Explosion hazard caused by dust
Lessons from Losses 2/2005
A short circuit occurred in an electric box in the facilities
of a company manufacturing wooden items, causing the dust layer
on top of the box to catch fire. An engine operator working nearby
noticed the fire, fetched the nearest portable fire extinguisher
and rushed to the scene. The strong discharge from the extinguisher
created a dust cloud which burst into flames. The fire spread
rapidly in the dusty production facility and employees had to
run to escape the conflagration.
Explosion hazard caused by dust
Dusting substances used in production and dusty by-products cause
fire and explosion risks that can easily go unnoticed. Dust transfer
and handling systems often contain continuously explosive dust-air
mixtures, but even a seemingly innocent-looking dust layer may
cause a fire and explosion hazard.
A dust explosion is often described as an occurrence in which
a pressure increase causes structural damage and a loud noise.
However, this is not necessarily the case, since dust explosions
also occur when the entire dust-air mixture ignites. Numerous
major incidents have started from small, local dust-air mixture
ignitions and flare-ups that have caused new, stronger dust explosions
and the rapid spread of fire.
Such an explosion hazard arises from dust originating in a combustible
substance whose particle size is typically less than 0.5 millimetres.
When dust and air form a mixture, the concentration of which exceeds
the lower explosive limit, the dust-air mixture becomes explosive.
Dust-air mixture explosion limits are substance-specific, but
often only a few dozen grams of dust per cubic metre of air suffices
to generate an explosive mixture. The explosion is caused by an
ignition source which can contain remarkably little energy, the
finest dusts igniting due to as little energy as is required for
Formation of a dust explosion hazard
Dust transfer and handling systems may contain explosive dust-air
mixtures on a continuous basis. Since these are often closed systems,
an explosion may cause structural damage and hurl splinters into
the air that are very dangerous to those working nearby. Moreover,
should a dust explosion occur in a dust transfer system, burning
substances may burst into production facilities situated far from
the original site of the incident, causing fires in several locations.
The threat posed by dust layers often goes unnoticed, a mere one-millimetre
layer of dust originating from a combustible substance being sufficient
to cause a hazard. When a dust layer mixes with air, the dust
cloud concentration is sufficient to cause a dust explosion if
a suitable ignition source comes into contact with the dust-air
mixture. An explosive dust cloud can be generated, for example,
by an air current or when cleaning using compressed air. Vibration
may also shake off dust layers formed on structures, generating
an explosive dust-air mixture.
For instance, a mechanical spark, hot surface, a spark generated
by an electronic device or static electricity can act as an ignition
source. The first dust explosion occurring in the premises may
generate new dust clouds that can cause even more destructive
dust explosions. An unexpected hazard often arises from attempts
to extinguish the fire: if this is performed too close to the
fire using a hand extinguisher discharge which is too powerful,
the dust layer can create a dust-air mixture causing a major conflagration.
Areas where hazardous dust-air mixtures may occur must be categorised.
Based on such categorisation, special requirements apply to equipment
construction, installation, use and protection systems in order
to ensure safe use in the said area. Areas where dust layers build
up must often be categorised as hazardous areas on account of
their explosive atmosphere, unless dust layers are removed through
Special requirements apply to electrical and mechanical equipment
situated in dusty facilities. Dust build-ups must be prevented
from catching fire by ensuring that the surface temperatures of
such equipment do not rise. Dust ignition temperature is often
lower than that of the substance from which the dust originates.
Since dust also acts as an insulant, a dusty device, even if operating
normally, may heat up to the extent that the dust on the device
catches fire, unless this hazard was taken into account when selecting
the device. Neither may a device located in dusty facilities generate
other ignition sources that may cause the ignition of the dust-air
mixture. For example, the device can be enclosed so that dust
does not come into contact with its inner parts. Filters, silos
and pipe work must be equipped with explosion pressure release
systems that safely discharge pressure. Damage caused by a dust
explosion and the resulting fire can be limited through structural
measures and by implementing fire-extinguishing systems. Spark
detection and extinguishing systems have proven efficient protection
systems since they are able to detect and extinguish hot particles
within the process before they pose a risk.
A dust explosion hazard can be eliminated or decreased as follows:
– Identify the production facility areas and parts of the
production process involving a dust explosion hazard in order
to target preventive measures at the places where they are most
– Eliminate or limit the generation of explosive dust-air
– Categorise areas at risk of explosion and use only electrical
and mechanical devices suitable for such areas.
– Implement protective measures to prevent the ignition
of explosive dust-air mixtures.
– Implement protective measures to limit the consequences
of a potential dust explosion.
The party responsible for the operations must draw up an explosion
protection document that demonstrates how the explosion hazard
has been taken into account and prevented in accordance with State
Council Ordinance No. 576 of 2003 on the Prevention of Hazards
to Workers Caused by Explosive Atmospheres.