Combustible Dust Basics and Classification|
Article- March 2012 By Larson Electronics.com
Larson Electronics EPL-EMG-48-4L-T5HO Explosion Proof Fluorescent Light Fixture with Emergency Battery Backup
Combustible dusts represent a clear potential for explosions and fire when present in sufficient quantities and within close proximity to sources of possible ignition. Combustible dusts are flammable materials that have been ground, cut or otherwise separated into fine particulates which promotes the rapid and explosive combustion of these materials. Combustible dusts consist of electrically conductive and non conductive materials and can be formed from a wide variety of materials including wood, aluminum, plastics, paper, magnesium, sugar, and many more. Many materials that do not normally pose a threat of combustion in a normal solid form can take on explosive potential when divided into particles finer than .420 microns in size.
As a result, the danger posed by these dusts is often difficult to recognize and easy to overlook. Areas where combustible dusts are present are designated as Class 2 hazardous locations with the severity of explosive potential rated according to material types and the frequency of their presence denoted by Division and Group classifications. Dust must be finely divided and present in sufficient quantities in order for a location to be given a Class 2 designation. Due to their unique properties, combustible dusts are classified by potential or likelihood of fire or explosion as noted by Divisions 1 and 2, and subcategorized by material types into groups E, F and G.
A Class 2 Division 1 hazardous location designation denotes an area where combustible dusts are normally present in quantities sufficient to promote ignition. Class 2 Division 1 locations also include areas where malfunctioning equipment or abnormal conditions could cause the formation of an ignitable dust contaminated atmosphere, or electrically conductive dusts may be present in sufficient amounts to cause ignition or equipment electrical failure through ingress into equipment housings.
A Class 2 Division 2 hazardous location designation denotes an area where combustible dusts are not normally present in sufficient quantities to produce ignition or explosion, and accumulation is not normally adequate to cause interference with the normal operation of equipment. Divison 2 locations also denote areas where dusts may be suspended in the local atmosphere due to infrequently malfunctioning equipment or may settle onto or near equipment in large enough amounts to cause equipment to overheat or malfunction resulting in ignition. Division 2 locations are considered abnormal and not expected to frequently be present.
Combustible dusts can be made up of a wide variety of materials. Materials that are normally difficult to burn can possess explosive potential when ground, chopped or otherwise divided into fine enough particulates. The details can get complicated, but it basically breaks down to the fact that the smaller a piece of material is, the less heat it can absorb before combusting. As a result, readily flammable objects become extremely flammable when ground into fine particulates, and even materials such as aluminum and iron become potentially explosive when ground to particles less than .420 microns in size. Thus, the particle type and size contributes to the overall ignition temperature of the dust in question.
Complicating matters further, dusts pose an additional danger through the insulating and sealing effects they have when layered over top of machinery or clogging the ventilation openings intended to help cool internal working parts. Particularly in areas where the creation of dust is a normal occurrence, dust can settle on top of machinery and light fixtures, as well as accumulate in hard to observe and reach areas, eventually causing equipment to overheat and thus cause fire. Conductive combustible dusts such as iron or aluminum pose even more risk due to their ability to settle inside of equipment and cause shorts, overheating and the creation of sparks. Due to their widely varied properties, combustible dusts are further classified into groups.
Group “E” dusts include the metallic dusts such as iron, aluminum, magnesium, steel and others. These dusts pose all of the problems noted above including conductivity, thermal layering, and flammable or explosive potential. The ferrous metal dusts are magnetically reactive and can readily accumulate or penetrate into the internal workings of motors that rely on magnets for their operation. These dusts also pose an abrasive threat which can also cause overheating and high operating temperatures in machinery.
Group “F” dusts are carbonaceous, with coal being the most notable combustible dust of this group. Dusts in this group may be poorly conductive, and pose less conductivity risk than group E dusts and generally do not pose a threat when exposed to equipment rated below 600 volts. Group F dusts have higher insulating values than Group E dusts and lower ignition temperatures, making control of equipment temperature and dust accumulation more important to safety than with group E dusts.
Group “G” dusts include food stuff dusts such as flour and sugar, plastics and composites, and chemical dusts. These dusts have the highest thermal insulating properties and the lower ignition temperatures, making control of accumulation and management of equipment temperature critical to safety. These dusts are not electrically conductive however, so equipment voltages play little role in their management.
The temperature at which a material ignites is known as the auto-ignition temperature and for classification purposes is the lowest temperature at which a type of material will ignite without an obvious ignition source such as flame. Auto-ignition temperatures vary for each type of material and normally cover a small ranging scale. For classification purposes, the lowest auto-ignition temperature is used to ensure the most effective equipment choices. Auto-ignition temperatures also vary according to the physical properties of dust, ie layered/accumulated or in atmospheric suspension. Dusts that are suspended or in “cloud” form generally pose a higher explosive risk than layered dusts. However, layered dust pose a dual fire/explosion threat due to the fact that a primary ignition or small explosion can often send these dusts back into suspension, resulting in a secondary explosion of much higher force after primary ignition.
Much like gases, vapors and liquids, dusts are classified according to their physical properties, explosive and flammability potential, and threat likelihood. For almost all applications where dusts are created in any quantity, it is safest to assume that the potential for fires or explosions exists, regardless of the type of materials being processed.