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Chapter 2
INVESTIGATIVE METHODOLOGY
NATURE OF EXPLOSIONS
The investigation of dust explosions begins with a basic understanding
of what 16 involved in the explosion process. The best investigation plan
i8 based on prior knowledge of the probable chain of events involved in the
explosions. Although there are differences in explosion patterns at
different locations, there is a general pattern that is repeated over and
over again. Events at different locations often differ only in minor
details.
Dust explosions occur when structures rupture due to the pressure
generated by the very rapid combustion of suspended dust in air after -
ignition by a source with energy sufficient to ignite dust. In most cases
ignition sources with energies of at least 10 to 20 millijoules are
required, but, upon investigation, one finds that the primary ignition
sources have generally many times the minimum energy required. When dust
burns in a confined space in air, pressures of 80 to 120 psig are attained.
In most cases the structure explodes since the most common membrane
materials used in the construction of grain elevators fail at pressures
ranging from 2 to 3 psig up to 30 psig (Brasie 1979~. As the combustion
proceeds, the pressure in the building and all the interconnected spaces
increases at a rate that is a function of the type of fuel, the amount of
fuel per unit volume, and the size of openings or vents that permits the
pressure to be relieved. Whether or not a structure ruptures is then simply
a function of the competition between the rate of pressure increase and the
ability of the vent spaces to keep the pressure from rising above the
failure point. Unfortunately, the basic design in most grain elevators is
such that most of the confined spaces (for example, tunnels and legwells)
have virtually no vent area. Some headhouses are virtually windowless, and
combustions originating in those headhouses generally cause partial if not
complete destruction.
There is no "explosion" until some part of the structure actually
ruptures. The boom or noise heard exterior to the exploding structure is
the noise caused by the air returning to the reduced pressure zone created
by the explosion. In very large, complex structures, particularly those
found in terminal or export elevators, the volumes of the interconnected
spaces are very large. Tunnels running underneath silos may be hundreds of
5
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1
6
feet long. Galleries are of comparable lengths. The interior design of
headhouses varies from some in which the bucket elevators are in steel
enclosures in open areas to those in which the elevation equipment is inside
hollow concrete wells or shafts. All elevators contain storage bins, either
for primary storage or for use as "working bins, that are generally within
the framework of the headhouse. Since the purpose of the elevator is to
move grain into and out of all of these places, fire and pressurized
combustion products can go through the various interconnections and reach
many of the bins and working spaces. Since the rate of.flame propagation.
and movement of pressurized gases is finite, an explosion in a given
building complex may actually be a series of explosions with time intervals
of fractions to greater than l second between them, depending on the
distances and the precise circumstances.
In the most general scenario a bucket elevator casing will explode
because of a fire inside and the resulting overpressurization. The
resulting fireball.that.is emitted causes air movement, which raises the
local dust that is lying around. This suspended dust burns, causing a
greater fire and the acceleration of the movement of air throughout the
facility, thus increasing the combustion zone volume. The hot gases
generated can penetrate up, down, or laterally in the elevator complex,
depending on the initial location. For example, the combustion front may
traverse to the top of the headhouse, down along the gallery and into empty
bins where new dust "fuel" may be found.. One or more bins may then explode,
generating more combustion gases, which then may enter the tunnel and
proceed in both directions down the tunnel, enter more bins through spouting
or perhaps proceed to other sections of the elevator complex. The entire
process can easily last several seconds. Sturdier parts of the elevator may
sustain pressures above 10 or even 20 psi. Other portions of less rugged
construction may fail at lower pressures. When self-venting occurs, i.e.,
rupture of membranes to the exterior, the rate of expansion of the
combustion gas is Slowed down substantially and penetration along further
interconnected spaces may not occur, particularly if there is insufficient
fuel to be suspended to keep the concentration above the lower flammability
limit of that particular dust combination. .
One cannot predict with any accuracy what the pattern will be in a given
elevator complex. However, an investigation of the normal amount of ambient
dust in different locations (which is a.function of housekeeping and dust
collection procedures) will give some indication of the potential for an
explosion and the likely extent of damage. The.amounts and location of
ambient dust that survive the fire and explosion process is sometimes a
valuable clue in helping to pinpoint the original source. Sometimes flame
front edges may be seen along dusty walls or on other surfaces delineating
the size and extent of flame propagation. Obviously, if a flame front dies
partway down a tunnel the origin of the flame must be at the burned end of
the structure. These types of clues may be helpful in pinpointing the .
Origin.
The above remarks are intended to help clarify what i s a dust
~~explosion" in a grain-handling complex. Knowing that the basic process i s
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7
one of pressurization with outward movement of gases towards available
openings helps the investigator understand from which direction the
pressurization occurred. In the cases in which an exploalon reaches
shock-speed levels, the propagation of the shock wave radially away from the
source may be quite evident by the deformation of structural members in the
path of the expanding high-speed shock wave. Hence, it is fairly important
that the investigator have a mental picture of what the physics of the
explosion process were in order to help locate the origin and identify the
directions of propagation. ,
METHODOLOGY
The purpose of the investigation is to identify the original source of
ignition and couple this with information concerning the available dust
loading (i.e., quantity of layered dust per unit volume) to describe the
explosion event in appropriate detail. Many different sources of facts must
be investigated in order to put together a coherent, logical and defendable
scenario for the explosion. There are several basic mechanisms: -
1. The direct investigation of the site and examination of all of the
areas and identif ication of those clues and pieces of information
that help in describing the basic explosion process at this site.
Interviewing and talking with anyone who can provide some
information on the event. These people include operating personnel
on the site, people in nearby buildings such as offices, homes, or
other industrial facilities, and even passers-by.
3. The post-explosion examination of individual items in a
.
laboratory. This step might include, for example, the
investigation of marks, fracture surfaces, deformation of parts, or
things related to electrical components. In this phase, care must
be taken to determine whether the damage to the individual
components being examined resulted from the explosion process being
investigated or from some earlier event. Individual judgment must
be used in these examinations.
The general approach to investigation is to have a team of experienced
investigation people. The tasks of the specific investigation are logically
divided into the areas discussed above. Depending on the circumstances and
the availability of witnesses, it may be desirable to have one or two team
members do all the appropriate interviewing. Interviewing is not without
its-difficulties since witnesses may assume that an investigator is an
adversary. In some cases it may not be possible to determine anything
because of the potential for litigation.
Site Investigation
Those directly involved in looking at the structure and surroundings
have a complex task. It is desirable to meet first with the owners and
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managers of the facility and local officials Deco apprise them of the mission
and intent of the investigating team. The team can establish rapport with
management by stating who they are, why they are at the facility, and what
they plan to do and accomplish. After their preliminary investigation the
team should once again meet with management to share information and
findings. This will be helpful in the event that the team wishes to revisit
the damaged site to witness, for example, salvage operations.
. .
If possible, the team should obtain a building or drawing plan showing
the various spaces and interconnecting design. The layout of the complex is
sometimes not obvious if destruction is severe. Needless to say the
investigating team should be properly prepared from the standpoint of
safety. Hardtoed.shoes, protective clothing, and hardhats are mandatory.
Gloves should be worn. Excellent, portable lighting is extremely
desirable. A powerful six-cell focusing flashlight is invaluable since
there is often no power at the site and at night extra illumination is
always needed. Furthermore, tunnels, the inside of bins, and galleries may
be dimly lighted, if at all. The investigative team should have.its own
liability insurance and should assure management that the team is
participating in the investigation at its own risk. Rules and restrictions
of local officials, such as fire marshals or other safety people, should be
followed. it may be necessary, however, to negotiate with these officials
if the restrictions are arbitrary and unnecessarily severe. In some cases
there is a preoccupation with immediate removal of rubble and wreckage to
clean up the site. It is desirable to photograph and examine the wreckage
before rubble and wreckage are removed, unless there is a time consideration
involved (for example, injured personnel in the wreckage).
The site investigative team should go through the entire structure
systematically from top to bottom.and from end to end as accessibility
permit s. Photographs should be taken of any places on ache site that have
any possible c rues. A record of the location of the photographs should be
maintained as they are obtained. It is desirable, if possible, to indicate
the location of photographs on a site or elevation drawing. There are many
things for the investigating team to seek. Some of the important clues
include the direction rubble is thrown and the extent.of damage of various
structural elements such as I-beams, concrete walls, reinforcing bars, or
other items.
In locations such as tunnels or galleries, where there may be light
construction items such as spouting, particular attention should be paid to
the extent of deformation and indications of the direction of the pressure
wave. Even small details should not be overlooked, such as which pieces of
rubble or wreckage are on top of which other pieces, whether glass is blown
in or out, and whether the roof has been lifted.and resettled.
Generally, after several tours through the accessible parts of the
complex the basic explosion pattern should be discernible and the point of
origin of the combustion can be determined. It is then important to go back
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to the area of origin and look for possible sources of ignition.
Unfortunately, the damage in the area of origin may be so bad that it is not
possible to find the critical evidence before clearing away the debri 8.
Bucket elevator casings, buckets, belts, etc. may be buried under tons of
wreckage. For example, the bottom bearing in the boot, which might be
suspected as an ignition source, frequently is not accessible. In some
Cases the boot not only may be under wreckage, it may be under water from
the firefighting action. In such cases, cooperation with those involved in
removing the wreckage or demolishing the facility is required to be sure
that at the time the critical elements are exposed someone is available to
obtain them for investigation before they are thrown away. It may not
always be easy to obtain this cooperation.
The second phase of the initial investigation can be described as
looking for small details. This phase may include the collection of pieces
and parts, (for example, components of electrical systems) or other things
that may require detailed off-site examination. At the site the team should
meticulously go through the wreckage and note minute details of the
placement and location of wreckage with respect to the sequence in which the
items are piled on top of each other. The team should carefully look for
and examine evidence of flame fronts and the direction in which the flame
fronts advanced. For example, frequently lights in tunnels will show char
markings on one side of the light only. The deformation, bending or
twisting of light-gauge metal objects is often a clue to the direction from
which forces were applied. Photographs of these details are extremely
valuable and sufficient records should be kept to indicate the location of
these items. - -
Frequently the failure of electric components may be judged to be the
cause of the initial ignition. However, one must be careful to,be sure ~.~-
broken or damaged electric components show positive evidence that they
failed prior to the explosion. It is easy to misjudge a smashed,
shorted-out section of conduit or wire as being responsible for the ignition
when in fact the damage was done during the explosion process.
The placement and layering of rubble, as mentioned before,-can provide
important clues as to'the sequence in which different portions of structures
failed. Damage exterior to the main structure may also provide clues in
some cases. Differences in damage adjacent to various openings in the main
elevator structure can provide clues to the intensity of the pressure wave
coming from the various apertures. This in turn can provide insight into
the direction and strength of the pressurization process during the main
explosion events.
Although these comments are broad and general, once some field
experience is developed, it becomes relatively easy to sort out the various
factors and to begin to pinpoint the probable initial zone of the explosion
process. It is then important to try to estimate what was the single
initial f ire-explosion event that started the whole sequence. The
experience of the investigating subpanel was that this was a discernible
event in almost all of the cases investigated.
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Sometimes the ignition source is a factor external to the basic
operation of the terminal itself. For example, the leakage of propane into
parts of structures associated with grain elevators and the subsequent
ignition of the propane were the first steps in the destruction of a
facility. The elevator, primarily of wood, was burned to the ground leaving
no evidence of blast effects that may have occurred due to an initial dust
explosion. However, from later examination and information provided by
people associated with the operation, it was determined that there was an
initial leakage of propane--from a line recently installed to provide fuel
for a dryer--into an adjacent structure where ignition occurred during grain
loading and unloading operations. The propane explosion propagated into the
elevator proper and the rapid expansion of the propane fireball initiated a
dust explosion. Indeed a dust explosion occurred and did heavy damage to
the elevator, which, being made of wood, was soon totally engulfed in flames
and destroyed. In a case like this, one must look at the operation of a
total elevator complex to include not just those things associated with the
movement or processing of grain but all those elements that are required to
operate the complex.
Interviewing of Witnesses
. _
Another major portion of the investigation is the interviewing of all
witnesses who may have information leading to the determination of the cause
of the explosion. Those who should be interviewed include operational
people directly on the site? people in nearby offices or other functional
buildings, truck drivers or railroad employees who may have been in the
vicinity, maintenance people, people who may have been driving by or who
live or work nearby. When an approximate timetable of the events leading up
to the explosion has been determined, it may be advisable to -id ~ --
some of the people in order to fill in details as the total pit: ~ -:_
to emerge. Hence, several different interviews may be desirable with
various people. It is particularly desirable to interview people one at a
time and to the extent possible not reveal statements of one individual to
other witnesses.
After the first round of interviewing, it is important to try to
reconcile the stories and viewpoints expressed by those interviewed.
Discrepancies in descriptions of what happened will frequently occur and
need to be resolved. The resolution of discrepancies generally requires
very tactful reinterviewing. Witnesses may have hazy recollections and may
exaggerate or present faulty opinions. They may collaborate on a "safe"
scenario (i.e., nonincriminating). Therefore, to get the best cooperation
from all concerned, it is important to convey to the interviewees that the
interviews are nonpunitive and nonthreatening.
Other sources of information are those involved in the emergency
processing after the explosion. They include firefighters, police or other
local officials, and those who have treated the injured in cases where
injuries occur. Frequently information is passed from those injured to
those helping, such as ambulance drivers. The subpanel noted that at later
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times injured witnesses have a tendency to be nontalkative, particularly
there is any perceived liability threat. Witnesses tend to be vocal and
cooperative 600n after the event but tend to become more reticent as time
pa 8Se8 ~ -
After interviewing, an integration session should be held in which all
of the investigators pool the information obtained. At this time, it may be
possible to construct a scenario that appears to be reasonably verifiable.
However,-discrepancies frequently exist and it may be that no specific
scenario can be stated.- It is then necessary to go back for additional site
investigation or investigation of pieces or parsecs that may supply ache
missing clues and to conduct additional interviews to try to f ill in the
gaps.
In some cases it may not be possible to pinpoint the actual initial
event until rubble and wreckage are removed from key parts of the equipment
that may yield clues. The removal may take place within hours or may be
delayed for weeks for various reasons. If the original investigators cannot
be present when the rubble is removed, the cooperation of some local person
in segregating key evidence is important. Management personnel, if
cooperative, may serve this function, or OSHA field people may be
available. Insurance people may also want to be involved.
Laboratory Investigation
In some cases the critical evidence from items obtained from the site
just after the explosion or when the rubble is cleared cannot be ascertained
by visual examination alone. Laboratory tests may be necessary to determine
fracture mechanisms, burn sequence, etc. It is important that appropriate
laboratory facilities be available to the investigating team. The
interpretation of laboratory data, of course, requires professional Judgment
and correlation with the rest of the information from the explosion site.
A report is the logical output of an investigation. Such a report
should document the data and logic that led to the conclusion on probable
chain of events. Photographs, site drawings, or related graphical
information are valuable portions of such reports. Reports issued by the
National Transportation Safety Board (NTSB) on transportation disasters,
pipeline explosions, etc. are good models to follow. The format of the NTSB
report is described in an earlier report by the panel (National Materials
Advisory Board 1980~.
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Representative terms from entire chapter:
elevator complex
