Flammability of dusts

The limits of flammability of dusts have been investigated (30G, 34G, 35G). Using various metal powders, Boyle and Llewellyn (15G) have found the minimum ignition energy to be proportional to particle size. Browning, Tyler, and Krall (10) report a variation of the lean limit as a function of particle size particles of about 60-micron diameter give the leanest combustible mixtures. 

Chim Ind (Paris)74, 961 (1955) CA 51, 18609 (1957) (An opinion on experimental values of the lower limit of flammability of dust-air mixts on the basis of thermochemical calcns) W)F.Otas6k, Uhli (Czech) 5,... 

As might be expected, particle size is an important factor in assessing the flammability of dusts. It appears that above some size, dust particles are relatively ineffective as contributors to dust explosions. Figure 20.4 shows this effect for several dusts. In this figure only the mean particle size is given. However, it is expected that particle size distribution would also be a complicating factor in assessing the flammability of dusts. 

Flammability = 4, ie, very flammable gas, very volatile, and materials that in the form of dusts or mists form explosive mixtures when dispersed in air Health = 2, ie, hazardous to health, but may be entered freely with self-contained breathing apparatus Reactivity = 0, ie, is normally stable when under fire-exposure conditions and is not reactive with water... 

Flammability information Flash point Fire point Flammable limits (LEL, UEL) Ignition temperature Spontaneous heating Toxic thermal degradation products Vapour pressure Dielectric constant Electrical resistivity Electrical group Explosion properties of dust in a fire... 

Hybrid mixture A suspension of dust in air/vapor. Such mixtures may be flammable below the lower explosive limit of the vapor and can be ignited by low energy sparks. 

Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Increasing the surface area of a combustible solid enhances the ease of ignition. Hence dust burns more rapidly than the corresponding bulk solid combustion of dust layers can result in rapid flame spread by train firing . Solid particles less than about 10 pm in diameter settle slowly in air and comprise float dust (see p. 51 for settling velocities). Such particles behave, in some ways, similarly to gas and, if the solid is combustible, a flammable dust-air mixture can form within certain limits. Larger particles also take part, since there is a distribution of particle sizes, and ignition can result in a dust explosion. 

Flammability of some commercial zinc dusts or powders is attributed to presence of zinc chloride. 

An analysis of static ignition hazard should start with data on the ignition sensitivity of the particular flammable material at its most flammable concentration in air, i.e., its MIE. This is especially important for dusts. It is prudent to determine this value on fines of the specific dust of interest, rather than to rely on published data. Hybrid mixtures, i.e., mixtures of dust and vapor for which vapor concentrations may be below their lower flammable limit, can be ignited by smaller discharge energies than might be expected. 

During the development of a new facility or process, or when introducing a new process into an existing facility for the first time, an inherent safety review can be conducted to understand the chemical reactivity hazards and explore hazard reduction alternatives. The review need not be limited to chemical reactivity hazards. It can be used to address all other types of process hazards at the same time, including flammability/ combustibility dust or mist explosibility elevated or reduced pressures or temperatures phase differences and health hazards such as toxicity, corrosivity, and asphyxiation. 

The MIE of gas — air or vapour—air mixtures can be determined from the structural formula and the molar heat of combustion of the compounds studied, and equations for the calculation are presented. The method is stated to give more accurate results than conventional methods used to assess flammability of mixtures of gas or vapour with air [1], It is claimed that in oxygen MIEs are about a hundredfold lower than in air [2], A study of the ignition behaviour of dusts, including correlation of electrical and mechanical minimum ignition spark energies and ignition temperature is made [3],... 

Ammonium dichromate is an irritant to skin. Inhalation of dusts can cause pulmonary irritation, perforation of the nasal septum and chrome sores. Ingestion can cause ulceration. It is also a flammable salt. 

As in the case of flammable liquid fires and explosions, the control of dust explosions is based upon prevention of ignition and secondarily limitation of damage in the event ignition does occur... 

Flammability and explosibility of dusts 4 D154-D163 5 D1578-D1579... 

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