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Health & Safety
ignition source detectors and extinguishing devices. The various temperature ranges and types of location, both enclosed
detectors should be linked to a control panel to ensure that all and exposed to ambient light, is available. Flame detectors
signals are processed. Spark and glowing ember detectors are suitable for use in daylight conditions, sophisticated control
usually infra-red devices designed to detect ignition or ignition systems, a variety of extinguishing devices making use of full-
sources both within a process and in an open area, such as a cone water sprays, water mist, steam or inert gas, and mechanical
machinery enclosure, surrounding that process. Ideally these diversion and isolation equipment complete the picture.
detectors should be insensitive to ambient light in order to avoid Dust extraction filters, cyclones and silos are necessarily used
false alarms. in all the industries, in which the equipment employed has the
potential to generate sources of ignition, and in which flammable
The key FPA recommendations are as follows: dust may be present. They and their associated ducting are
• A comprehensive risk assessment for the whole plant should probably the plant locations, where most dust explosions occur.
be carried out before any detection and suppression equipment The extractor fan itself can be a source of trouble, if
is installed maintenance is poor. Other causes may be mechanical
• Detectors must be able to cover the whole area being breakdown, foreign bodies or an electrical failure. Accumulated
monitored dust can ignite and travel in the form of a glowing ember or spark
• They must be able to detect the smallest spark or other along the ducting to the filter. Detectors, which will react to
ignition source considered to be a risk particles at temperatures dictated by the materials being
• They should be able to assess the energy level of the ignition processed, are, ideally, installed in the ducting just after the fan
source and to trigger a proportionate response and linked to an extinguishing zone in the ducting prior to the filter.
• They should be stable in performance, that is they should not The distance between the detection and extinguishing zones is
trigger suppression unless an ignition source is present and calculated in relation to the velocity of air within the ducting in
should not allow a source to go undetected order to allow sufficient time for the control system to react to an
ignition source and effectively trigger the extinguishing system.
Fortunately, successful prevention systems are readily available, Where there is insufficient distance between the fan and the filter,
typical ones being provided, for instance, by the Swedish as is often the case, other measures using additional detection
company, Firefly AB, which has worked closely with various and extinguishing can be applied.
industries, not only in its own country but throughout the Fires can be ignited in open areas, where flammable dust has
industrial world, to develop equipment and application methods accumulated, usually around process machinery but sometimes
to meet their individual potential dust explosion hazards. Firefly more generally. If dust is present in the atmosphere in a process
always installs complete systems and will not proceed with a plant in a sufficient concentration, a rapidly spreading flash fire
project without firstly carrying out a thorough on-site may also occur. Ignition sources may be overheated machine
assessment of risk. components, sparks generated by the machinery itself or sparks
A range of spark and hot particle detectors designed to match arising from maintenance work being carried out in the vicinity.
Flame detectors, which are
Explosion characteristics of various dusts
designed to react to the radiation
Type of Dust Minimum ignition Minimum spark energy Minimum explosive emitted by flames but not to
temp. of dust cloud needed to ignite cloud concentration daylight or other forms of ambient
(
o
C) (millijoules) (g/litre) lighting, when suitably placed and
Agricultural Products linked with appropriate
Cocoa 500 120 0.065 suppression systems, provide an
Coffee 360 160 0.085 effective counter-measure against
Corn starch 390 30 0.040 these hazards.
Grain dust 420 25 0.045 Comprehensive, reliable
Flour 380 50 0.050 equipment for the prevention of
Metal Powders dust explosions and fires in all
Aluminium (atomised) 640 50 0.045 possible locations within
Magnesium 560 40 0.030 processing plants is readily
Titanium 375 15 0.045 available along with a wealth of
Uranium 20 45 0.060 written guidance, expert on-site
Plastics risk assessment, commissioning,
Cellulose acetate 320 10 0.035 training, and after sales service.
Methyl cellulose 360 20 0.030 There is, therefore, no reason why
Nylon 500 20 0.030 measures to reduce fire and
Phenolic resin 460 10 0.015 explosion risk to the absolute
Polyethylene 390 10 0.020 minimum should not be universally
Urea resin 430 34 0.020 adopted within process industries.
Miscellaneous For more information contact
Coal (high volatiles) 610 60 0.055 Orthos on tel: +44 (0)1858
Cork 470 35 0.035 464246 or visit:
Paper 440 60 0.055 www.orthos.uk.com
56 Solids & Bulk Handling • August 2007 www.solidsandbulk.co.uk
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