Puff 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. 

The diffusion of individual plume elements, according to Gifford (1959) can be deiernnned from the general Gaussian diffusion model. The model is usually in two basic forms, a puff release - appropriate for an accident, and a continuous release for "routine" release or an accident of long duration. 

Technica, 1988 Wheatley, 1988 Ziomas, 1989). Plume dispersion consists of two phases the gravityslumping and passive dispersion period. Continuous releases with any time-profile and puff releases as of short duration continuous releases are treated using the release category and th r... 

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. 

On Figure 14 the sublayer region corresponds closely to the values deduced from Aleksandrova s formulas and mast measurements for the precipitation of a puff released from a point source (72) (Reference 72 gives a formula D = b uz where u is mean wind speed. For Group 2 stability (unstable layer below inversion in our case) b averages out to 0.064 for 18 experimental observations. The standard deviation is 0.015.) The values reported by Lettau (73) (as Austausch coefficients) were obtained from Leipzig wind profile measurements. The data of Ivanov... 

If someone is working outdoors and they are exposed to a toxic gas, it is generally a good idea to get indoors. As a mle of thumb the concentration of toxic gas inside a building is around a tenth of the concentration outside. In principle, were the gas release continue for a long time, the concentration of the gas in the building would evenmally increase to the level outside. However, most gas releases do not go on for a long time. After the initial puff release, the amount of gas reduces, either because the inventory is exhausted or because the affected unit is isolated by the operator or by the safety instrumentation. 

Table 2.36 Probabilities of death after a puff release of CO for different building qualities and mitigation strategies... 

Releases can be instantaneous and last for but a short time ( puff release ) or continuous, which then lead to plume formation. 

Example 10.14 Effects of a puff release of carbon monoxide... 

The original mixture of gas and air, which results from a puff release, is supposed to have cylindrical shape. Usually a ratio of diameter to height of 1 is assumed. 

Table 10.7 Results of the dispersion calculations for a puff release of chlorine...

The literature indicates numerous, often widely different values for assigning probabilities to scenarios such as those of Figs. 10.2, 10.3 and 10.4. This reflects uncertainties due to lack of knowledge (epistemic) and stochastic (aleatory) effects. If properly treated these uncertainties should be represented by probability distributions. Table 10.15 gives an overview of conditional probabilities for the consequences of a puff release of a pressurized flammable gas from various sources. Not only are the differences in probabilities evident but also the differences as to the endpoints. 

Table 10.15 Conditional probabilities for the event tree instantaneous (puff) release of a flammable gas under pressure (from [75])...

If a liquid is released vaporization is assessed with simple models. In any case the dispersion calculations are performed conservatively assuming a puff release, even if the vaporization process extends over considerable time spans. 

Puff releases are assumed for the dispersion calculations, since a time distribution of the released quantities would require arbitrary assumptions of release duration and mass flow rate. Furthermore, puff releases lead to higher atmospheric concentrations and hence stronger effects than time-dependent releases of the same quantities. 

Two types of dispersion models are used to describe these releases when the puff or plumes are neutrally or positively buoyant. When there is an instantaneous release or a burst of material, we make use of a puff model. In this model the puff disperses in the downwind, cross wind, and vertical directions simultaneously. Computer codes written for puff models usually have the capability of tracking multiple puff releases over a period of time. When the release rate is constant with time, the puff model can be mathematically integrated into a continuous model. In this case, dispersion takes place in the cross wind and vertical directions only. The mathematical expressions are those discussed in Section III. The dispersion coefflcients used, however, may differ from those described in Section IV. Plume rise equations from Section V for positively buoyant plumes may be used in conjunction with these dispersion models. The equations of current models indicate that they are well formulated, but the application of the models suffers from poor meteorological irrformation and from poorly defined source conditions that accompany accidental releases. Thus, performance of these models is not adequate to justify their use as the sole basis for emergency response planning, for example. 

FIGURE 2.35. Spreadsheet output for Example 15a, b Puff release. 

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