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Dust Related Explosion and Fire Hazards
Inflammable and Explosive Dusts
The three ingredients that cause fires are heat,
oxygen and combustible material. When all three components are present,
combustion takes place. Many dusts and powders that emanate out of the
industrial processes are easily
flammable. These dusts carry characteristics that differentiate
them such as:
- Dusts that explode
easily. Since they carry an
oxidant,
they do not need oxygen from the atmosphere to explode. (Eg. Chemicals that
pump up automobile air bags, gunpowder)
- Dusts that ignite
easily. They require little or no external heat to ignite.
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An explosion is defined as the process in which combustion
occurs and spreads so rapidly as to create a high pressure. In this case, the
fire expands from a source of ignition and develops high pressure when
restricted in an area.
Explosions have lower and upper limits (of dust
concentration). A lower limit of explosion indicates that the concentration of
dust particles is below the lower limit of explosion. Below this level, the dust
concentration will not explode even on ignition. This happens because the heat
produced in such a circumstance is not sufficient to affect other dust
particles. Alternatively there could be a dust concentration of explosive levels
but there may not be enough oxygen to start the fire.
To counter the hazards of fire and explosions, it is necessary to know the
characteristics of the dust. For example, fine aluminum dusts explode at very
low limits of explosion whereas coarse aluminium dusts do not catch fire even
under the influence of another source of heat.
Looking for other
dust collection information topics? Mouse over the resource section
(at the bottom) of the left hand navigation.
Understanding Explosions
By mixing gas (air) and compound (dust) in concentrations that create an
explosion, combustion proceeds at high speed (sonic speed or supersonic speed)
from the point of ignition. The face of this growing ball of fire is called
‘flame front. If there are no obstructions, this high pressure flame burns
itself out after traveling a distance. But if it restricted in any way, the
pressure increases tremendously. By knowing the rate of increase of this
pressure for a particular compound, the hazard that it can cause can be known.
The baghouse has an inherent advantage with its air cleaning process
that may keep dust concentrations below the lower limit of explosion. On the
other hand, shaker dust collectors are susceptible to explosions since static charges
could set off a spark while ‘shaking’ or bag filter cleaning is in process.
Explosions are of two types. Primary explosions are those ignited by dust
concentrations being at explosive levels. These primary explosions could take
part in one part of the system. But then they move rapidly along the system
ductwork. When the flame front reaches the dust collector it could cause further
explosions (by creating explosive dust concentration by unsettling all dust
particles on the filter bags). This would cause a secondary explosion.
Explosion Vents
To reduce the damages that could be caused by explosion, dust collectors are
provided with exhausts or vents that allow the explosion to go out of the system
before further damage to other parts. Common designs of explosion vents are:
- Membrane: A membrane that breaks at a preset
pressure on the explosion vent to allow the flame front to exit from the vent in
high pressure situations (explosion)
- Hinged/ Restrained panels: By installing Hinged or
Restrained panels with springs that open at preset pressure, the flame front is
allowed to exit the system
NFPA – 68 Guide for Venting Deflagrations
Information on controlling explosions and reducing their structural effects
on dust collectors can be obtained in the NFPA – 68 guide ‘Guide for venting of
Deflagrations’ prepared by the National Fire Protection Association.
Deflagration is defined as the propagation of a combustion zone at a velocity
that is less than the speed of the unreacted medium. Simply put, deflagration is
nothing but burning with great heat.
Early vent designs used an assumed vent ratio of 40:1 (vent ratio is). In time
however, many dusts and powders have been analyzed for their individual rate of
combustion (burning) and combustibility. Based on these analyses, a value of
‘Kst’ has been assigned to each dust, which indicates the deflagration index of
the dust (expressed as bar-m/second). Higher ‘Kst’ value indicates faster rate
of combustion.
The NFPA – 68 guide is a useful document that provides ‘Kst’ values for
varieties of dusts and powders. It also provides the method to calculate vent
area for various applications.
Shaker Collectors and Explosions
Shaker collectors carry the hazard of explosions due to the ‘shaking’ of the
bags that could cause static charge on the bags to set off a spark, leading to
an explosion. Since dust concentrations invariably reach explosive limits during
cleaning operations in shaker collectors, precautions need to be taken to avoid
explosions. A common method is by coating bag fibers with a conductive coating
that grounds the static charge thus neutralizing danger from static charge.
Shakers are prone to secondary explosions as well because these bags tend to
accumulate more dust (combustible material or fuel) when compared to other
collectors.
Fabric Jet Collectors and Explosions
These collectors posses an inherent advantage in mitigating explosion risk
due to static charge since their design uses reverse flow of air. This reverse
flow of air (that is not ionized) through the dust, depletes all static charge
from the dust particles. The cleaning system is thus, constantly cleaning up any
charges from building up as well. Fabric jet collectors also has the design
capability to retard secondary explosions.
| Assume a
cylindrical bag in a high ratio reverse air system (4 ½ inch
diameter and 96 inch length). The total air pumped into the bag
during a cleaning cycle is one cubic foot. This air forms a hollow
cylinder, lined with one-inch thick dust. |
Controlling Secondary Explosions
The energy produced in the primary deflagration must be reduced to control
secondary explosions. This can be achieved by controlling factors that
contribute to explosion i.e fuel and oxygen. Some of the methods are:
- Reduce oxygen in the air:
Use of new generation compact high ratio fabric designs and use of multiple
hoppers (reduces hopper volume at product end) reduces oxygen levels in the
system considerably.
- Reduce fuel (dust particles) in the air:
Smooth finish filter media (such as eggshell or singed) on bags, reduces dust
accumulation on bag surfaces. Use of PTFE laminated bag with frequent
cleaning reduces dust concentrations.
It is also advisable to remove dust from hopper as it could form a potential
concentration for explosive levels. Another method of reducing dust
concentration (fuel) is by maintaining a low pressure drop (indicated by the
magnehelic gage) through the filter media by frequently pulsing air cleaning
valves. In centrifugal and pleated bag media the pressure drop must be kept low
by cleaning the media frequently.
- Other techniques:
By mixing inert air produced in another part of the system with the combustible
dust of another machine a less combustible mixture can be formed.
Fire Hazards
Collectors are highly susceptible to fires due to the presence of fuel (dust
particles) and oxygen. Collectors have fan flow that further contributes to the
fanning of a fire and helping it grow. Since collectors contain all these
fire-friendly characteristics it is necessary that proper filter media is chosen
and operating techniques are followed with care. There are many dusts that burn
(form a fire) but they have a low rate of combustion.
Causes of Fire
The main causes of fires are (i) sparks and (ii) spontaneous combustion.
Sparks:
Sparks find their way into dust collector systems from the industrial process.
They enter through the hoods and ducts in the collector system. One way of
tackling sparks was by having a long duct. Traveling along the long duct would
enable the spark to burn itself out or cool off. However it is not an effective
solution since sparks are known to travel over a 100 feet and survive the
cyclone before igniting the bags and dust in the dust collector. Fires in dust
collectors start when the system is in process.
To understand the nature of the sparks we may look at the example of a campfire
where sparks fly over the fire. Though sparks consist of a heavy particle, they
‘fly’ because they are surrounded by a layer of ‘hot air’. This layer of hot air
around it makes the spark behave like a hot air balloon and it travels long
distances easily. Dust collector systems have a smooth flow duct design which
helps sparks to travel unaffected by gravity and centrifugal forces of the
system.
Spontaneous Combustions
Fume dusts are a classic case of fine dusts that have large surface areas.
Fume dusts are known to oxidize and the process of oxidation produces heat which
is a factor that can start a fire. Oxidation does not cause a problem when the
dust collector is in operation since the heat is removed by the flow of dust
through elements. However when the dust collector is not in operation, heat
generated by the oxidation of these dusts forms hot spots on the filter cake.
These hot spots in the filter cake could ignite when the flow is restarted in
the system. Once ignited the fires are fanned by the airflows and cause
extensive damage.
Ways to Control Sparks
To extinguish sparks, it is essential that the layer of hot air surrounding
the spark is removed. This can be achieved by creating a change in air velocity.
The change in air velocity creates eddies in the air stream and removes hot air
from the spark. Once the layer is disturbed, sparks can be cooled in a fraction
of a second. The eddies can be created by
- abrupt change in duct sizes in the system
- single/multiple plates with orifices placed near the
hood inlet
- change in direction of duct such as a square elbow
instead of a smooth turning vane
- spark trap
Sprinkler Systems
Water sprinkler systems could be installed in collectors in the area where
clean air is recycled into the work place. However, one must remember to turn
off the water sprinkler system when the fire is put off because water could
collect in the hopper and bins. If a large amount of water accumulates, the
structure of the dust collector could be affected badly involving expensive
reconstruction.
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