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Health & Safety
Orthos bites the dust
Dust explosions are common in processing industries. Orthos gives an
insight into readily available prevention systems
A large number of dust explosions
occur in UK industry each year: these
take place in all industries and
locations, in which flammable powders
are being handled or processed.
Industries particularly at risk, because
of the materials involved and the
equipment used, include organic
chemicals production, flour and grain
milling, animal feeds manufacture,
metal powders, plastics, solid fuels
processing, woodworking and paper,
especially tissue, manufacture.
Three conditions need to exist
simultaneously for a dust explosion,
which is the rapid burning of a dust
cloud, to occur. They are:
• A flammable dust cloud
• An ignition source which contains
A catastrophic industrial
enough energy to trigger combustion fire resulting from a
• An atmosphere which supports
dust explosion
combustion
If these conditions arise in a confined
space such as a silo, cyclone or dust
filter, the rise in temperature and the
consequent rapid increase in pressure can cause the rupture of Possible ignition sources include:
the walls of the equipment; in other words, an explosion. The • Friction or mechanical failure
pressure wave generated by this initial explosion can also disturb • An overheated surface, e.g. a faulty bearing
dust lying in the surrounding area and give rise to a secondary • A glowing ember, e.g. a burning wood shaving, paper fragment
explosion, which could have severe consequences for the plant or small dust ball
and, more importantly, its operators. • Tramp metal or stone causing a spark
Dusts in a filter can ignite at temperatures ranging from as • Welding or cutting
high as about 600°C. for, say, aluminium powder down to 20°C • Static electrical discharge
for zirconium powder. In a silo, however, the ignition • Electrical failure
temperature for a given powder may be lower. This lower They arise typically in association with dust extraction fans,
threshold may be attributed to several factors, the main one crushers, grinders and pulverisers, dryers, woodworking
being that, where dust is in layers and compressed, ignition machinery, paper web handling and cutting equipment and
temperatures are generally significantly reduced. tobacco shredders.
To put these figures into context; a newly extinguished match Recently the Fire Protection Association (FPA) published a set
head, which has a temperature of about 500ºC, is a good example of guidelines entitled “Recommendations for spark detection
of a black body, which has sufficient energy to ignite a dust cloud. and suppression systems on pneumatic conveying installations”.
Apart from the minimum ignition temperature the other two This publication, RC28, is an attempt to detail system
conditions needed for a flammable dust to ignite are a defined requirements and procedures and is broadly valid, not just for
minimum dust concentration and a heat source containing the pneumatic conveying systems, but also for most situations, in
necessary minimum ignition energy. These parameters vary which the various ignition sources mentioned above may be
depending on the material, and are also influenced by other present and in which there is a risk of dust ignition. There is
factors such as particle size and moisture content. The table insufficient space in this article to allow much of the publication
provides a selection of typical temperature, energy and dust to be quoted. It is recommended, therefore, that RC28 be
concentration figures for various materials. These are mostly obtained and studied in detail.
common and often innocuous-seeming products. The equipment recommended comprises suitably located
www.solidsandbulk.co.uk August 2007 • Solids & Bulk Handling 55
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