Pasquill-Gifford Dispersion Modeling

Pasquill-Gifford or Gaussian dispersion applies only to neutrally buoyant dispersion of gases in which the turbulent mixing is the dominant feature of the dispersion. It is typically valid only for a distance of 0.1-10 km from the release point. 

RMP Offsite Consequence Analysis Guidance (Washington, DC US Environmental Protection Agency, 1996). 

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TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

FIG. 26-54 Horizontal dispersion coefficient for Pasquill-Gifford plume model, Reprinted ffomD. A. Ct owl and J. F. Louvar, Chemical Process Safety, Fundamentals with Applications, Z.9.90, p. 138. Used hy permission of Ft entice Hall)... 

Figure 5-11 Dispersion coefficients for Pasquill-Gifford plume model for urban releases.

PasquiU Atmo.spheric Diffusion, Van Nostrand, 1962) recast Eq, (26-60) in terms of the dispersion coefficients and developed a number of useful solutions based on either continuous (plume) or instantaneous (puff) releases, Gifford Nuclear Safety, vol, 2, no, 4, 1961, p, 47) developed a set of correlations for the dispersion coefficients based on available data (see Table 26-29 and Figs, 26-54 to 26-57), The resulting model has become known as the Pasquill-Gifford model. 

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

The equations for cases 1 through 10 were rederived by Pasquill8 using expressions of the form of Equation 5-37. These equations along with the correlations for the dispersion coefficients are known as the Pasquill-Gifford model. 

The Pasquill - Gifford model PUFF is suited for dispersion modeling in instantaneous outflow conditions. It is a dispersion model with normal (Gauss) distribution of concentrations and Lagrange approach, which consists in gas element move monitoring in wind field. The gas cloud spreads in the wind direction. At first the cloud grows and the gas concentration sinks. Later the cloud volume decreases, because more and more gas disperses in insignificant concentrations outside the cloud. 

The flash fire can occur as a result of delayed ignition of a cloud of gas. In this case a time-limited steady release of gas is simulated as series of puffs, each of which is considered as a separate release described by Pasquill-Gifford model. Both atmosphere stability and direction and speed of wind are taken into account, as the gas dispersion depends on these factors. The computation of individual risk for flash fire reflects the fact that the heat flux, the probability of ignition of dispersed gas and exposure time vary in time and space. 

The atmospheric dispersion is modeled by a modified Gaussian plume equation which estimates the average dispersion of the contaminants released from the source in each wind direction. The Gaussian model of a plume dispersion accounts for the gaseous contaminant transport from the source area to a downwind receptor and is represented by the equation of Pasquill as modified by Gifford (Chacki, 2000) ... 

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