Dispersion 17
Dispersion 17 : When a hazardous substance is released it may explode or burn at the point of release, or it may travel away from the source. During such travel it becomes progressively diluted with the surrounding fluid. This is the process of Dispersion. A Gas or vapour may mix with air and form a Gas Cloud, which drifts downwind; a liquid may fix with water and disperse along any currents present. If dispersion proceeds for a long enough time the concentration of hazardous substance will fall below that required to cause harm. For a flammable substance, the concentration will eventually fall below the lower flammable limit; for a toxic substance, the level will fall below the threshold of significant toxicity. The progress of dispersion to such levels may be estimated quantitatively using a dispersion model which predicts Peak or mean Concentration at a point of interest, or Isopleths showing the extent of the effects of a gas cloud. Great care is necessary in interpreting the results of model calculations, particularly with the definition of peak or mean concentration. Dispersion is a stochastic phenomenon, since it is determined by turbulence. In principle, concentration should be defined in statistical terms, taking specific account of frequency and duration of sampling, number of samples, volume of samples, duration of event. In practice, for toxic substances a time-averaged mean concentration or integrated dose is used, so statistical details are neglected. For flammable substances, real difficulties can arise, since the "instantaneous" concentration determines whether ignition is possible. The dispersion behaviour of a gas cloud is determined by its intrinsic properties as well as local external conditions. Differences arise between Dense, Neutral Density and Buoyant Gas Clouds. A dense gas cloud may be formed from a Dense Gas or because it is associated with a cold vapour release. Such gas clouds, which may also be called heavy or non-buoyant, tend to slump towards the ground in the early stages of dispersions. A buoyant gas cloud, often associated with combustion, tends to rise. A neutral density gas cloud has the same density as the atmosphere so it follows the turbulence pattern of the atmosphere. This is known as Passive Dispersion. The external conditions affecting dispersion include Wind Speed, Surface Roughness and the stability of the atmospheric boundary layer, which is characterised by Weather Category. A gas cloud may behave as a discrete Puff from a brief (instantaneous) release, or a PLUME may form during a continuous release. The time-averaged shape of a plume is a cigar-Shape downwind, but an instantaneous picture may show a complex shape. Time-averaged plume dimensions are usually of most interest for toxic materials, but instantaneous dimensions are of interest for flammables (See below)
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