Combustion dusts

Common materials—such as textiles in the form of fibers or fabrics, foamed rubber, foamed plastics, thin sheets of plastic, paper, corrugated cardboard, combustible dusts, dry grass and twigs, and wood shavings—are all examples of materials with large sutface areas in relation to their volumes. In a well-estabhshed fire, materials with relatively small surface areas, such as chunks of coal or logs, burn readily. 

Combustible Dusts Dusts are particularly hazardous they have a very high surface area-to-volume ratio. When finely divided as powders or (dusts, solids burn quite differently from the original material in the bulk. Dust and fiber deposits can spread fire across a room or along a ledge or roof beam very quickly. On the other hand, accumulations of dust can smolder slowly for long periods, giving little indication that combustion has started until the fire suddenly flares up, possibly when no one suspects a problem. 

Many combustible dusts produced by industrial processes are explosible when they are suspended as a cloud in air. A spark may be sufficient to ignite them. After ignition, flame spreads rapidly through the dust cloud as successive layers are heated to ignition temperature. 

From a safety standpoint, priority must be given to the measures in item 1. Group 2 cannot be used as a sole protective measure for flammable gas or solvent vapors in industrial practice with sufficient reliability, but can be applied as the sole protective measure when only combustible dusts are present if the minimum ignition energy of the dusts is high (>10 mj) and the operating areas concerned can easily be monitored. 

For combustible dusts, the explosibility limits do not have the same meaning as with flammable gases and flammable vapors, owing to the interaction between dust layers and suspended dust. This protective measure can, for example, be used when dust deposits are avoided in operating areas or in the air stream of clean air lines after filter installations WTiere in normal operation the lower explosibility limit is not reached. However, dust deposits must be anticipated with time. When these dust deposits are whirled up in the air, an explosion hazard can arise. Such a hazard can be avoided by regular cleaning. The dust can be extracted directly at its point of origin by suitable ventilation measures. 

TABLE 26-20 Influence of Relative Circumferential Speeds on Danger of Ignition for Combustible Dusts... 

Several general principles may be applied to equipment handling combustible dusts ... 

ASTM E 1226. Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts. 

NFPA 499 Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, 1997 edition. National Fire Protection Association, Quincy, MA. 

Minimum Explosible Concentration (MFC) The lowest concentration of combustible dust necessary to produce an explosion. 

Electric cyclones, which utilize an electrode in the center of the cyclone to establish an electric field within the cyclone body. This device is more efficient than the standard cyclone. It is probably more applicable to mists and droplets than to dry particulates, due to possible fire or explosion hazards with combustible dusts. 

There is a critical oxygen content below which ignition of combustible dusts or gases will not occur and this can provide a means for safe operation under an inert atmosphere, i.e. inerting . 

HS(G)103 Safe handling of combustible dusts precautions against explosions... 

Withdrawn) 1990 AMD 1 Half masks and quarter masks for respiratory protective devices (AMD 7113) dated 15 November 1992. Withdrawn, superseded by BS EN 140 1999 Guide to the use of electrical apparatus complying with BS 5501 or BS 6491 m the presence of combustible dusts... 

AMD 1 Electrical apparatus for use m the presence of combustible dust. Part 1 Electncal apparatus protected by enclosures - Construction and testing (AMD 10764J dated October 1999. Partially superseded BS 6467-1 and 2 which remain cunent... 

Combustible dusts include metal dust (e.g., aluminum, magnesium, and their commercial alloys), carbonaceous dust (e.g., carbon black, charcoal, and coal), flour, grain, wood, plastics, and chemicals. 

Flammable gases and vapors or combustible dust may be present in quantities sufficient to produce explosive or ignitable mixtures due to ... 

Energy on tlie Explosion Data of Combustible Dusts (1-m ... 

Rapid release of energy dirough die ignition of atmospheric mixtures of flanuiiable gases, vapors or combustible dusts widiiii the explosive range... 

Using the cubic equation presented earlier, Bartknecht [54] developed for vessels of different sizes for the same process system in closed or vented vessels, valid for flammable gases and combustible dusts ... 

Scope—Articles 500 Through 505. Articles 500 through 505 cover the requirements for electrical equipment and wiring for all voltages in locations where fire or explosion hazards may exist due to flammable gas or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings. 

Location and General Requirements. Locations shall be classified depending on the properties of the flammable vapors, liquids or gases or combustible dusts or fibers that may be present and the likelihood that a flammable or combustible concentration or quantity is present. Where pyrophoric materials are the only materials used or handled, these locations shall not be classified. 

Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. 

Group G. Atmospheres containing combustible dusts not included in Group E or F, including flour, grain, wood, plastic, and chemicals. 

Where flammable gases or combustible dusts are or may be present at the same time, the simultaneous presence of both shall be considered when determining the safe operating temperature of the electrical equipment. 

Class II Locations. Class If locations are those that are hazardous because of presence of combustible dust. Class If locations shall include those specified in (a) and (b) below. 

FPN No. 1) The quantity of combustible dust that may be present and the adequacy of dust removal systems are factors that merit consideration in determining the classification and may result in an unclassified area. 

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