Product category:
Fire, smoke and gas leak detection
News Release from: Orthos (Engineering) | Subject: Firefly
Edited by the Processingtalk Editorial
Team on 29 June 2007
Dust explosion hazards in processing
industries
A large number of dust explosions occur in UK industry each year: these take place in all industries in locations where 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
This article was originally published on Processingtalk on 6 Dec 2006 at 8.00am (UK)
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They are:.
* A flammable dust cloud.
* An ignition source which contains enough energy to trigger combustion.
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* An atmosphere which supports 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 the walls of the equipment; in other words, an explosion.
The pressure wave generated by this initial explosion can also disturb dust lying in the surrounding area and give rise to a secondary explosion, which could have severe consequences for the plant and, more importantly, its operators.
Dusts in a filter can ignite at temperatures ranging from as high as about 600C for, say, aluminium powder down to 20C for zirconium powder.
In a silo, however, the ignition temperature for a given powder may be lower.
This lower threshold may be attributed to several factors, the main one being that, where dust is in layers and compressed, ignition temperatures are generally significantly reduced.
To put these figures into context; a newly extinguished match head, which has a temperature of about 500C, is a good example of a black body, which has sufficient energy to ignite a dust cloud.
Apart from the minimum ignition temperature the other two conditions needed for a flammable dust to ignite are a defined minimum dust concentration and a heat source containing the necessary minimum ignition energy.
These parameters vary depending on the material, and are also influenced by other factors such as particle size and moisture content.
The following few examples of ignition data for dust clouds of innocuous-seeming materials illustrate how easily the conditions required to trigger a dust explosion can arise.
The figures are Minimum Ignition Temperature (degrees C), Minimum Ignition Energy Needed (millijoules) and Minimum Explosive Concentration in each case.
Wheat Flour : 380; 50; 0.050.
Milk Powder: 490; 50; 0.050.
Nylon : 500; 20; 0.030.
Paper : 440; 60; 0.055.
Wood Dust : 430; 20; 0.050.
Possible ignition sources include:.
* Friction or mechanical failure.
* An overheated surface, e.g a faulty bearing.
* A glowing ember, e.g a burning wood shaving, paper fragment or small dust ball.
* Tramp metal or stone causing a spark.
* Welding or cutting.
* Static electrical discharge.
* Electrical failure.
These sources arise typically in association with dust extraction fans, crushers, grinders and pulverisers, dryers, woodworking machinery, paper web handling and cutting equipment and tobacco shredders.
Recently the Fire Protection Association (FPA) published a set of guidelines entitled "Recommendations for spark detection and suppression systems on pneumatic conveying installations".
This publication, RC28, is an attempt to detail system requirements and procedures and is broadly valid, not just for pneumatic conveying systems, but also for most situations, in which the various ignition sources mentioned above may be present and in which there is a risk of dust ignition.
There is insufficient space in this article to allow much of the publication to be quoted.
It is recommended, therefore, that RC28 be obtained and studied in detail.
The equipment recommended comprises suitably located ignition source detectors and extinguishing devices.
The detectors should be linked to a control panel to ensure that all signals are processed.
Spark and glowing ember detectors are usually infra-red devices designed to detect ignition or ignition sources both within a process and in an open area, such as a machinery enclosure, surrounding that process.
Ideally these detectors should be insensitive to ambient light in order to avoid false alarms.
The key FPA recommendations are as follows:.
* a comprehensive risk assessment for the whole plant should be carried out before any detection and suppression equipment is installed.
* detectors must be able to cover the whole area being monitored.
* they must be able to detect the smallest spark or other ignition source considered to be a risk.
* they should be able to assess the energy level of the ignition source and to trigger a proportionate response.
* they should be stable in performance, that is they should not trigger suppression unless an ignition source is present and should not allow a source to go undetected.
Fortunately, successful prevention systems are readily available, typical ones being provided, for instance, by the Swedish company, Firefly, which has worked closely with various industries, not only in its own country but throughout the industrial world, to develop equipment and application methods to meet their individual potential dust explosion hazards.
Firefly always installs complete systems and will not proceed with a project without firstly carrying out a thorough on-site assessment of risk.
A range of spark and hot particle detectors designed to match various temperature ranges and types of location, both enclosed and exposed to ambient light, is available.
Flame detectors suitable for use in daylight conditions, sophisticated control systems, a variety of extinguishing devices making use of full-cone water sprays, water mist, steam or inert gas, and mechanical diversion and isolation equipment complete the picture.
Dust extraction filters, cyclones and silos are necessarily used in all the industries, in which the equipment employed has the potential to generate sources of ignition, and in which flammable dust may be present.
They and their associated ducting are probably the plant locations, where most dust explosions occur.
The extractor fan itself can be a source of trouble, if maintenance is poor.
Other causes may be mechanical breakdown, foreign bodies or an electrical failure.
Accumulated dust can ignite and travel in the form of a glowing ember or spark along the ducting to the filter.
Detectors, which will react to particles at temperatures dictated by the materials being processed, are, ideally, installed in the ducting just after the fan and linked to an extinguishing zone in the ducting prior to the filter.
The distance between the detection and extinguishing zones is calculated in relation to the velocity of air within the ducting in order to allow sufficient time for the control system to react to an ignition source and effectively trigger the extinguishing system.
Where there is insufficient distance between the fan and the filter, as is often the case, other measures using additional detection and extinguishing can be applied.
Fires can be ignited in open areas, where flammable dust has accumulated, usually around process machinery but sometimes more generally.
If dust is present in the atmosphere in a process plant in a sufficient concentration, a rapidly spreading flash fire may also occur.
Ignition sources may be overheated machine components, sparks generated by the machinery itself or sparks arising from maintenance work being carried out in the vicinity.
Flame detectors, which are designed to react to the radiation emitted by flames but not to daylight or other forms of ambient lighting, when suitably placed and linked with appropriate suppression systems, provide an effective counter-measure against these hazards.
Comprehensive, reliable equipment for the prevention of dust explosions and fires in all possible locations within processing plants is readily available along with a wealth of written guidance, expert on-site risk assessment, commissioning, training, and after sales service.
There is, therefore, no reason why measures to reduce fire and explosion risk to the absolute minimum should not be universally adopted within process industries. Request a free brochure from Orthos (Engineering) ...
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