Explosions and Fires in
Baghouses and Cartridge Dust Collectors
by
Gary Berwick, P.Eng.
Excerpts from an article published in various
media.
Click
here to view (.pdf) and print the entire article
EXPLOSIONS
Dust explosions are possible whenever the process produces combustible
dusts... ...When a dust can sustain an explosion, the dust concentration
must be within the explosive limits....
Intensity of the explosion is dependent on the rate of pressure
rise and maximum pressure developed. ...the dust collector bags usually
obstruct the expansion of the explosive flame front.
Limiting exposure to hazards
- ...One source of ignition is sparks, often produced in the hoods
venting processing machines. Sometimes the machinery can be modified
to prevent spark generation. Another method is to install
spark suppressors
prior to the dust-laden gas entering a dust collector. ...
- Isolate Operations; The collectors may be located outdoors or
away from the main production areas.
- ...Inert dust can be introduced into the system so that the
lower explosive limit of the dust mixture
- ... Explosion vents can be provided on ducts and on the collector
housings. ...
- Changes in Design of collectors:... rectangular units can be
braced to withstand higher explosive pressures.
- ...Grounded bags can provide a false sense of security... When
the filter element is cleaned, sparks can be generated.
- There are no known cases where an explosion was generated within
a pulse jet dust collector.
- ... Referring to figure 1, which is a view of a cylindrical
bag in a Pulse jet collector.
The bag is the cylinder between the two dark hollow sections of
cylinders. When the bag is cleaned a small volume of dirty air is
propelled from the bags and extends a fraction of an inch between
the bags. This forms a hollow cylinder of dust laden agglomerated
dust... usually less than 3% of the housing volume so the explosion
would dissipate itself ...
- External sources of detonations in pulse jet cleaning collectors
which experience explosions are as follows:
-
An explosion front traveling through the duct and enters
the collector ...
-
Off- line cleaning (in which the fan is shut down) increases
an explosion hazard...
-
Hopper fires can occur if the hopper is not cleaned out
before de-energizing the fan... and the fire in the hopper provided
a detonation source ...
Click here
for information on the QUENCHER spark arrestor
Explosion hazards in other Dust Collectors
Mechanical shaker collectors are inherently more hazardous than
cylindrical bag collectors..Pulse Jet Cartridge (Pleated filter
element) Collectors ... a certain cleaning jet can only clean a
fixed area of media. The rest of the area is plugged and it holds a
lot of dust. This is discussed in the article (History
of Cartridge Collectors) ...plugged media contributes a lot of dust
to fuel a fire or explosion.
FIRES
... Fires in ducts are usually a result of poor duct design so that
dust drops out in ducts. Fires can occur in exhaust ducts as well as
inside dust collectors.
Requirements of fires or any combustion process are:
-
Fuel, in gas, liquid or solid form.
- Oxygen (Atmosphere consists of 20 per cent oxygen)
- Fuel must be raised to the ignition temperature to start burning.
Sources of ignition include:
Overheating of coils, motors, friction, spontaneous combustion,
static discharge, burning debris drawn into the vent system.
Spontaneous combustion occurs when dust slowly oxidizes in a collector
or in any accumulated pile.... A “hotspot “ develops. ...
Static discharge - Generally speaking static built up in a collector
is reduced or eliminated by the jet cleaning system...not so for mechanical
cleaning (shaker) collectors.
Burning debris drawn into the exhaust system can be a source
of ignition.
Transport of sparks through ducts. ...This spark and the hot gas
associated with the spark can travel hundreds of feet in a duct... Spark
suppressors are placed in the duct to change the laminar flow to turbulent
(coarse) flow. This agitation or turbulence strips the air from around
the ember and cools the spark below ignition temperature.
Prevention
...eliminating the causes of ignition ...Spark traps;
...design for proper dust transport velocities. Install a pneumatic
actuated duct booster to flush dust into the dust collector and remove
electrostatic charges on duct.
...Spontaneous combustion in Pulse Jet Collectors can be prevented by
pulsing collector when the system is idle. ...
Click here
for information on the QUENCHER spark arrestor
Click here for information
on the BOOSTER - Duct Cleaner
Putting out fires
...Cooling below ignition temperature, Cutting off fuel supply,
Cutting off oxygen supply.Water Hose and nozzles. This is
an attempt to cool the solid fuels below the ignition temperature and
to cut off the flow of oxygen to the fuel. It also takes away heat by
turning water into steam. ... The hoppers should have automatic drains
to prevent the water from doing structural damage. ...
Inert gas systems such as carbon dioxide or nitrogen gas are
sometimes provided...
Fan Operation during a Fire Whether to shut down a fan on a dust
collector because of fire can be a difficult decision ... If the gas
stream was re-circulated the decision is of course to shut down the
fan.
Other fire extinguishing systems
Manual Fire extinguishers are usually either inert gas like carbon dioxide
or inert powders... The manual gas extinguishers should not be operated
through doors of the dust collectors ...
There a several other systems to fight fires in
dust collectors which
will not be covered in this article.
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and print the entire article
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