Explosions and Fires; Baghouses, Cartridge Dust Collectors

by:
Gary Berwick, P. Eng.
Quality Air Management
June 2006

Excerpts from an article published in different media.

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:
1. An explosion front traveling through the duct and enters the collector ...
2. Off- line cleaning (in which the fan is shut down) increases an explosion hazard...
3. 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 ...

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.

Transport of Sparks Through Ducts

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. ...

QUENCHER spark arrestor (trap)

Duct Cleaner - BOOSTER

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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