Dispersion in air

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... 

Measurements of the concentrations and characteristics of dust dispersed in air or other gases may be necessary (1) to determine the need for control measures, (2) to establish compliance with legal requirements, (3) to obtain information for collector design, and (4) to determine collec tor performance. 

Vapor cloud explosions can result if clouds of flammable vapor in air are formed. It is important to understand how hquids and gases flow through holes in equipment and how resulting vapor or gas clouds are dispersed in air. 

Materials that on shor t exposure could cause death or major residual injury 4 Materials that will rapidly or completely vaporize at atmospheric pressure and normal ambient temperature, or that are readily dispersed in air and will burn readily 4 Materials that in themselves are readily capable of detonation or of explosive decomposition or reaction at normal temperatures and pressures... 

Explosion testing should be performed to establish safe operating limits. Dust explosibility and ignitability are measurements of the potential for a combustible material, in dust form, to explode or ignite. Any combustible material has the potential to cause a dust explosion if dispersed in air as a dust and ignited. Further details on explosibility testing can be found in Palmer (1973), Bartknecht (1989) and Eckhoff (1997). 

All eombustible solids ean ereate a dust explosion hazard if dispersed in air as a fine dust within eertain eoneentration limits. Refer to Table 6.2. The hazard inereases with deereasing partiele size. 

The rates of gas-solid reaetions are surfaee area dependent, so finely-divided metals, eoal ete. may be prone to oxidation leading to spontaneous eombustion. A eombustible dust will burn mueh more rapidly than the bulk sold, and if dispersed in air eause a dust explosion (refer to Table 6.2). 

Finely divided aluminium powder or dust forms highly explosive dispersions in air [1], and all aspects of prevention of aluminium dust explosions are covered in 2 recent US National Fire Codes [2], The effects on ignition properties of impurities introduced by recycled metal used to prepare dust were studied [3], Pyrophoricity is eliminated by surface coating aluminium powder with polystyrene [4], Explosion hazards involved in arc and flame spraying of the powder are analysed and discussed [5], and the effect of surface oxide layers on flammability was studied [6], The causes of a severe explosion in 1983 in a plant producing fine aluminium powder are analysed, and improvements in safety practices discussed... 

Dust explosions demonstrate unique behavior. These explosions occur if finely divided particles of solid material are dispersed in air and ignited. The dust particles can be either an unwanted by-product or the product itself. 

Lewis RG, Lee RE Jr. 1976. Air pollution from pesticides sources, occurance, and dispersion. In Air Pollution from Pesticides and Agricultural Processes. Ed RE Lee, Jr. CRC Press, Inc. pg 18. 

Substantial research on environmental pollution plume persistence and dispersion in air has produced a classification system called Pasquill s stability classes for plume stability in air. Unfortunately, the system does not apply as directly as we might wish since it applies at the much larger scale of stack exhausts and the like. However, some insight from the system is available. 

The pyrotechnic phenomena of heat, smoke, noise, and motion are reasonably easy to comprehend. Heat results from the rapid release of energy associated with the formation of stable chemical bonds during a chemical reaction. Smoke is produced by the dispersion in air of many small particles during a chemical reaction. 

More recent development has centered on detonation of liquid fuel drops dispersed in air, although fuel films will also support detonation (Ref 9). In the fuel drop-air explo-. sive, it is necessary that relatively large... 

Measurement and analysis of trash and dust in cotton is important in the prediction of dust generation potential, in relation to byssinosis (14) and rotor spinning performance (15). By definition, trash is the nonlint particles including dust which settle under the influence of gravity in the processing of cotton. Dust is the finer material capable of being dispersed in air. Particulate burden is the combined trash and dust level in cotton. 

Materials which will rapidly or completely vaporize at atmospheric pressure and normal ambient temperature, or which are readily dispersed in air and which will burn readily. 

Some forms of these products may contain sufficient fines to be considered nuisance dust and present dust explosion potential if sufficient quantities are dispersed in air. Unformulated PVB and PVF resins have flash points above 370°C. The lower explosive limit (lei) for PVB dust in air is about 20 g/m2. 

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