A dust explosion is the rapid combustion of fine particles suspended in the air,
often but not always in an enclosed location. Dust explosions can occur
where any dispersed powdered combustible material is present in high enough
concentrations in the atmosphere or other oxidizing gaseous medium such as
oxygen. Dust explosions are a frequent hazard in
underground coal mines, in grain elevators, and other industrial
environments. On the other hand, they are also commonly used by special
effects artists, filmmakers, and pyrotechnicians, given their spectacular
appearance and ability to be safely contained under certain carefully
controlled conditions. Terminology
If rapid combustion occurs in a confined space, enormous overpressures can build
up, causing major structural damage and flying shrapnel. The sudden release of
energy from a "detonation" can produce a shockwave, either in open air or in a
confined space. If the spread of flame is at subsonic speed, the phenomenon is
sometimes called a "deflagration", although looser usage calls both
phenomena "explosions". Dust explosions may be classified as
being either "primary" or "secondary" in nature. Primary dust explosions may
occur inside process equipment or similar enclosures, and are generally
controlled by pressure relief through purpose-built ducting to the external
atmosphere. Secondary dust explosions are the result of dust accumulation
inside a building being disturbed and ignited by the primary explosion,
resulting in a much more dangerous uncontrolled explosion inside the
workplace. Historically, fatalities from dust explosions have largely been the
result of secondary dust explosions. Conditions required
There are four necessary conditions for a dust explosion:
A combustible dust The dust is suspended in the air at a
sufficiently high concentration There is an oxidant
There is an ignition source In some traditional analyses of dust
explosions, confinement is considered to be a fifth requirement; this is not an
essential condition, but can greatly aggravate the physical damage that
results. = Sources of dust =
Many common materials which are known to burn can generate a dust explosion, such
as coal and sawdust. In addition, many otherwise mundane organic materials can
also be dispersed into a dangerous dust cloud, such as grain, flour, starch,
sugar, powdered milk, cocoa, coffee, and pollen. Powdered metals can form
explosive suspensions in air, if finely divided.
Explosive dust can arise from activities such as transporting grain, and grain
silos have often been demolished violently. Mining of coal leads to coal
dust, and flour mills likewise have large amounts of flour dust as a result
of milling. A gigantic explosion of flour dust destroyed a mill in Minnesota
on May 2, 1878, killing 14 workers at the Washburn A Mill, and another 4 in
adjacent buildings. A similar problem occurs in sawmills and other places
dedicated to woodworking. Although not strictly a dust, paper
particles emitted during processing - especially rolling, unrolling,
calendaring/slitting, and sheet-cutting - are also known to pose an explosion
hazard. Enclosed paper mill areas subject to such dangers commonly
maintain very high air humidities to reduce the chance of airborne paper dust
explosions. In special effects pyrotechnics,
lycopodium powder and non-dairy creamer are two common means of producing safe,
controlled fire effects. Thermobaric weapons, depending upon their fuel, are
also a potential and intentional source of dust.
To support rapid combustion, the dust must consist of very small particles
with a high surface area to volume ratio, thereby making the collective or
combined surface area of all the particles very large in comparison to a
dust of larger particles. Dust is defined as powders with particles less
than about 500 micrometres in diameter, but finer dust will present a much
greater hazard than coarse particles by virtue of the larger total surface area
of all the particles. = Concentration =
Below a certain value, the lower explosive limit, there is simply
insufficient dust to support the combustion at the rate required for an
explosion. A combustible concentration at or below 25% of the LEL is considered
safe. Similarly, if the fuel/air ratio increases above the upper explosive
limit, there is insufficient oxidant to permit combustion to continue at the
necessary rate. = Oxidant =
Typically, normal atmospheric oxygen can be sufficient to support a dust
explosion if the other necessary conditions are also present. High-oxygen
or pure oxygen environments are considered to be especially hazardous,
as are strong oxidizing gases such as chlorine and fluorine. Also, particulate
suspensions of compounds with a high oxidative potential, such as peroxides,
chlorates, nitrates, perchlorates, and dichromates, can increase risk of an
explosion if combustible materials are also present.
= Sources of ignition = There are many sources of ignition, and
a naked flame need not be the only one: over one half of the dust explosions in
Germany in 2005 were from non-flame sources. Common sources of ignition
include: electrostatic discharge
friction electrical arcing from machinery or
other equipment hot surfaces
fire However, it is often difficult to
determine the exact source of ignition when investigating after an explosion.
When a source cannot be found, ignition will often be attributed to static
electricity. Static charges can be generated by external sources, or can be
internally generated by friction at the surfaces of particles themselves as they
collide or move past one another. Mechanism
Dusts have a very large surface area compared to their mass. Since burning
can only occur at the surface of a solid or liquid, where it can react with
oxygen, this causes dusts to be much more flammable than bulk materials. For
example, a 1 kilogram sphere of a combustible material with a density of 1
g/cm3 would be about 12.4 centimetres in diameter, and have a surface area of
0.048 square metres. However, if it were broken up into spherical dust particles
50 µm in diameter it would have a surface area of 120 square metres. This
greatly-increased surface area allows the material to burn much faster, and
the extremely small mass of each particle allows them to catch on fire
with much less energy than the bulk material, as there is no heat loss to
conduction within the material. When this mixture of fuel and air is
ignited, especially in a confined space such as a warehouse or silo, a
significant increase in pressure is created, often more than sufficient to
demolish the structure. Even materials that are traditionally thought of as
nonflammable, or slow burning, can produce a powerful explosion when finely
divided, and can be ignited by even a small spark.
Demonstration of an open-air dust explosion
Effects A dust explosion can cause major damage
to structures, equipment, and personnel from violent overpressure or shockwave
effects. Flying objects and debris can cause further damage. Intense radiant
heat from a fireball can ignite the surroundings, or cause severe skin burns
in unprotected persons. In a tightly enclosed space, the sudden depletion of
oxygen can cause asphyxiation. Protection and mitigation
Much research has been carried out in Europe and elsewhere to understand how
to control these dangers, but dust explosions still occur. The alternatives
for making processes and plants safer depend on the industry.
In the coal mining industry, a methane explosion can initiate a coal dust
explosion, which can then engulf an entire mine pit. As a precaution,
incombustible stone dust may be spread along mine roadways, or stored in trays
hanging from the roof, to dilute the coal dust stirred up by a shockwave to
the point where it cannot burn. Mines may also be sprayed with water to
inhibit ignition. Some industries exclude oxygen from
dust-raising processes, a precaution known as "inerting". Typically this uses
nitrogen, carbon dioxide, or argon, which are incombustible gases which can
displace oxygen. The same method is also used in large storage tanks where
flammable vapors can accumulate. However, use of oxygen-free gases brings
a risk of asphyxiation of the workers. Workers who need illumination in
enclosed spaces where a dust explosion is a high risk often use lamps designed
for underwater divers, as they have no risk of producing an open spark due to
their sealed waterproof design. Good housekeeping practices, such as
eliminating build-up of combustible dust deposits that could be disturbed and
lead to a secondary explosion, also help mitigate the problem.
Best engineering control measures which can be found in the National Fire
Protection Association Combustible Dust Standards include:
Wetting Oxidant concentration reduction
Deflagration venting Deflagration pressure containment
Deflagration suppression Deflagration venting through a dust
retention and flame-arresting device Notable incidents
Dust clouds are a common source of explosions, causing an estimated 2,000
explosions annually in Europe. The table lists notable incidents worldwide.
See also Air to fuel ratio
Coal dust Combustion
Explosion vent Flour bomb
References John Barton: Dust Explosion Prevention
and Protection Institution of Chemical Engineers, Rugby 2002, ISBN
0-85295-410-7. Rolf K. Eckhoff: Dust Explosions in the
Process Industries, 2nd ed. Butterworth-Heinemann, Oxford 1997, ISBN
0-7506-3270-4. External links
Incidents in France and the US: Combustible dust explosion investigation
products from the Chemical Safety Board Combustible Dust Policy Institute-ATEX
OSHA case studies of dust explosions Protecting process plant and grain
handling facilities from the risk of dust hazard explosions:
Dust Explosions - The Dangers, and The Precautions Needed To Avoid Them
Hazard Monitoring Equipment - Selection, Installation and Maintenance
Seminars for Combustible Dust Safety FAQs: Solutions for Minimizing Dust
Explosion Risks