McQuaid

A complete analysis of dense gas dispersion is much beyond the scope of this treatise. More detailed references are available (Britter and McQuaid, Workbook on the Dispersion of Dense Gases, Health and Safety Executive Report No. 17/1988, England, 1988 Lees, 1986, pp. 455 61 Hanna and Drivas, 1987 Workbook of Test Cases for Vapor Cloud Source Dispersion Models, AlChE, 1989 Guidelines for Chemical Process Quantitative Risk Analysis, 1989, pp. 96-103). 

Many computer codes, both public and private, are available to model dense cloud dispersion. A detailed review of these codes, and how they perform relative to actual field test data, is available (Hanna, Chang, and Strimaitis, Atmospheric Environment, vol. 27A, no. 15, 1993, pp. 2265-2285). An interesting result of this review is that a simple nomograph method developed by Britter and McQuaid (1988) matches the available data as well as any of the computer codes. This method will be presented here. 

The Britter and McQuaid model is not appropriate for jets or two-phase plume releases. However, it would be appropriate at a minimal distance of 100 m from these types of releases since the initial release effect is usually minimal beyond these distances. 

McQuaid (1991), CCPS (1993a), and Hendershot (1995a) review a number of specific examples of inherent safety tradeoffs. These include ... 

McQuaid, J. (1991). Know Your Enemy The Science of Accident Prevention. Trans. IChemE 69, Part B (February), 9-19. 

FIGURE 19-4. Treatment algorithm for active gastrointestinal bleeding resulting from portal hypertension. (From Schianotd, Bodenheimer HC. Complications of Chronic Liver Disease. In Friedman SL, McQuaid KR, Grendell JH (eds.)... 

R. E. Britter and J. McQuaid, Workbook on the Dispersion of Dense Gases (Sheffield, United Kingdom Health and Safety Executive, 1988). 

Figure 5-13 Britter-McQuaid dimensional correlation for dispersion of dense gas plumes.

Table 5-4 Equations Used to Approximate the Curves in the Britter-McQuaid Correlations Provided in Figure 5-13 for Plumes...

The Britter-McQuaid model is a dimensional analysis technique, based on a correlation developed from experimental data. However, the model is based only on data from flat rural terrain and is applicable only to these types of releases. The model is also unable to account for the effects of parameters such as release height, ground roughness, and wind speed profiles. 

Use the Britter-McQuaid dense gas dispersion model to determine the distance to the 1% concentration for a release of chlorine gas. Assume that the release occurs over a duration of 500 s with a volumetric release rate of 1 m3/s. The wind speed at 10 m height is 10 m/s. The boiling point for the chlorine is —34°C, and the density of the liquid at the boiling point is 1470 kg/m3. Assume ambient conditions of 298 K and 1 atm. 

General References Britter and McQuaid, Workbook on the Dispersion of Dense Gases, Health and Safety Executive Report 17/1988, Sheffield, U.K., 1988. Mannan, Lees Loss Prevention in the Process Industries, 3d ed., Chap. 15, Elsevier Butterworth-Heinemann, Oxford, U.K., 2005. Panofsky and Dutton, Atmospheric Turbulence, Wiley, New York, 1984. Pasquill and Smith, Atmospheric Diffusion, 3d ed., Ellis Horwood Limited, Chichester, U.K., 1983. Seinfeld, Atmospheric Chemistry and Physics of Air Pollution, Chaps. 12-15, Wiley, New York, 1986. Turner, Workbook of Atmospheric Dispersion Estimates, U.S. Department of Health, Education, and Welfare, 1970. 

Empirical modek Empirical models rely on the correlation of atmospheric dispersion data for characteristic release types. Two examples of empirically based models are the Pasquill-Ginord model (for passive contaminants) and the Britter-McQuaid model (for denser-than-air contaminants) both of which are described below. Empirical models can be useful for the validation of other mathematical models but are limited to the characteristic release scenarios considered in the correlation. Selected empirical models are discussed in greater detail below because they can provide a reasonable first approximation of the hazard extent for many release scenarios and can be used as screening tools to indicate which release scenarios are most important to consider. 

Britter and McQuaid provide correlations for denser-than-air (continuous) plumes and (instantaneous) puffs released at ambient temperature. Since many materials of practical interest are released below ambient temperature, Britter and McQuaid provide guidance as to how to predict the limiting cases for such releases. 

Britter and McQuaid report that the averaging time ti for the plume correlation is 10 min, and Eq. (23-77) should be used with p = 0.12 for other averaging times (and limited to averaging times no shorter than about 20 s as for relases). 

Denser-than-air puff or plume Britter and McQuaid use the ratio of the source duration to the travel time to distinguish between plumes and puffs with a slightly different definition of travel time tt = xe/(0.4ur). The release can be considered a plume if ts > tt, where ts is the source time scale defined above, and the release can be considered a puff if ts < tJ4. For tt/4 plume models are entirely appropriate the predicted concentration is considered the largest of the puff and plume predictions. 

When denser-than-air effects are important, use the Britter-McQuaid (plume or puff) models. Otherwise, assume the release is passive and use the Pasquill-Gifford (plume or puff) models. Adjust values for the virtual source correction s) as appropriate. 

Dethier MN, Williams SL, Freeman A (2005) Seaweeds under stress manipulated stress and herbivory affect critical life-history functions. Ecol Monogr 75 403 418 Diaz E, Guldenzoph C, Molis M, McQuaid C, Wahl M (2006) Variability in grazer-mediated defensive responses of green and red macroalgae on the south coast of South Africa. Mar Biol 149 1301-1311... 

McQuaid CD, Froneman PW (1993) Mutualism between the territorial intertidal limpet Patella longicosta and the crustose alga Ralfsia verrucosa. Oecologia 96 128-133 Menge BA (1991) Relative importance of recruitment and other causes of variation in rocky intertidal community structure. J Exp Mar Biol Ecol 146 69-100... 

Hirsch KS, Jones CD, Audia JE, Andersson S, McQuaid L, Stamm NB, Neubauer BL, Pennington P, Toomey RE, Russell DW (1993) Proc Natl Acad Sci USA 90 5277 Ferrali A, Menchi G, Occhiato EG, Danza G, Mancina R, Serio M, Guarna A (2005) Bioorg Med Chem Lett 15 145... 

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