Atmospheric Vapor Cloud Dispersion

Chapter 2 discussed the possible influence of atmospheric dispersion on vapor cloud explosion or flash fire effects. Factors such as flammable cloud size, homogeneity, and location are largely determined by the manner of flammable material released and turbulent dispersion into the atmosphere following release. Several models for calculating release and dispersion effects have been developed. Hanna and Drivas (1987) provide clear guidance on model selection for various accident scenarios. 

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

The flash point of a liquid is the minimum temperature at which its vapor pressure is sufficiently high to produce a flammable mixture with air above the liquid. Therefore, the generation of a flammable gas or vapor cloud for liquids whose flash points are above the ambient temperature, e.g., xylene (see Table 3.1), is only possible if they are released at elevated temperatures or pressures. In such 

TABLE 3.1. Explosion Properties of Flammable Gases and Vapors in Air at Atmospheric Conditions  

Gas or Vapor Flammability Limits (vol. %) Flash Point rc) Autoignition Temperature rc) Laminar Burning Velocity (mis) 

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A fuel-air mixture is detonable only if its composition is between the detonabil-ity limits. The detonation limits for fuel-air mixtures are substantially narrower than their range of flammability (Benedick et al. 1970). However, the question of whether a nonhomogeneous mixture can sustain a detonation wave is more relevant to the vapor cloud detonation problem because, as described in Section 3.1, the composition of a vapor cloud dispersing in the atmosphere is, in general, far from homogeneous. 

If environmental and atmospheric conditions are such that vapor cloud dispersion can be expected to be very slow, the possibility of unconfined vapor cloud detonation should be considered if, in addition, a long ignition delay is likely. In that case, the full quantity of fuel mixed within detonable limits should be assumed for a fuel-air charge whose initial strength is maximum 10. 

Guidelines for the Use of Vapor Cloud Dispersion Models, the associated Workbook of Test Cases for Vapor Cloud Source Dispersion Models and research now in progress are directed toward a more complete understanding of the geographic areas affected by a release to the atmosphere. 

Guidelines for Safe Storage and Handling of High Toxic Hazard Materials Guidelines for Use of Vapor Cloud Dispersion Models Understanding Atmospheric Dispersion of Accidental Releases Expert Systems in Process Safety... 

General References Crowl and Louvar, Chemical Process Safety Fundamentals with Applications, Prentice Hall, Englewood Cliffs, NJ, 1990, pp. 121-155. Hanna and Drivas, Guidelines for Use of Vapor Cloud Dispersion Models, AIChE, New York, 1987. Hanna and Strimaitis, Workbook of Test Cases for Vapor Cloud Source Dispersion Models, AIChE, New York, 1989. Lees, Loss Prevention in the Process Industries, Butterworths, London, 1986, pp. 428-463. Seinfeld, Atmospheric Chemistry and Physics of Air Pollution, Chaps. 12, 13, 14, Wiley, New York, 1986. Turner, Workbook of Atmospheric Dispersion Estimates, U.S. Department of Health, Education, and Welfare, Cincinnati, 1970. 

The output of evaporation models is the time-dependent mass rate of boiling or vaporization from the pool surface. These models rarely give atmospheric vapor concentrations or cloud dimensions over the pool, which may be required as input to dense gas or other vapor cloud dispersion models. 

The basis for the protective action distances given in Table 1 is based on analysis using state-of-the-art source term and vapor cloud dispersion modeling, probabilistic application of acmal atmospheric data, and information on toxicological exposure guidelines for each chemical. 

The effect of atmospheric dispersion on the structure of a vapor cloud may be summarized as follows. In general, the structure of a vapor cloud in the atmosphere... 

Local partial confinement or obstruction in a vapor cloud may easily act as an initiator for detonation, which may propagate into the cloud as well. So far, however, only one possible unconfined vapor cloud detonation has been reported in the literature it occurred at Port Hudson, Missouri (National Transportation Safety Board Report 1972 Burgess and Zabetakis 1973). In most cases the nonhomogeneous structure of a cloud freely dispersing in the atmosphere probably prevents a detonation from propagating. 

Zeeuwen et al. (1983) observed the atmospheric dispersion and combustion of large spills of propane (1000-4000 kg) in open and level terrain on the Musselbanks, located on the south bank of the Westerscheldt estuary in The Netherlands. Thermal radiation effects were not measured because the main objective of this experimental program was to investigate blast effects from vapor cloud explosions. 

Atmospheric dispersion of any rupture disc discharges would result in a vapor cloud with gas concentrations above the lower explosive limit. Thus, such releases must be avoided, and other mitigation procedures should be used. However, as an additional check on the situation, mapping of the potential gas cloud versus the plant layout was conducted with the conclusion that no ignition sources were likely to be present in the region where the vapor cloud would be flammable. 

If the material released to the atmosphere is not ignited, the spill can be accompanied by flash vaporization, liquid entrainment, and/or liquid accumulation (with pool formation and evaporation), and associated vapor dispersion. Absence of an immediate ignition source allows a vapor cloud to form as the vapors disperse downwind. A portion of this vapor cloud may be flammable, and if the gas has any toxic components, it can also pose a toxic hazard. The downwind extent of the flammable hazard depends on the size of the release, the upper and lower flammability limits of the material, and the air entrainment rate. 

When a large amount of volatile material is released rapidly to the atmosphere, a vapor cloud forms and disperses. If the cloud is ignited before it is diluted below its lower flammability limit, an uncontrolled vapor cloud explosion will occur. This is one of the most serious hazards in the process industries. Both shock waves and thermal radiation will result from the explosion, with the shock waves usually the more important damage producers. UVCEs usually are modeled by... 

Furthermore, the spontaneous surface ejection of kinetic protons might leave the oceans at a negative electric potential. Trapped in the atmospheric vapor, the protons can be involved in the cohesive dispersion of clouds. Various lightening effects might therefore be due to massive F1+ currents. 

A solution of sulfur trioxide [7446-11-9] dissolved in chlorosulfonic acid [7990-94-5] CISO H, has been used as a smoke (U.S. designation FS) but it is not a U.S. standard agent (see Chlorosulfuric acid Sulfuric acid and sulfur trioxide). When FS is atomized in air, the sulfur trioxide evaporates from the small droplets and reacts with atmospheric moisture to form sulfuric acid vapor. This vapor condenses into minute droplets that form a dense white cloud. FS produces its effect almost instantaneously upon mechanical atomization into the atmosphere, except at very low temperatures. At such temperatures, the small amount of moisture normally present in the atmosphere, requires that FS be thermally generated with the addition of steam to be effective. FS can be used as a fill for artillery and mortar shells and bombs and can be effectively dispersed from low performance aircraft spray tanks. FS is both corrosive and toxic in the presence of moisture, which imposes limitations on its storage, handling, and use. 

Introduction Gas dispersion (or vapor dispersion) is used to determine the consequences of a release of a toxic or flammable material. Typically, the calculations provide an estimate of the area affected and the average vapor concentrations expected. In order to make this determination, one must know the release rate of the gas (or the total quantity released) and the atmospheric conditions (wind speed, time of day, cloud cover). 

After the rate of toxic material entry into a cloud has been determined (using the equations in the preceding section), the effects of atmospheric conditions on the dispersion (dilution) of the material— particularly gas or vapor—can be evaluated. 

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