Determination
of the origin of a fire is a necessary part of an investigation. Burn patterns
at the loss site typically aid in the determination of the fire origin. The
following are some thoughts to consider when assessing the origin of a building
fire based on burn patterns.
The concept
of low burn is often invoked as a way to determine the fire origin. The low
burn area usually is the fire origin since the fire typically travels upward in
a V shaped pattern. Figure 1 shows a low burn area where severe
Figure 1
damage
has occurred, characterized also by deep charring of the wood. The well known
phenomenon, that hot gases from a fire rise, accounts for the fire spreading
towards the upper portion of the building (V pattern). In Figure 1 the pattern
is typical of a single origin at the bottom of the V.
Figure 2
shows multiple V patterns in a building which is often construed to be multiple
origins. Since multiple accidental fire origins are unusual, suspicion of arson
is often opined. Other alternative possible causes should be investigated
thoroughly. During one fire, the fire department began to fight a fire in a residential
building until a substantial natural gas leak occurred. They were withdrawn
from the scene until the gas could be turned off. As a result of holes chopped
in walls and roofs by the fire fighters, the fire caused severe burnouts in
several locations looking like a
Figure 2
multiple
origin fire. The common characteristic of fire origins shown in Figures 1 and 2
are the lack of pattern distortion from other influences such as wind. Figure 3
shows burn pattern distortion from
Figure 3
wind. During
windy conditions, a fire can easily spread downwind yielding a distorted V
pattern. In some instances it is tempting to place the origin in the vicinity
of the center of the burn (False origin Figure 3). If high winds were present,
searching for an origin upwind from the center of damage may be fruitful.
Structural
interactions with the fire can also result in a burn pattern distortion and
false origin analysis. Figure 4a
Figure 4a
shows a
fire starting above the doorway. As the fire progressed, it weakened the roof
structure causing a collapse, resulting in the fire dropping to a lower level
(drop down fire). This can result in a false origin determination as shown in Figure
4b.
Figure 4b
Flammable
materials located in a building can cause distortion to a fire burn pattern. In
Figure 5 a fire started to the right of the main entrance. However heat
transfer from the fire ignited flammable materials nearby, causing a low burn
area that can be mistaken as the origin.
Figure 5
In other
instances, the fire may be so devastating that burn patterns may be inconclusive
as far as the origin is concerned. Figure 6 shows a severe burnout of a
building with no discernable pattern pointing to the origin. In these instances
some other means may be required to find the origin.
Figure 6
Burn
pattern recognition is quite useful in analyzing the origin of building fires.
Thorough examination of burn patterns is necessary to avoid some of the
pitfalls of false origin determinations.
An Important Ingredient in Fire Investigation
After a fire, thermal patterns remaining at a fire scene
are an important ingredient in fire investigation. Charred wood, soot
deposition, melting, spalling and structural deformation are examples of
thermal patterns caused by fires. When performing fire cause/origin analysis,
fire investigators often seek out thermal patterns to aid in determination of
the fire origin. The following article is a mini-encyclopedia of examples of
thermal patterns along with what can and cannot be gleaned from the analysis of
thermal patterns.
Figure 1a Classic V-pattern
Figure 1a depicts the classic V-pattern relied upon by
many investigators. When hot gases from a fire rise in a natural convection
mode, (as a result of heat related gas buoyancy) the gases tend to spread out
as they rise, forming a V. Consequently, the fire investigator looks to the
base of the V as the origin of the fire. In Figure 1a, the soot pattern, on the
walls of the home, has spread outward near the eaves, suggesting that the fire
origin is at grade level (arrow). Soot patterns inside the home were less
severe, suggesting that the fire started on the
Figure 1b Melted gas meter and pressure regulator
outside of the wall. A closer look, at the base of the V,
shows burned gas piping as shown in Figure 1b. Severe charring of wood and
melted aluminum casings are indicative of a fire origin in this vicinity. It
was determined that frost heaving caused movement of the natural gas piping, resulting
in gas leakage through a compression fitting. The exact ignition source was undetermined.
A possible ignition source could be a water heater, just inside the wall,
igniting the natural gas before an explosive volume of gas could have
accumulated. Air infiltration is a typical avenue for natural gas to
Figure 2 Kitchen fire
enter a home, especially in the winter. Figure 2 is a view
of another classic V-pattern in a kitchen. The pattern is typical of an
overheated appliance on the kitchen countertop. The arrow points to severely
burned wallboard at the base of the V-pattern where a decomposed, plastic
coffee pot housing was found, the apparent cause of the fire.
Propane gas is heavier than air and can cause random burn
patterns throughout a building, provided there is sufficient venting of the
deflagration so that an
Figure 3 Propane gas leakage causes a fire
explosion does not occur. In Figure 3, the propane
cylinder on the fork truck (far left arrow) leaked, resulting in pooling of
propane gas on the floor. A water heater pilot light is the likely ignition
source of the propane. After ignition, random pools of propane ignited, causing
multiple V-patterns as shown by the arrows on the right side of the photo. The
amount of propane released before ignition was small, unable to sustain an
explosion, but sufficient to cause random burning throughout the building, depending
on the amount trapped in areas throughout the building. Multiple V-pattern observation
is often a tenet of an opinion by an arson investigator that a fire of
incendiary cause has occurred. Although multiple V-patterns have occurred in
this example, the nature of the patterns is unique to propane gas accumulation
and is not necessarily an arson.
Machine shops often use numerically controlled, electro
discharge, milling machines. These machines use electrical arcs to erode away
metal on the work piece to manufacture a finished part. The electrode and work
piece are submerged in a dielectric oil for cooling and reduced oxidation of
the electrode head. The machines run on a computer program and are often left
running overnight, unattended. If the oil level drops below the electrical arc,
ignition of the oil can occur, causing a smoky fire. A float switch is designed
to shut off the machine if the oil level drops below a predetermined level.
Figure 4 is a view
Figure 4 Electro discharge milling machine
of an EDM machine that was left operating overnight. A
fire resulted, virtually destroying the building. Burn patterns on the side of
the oil reservoir reflect a low oil level (arrow). The oil in the reservoir
tends to cool the side of the tank, showing a different oxidation pattern than
the upper side of the tank where high temperature oxidation is noted. A failure
of the float switch is a possible cause of this fire.
Wood char depth is a burn-related pattern utilized by fire
investigators. In Figure 5 the charring of wood joists, often
Figure 5 Char depth
called alligatoring, is a clue to the fire origin. The
arrow shows the most severe char depth right above the furnace flue pipe, a
typical consequence of flue pipes situated too close to combustible materials.
Heating of the wood, over time, tends to lower its ignition temperature until
it is ignited as a result of heat transfer from the flue pipe.
Figure 6a Before the fire
Figure 6a is a view of a propane gas fired heater in a
produce truck, used to keep produce from freezing in the winter. The arrow
points to pyrophoric decomposition of the wood near the heater exhaust. The
discoloration of the wood is an indication that ignition of the plywood
sheathing is eminent. Figure 6b is a view of a heater in an identical truck
that caught fire, destroying the produce and part of the van. The arrow points
to the area where the plywood sheathing had been installed. In this vehicle,
pyrophoric decomposition of the wood has reduced the ignition temperature to
nearly that of the exhaust pipe, resulting in the fire.
Figure 6b After the fire
Figure 7 Passenger seat auto fire
Figure 7 is a view of the passenger seat in a 4-door
sedan. Burn patterns tend to focus on the seat as an origin of the fire. Melted
polymers, such as polyurethane, are evident on the seat and dash. The damage to
the dash is most likely a result of a fire on the seat. The vehicle was found
in this condition, with the fire self-extinguished from lack of oxygen (doors
closed). Arrows point to remains of emergency flares near the center of the
seat. There are no vehicle related ignition sources in that area. Is arson
ruled out in this case? Probably not, since spontaneous ignition of road flares
on seating in automobiles is unlikely.
Figure 8 Fuel line repair
Burn patterns may not always point to the actual square
inch where an ignition occurred. Fuel leakage in a vehicle, typically sprays
fuel throughout the engine compartment, causing a diffuse burn pattern.
Inspection of the fuel lines may show deficiencies that are causative to the
fire, as shown by the improper fuel tube repair in Figure 8.
Figure 9 Fire remains in a dishwasher
Smoke damage occurred to a home as a result of soot
evolution from a dishwasher. The dishwasher was in the drying cycle at the time
of the fire. A view inside the washing compartment yields thermal patterns that
are clues to the fire cause. The most severely burned area in the compartment
is shown near a collection of utensils. The upper arrow, in Figure 9, shows a
steak knife with the wood handle burned away. The lower arrow points to the
heating coil used in the drying cycle. Thermal patterns are consistent with one
of the steak knife handles being ignited by the hot drying coil in the washer.
Figure 10 Extension cord coil
Figure 10 shows a 100-foot extension cord wrapped in a
neat coil and placed on a rug. The homeowner had connected a new dehumidifier
to the cord in a finished basement. Thermal pattern analysis indicated melting
of insulating material on the extension cord. There was smoke damage to the
home, but no other areas suffered the degree of melted polymer than the
electrical cord. Apparently, the extension cord was too long for the appliance,
causing a voltage drop and resistive heating. The coil itself limited heat
transfer from the wiring. The consequences were increased heating and increased
electrical resistance, resulting in ignition of the insulation material and
carpeting.
Burn patterns on water heaters are dramatic, in that one
views a gradation of thermal patterns from the bright white of the original
painted surface to the severely corroded, oxidized, sheet metal. Figure 11
depicts a typical V pattern with the base of the V near the gas valve. Caution
should be exercised when interpreting such a thermal pattern. Possibilities of
the fire cause are gas valve failure, gas pipe leakage, combustible materials
placed too close to the heater, and
Figure 11 Typical gas fired water heater
flame rollout. In the case of Figure 11, flame rollout
ignited nearby garbage bags.
One large loss involving a 2-story home occurred during
the winter, while the owners were on vacation. Approximately 6 inches of water
was found in the basement as well as water vapor throughout the home. Wallboard
had deteriorated and fallen from wall studs exposing several burned areas, like
that shown in Figure 12. Some investigators were interpreting the burned areas
at the pipe joints to be a result of a malfunction of the water heater. What
had actually occurred was a freeze failure of a few wall-mounted water pipes
during unusually cold weather. This caused hot water to run through the pipe
fractures into the basement, generating the large amount
Figure 12 Burn pattern near pipe joint
of water vapor that damaged the drywall. The burn patterns
at the pipe joints were formed during construction of the building when
plumbers soldered the pipes with torches and were unrelated to the loss.
Figure 13 The grounder
Some thermal patterns yield inconclusive results, such as
the “grounder’ shown in Figure 13. The destruction is so widespread that
pinpointing the fire origin, based on thermal patterns, is difficult. Other
thermal patterns can be more definitive, as evidenced by previous examples.
Thermal pattern recognition and interpretation will most likely remain a
significant ingredient in fire cause/origin analysis.
During
the heating season, many fire losses will occur with origins in building
mechanical equipment. A thorough investigation requires information on the
probable cause of the fire. The discovery of defective mechanical equipment design
or installation will certainly enhance the chances of monetary recovery. The
following are examples of fire causation often found in mechanical equipment in
buildings.
Boilers
Figure 1
shows a badly overheated boiler that was the cause of a fire in a small
apartment building. The building was nearly destroyed. The cause of the fire
was the overheated boiler that ignited
Figure 1
floor
joists in the vicinity of the flue pipe. A clogged low water cut-off control
did not sense the drop in water level in the boiler, resulting in the overheating.
The maintenance service hired by the insured did not follow appropriate
blow-down procedures that would have prevented the malfunctioning.
Boiler
control malfunctions can cause fires. The controls should be carefully removed
and tested. Boiler installation should be inspected for proper clearances to
combustible materials. Since boiler flue temperatures often reach 500 to 600
degrees Fahrenheit, a properly operating boiler can cause a fire if the flue
pipe is near combustible materials such as wood structural components. Finally,
operational procedures should be reviewed to ascertain the effect of the behavior
of boiler operating personnel on the probable cause of the loss.
Furnaces
A
significant number of furnace malfunctions occur in the flue pipe system. Flue
blockage, insufficient flue distance from combustibles and improper flue assembly
usage are typical causes. Control malfunctions may result from either gas
valves or limit switches. In some instances, the limit switch failed causing
overheating of the furnaces. Electrical faulting in wiring connected to furnace
controls has been known to occur, thereby initiating a fire.
Water Heaters
Figure 2
shows a burned water heater that was the origin of a fire. Food grease from an
overflowing trash container near the water heater had spilled onto the heater
and drained down the side into the
Figure 2
burner
area, resulting in the fire. Careless placement of combustible debris in the
vicinity of water heaters is a common cause of fires. Other causes include
malfunctioning gas valves and flue blockage.
Kerosene Heaters
Figure 3
shows a burned kerosene heater at the origin of a fire. Typical causes of
kerosene heater fires are usage of gasoline as a fuel, spillage of fuel from
the heater while ignited, placing the heater near combustible materials, and improper
adjustment of the heater. Improper adjustment of the heater can result in a
sooty flue product from the heater causing significant smoke damage without a
fire. In such an instance, a defective wick adjustment mechanism was discovered
which did not allow the insured to properly adjust the heater.
Fireplaces
Fireplace
fires are know to be caused by defective flue liners, improperly installed flue
liners, creosote build-up in the chimney, blocked flue pipes (bird nests),
insufficient screening of the fireplace, gas igniter malfunction, and insufficient
hearth area in front of the fireplace. Check local building codes for installation
deficiencies that could have caused the fire. Check to see if chimney sweeps
had cleaned the fireplace flue. In one instance, a chimney sweep reassembled a
flue liner improperly resulting in a gap that allowed hot debris to enter the
attic.
Several
causes of fires due to mechanical equipment have been reviewed. Many more can
occur, depending on the complexity of the mechanical equipment.
Figure 3
Fires
started by furnaces have been a major factor in losses associated with residential
and commercial property. The beginning of the heating season usually brings a
rash of fire losses as marginal equipment that survived the previous season
fails, causing a fire. During the heating season as cold spells occur, marginal
equipment is often over stressed, resulting in a fire. After a thorough burn
pattern analysis is performed on the building and a preliminary assessment
indicates a furnace related fire, a closer look at the furnace installation may
be fruitful. The following is a listing of some typical fire causation modes associated
with furnace (and boiler) fire losses that may be helpful in a causation
analysis:
Combustible Material near the Furnace
Some
furnace rooms act as a depository for cardboard boxes, garbage and other combustible
materials. Placing these items close to a furnace can cause a fire as a result
of heat transfer from the furnace or flame rollout from the draft diverter.
Check for the remains of unusual combustible material near the furnace. Inquire
as to what was in the furnace room at the time of the fire.
Improper Furnace Installation
Furnaces
and furnace flues are often required to be installed with certain clearances
from combustible building materials. Figure 1 shows the remains of a single
wall flue vent installed near a floor joist causing the fire. The flue vent
should have been a double wall design that reduces heat transfer from the hot
flue gases to the floor framing.
Figure 1
Control Malfunction
Several
controls are installed in a furnace to reduce the chance of a fire. Such
controls are the high temperature limit switch, thermocouple pilot flame
failure sensor and flue spillage sensor. Failure of any of these could result
in a fire. Figure 2 shows a typical furnace burner
Figure 2
with the
gas control valve indicated by the arrow. The gas control valve houses several
safety features that should be checked during examination. Figure 3 shows a
typical high temperature limit switch that is designed to turn off the furnace
when plenum overheating occurs. If a control malfunction is suspected,
laboratory testing should be performed on the suspected control mechanism if
possible. Handle the controls with care. Jarring a sticky gas valve could
result in loss of evidence.
Figure 3
Figure 4
Cracked Heat Exchanger
Figure 4
shows a cracked heat exchanger brought on by cyclic thermal stresses. Figure 5
is a close-up of the crack. As the crack develops, hot combustion products can
impinge on combustible materials causing a fire. Heat exchanger cracks can also
cause indoor air pollution with sometimes serious health effects.
Figure 5
Improper Wiring
Although
rare, improper wiring of furnace control systems can cause a fire. One recent
loss was a result of an improperly connected wire that bypassed overheating
safety devices. When the fan motor failed, the furnace overheated since the
high temperature safety devices were ineffective, resulting in severe damage to
the building.
Figure 6
Sludge
Accumulation
of debris or sludge in boilers can cause over- heating and fire. Figure 6 is a
section of a boiler heat transfer tube that is partially blocked with calcium
deposits as indicated by the arrow. Note the burned heat transfer fins. This is
usually a result of poor maintenance on the boiler system.
Furnace
and water heater related fires in residential buildings continue to be a cause
of loss to many insurers. The previous article: “Furnace Related Fires,” reviewed
several examples of furnace related fires. The following are some additional
case studies. Figure 1 shows a furnace that sustained a relatively minor
Figure 1
fire
that initiated in the vicinity of the high voltage module that drives the spark
ignition system. The arrow in Figure 2 points to the badly damaged module which
was the ignition source of the fire. Although the direct damage to the home
from the fire was minimal, the induced damage was substantial. The home was
unoccupied while the insured was on vacation during the winter. The
Figure 2
furnace
caught fire but fortuitously self extinguished from lack of fuel in the vicinity
of the fire origin. This caused the furnace to be inoperative, resulting in
severe water damage to the home once the frozen water pipes thawed. The exact
cause of the failure of the ignition module could not be determined because of
severe damage to the unit from the fire. Figure 3 is a view of an old forced
air furnace in a home that was badly damaged by a fire. Figure 4 is a view of a
typical floor heating duct register that is badly burned. Figure 5 is a view of
another register that is connected to the forced air system and is in good condition.
Did the furnace overheat and cause a fire? The controls and burner which were
located at the lower portion of the furnace were in good condition. Testing
showed no malfunction in the burner gas valve or limit switches. The most
Figure 3
Figure 4
severely
burned areas were near a few registers on the forced air ducting that was far
away from the furnace. If the furnace was the heat source causing severe
pyrolysis at the registers, how come the wood near the burners or the wood near
some other registers is not pyrolyzed. This is a violation of the principles of
heat transfer since heat does not naturally flow from a cold source to a hot
source. Chemical analysis in the vicinity of the badly burned registers
Figure 5
Figure 6
confirmed
the existence of an accelerant. Figure 6 is a view of a home after a natural
gas explosion had occurred. A fire resulted but was quickly extinguished by
Figure 7
the fire
department. The furnace and water heater were suspected as having malfunctioned
but no apparent deficiencies were found. Within a week, two more homes on the
same block suffered explosions. Suddenly the focus of the investigation was
directed toward the gas utility piping. The gas main was excavated and several
cracks were found in the piping like that shown in Figure 7. Figure 8 is a view
of a typical part of the gas main showing markings indicative of having been
struck by an excavator.
Figure 8
About 10
to 15 years before the loss, water pipes had been placed in the ground next to
the gas main. Apparently the excavator had struck the gas piping during
installation causing random cracks throughout the piping system. Seasonal soil
movements enlarged the cracks which began to leak natural gas more profusely.
Some of the residents noticed brown areas on their lawns despite every effort
to properly care for the lawn during the summer. This resulted from natural gas
filtering through the soil to the atmosphere. However, when the winter frost
layer developed, the avenue of escape for the natural gas was blocked. The
natural gas then moved horizontally, entering voids near sewer lines and other
utilities, causing gas accumulation in some homes. The gas was most likely
ignited by pilot systems of gas fired equipment.
In
another case, a water heater was involved in an explosion when the insured
attempted to light the pilot after the propane fuel cylinders were replaced.
The insured indicated that he did not smell any gas. Chemical analysis of gas
samples from the cylinders indicated very little or no odorant was added to the
gas. The gas valve on the water heater was apparently on recall as a result of
a propensity to remain open when the pilot light extinguished. The insured
typically would let the gas run out before replacement of the fuel cylinders,
relying on the gas valve safety system to close the valve when the pilot flame
was extinguished. When the new propane cylinders were connected, gas
accumulated in the basement that could not be smelled by the insured because of
lack of odorant. The interaction of a sticky gas valve with the lack of odorant
in the gas precipitated a hazardous environment, resulting in the loss when the
insured attempted to light the pilot.
It
should be noted that in two of the mentioned case studies the furnace or water
heater did not cause the loss. It is not unusual for the furnace or water
heater to be blamed for a loss, despite a lack of supporting evidence. Each of
the case studies, however, had sufficient information to evaluate the role that
the furnace or water heater played in the event.
As the heating season arrives, so do the fire losses
originating in or near chimneys, fireplaces and flues. Typical causes include
deficiencies in installation, deficiencies in manufacture, wear-out, biological
(flue blockage from bird nests), lack of maintenance and improper usage. The
following case studies serve to illustrate typical fire origin scenarios along
with hints on what to look for when performing a cause and origin analysis.
Figure 1
Figure 1 is a view of a new wood stove in the middle of a
family room, which was being used for the first time when a fire developed at
the ceiling joist near the flue. Figure 2 is a view of burned ceiling joists
where a single wall flue had been installed with little clearance between it
and ceiling framing. Wood stove flue gases can be quite warm when compared to
fuels such as natural gas or oil. Consequently, multiple wall flues are
required or significant clearance between the flue and combustible material
must be maintained. Since the fire occurred on the first ever usage and the
owner had fueled the stove properly, an installation deficiency was suspected
as born out by the discovery of a single wall flue with insufficient clearance.
Figure 2
Figure 3
Figure 3 is a view of a badly burned condominium unit with
a fire origin near the fireplace. The owner indicated that he had smelled
natural gas odor near the fireplace for several weeks prior to the fire. Why he
did not attempt to correct the problem prior to the loss remains a mystery. Figure
4 shows a gas shut-off valve removed from the gas pipe that supplied natural
gas to the fireplace burner. The gas valve had been installed backwards in the
gas line. On certain gas valves an arrow indicates the direction of flow so
that when the gas is shut off the
Figure 4
valve stem seal (indicated by the arrow in Figure 4) is
not pressurized. Testing indicated that the valve stem seal leaked significantly.
If the valve is installed backwards, the stem seal is then pressurized when
shut, which can result in gas leakage and a possible fire or explosion. During
operation of the burner, the pressure drops significantly in the valve resulting
in virtually no leakage through the valve stem. In this case an installation
deficiency caused the fire.
Figure 5
Figure 5 is a view of a metal flue pipe showing holes that
have been drilled by the installer to secure a bracket for mounting in a wood
frame chase. Apparently these holes were not in the proper location and the
bracket was secured several inches above. Burn patterns suggest that the origin
of the fire was at wood framing that was about 1 inch from these holes. The
drilled holes penetrated both walls of the double wall flue. The fire occurred
on a very cold day when the gas fired furnace was working at or near capacity.
The holes offered an avenue for hot gases to exit the flue and come in contact
with wood framing; setting in motion pyrolysis of the wood and eventual
ignition. The penetration of the flue liner is an obvious construction
deficiency.
Figure 6
On a cold autumn evening a homeowner noticed a sudden
roaring sound coming from the fireplace after a fire was started in the
fireplace. Shortly afterward, severe cracking was noticed in the chimney tile
as shown by the lower arrow in Figure 6. The upper arrow points to severe
sooting and creosote buildup in the chimney. The owners burned soft wood
regularly and had not cleaned the chimney in 2 years. This is a classical chimney
fire where creosote deposits, built up over the years, ignited, causing severe
thermal damage to the chimney. This usually manifests itself in cracking of
chimney tile and brickwork. Here lack of maintenance is the deficiency that
caused the fire.
Figure 7
Figure 7 is a view of a fireplace showing the origin in
the vicinity of the ash pit. Figure 8 is a view of the ash pit, which was
improperly constructed since the hot
Figure 8
ash pile contacted combustible floor framing. This is a
construction related deficiency.
Bird nests sometimes cause havoc with chimneys either by
blocking the flue or by contacting hot metallic components.
In some instances owners use fireplaces as an incinerator,
burning garbage and flammable waste.
A statement from the owner is often helpful in
establishing the nature of the loss such as misuse, improper maintenance or a
construction deficiency. Inspection and documentation of the scene is necessary
especially in those matters where subrogation is contemplated. Even though the
building may have been constructed several years before a fire occurred, an
installation or design related deficiency would not necessarily cause a fire
immediately. The fire depicted in Figure 5 did not manifest itself until one of
the coldest days since the home was built was experienced. Finally, keep in
touch regarding the latest developments in spoliation of scene evidence.
Damaged evidence or failure to notify potential litigants regarding inspections
and testing can limit monetary recovery potential.
Good
design practice and local codes require that water heating boilers be equipped
with a low water cutoff control. This device signals the burner to shut down
when the water level in a boiler decreases to a predetermined level. An alarm
then sounds for the boiler attendant to provide manual feed water (more water),
or an automatic feed water control activates, bringing the water level to the
appropriate height. The water level may decrease, due to small leaks in the
system, causing overheating of the boiler if the low water cutoff malfunctions.
The boiler can be damaged and the heat from the upper portion
Figure 1
of the
boiler can ignite combustible materials such as wood joists in the boiler room.
Figure 1 shows a boiler that has experienced overheating. Burn patterns are
severe on the top side of the boiler
Figure 2
while
the lower portion is unscathed. Figure 2 shows the deepest char depth, which is
just above the boiler toward the flue end, a place expected to be quite hot
from an overheating boiler. This particular boiler had a float type low water
cutoff control. Electrical measurements were performed on all controls. Testing
indicated that the low water cutoff control was still signaling the burner that
the water level was within limits yet there was no water in the boiler. This
testing
Figure 3
was performed
with the control on the boiler. Figure 3 shows the low water cutoff after
removal from the boiler. Notice the severe sludge deposits inside the control
as shown by the arrow. This prevented the float from descending and opening the
circuit to the burner. After the control was cleaned it worked properly. The
probable cause of this fire was lack of maintenance causing a low water cutoff
control to malfunction. The manufacturer of this control installed a warning
tag on the unit stating that the operator should blow down (clean out) the
control every week. It appeared that this had not been done for several months.
The system had no water treatment apparatus installed.
While
analyzing a fire in a building housing a boiler, the following characteristics
may indicate a low water cutoff control related fire:
a. The fire started in the boiler room
or along a passage near the boiler flue.
b. The boiler is badly damaged especially
on the upper portion near the breeching.
c. The boiler tubes on top are
severely distorted while those on the bottom are not.
d. There is no water treatment on the
system to reduce the accumulation of contaminants entering the boiler water
circuit.
e. There appears to be generally poor
housekeeping and poor maintenance in the boiler room.
f.
The
boiler fluid delivery piping was known to have leaked extensively.
If a
malfunction of the boiler is suspected, DO
NOT remove the boiler controls until they are tested in place. A slight movement
of a sticky low water control can cause it to return to an operational
condition resulting in loss of important evidence. The remaining controls
should also be tested to rule out any additional or aggravating causes such as
a malfunctioning gas valve or temperature limit device.
Gas valves are installed in many appliances such as
dryers, water heaters, furnaces, and industrial machinery. The purpose of the
gas valve is to reduce the gas line pressure to a level required by the using
appliance. The gas valve also acts as a safety device to shut off the gas
automatically in case of flame failure. Modern gas valves incorporate pressure
regulation for reliable burner operation. A diaphragm connected to a valve controls
gas flow and pressure. A thermocouple senses heat from a pilot flame and
generates a current that holds open a solenoid valve. If the flame near the
thermocouple is extinguished, the current from the thermocouple stops and the
solenoid valve closes off gas flow.
Although relatively rare, gas valves can fail causing a
fire or explosion. Gas valves can fail from a variety of causes including
contamination, product deficiency, and misuse.
CONTAMINATION
Most building mechanical codes require a sediment trap in
the gas line, often called a drip leg. Figure 1 is a view
Figure 1
Figure 2
of a typical drip leg. Theoretically any debris that flows
toward the gas valve will be snared by the drip leg. Figure 2 is a view of
contaminants in a water heater gas valve. These contaminants accumulated on
valve seats allowing gas flow when the heater was in the off position. This
resulted in gas leakage and an eventual explosion. The contaminants were
analyzed and found to be copper sulfide, a byproduct of the reaction between
odorant in the gas and copper. It was found that a copper tube was used to
connect the gas service to the water heater without using a drip leg. In this
case, the two deficiencies were the use
Figure 3
of a copper line in gas service with a sulfur based
odorant and the absence of a drip leg. Figure 3 is the radiograph (x-ray) of
the water heater gas valve showing internal components. An x-ray is a valuable
tool in analyzing the condition of a gas valve prior to disassembly since it is
a nondestructive testing method.
Figure 4
PRODUCT DEFICIENCY
Figure 4 is a view of a home that sustained an explosion
in the basement. The arrow points to structural damage at the wall. Figure 5 is
a view of the water heater which was being lit by the home owner when the
explosion occurred, causing serious burn injury. Apparently the home owner was
in the habit of allowing the propane gas tanks to become
Figure 5
empty before calling for a refill. The gas valve on the
water heater was an old design that had been recalled because of intermittent
operation of the flame failure safety shut off. With this particular valve, one
could turn the gas valve control knob from pilot to on when the pilot light was
out. This caused damage to the valve which did not close during flame failure.
When the homeowner went into the basement to light the water heater, he was
unaware that gas was flowing through the gas valve due to an open main valve.
An unfortunate synergism had occurred in that the propane gas that had just
been delivered was not supplied with odorant. Consequently, he did not smell
the leaking propane. Since propane is heavier than air, a pool of gas had accumulated
on the floor which initiated an explosion when a flame was struck to light the
water heater.
Figure 6
JURY-RIGS
Some gas valve related failures are a result of field
expedients often called "jury-rigs.” Figure 6 shows a flattened tin can being
used as a baffle to prevent the pilot flame from being extinguished by down
drafts. The problem with this "new design" is that when the
thermostat calls for heat and the gas valve opens, it becomes very difficult
for the pilot flame to communicate with the raw natural gas at the burners.
Consequently a large amount of natural gas flows from the burners before
ignition occurs. This results in several large flash ignitions of the natural gas,
with one such ignition finally igniting nearby combustible materials.
Gas valve malfunctions can cause property loss and
personal injury. Statements as to activities by people near the appliance can
be helpful such as the information on the habit of the homeowner who allows the
gas to run out before reordering. Testing for the existence of odorant in the
gas may explain why the leakage may not have been detected. Review of product recalls
may show a deficiency in the gas valve. Finally, testing of the valve may be
required for further insight as to the cause of the accident.
Hydrogen peroxide (H202) is a widely used chemical in the
minerals, food processing, paper pulp, cosmetic, pharmaceutical and textile
industries. Hydrogen peroxide is typically provided in a solution with water
ranging from 2-70% concentration, depending on application. Hydrogen peroxide
is a strong oxidizer and will initiate or sustain a combustion process quite
readily. (Oxidizers are chemicals that release oxygen during a reaction.) If a
hydrogen peroxide solution evaporates on a combustible material such as
clothing, a fire may result spontaneously, without the need for an ignition
source. Hydrogen peroxide was also used as an early rocket fuel oxidizer.
The following case study is illustrative of an oxidizer
related fire. Figure 1 depicts the roof of a warehouse, the arrow pointing to a
severely burned area of the metal deck. Looking below this burn pattern yields
the photo shown in Figure 2. The arrow points to a badly decomposed mass of
plastic bottles, packaging
Figure 1
Figure 2
material and wood pallets. The fire occurred early in the
morning, before the dayshift arrived. The apparent source of the fire was
several polyethylene bottles of hydroxide bleach, an example of which is shown
in Figure 3. Conversations with the manufacturer of the bleach
Figure 3
product indicated that the concentration of peroxide in
water was approximately 25%. The US Department of Transportation requires a DOT
Oxidizer label on a product container for concentrations of
8 to 52% peroxide. (49CFR 172.101) The product involved in
the fire did not have such a hazard label. Prudent packaging engineering dictates
that the bottle for the 25% peroxide solution be opaque and have a vented cap.
Sunlight can cause decomposition of the peroxide solution, resulting in oxygen
evolution and
Figure 4
high pressures. A vented bottle cap is required to relieve
pressures in the storage containers to prevent a catastrophic failure of the container.
Figure 4 shows the type of opaque bottle that should have been used. A venting
cap is not shown but is also required.
A reconstruction of the fire scenario is as follows:
Peroxide solution (25% in water) was being stored in clear polyethylene
bottles. Either as a result of agitation, sunlight, or contamination, the
pressure increased in the peroxide laden bottles. Since there was no means of
pressure release, the pressure in the bottles continued to build until they failed,
releasing the peroxide. As the solution evaporated, the peroxide reacted with
nearby combustible materials causing a fire. Spontaneous ignition of combustible
materials near the oxidizer suggested leakage of the solution.
Storage and handling of hydrogen peroxide solutions is
well regulated with special safety features mandated for containers. Product
chemical analysis acts as a starting point to determine the cause of a fire
whose characteristics mimic those found in this case study.
Pose
Fire Control Problem
A
technical report entitled "Exploratory Testing of Flammable Liquids in
Plastic Containers" (FMRCJ.10NOE4.RR) was published by Factory Mutual
Research Corp., Norwood, Mass.
on April 6, 1986. The
article is somewhat technical, requiring a BS degree in science or engineering
to totally understand the intricacies of the testing. The report is a preliminary
study of the hazards of combustible and flammable liquids stored in plastic containers.
Although many flammable liquids are stored in plastic containers, the study was
limited to three typical liquids: mineral spirits, alcohol and corn oil. Polyethylene
containers were used for testing because of their widespread use in packaging
flammable liquids. Three sets of tests were performed. The first test dealt
with characterizing the flammable materials from product data sheets and
laboratory bench tests. Technical data such as specific gravity, heat of
combustion, flash point, fire point, etc. was obtained and reported. The next
two tests dealt with measurements of heat release rate, flame height, thermal
radiation, temperature and visual observations of the flammable liquids in
their plastic containers after being subjected to a heat load.
After
this preliminary testing, Factory Mutual Research concludes that fires involving
mineral spirits and alcohol products in their plastic containers will be
difficult to control using conventional sprinkler technology. Corn oil testing
indicated a better chance of control using conventional sprinkler systems. It appears
that a way to analyze the flammability of liquids in plastic containers can be
developed with additional testing. Implications are that flammable liquids in
plastic containers pose a different type of fire hazard that may be difficult
to control. The report may be useful to the underwriting department to make
sure that they are aware of additional hazards of insureds utilizing flammable
liquids in plastic containers.
While
adjusting a loss, analyze the involvement of flammable liquids in plastic
containers. They may have contributed to the loss, which could play a significant
role in subrogation efforts.
While investigating a fire loss,
the possibility of an electrical malfunction causing the fire is often at the
top of the list of most analysts. It is not uncommon for an investigator to
blame a fire on an electrical deficiency after ruling out other causes, even
though there is insufficient evidence to formulate an opinion on an electrical
origin. Care should be taken when analyzing electrical equipment to avoid the
pitfalls and embarrassment of the "false origin." The following are
some case studies that give insight into analyzing electrical malfunctions as a
fire cause. Electrical short
Figure 1
circuits or faults typically
generate heat and high temperatures that tend to melt wiring. The molten wire
often forms spherical droplets before solidification occurs. This is typically
called beading or balling. Figure 1 is a photograph of a typical electrical
fault in a wire resulting in beading. A fault has occurred between two wires
generating significant heat and melting of the wires. Does beading necessarily
mean that one has found the cause of the fire. Not necessarily. Figure 2 shows
several areas of beading on an
Figure 2
electrical conductor found in a
fire. In many instances the beading may be a result of a fire that burns
through electrical insulation causing a fault. At this point burn pattern
recognition in the vicinity of the beading is important. If there is uniform destruction
throughout the building with evidence of beading throughout the building, the
beading is probably a result of the fire. However, if a precise area of fire
origin is identified from burn patterns and a single wire is beaded in this
area where no other ignition sources exist, it is likely that the electrical
fault is a cause of the fire.
Electrical faults can also exist
without beading. Figure 3 is a view of a wire that
Figure 3
short-circuited with insufficient
resistance to cause arcing or beading. The fault developed and melted a portion
of the wire without telltale beading. This does not necessarily mean that
beading did not exist in the primary event. It means that the increased heating
has caused the liquid copper droplets to coalesce into the wire strands causing
a smooth solidified surface at the end of the wire. This is typical of
smoldering faults which slowly heat and cause ignition.
In some instances the arcing
generated from a fault can cause arc erosion. The discharge of electricity
through the wires can erode away metal with disastrous consequences. Figure 4
is a view of a wire that was resting against a fuel line in a large tractor.
During operation of the tractor, the wire insulation rubbed against the fuel
line causing breakdown of insulation. As the electrical wire faulted, it
generated a hole in the fuel line causing the fuel to be ignited by the arc.
Severe damage was sustained by the tractor as a result of the fire.
Figure 4
Faulting in electrical wiring may or
may not be a cause of a fire. Care should be taken in analyzing burn patterns
and other ignition sources at the origin of the fire.
Chafing is the failure of wire
insulation, usually as a result of mechanical means. Figure 1 shows typical
examples of wire chafing. Figure 1A depicts the common "cut through"
of the insulation from a sharp object. Wiring is often erroneously
Figure 1
routed over sharp edges that cut
the insulation, causing a short-circuit and possible fire. Figure 1B shows the
classical "wear through" configuration, a condition where wiring is
in contact with an oscillating or vibrating part. The mechanical motion wears
the wire insulation until a short circuit occurs along with a possible fire. In
Figure 1C, foot traffic has caused a crush like failure to wiring routed under
a carpet. This failure mode is known in the motor vehicle industry. Figure 1D
is the pinch failure mode where the wire insulation is damaged by two
mechanical parts acting like shears. A related failure mode is chafing as a
result of rodent chewing.
Figure 2 is a view of a heating
unit in a high rise building that suddenly started generating smoke. After the
fire department had left, claims investigation began with the heating unit and associated
wiring. There was severe faulting of the
Figure 2
Figure 3
power cable in the vicinity of a
metallic enclosure inside the unit. Figure 3 is a view of the faulted power
wire that had been removed from a flexible metallic conduit. Burn patterns
confirmed that the electrical fault was the likely cause of the fire. In the apartment
there were two additional identical units, certainly a fortuitous find since
comparison with an exemplar usually aids in the investigation. Figure 4 is a
view of the power wiring of the exemplar showing the flexible
Figure 4
metal conduit with the power wiring
entering a metallic enclosure. Surprisingly, none of the other units in the
apartment were equipped with a connector to connect the flexible metal conduit
to the enclosure. The flexible metal conduit was unrestrained, relying on the
wire to support it at the entry into the enclosure. The enclosure was
constructed of a thin gauge of galvanized steel with sharp edges at the
knockouts. The power wire rested against the sharp edge supporting the flexible
cable. During heater operation, fan vibration caused the flexible metal conduit
to oscillate, resulting in severe chafing of the wiring. Wires in the exemplar
units were in various stages of chafing and on their way to eventual failure.
This installation was deficient in that a connector should have been installed
to stabilize the wiring and act as a protector from vibration and chafing.
Identification of the chafing
failure mode obviously depends on the quality of the evidentiary remains.
Usually the cause of failure is installation related such as improper securing
of wires or lack of hardware. Sometimes a design deficiency arises through improper
selection of wire insulation. There are various grades of wire insulation, some
of which are chafing resistant. A review of the quality of wiring used in an
installation may demonstrate that substandard wire insulation was the ultimate
cause of the failure.
Fire development in clothes dryers
causes a range of losses from severe smoke damage to the building interior to
disruption of the entire building. The typical clothes dryer is composed of a
large rotating drum which provides an environment of warm air and moisture
removal. The drum is supported on bearings and usually powered by an electric
motor. The heat source is usually an electric heating coil or a gas fired
burner. Temperature limit switches are placed at strategic locations throughout
the dryer to provide input to temperature control devices. There are a variety
of causes of dryer fires with the following being typical examples.
Figure 1 is a view of an electric
dryer
Figure 1
Figure 2
that sustained a fire in the drum.
The origin of the fire appeared to be near electric heating coils as shown in
Figure 2. Combustible debris such as floor dust appeared to be contacting the
heating coil. A look under and around the dryer revealed a large accumulation
of dust and lint. Ingestion of combustible debris into the dryer which contacts
heating coils is a common cause of fires.
Figure 3
Figure 3 is a view of a gas dryer
with a relatively low burn pattern in the vicinity
Figure 4
of the burner. Figure 4 is a view of
one of the temperature limit switches that had parted from its mount. The small
plunger that was a part of the limit
Figure 5
switch had fallen out (Figure 5), rendering
the high temperature limit function ineffective. This resulted in overheating,
causing the fire. This appears to be a manufacturing related deficiency. Figure
6 is a view of a gas dryer that caught fire
Figure 6
shortly after the owner heard a
scraping sound. Figure 7 is a view of a failed support bearing which caused the
drum to wobble and scrape the back of the dryer. This opened a gap between the
drum and
Figure 7
dryer housing, resulting in clothes
falling out of the drum onto the bottom of the dryer.
Figure 8
Figure 8 shows clothing tangled in
the other support roller as shown by the right arrow and clothing contacting
the burner shroud as shown by the left arrow. The bearing failure appears to be
an end of useful life failure, as the dryer is old.
Figure 9
Figure 9 is a view of a large
industrial dryer that sustained a fire in the vicinity of the air circulating
impeller, which was constructed of a magnesium alloy (Figure 10). The magnesium
alloy had
Figure 10
ignited and had to be extinguished
with a chemical agent. The impeller was partially consumed, yet one interesting
Figure 11
piece remained. In Figure 11 we see
a lump of magnesium with a bent paper clip lodged inside. Scrape marks on the
paper clip suggest that it became engaged in the gap between the rotating
impeller and shroud, causing frictional heating that ignited the magnesium.
Some explosive events occur in
dryers, usually as a result of volatile cleaning chemicals entering the dryer
or, in some instances, the ignition of butane cigarette lighters in the dryer
drum.
Some items to consider when
attempting to determine the cause of a dryer fire are statements from
witnesses, the physical handling of the dryer after the fire, storage of the
dryer and the fire origin. In some instances, the burned dryer may be a
"victim" of the fire and not a cause. The typical burn pattern
analysis and witness information will aid in this determination. Burned dryer
components are often delicate and rough handling, as a result of moving, can
cause a loss of evidence. Finally, analyze the dryer as soon as possible since
environmental influences such as corrosion and animal infestation can cause evidence
deterioration.
Electric blankets are a useful
product and have been well accepted nationally, by consumers. Electric blanket
manufacturers and Underwriters Laboratories have worked together to improve product
safety. Despite these efforts fires do occur in relatively few of the blankets
in use. Typical failure modes that cause electric blanket fires are often
difficult to analyze due to the inherent destruction of the blanket once a fire
has started. Nevertheless, current thought suggests the following causes of
electric blanket fires:
1.
Thermostat
malfunction
2.
Control
unit malfunction
3.
Wiring
faults
4.
Product
abuse (improper laundering, leaving the blanket heating in a bunched condition,
smoking)
All manufacturers provide some
thermostatic mechanism in the electric blanket wiring as mandated by UL
standards. Figure 1 is a photograph of a typical thermostat used in electric
blankets. This
Figure 1
unit, which is one of several,
interrupts the power to the blanket if overheating results when the blanket
cannot dissipate the heat that is being generated.
When investigating a fire where a
thermostat malfunction is suspected, it is useful to delicately recover the
electric blanket and have the thermostats radiographed to ascertain their
positions. Radiography is also helpful in analyzing malfunctions of control
units. Figure 2 shows a typical control unit. Wiring
Figure 2
faults due to insulation failure
can be detected by visual examination of the wiring. However a wiring fault can
be a result or a cause, so interaction with the environment around the blanket
must be studied carefully. Product abuse is an important contributor to blanket
malfunctions. Improper laundering at high temperatures often violates manufacturer's
instructions, causing thermostat failure. Check for fabric shrinkage or water
inside the thermostat housings as evidence of having been damaged during
laundering. Check out smoking habits of the blanket user. It is thought that
leaving the electric blanket on in a bunched condition could cause a fire.
Although unlikely (thermostats should protect the blanket from overheating)
check the burn patterns on the blanket to determine if multiple burns are found
at several positions on the blanket. Figure 3 shows multiple burn areas which
suggest a bunching related fire. In this case the overheating occurred as a
result of a damaged thermostat. Finally, your investigation of an electric blanket
fire may result in a conclusion of 'undetermined' due to the all too often occurrence
of severe damage that masks the evidence of fire causation.
Figure 3
Figure 1 is a view
of a combination excavator loader that was part of a rental fleet. Typically
these vehicles remain idle through the winter but with the onset of spring,
rental demand returns. The vehicle was started and warmed up by the renter at a
construction site when a fire developed in the engine compartment according to
witnesses. Attempts to extinguish the fire were unsuccessful and a near total
loss of the vehicle resulted. The vehicle was new with no service history
problems and no recalls.
Figure 1
Figure 2
Burn patterns in
Figure 2 tend to confirm a fire origin in the engine compartment. The burn was
low with classic V-pattern. Opening the engine compartment cowling yielded the
photo show in
Figure 3
Figure 3. The
corroded turbocharger housing (left arrow) is quite evident having achieved
normally operating temperatures in excess of 500F as a result of hot exhaust
gasses that drive the unit.
Figure 4
The right arrow
points to combustible debris on top of the engine near the turbocharger. No electrical or mechanical deficiencies were
found at the origin that could have caused a fire. Figure 4 is another view of
the debris pattern on top of the engine. Figure 5 is a close-up of an eggshell
in the middle of the debris which was mostly straw and twigs. Skilled analysts would opine that a bird
(North American Robin, Turdus Migratorius) had made a nest in the engine
compartment. The spring had been late that year and cover for birds was at a
premium since the trees had not yet
Figure 5
leafed out. There
was easy access to the rain free engine compartment through the exhaust stack
clearance hole shown in Figure 6. The massive steel engine
Figure 6
provides heat
capacity and a stable temperature in the engine compartment. After the birds parted, combustible nesting
materials remained and were ignited by the hot turbocharger surface once the
vehicle warmed up. Turbocharger surface temperatures are typically high enough
to easily ignite typical nest materials.
Bird problems are
nothing new to the claims industry. Bird nests in chimney flues which offer a
warm environment have resulted in flue fires and heating equipment malfunctions
as illustrated by the bird remains causing overheat damage to the boiler in
Figure 7. Equipment that is not being used during the spring is a prime
candidate for a bird nest site which can be constructed in days. Screen
guarding of access openings is a typical preventive measure for bird entry.
Origin of fire:
engine compartment, cause of fire: biologic intervention.
Figure 7
Figure 1 is a photograph of the
remains of a building after an explosion has occurred. Explosions often result
in severe damage to a building from the initial blast as well as the ensuing
fire. Losses can reach several millions of dollars.
Figure 1
The adjuster may be one of the
first of a long line of personnel involved in the analysis of the explosion and
resulting subrogation that may last for several years. Quick action on
collecting data concerning the explosion results in better documentation of the
accident scene for future analysis. Here are some thoughts that may help in the
collection of data.
Explosions often result in debris
and damage patterns around the building that are easily identifiable. A debris
pattern reflects the rest positions of objects that were hurled away from the
explosion initiation area. A damage pattern is the configuration of the
building that exploded as well as neighboring buildings that were damaged by
the explosion. This information should be photographed immediately from the
ground and from the air if the patterns are widespread. Explosive debris and
minor damage to other buildings is often cleaned away or repaired quickly,
resulting in a loss of valuable information to the explosion analyst. The
explosion analyst uses this information in an attempt to identify the
initiation point of the explosion - the point at which the explosion started.
After an assessment of the seriousness of the loss, explosion analysts may be
required. If so, get the analysts and experts in right away while the scene is
relatively undisturbed. The experts will usually try to identify the initiation
and ignition points along with the location of explosive fuel. The ignition
point is the location where explosive fuel was ignited and may differ from the
initiation point. In vapor cloud explosions, the initiation and ignition points
are often different locations. Photographic documentation of the interior of
the building is a must. The building should be secured so that the scene is
undisturbed. Analysts should be brought in to identify the ignition, initiation
and fuel source locations. This may mean stationing a full time guard at the
scene to prevent loss of valuable evidence. Having established the initiation,
ignition and fuel source locations, the next area of inquiry is why the
explosion occurred. Was the release of an explosive mixture the result of
carelessness of the insured or was this a result of a component or system
defect. The identification of a component defect that caused the explosion
significantly affects subrogation interests. Analysis and inspection of
building systems including electrical gear, process systems and environmental
systems should be performed to isolate the probable cause. After determining
the probable cause of the explosion, artifacts that were removed should be identified
and documented so that the chain of evidence is preserved. Since the building
is likely to be torn down and rebuilt, it is necessary to remove certain
critical pieces of hardware to the evidence storage locker.
The following sums up some ideas on
how the adjuster can effectively influence the analysis of an explosion. Photograph
debris and damage patterns, secure the scene, call in the experts immediately
if required, and document the removal of evidence for possible future
litigation.
When analyzing the cause of a fuel
gas related fire or explosion, examination of flexible gas connections in the
building is advisable. Leaks in the flexible gas connector can be caused by
improper installation, excessive bending from movement of the appliance,
defective design or manufacture and environmentally assisted cracking. Improper
installation such as cross threading of connectors and insufficient torquing of
the connection can cause leakage. Inexperienced installers are likely to be at
the root of poor installations. Excessive bending of the gas line can occur by
forcing the appliance against an immobile object, such as a wall, causing
crimping of the tube. It does not take too many cycles of excessive bending to
cause partial or total failure of the tube. Older homes and installations are
candidates for this type of failure since remodeling and other rearrangements
of appliances over the years have taken their toll.
Figure 1
A typical manufacturing or design
defect is insufficient soldering of the tube assembly to the end ferrule.
Explosions have occurred from leaks caused when the solder connection failed
allowing a large crack near the end of the connector. Utility companies often
compile lists of flexible connectors with poor performance histories.
Environmentally assisted cracking
(stress corrosion cracking) occurs in certain types of brass used in flexible
connectors. Figure 1 is a view of a typical flexible gas connector. Figure 2 is
a view of a crack in a flexible connector caused
Figure 2
by environmentally assisted
cracking. The crack is in a stressed section of the tube. Chemical analysis of
the brass showed it to be Cartridge Brass (nominally 70% copper and 30% zinc).
Brasses containing more than 20% zinc are highly susceptible to stress
corrosion cracking. Figure 3 is a photomicrograph of a cross-section of the
tube showing the grain structure of the brass and severe intergranular corrosion
in the tube, characteristic of stress corrosion cracking. The corrosion is
aggravated by household cleaning chemicals such as
Figure 3
ammonia. The cracking can occur
with very little movement of the tube and is often a subtle failure.
These are a few examples of
deficiencies that can occur in flexible brass connector installations which can
result in substantial property or casualty loss. Clues to causation of the loss
may lie in the detailed examination of flexible brass connectors attached to
gas appliances.
Periodically, an explosion loss
occurs that is related to the flexible gas connector used to transport fuel gas
to various appliances. A unique failure of the brass flexible connector has been
observed
Figure 1
involving the solder joint near the
flare/threaded nut end. Figure 1 is a cross-sectional view of an old flexible
connector design where the corrugated flexible brass tubing is soldered to the
end flare. The end flare is the mating surface with the appliance fitting and
is secured by the threaded nut. Apparently, over time, the solder joint fails
and a crack forms at the solder/brass interface. Newer connectors do not have
this reliability problem.
Figure 2
Figure 2 is a cross-section of
the more modern connector with a flare that is formed as an integral part of
the connector. The absence of the solder joint enhances flexible connector
reliability.
Figure 3
Figure 4
Figures 3 and 4 show two
different solder joint connectors that failed causing an accumulation of
natural gas in a home and an explosion. The characteristic solder fracture
surface is apparent in each of the connectors. The flexible connectors showed
little sign of excessive bending or strain. In each case the connectors were
over 30 years old.
Figure 5
Figure 5 is a photograph of a
modern flexible connector with a protective
Figure 6
coating. Figure 6 is an end view
of the integral flare. Figure 7 is a view of the manufacturer’s identification
band. The apparent cause of the failures is an aging effect, characteristic of solders.
Solders are multi-phase lead/tin alloys, which tend to precipitate over time,
causing a reduction in fatigue strength. As a result of cyclic loading brought on
by periodic forces such as thermal expansion and contraction of the solder joint,
metal fatigue initiates a crack that eventually propagates through the solder joint
causing the gas leak and possible fire or explosion.
Figure 7
When analyzing an explosion or
fire, which is a result of a gas leak in a flexible connector, determine if the
manufacturing method involves a solder joint. If a fractured solder joint is
found, then metallurgical analyses may be required to verify the failure mode.
The age related failure of the old solder joint technology is tantamount to a
"time bomb" in that it can occur without warning and at virtually any
time.