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Typical manufacturing, mining and material handling
operations produce various types of dust and other
contaminants. These contaminants may be quite toxic
when they enter the workers’ lungs. Protection
systems involve either suppressing the mostly solid
particulate contaminants generated or the venting
and collection of the contaminants.
Dust Suppression
The first approach is to prevent the generation of these
toxic contaminants before they enter the work
environment. This can be accomplished by dust
suppression systems. An example of this would be a
rock quarry or coal mine. As the gravel or coal is
processed into smaller usable sizes by crushing and
placed on belt conveyors to be delivered to trucks
or railroad cars, and then delivered to further
processing. All of these operations produce large
quantities of dust if they are not controlled in
these operations. Dust suppression is achieved by
spraying liquid over the rocks and coal so that the
operations do not produce dust. In dust suppression
technology compounds are added to the water that
eliminates the surface tension of the water so that
the liquid coating is spread over a much larger
surface area. The dust then stays attached to the
pieces and to adjoining particles to prevent dust
generation. The spreading of presumably water based
solution increases the rate of evaporation. In
colder climates the dust and product does not allow
the material to freeze so that loading and unloading
are facilitated.
Venting of Dust Producing Operations
The most common contaminant in industrial manufacturing
operations is solid particulate. First the
contaminant is contained by putting enclosures or
hoods around the dust generating machines that will
allow access for the workers. The hood is then
ventilated and the contaminated gas stream is
cleaned by a dust collector. The dust is separated
from the gas stream and the gas stream is vented to
the work environment or outdoors. The process of
collecting this dust, disposing of it, maintaining
and servicing the dust collector equipment can
expose the workers to serious hazards:
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The first hazard often present is
accumulation of
dust in the bottom portion of the horizontal
duct runs.
Most well designed vent ducts have cleanout
doors every several feet. These ducts are near
the ceiling usually 12 or more feet above the
floor. The cleanout doors are located on the
lowest underside of the duct and when the doors
is opened dust will pour out towards the floor
exposing the worker and environment to dust that
might be inhaled. Special man hoists are
recommended and breathing masks are indicated. A
better approach is to install pneumatically
actuated Duct Cleaner-Boosters in the system.
These will momentarily increase the velocity in
the ducts pushing the dust accumulation toward
the dust collector. It makes the duct cleaning
operation automatic and safe, with minimum
exposure to the dust.
Contact QAM toll-free (800) 267-5585, e-mail: info@qamanage.com
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The next hazard is when flammable
dusts are produced in the machine being vented
to the dust collector. If dry dust is collected,
sparks can be entrained from the hood(s), and
carried into the collector. There is a coating
of flammable dust on the filter elements. The
velocities through the filters are much lower
than in the duct and if a spark reaches the
filter elements, the dust may reach the ignition
temperature start a fire. In well designed
ductwork the flow is designed to be laminar.
Sparks may be transported for more than a
hundred feet. To guard against this occurrence,
an in-line spark suppressor with a duct cleaner
– booster should be installed. The suppressor
device will induce extreme turbulent flow which
cools the spark below the ignition temperature
and protects against fires.
Contact QAM toll-free (800)
267-5585, e-mail:
info@qamanage.com
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Explosions are another operating hazard. To
have an explosion the concentration of dust in the
housing or duct must be between the lower and upper
explosive concentrations and a spark must be
present. In mechanical cleaning (shaker) collectors,
the flow is stopped in the filter compartment and
the filter elements are agitated all at the same
time. A potential for an explosion occurs since the
concentration will likely pass through the explosive
limits during this action. Protection consists of
grounding the filter elements to prevent sparks
generated during cleaning. Additional explosion
rupture panels are installed and vented outdoors. In
continuous cleaning pulse jet collectors small
sections of the collector are reverse flushed.
Around each bag in the cleaned section is a very
small volume of air which can pass through the
explosive limits. Even if a spark is present, an
explosion would be dissipated without danger. If the
collector filters are to be replaced the first
procedure is to remove as much flammable or
explosive dusts from the filters as possible. The
exhaust fan’s direction is reversed to maintain a
low flow and prevent dust from returning to the
hood. The collector is cleaned one section at a time
allowing time for the dust to settle into the
collection hopper. After several complete cleaning
cycles a large portion of the dust will be ejected.
This lowers the exposure of the worker in handling
the filter elements. There are two general types of
filter elements; those with smooth surfaces usually
cylindrical or oval with smooth surfaces, and,
pleated filter elements. There is a potential for
pleated filter elements to bridge and have dust
collected in the valleys of the pleats. Even if a
reverse pulse collector is cleaned slowly with the
fan reversed, considerable dust may be present in
the valleys. Recent new technology provides for
wider pleat spacing and stiffer filter media which
allows off line cleaning as described above to be
effective.
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There are some contaminants either
liquid or solids which are not suited to fabric
media collectors. They will not form a filter
cake or the dust is very unstable. Gunpowder and
the propellant for inflating bags in an
automobile are two common examples. The most
common approaches are gas washers that scrub the
contaminant from the vent stream. Another
approach is to mix inert dust into the vent
system so the dry powder mixture is no longer
flammable or explosive. Some operations will
produce dust that is so wet that it will quickly
turn the filter cake into a mud which will blind
the filter elements.
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Wet Dust Collectors have a variety
of designs and must deal with the problem of
surface tension of the water which is used to
clean the gas. To get adequate collection
efficiency, historically designers have resorted
to higher pressure drop designs so that the
solid and liquid contaminants might penetrate
through the scrubbing surfaces. The same types
of compounds, as in dust suppression systems,
allow operating at very high efficiencies with
minimal power consumption. It was necessary to
design special multi-pass mist eliminators to
collect the overspray from the scrubbing
surfaces. Therefore gas leaving the collector is
often below saturation from the heat regain as
it passes through the exhaust fan.
Contact QAM toll-free (800)
267-5585, e-mail:
info@qamanage.com
Other Hazards and Considerations
Rotary Feeders located in the bottom of a
collection hopper can pose a danger to maintenance
personnel. There are instances where the feeder
fails and dust builds up in the hopper. This dust
can ignite and burn. The main approach is to
shutdown the section of the collector and let it
burn depriving it from getting oxygen. Normally, it
will self extinguish. Spraying in water can create
an explosion as the water displaces the dust with
steam and will go through the explosive limits. On a
power plant boiler, a maintenance man decided to
pour water into the hopper. The dust was agitated
and an explosion occurred.
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It is preferable to install
collectors where the filter elements can be
changed from outside the collector housing.
Collectors with removable roof doors are widely
available. When a filter element extends 8 feet
above the doors, even a moderate wind is a
problem and workers must plan to protect
themselves from these forces.
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Another type of design is
walk-in-plenums. There is a housing tall enough
to change the filters, out of the weather
elements. The entrance is usually a hinged
access door. Rarely do these chambers have
lighting. A particular hazard is when in the
housing the door slams shut. It is hard to find
the door in the dark and a wind can keep it
closed. It is advisable to clamp the door with a
cable and lock it. Remember the bags and cages
must be lowered to the ground and hauled up
again and fed to the collector through the
access door. The pulse pipes must be removed and
temporary storage space provided. These pulse
pipes can fall through the holes in the filter
mounting plates, forcing a man to enter the
collector. Toxic gases can seep into the clean
air plenum, so, the breathing zone should be
monitored. When a collector is initially
started, dust will seep into the plenum until a
filter cake is formed. Personnel should not
enter the plenum until this conditioning of the
filters is completed.
Contact QAM toll-free (800)
267-5585, e-mail:
info@qamanage.com |