively by the subpanel, was not good.
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Observations
The explosion occurred at mid-afternoon. Moderate damage was done to.
the metal-clad, frame-structure elevator buildings. The one annex building
seemed to suffer slightly more damage than the other. The bins were
essentially empty. An explosion propagated down the tunnel destroying the
door.at the end. Some of the bin bottoms were blown inward. The explosion
was able to enter the empty bins in this fashion. The metal tank beyond the
door was slightly burned on it8 side. The gallery and bin roof on this
annex building were mostly destroyed. It had been displaced to the side and
some of it was on the ground. The wooden bins were still intact but showed
'evidence of internal burning. The bin roof and gallery of the other annex
were still intact but the gallery showed,some bulging. This annex was
mostly full and the explosion did not enter the bins. The two sides of the
headhouse that did not face the annexes were blown off from the bin floor
level to the roof. There was little damage to the bucket elevators and
related equipment. A slight amount of burned grain was present. Before the
'explosion, the elevator had been unloading trucks of soybeans. Someone
smelled smoke coming from the boot pit area that was covered by a hatch on
the elevator work floor. The fire department was called and the elevator
was evacuated. Upon the arrival of the fire department, three elevator
employees and three firemen lifted the boot pit cover and observed flames.
The fireman with a baseline set the nozzle on fog and started to spray water
into the burning boot pit. A "woosh" was heard and a ball of flame shot out
of the pit'. The people who were around this area promptly exited the
elevator and after they had cleared the facility a blast was heard coming
from the elevator. The fireman began to put water on the resulting fires,
which were mainly confined to the grain. The watering was stopped when it
was decided that the burning grain could ' tee shoveled from the elevator.
There was little fire damage to the structure.
Of the six persons standing around the boot pit only one was injured
during the attempt to escape. His exit was by an indirect path to the
outside that took him through part of the elevator, rather than by the
direct exit door to the outside that the others used. This slight time
difference was apparently enough to allow the,expanding fireball to catch up
with him.
Scenario
The boot pit area beneath the work floor had not contained a light ~
fixture and a regulatory agency had ordered the installation of one. This
area had not been recently cleaned (probably for a month) and the
accumulated dust had buried the light fixture and the light was turned on.
The hot fixture, well insulated by the grain dust, caused the dust to
ignite. The application of the fog water stream dispersed the burning and
unburned dust into the air and an explosion occurred. This partially vented
into the work floor area but also traveled up the two bucket elevators to
the top of the headhouse, into the two galleries, and down the one tunnel.
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Conclusions
.
Proper and correctly installed electrical equipment can cause problems
if the surrounding environmental conditions change. The accumulation of
dust not only presents a fire hazard but also an explosion hazard. In
elevator designs, all areas, boot pits included, must have easy access, be
well lighted, and be conducive to the performance of routine tasks such as
housekeeping. Dust must not be allowed to accumulate in hidden areas where
its danger goes undetected.
- The fighting of elevator fires is a sensitive procedure. There seems to
be a general belief that a fog stream will not disturb the dust and lead to
an explosion. Until better methods are devised it is probably best to
remove burning grain and dust from the elevator by the use of buckets ant
shovels. Fire department personnel in areas where grain elevators are
located must be educated concerning tactics for combating fires in grain
elevators. Wooden elevators would appear to be leaky enough that the
overpressure resulting from the explosion cannot build up to a high enough
level to do major structural damage. Therefore, if the few resulting fires
from the explosion can be initially controlled, the wooden structure need
not burn down. The response time of the fire department is critical. When
bins are empty the most severe explosion damage may occur. Every effort
should be made to ensure that the bins are substantially sealed off from the
tunnel, that the bin bottoms cannot collapse upward, and that the bins are
sealed off from the gallery or distributor.
Although the employees were aware of the possibility of explosions, none
of them seemed aware that a dust explosion could literally tear an elevator
apart. It is important that the grain industry trade organizations, the
agricultural extension services, and the government regulatory agencies
publicize this-problem. ~ -
Incident No. 3
A fire and explosion occurred at a small country elevator and mill
combined in the same wooden structure. There were rail loading and truck
dump and truck loading facilities, the latter of the drive-through type. A
wooden flat storage shed and an office building were located near the
elevator mill building. A metal bucket elevator with twin legs ran through
the central part of the structure. There was no headhouse structure as the
head pulley was located on the roof. The bins had open tops. The level of
housekeeping, as judged qualitatively by the subpanel, was not good.
!
Observations
The explosion occurred late at night. Minor structural damage was done
to the wooden structure by the blast wave in the mill area, which was not
well vented. A slight separation of the roof and walls occurred.
Substantial fire damage occurred in the cupola where there was a sustained
fire before and after the explosion. Slight charring of the wooden
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structure occurred in the truck dump area and in the mill. Substantial
structural damage due to the blast occurred to the 'lea caning in
approximately the lower one-half portion. The belt had parted and dropped.
It had burned through, but did not look as if it had caught on fire from
frictional heating. Another area on the belt also showed evidence of
burning. Prior to the explosion, milo had been ground and screened,
mechanical repair n had been made on the screener, and wheat had been loaded
into a truck. This activity had gone on from mid-afternoon until late
evening, during which time the lights in the elevator had been left on
continuously. At the completion of the loading, the 2 workers closed the
elevator and went to the office to relax before going home. As they
prepared to leave the office they noticed a flickering light in the cupola
window and identified it as a fire. The workers called the fire department
and proceeded to the roof (approximately'30 feet off the ground) on the
external ladder to fight the fire with small hand fire extinguishers. When
they looked through the window of the cupola they saw a fire raging on the
bin floor ant on one wall where there had been a lighting fixture. They
discharged the fire extinguishers with ' little effect and the one worker left
to get more. An explosion occurred. At approximately this time, the fire-
department arrived and successfully suppressed the continuing fire in the
cupola. A fireman who was responding to the alarm was approximately 100
yards away from the elevator at the time of the explosion. He heard a
"woosh" sound and saw a fireball come out of the cupola', out of the one open
truck dump door, and out from around a large banging door on the feed mill.
The resulting fire was successfully suppressed.
Scenario
A vapor-proof light fixture was installed on the cupola wall with its
axis of symmetry perpendicular to the wall. There was as mucks Hi; `-- !
inches of dust-on horizontal surfaces including the top of the 'sit_
f ixture . The light f ixture became hot enough to ignite layered dust and
this in turn fell onto the dust layer on the floor and ignited it. The
wooden bin floor surrounded the metal leg casing. The fire surrounding this
casing caused the bucket elevator belt to catch fire and burn through.
Since the one side of the leg was closer to the ignition source than the
other 8 ice, the latter had only started to burn by the time the belt
dropped. When the belt with its metal buckets fell down the legs
accumulated dust was disturbed, creating a combustible dust-air mixture in
the presence of burning belting or sparking metal cups. The leg casing
exploded, discharging a fireball into the truck dump area. This fireball
vented out the one open door and into the feed mill where additional layered
dust was stirred up, producing a secondary explosion, which vented through
the hanging door on the feed mill. The fire in the cupola still continued
out of control.
Conclusions
An accumulation of dust on the vapor-proof light fixture probably led to
this accident. Presumably problems had not occurred before because the
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lights had not normally been left on for this length of time. Had there
been a sprinkler system in the wooden elevator, the fire may have been
initially contained. Had not the leg belt dropped, an explosion probably
would not have occurred. In this case an outside ladder allowed the workers
access for fire fighting. Perhaps they would have been more successful in
fighting the fire had a standpipe been available. Considerable care must be
exercised in the fighting of dust fires because it is easily possible for
the fire to become an explosion. The firemen were successful in containing
the fire because they were equipped with a snorkel truck and the elevator
was not very tall, approximately 30 feet. The blast damage to the structure
was not severe because of the large venting area afforded by the one open
truck dump door and the hanging door. Also, wooden structures are for the
most part rather porous. Because of its low density, wood does not make
effective projectiles. -
The subpanel investigators arrived at this elevator a week after the
explosion had occurred. It had not been reported to federal authorities.
The accident was noted in news accounts. It is necessary that a system be
instituted so that the occurrence of an accident is promptly reported. In
some investigations equipment other than that normally carried by the
investigators is required--ladders, boots for deep water, and winches. (In
this particular case the fire department provided such equipment.) It is
desirable not to let witnesses confer before they are interviewed. It is
also best to interview witnesses first separately and then as a group. The
investigators should be able to spend as much time at the site as required
for a thorough analysis. Some investigation tasks, such as the removal of a
dropped belt, are physically impossible for investigators to attempt.
Therefore, it is important that an investigating team have the means to
obtain the necessary assistance to handle heavy manual tasks.
Incident No. 4
An explosion occurred at a country elevator of medium size with an
associated mill. The elevator was of concrete construction. It had truck
loading and unloading facilities and rail loading facilities. There was a
single set of silos with a large Butler bin at one end and the headhouse and
mill building at the other. There was no gallery. There was a screw
conveyor in the tunnel. Housekeeping, as judged qualitatively by the
subpanel, seemed adequate.
Ob servations
The explosion occurred during morning operations. Severe structural
damage was limited to the concrete structure under the headhouse. This
structure was external to the grain silos and contained the manlift shaft
and the legwells. Approximately the top one third of two of the walls had
been totally blown away. One of the damaged walls was opposite the bin wall
and the other orthogonal to it. Moderate structural damage was sustained by
the headhouse. The headhouse was connected to the leg wells and manlift
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shaft through hales in the floor. The walls of the headhouse were bulged
and the casement windows in the headhouse were broken and blown open. The
boot area was full of rubble resulting from the collapse of the upper leg-
wells and manlift shaft. The tunnel area showed evidence of burning and the
door at the far end of the tunnel was destroyed by the overpressure. At the
time of the explosion a rail car was being loaded with corn, and alfalfa
pellets were being unloaded at the feed mill. The corn was being screened
and trouble had been reported with the screener clogging. Subsequently it
was reported that the entire system had plugged--screw conveyor, bucket
elevator, and feed spout. Several witnesses heard the explosion and said
that there was only one. Only one individual actually saw the explosion and
he stated that the north side of the structure under the headhouse blew out
spewing concrete and two bodies. The witnesses stated that 3 days before
the explosion a fatal accident had occurred causing substantial mechanical
damage to the bucket elevator. During the day preceding the explosion
considerable time was spent repairing that damage. Observations made by
OSHA employees subsequent to the team's first visit and during elevator
reconstruction revealed that the bucket elevator belt was covered by
concrete rubble in the boot area. It had metal buckets and was parted. At
the break in the belt there was evidence of combustion. The lagging in the
head pulley was only partially intact showing signs of being burned off.
Scenario
The bucket elevator was jammed either because of the mechanical damage
that had been previously done to it or because of a choke brought about by
the clogging of the screener. An attempt was made to "jog" the leg. It is
not clear who turned the motor on as the only controls were at ground level
outside the elevator near the boot. The two injured employees were blown
out of the headhouse and the employee in the feed mill stated that he had
not been in the elevator for the 5 minutes preceding the explosion. The
slipping belt on the head pulley caused a friction fire that burned through
the belt. The belt then fell down the legwells and dislodged accumulated
dust. The flaming belt end or the metal cups striking the concrete wall
acted as an ignition source for the suspended dust-air mixture. An
explosion then occurred in the legwell and propagated upward into the
headhouse. -The headhouse then exploded, venting through the headhouse
windows to some extent. Also, the leg explosion was partially reflected off
the headhouse floor causing the top portion of the legwells and manlift
shaft to fall. This vented the high pressure gases. The explosion also
traveled down the legwell into the boot. The explosion vented into the
tunnel, in which it appeared an explosion was not sustained.
Conclusions
- This accident may have occurred as a result of events 3 days before.
The elevator equipment had been damaged and the employees were still
thinking about the fatal accident and the funeral. Generally, accidents
occur more frequently when employees are under stress or distracted--Monday
mornings, Friday afternoons, shift changes, graveyard shifts, overtime, etc.
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If modern devices are employed it should be possible to prevent a leg
choke. The leg feed should be controlled by a torque-sensing device on the
drive motor. As the torque required increased, the feed rate would be
decreased. The jogging of legs should be physically prevented through the
use of interlock devices. The elevator boot should be easily accessible so
that if a choke does occur it can be cleaned out. Shovel size access ports
should be put in the boot, and the boot pit should be well lighted and free
of obstructions. Provisions must be made to remove the excess grain from
the pit.
Concrete legwells allow little explosion venting. Pressures up to one
atmosphere can probably develop before failure occurs allowing continued
propagation of the explosion. When they do fail, concrete fragments act as
projectiles and high pressure gas is released. In this case, however, since
the legwell was on the exterior of the structure, the damage occurred to the
surrounding area and not in the interior of the elevator. The truck dock
was unfortunately within the distance of travel of some of the fragments.
The destruction of the manlift shaft broke the only connection between
the top and ground level of the elevator. The only access to the roof was
by a crane bucket. Secure, blast-resistant means of egress must be provided
so that injured employees can be removed from the top of an elevator.
The bin bottoms and gates were of particularly substantial construction.
Although a combustion wave did travel down the tunnel, it did not get
into the silos and cause them to explode. Also, the absence of a gallery
to some extent prevented the explosion from entering the bins. It is
possible, however, for a distributor to propagate the explosion into the
bins through the spouts.
The arrival of the subpanel members within 24 hours of the explosion
aided considerably with the investigation. Some public safety
officials--State Fire Marshall--were still available. Furthermore, the
witnesses were still willing to talk freely and events were still quite
fresh in their minds. An investigator was able to return to the site
during the period of reconstruction. He was able to examine items of
interest as they were removed from the rubble. At the time of the first
visit the bucket elevator belt and the head pulley were not accessible.
During return visits it was also possible to visit with injured personnel
who were not initially available because of hospitalization.
Incident No. 5
An explosion occurred at a large concrete country elevator. There
were truck dump and truck loading and rail loading facilities. A very
large headhouse was located on the top of the silos and approximately in
the center. There were two concrete double-leg bucket elevators, one
used for unloading and one used for outloading. The truck dump was of
the drive-through type located toward one end of the structure. An
abandoned brick soy mill was at this end of the elevator. At the other
end of the ~
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elevator was a large steel storage tank. Along the one side of the elevator
was a wooden building used for storage. The office was in a separate
building some distance from the elevator. The level of housekeeping, as
judged qualitatively by the subpanel, was poor.
Observations
The explosion occurred in early afternoon. Slight damage was done to
the ad jacent wooden storage building whose end was parallel to the drive
entering the truck dump. The blast, which came out of the truck dump,
removed some of the weatherboard structure from the framing. The soy mill
also received slight blast and missile damage resulting from high pressure
gases venting from a personnel door on the end of the elevator. Fire,
blast, and missiles moderately damaged the hopper-bottom trailer that was
. . .
standing on the truck dump. The fire damage seemed to engulf the truck;
however, the blast and missile damage was the worst on the side toward the
bucket elevators. The truck dump area showed a uniform scorching due to
fire. Much grain from broken house bins had fallen into this area. The
elevator control room adjacent to the truck dock and the leg was also burned
by the passage-of a fireball. Both bucket elevators were severely damaged
by blast in the vicinity of the truck dump. The legwells for each elevator
were located on either side of the manlift-ladder shaft and the up- and
down-legs were separate. Both down-legs had shattered completely, spewing
concrete fragments into the control room and across the truck dump. The leg
casings showed decreasing damage to the level of the screener f lo or
approximately half way to the top of the elevator. The failure of the leg
casings made both the manlift and ladder nonoperative. Fumigant had been
stored near the legwells and as a result of the explosion had been dispersed
around the work floor and truck dump area. The tunnel under the bins toward
the steel storage tank showed slight evidence of an explosion. At the far
end some damage had been done to the auger for the steel storage tank.
Moderate damage was done to the tunnel area going to the other side. The
tunnel terminated in a room with a personnel door and this door had blown
off. It was clear that a rather large explosion had occurred in the truck
dump area but to a large extent was able to vent through the open entrance
and exit doors. Access to the upper part of the elevator was difficult as
-the ladder in the manlift shaft had been destroyed. Entrance was gained by
climbing to the top of a-grain drier that was located on the side of the -
elevator opposite that of the storage shed. At this level, which was
approximately 50 ft above the ground, there was also a window in the
elevator. A 20 It extension ladder was laid from the top of the grain drier
to the window. This was the level of the screener. Looking into the house
bins it could be seen that the legwells had also exploded into the bins.
The damage to the legwells was below this levele From this level it was
possible to use the ladder to the headhouse . At the bin f loor level there
was a Mayo spout coming from the distributor with a circle of holes for the
respective bins. The floor was covered with much accumulated grain and
dust. The walls were covered with dust to such a thickness that it almost
resembled a layer of cork. A ladder and catwalk encompassed the distributor
f loor and at the highest level within the headhouse was the head pulley and
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drive motor. The head pulley covers had been blown off. The lagging on the
unloading leg had bunched on one side of the head pulley. This caused the
belt to slide to one side and it and the metal buckets were rubbing against
the leg casing, which was metal at this point. The headhouse was moderately
damaged. The broken windows and bulged sides indicated that an explosion
had occurred within the headhouse. An inspection of the roof showed that
the headhouse had apparently separated from the roof and that it had slid
along spouts passing diagonally through the walls. It thus appeared as if
the headhouse had to some extent lifted vertically. The roof on the end of
the elevator toward the steel tank appeared to have hinged upward to relieve
the pressure when some of the tanks exploded. Witnesses reported that it -
neemed as if the entire roof of the elevator had lif ted . All of the bin
covers were displaced. The spout feeding the large steel tank had fallen.
Many of the spouts within the elevator were worn through and were patched
with rags, boards, and wire. Fires had occurred in many of the tanks. At
the time of the explosion rail cars were being loaded with corn and corn was
being unloaded from the hopper bottom truck. The truck driver was
apparently closing his hopper bottom from the side of the truck toward the
bucket elevators. An elevator employee was in the control room, and another
at the scales whose controls were located in the wooden storage shed.
Scenario
The metal cups rubbing the metal leg casing near the head pulley caused
either a hot spot or sparks and this leg exploded. This explosion then
propagated down the leg and into the distributor. After accelerating down
the leg, it blew out at the work floor and boot pit level causing additional
explosions in the truck dump area, the dump pit, and the two tunnels. From
the distributor it was able to get into the second leg, the headhouse, and
the various silos.
Conclusions
Out-of-alignment belts in bucket elevators may act as ignition sources
either because they rub on a metal casing or their metal buckets strike a
metal casing. Belt alignment monitors should be used and plastic buckets
should be considered. Regular inspections should be made of the head pulley
to check the conditions of the lagging. Inspection should be facilitated
through the use of conveniently located doors, catwalk, and ladders.
Through the application of suction it may be possible to remove the
explosive dust concentration within the leg so that if an ignition source
toes appear, an explosion need not result. Headhouses should be designed so
that all horizontal and vertical surfaces may be regularly swept to remove
dust.
Elevators should be designed so as not to have interior concrete
legwells. These allow explosion pressures to rise to a high level, vent
high pressure gases into confined spaces such as bins where additional
explosions occur and spew out concrete fragments. All too frequently these
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are slip-formed to include the elevator or manIift and ladder. These are,
of course, also destroyed when the casing fails. It is necessary to provide
ladders giving access to the top of the elevator in other locations.
Control rooms should not be located inside elevators especially next to
legwella. As far as possible personnel should not be in the truck dump
area. With hydraulic lifts this is possible to some extent. The elevator
roof should not be tied to the tank structure. It would then be possible
for the high pressure gases to lift the roof and not cause a failure in the
side of the tank. Open truck dump doors are very effective venting areas
for explosions.
. . .
Although some concern has been expressed about ignition by hot surfaces
of a running or recently run truck, it seems unlikely that a significant
hazard exists. A surface must exceed 400°C and be exposed to a cloud of
dust with' a concentration above 'the lower explosive limit. It seems
unlikely that this will occur when a truck dumps or is dumped because the
heaviest clouds are at the grating and the truck body shields the exhaust
system from the 'high concentration dust clouds. , '
Incident No. 6
An explosion occurred at a large concrete country elevator. There were
truck loading a,nd'unloading facilities as well as rail loading facilities.
There was a large flat storage shed near the elevator and the office
building and scales were separate from the elevator. The elevator was of
unusual design in that there was no headhouse or gallery and the silos were
arranged in a single circle about a steel-cased double-leg bucket elevator.
Another elevator was external to the bin structure. Additionally, the
entire elevator interior was slightly pressurized to help prev,=~ tne --e
of dust into the elevator. The level of housekeeping, as judged
qualitatively by the subpanel, appeared to be adequate. '
Observations
The explosion occurred in mid-morning. There was a fatality and several
injuries. Severe damage occurred to the elevator control room, the'
penthouse that covered the interior legwell, and the truck that was located-
on the truck dump. Moderate damage occurred to all leg casings and the flat
storage shed. A significant explosion occurred in the basement of the
elevator and the escape of these ' high pressure gases caused the severe
damage. Part of the gases went up the center of the circle of' tanks where
the outloading bucket elevator and the manlift were located. This destroyed
the penthouse, which was situated on top of the vertical shafte Six-inch
steel channels that supported the structure were bent by this blast.
Additional high-pressure gas escaped through the tunnel to the truck dump.
The elevator control room was located on top of this tunnel between the
elevator structure and the truck dump. The floor of the control room was
lif ted to almost ceiling height when the top of the tunnel failed. Burned
gases e scaped around the metal dump pit and through the hydraulic truck
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hoist and lif ted the truck that had just dumped . The remaining high-
pressure gases vented through a personnel door forming a high-velocity Jet
that impacted upon the flat storage shed approximately 75 feet away. Some
of the 2" x 4" structural members were broken and some of the sheet metal
covering wan torn off. The damage to the interior outloading leg was caused
by the belt dropping. The ends of the belt were badly burned and the head
pulley lagging was partially missing and showed evidence of combustion. The
outside unloading leg exploded causing the metal casing panels to separate
at the corners. Some of the bin covers had been lifted of f and there had
been fires in several of the bins.
Just before the accident, corn was being loaded into a string of rail
cars and corn had been dumped into the truck pit, but it was yet to be
elevated. The interior leg, which was feeding the rail cars at this point,
choked. Help was sought to dig out the choke, but none could be found. A
decision was made to Jog the leg and thus defeat the interlocks. A drop in
amperage of the bucket elevator motor caused the supervisor to go outside
and look at the head pulley and then go to the boot area and open the
inspection door to see if the belt had dropped. The belt had not dropped
and the supervisor took the manlift to the top of the elevator to inspect
the head pulley. At this time the explosion occurred. The supervisor was
approximately two thirds of the way to the top when struck by the blast. He
managed to get out of the manlift and up the inside ladder to the top of the
elevator and then proceeded to climb down the outside ladder. A farmer in
his truck cab was slightly injured when his truck was displaced by the
blast. The elevator employee at the truck tail gate was thrown into an
adjoining field by the blast. The elevator operator in the control room was
crushed when the control room floor was lifted to the ceiling. The local
fire company arrived at the scene, strung hose lines to the 100 foot high
elevator roof, and extinguished the bin fires by hosing down the grain.
Scenario
For some reason a choke occurred in the outloading leg. As help was not
readily available to dig out the choke, it was decided to jog the leg . This
caused a belt and lagging fire at the head pulley. The boot inspection door
was opened, allowing pressurized air from the elevator interior to flow into
the leg and escape to the outside. This flow of air intensified the belt
and lagging fire. The belt burned through and dropped down the leg. For
some reason the leg did not explode. The collapsing belt pushed a cloud of
dust out of the open boot inspection door and caused the leg casing deco
split. I Burning pieces of lagging and belt fell out of the casing and into
the dust cloud in the basement of the elevator, which subsequently exploded.
Cone fusions
The bucket elevator choked and in spite of interlock devices an attempt
was made to jog the leg. Chokes can be prevented with motor torque or belt
tension devices controlling the bucket elevator feed gate. The boot area
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was quite accessible, so it should have been relatively easy to have removed
the choke. Employees must be educated that jogging a leg may be a fatal
mistake .
The blast demolished the control room and the employee lunch and break
room. Areas where personnel normally congregate must be removed from the
elevator. Since escaping jets of gas from doorways and windows can travel
large distances, buildings should not be placed opposite the elevator.
It is clear that fighting fires on the top of an elevator is difficult
and may not be desirable from the point of view of grain salvage. Dry
standpipes to the top of the elevator are probably desirable, but for a
severely burning bin fire it is not clear that water is effective. Indeed
combustion of grain in a limited oxygen environment may produce carbon which
then reacts with water through the water gas mechanism to produce quite
dangerous carbon monoxide and hydrogen. It still remains to be determined
whether water will effectively penetrate a bin fire. The water-logget grain
also expands and if not quickly removed can place nonde sign mechanical loads
that lead to failure on the silos.
Bin covers may, if large enough, effectively vent the high-pressure
gases from exploding partially filled tanks. Silos that are nearly full do
not explode effectively. However, the bin covers must be securely fastened
to the bin tops with a short piece of cable or chain or they become
effective missiles. Distributors are effective in multiplying the paths of
explosion propagation as numerous spouts are interconnected at this point.
If an explosion comes down from the bucket elevator feed, it can usually get
into each bin. The truck dump is an effective large high-pressure vent for
the basement areas. (Personnel located in this area are likely to be injured
in an explosion. Therefore, only personnel actually participating in the
operations should be in this area. Truck drivers and visitors should be
encouraged to wait elsewhere.)
During this investigation, cooperation with personnel from the
investigative agencies was good. Also, additional people had been brought
in to assist with elevator operation and repair; thus, the work load was
considerably lightened and employees were able to spend much more time with
the subpanel investigators. Blueprints and flow diagrams provided to the
investigators aided the investigation considerably.
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1
Representative terms from entire chapter:
explosion occurred
