Vapor dispersion models

Steven R. Hanna and Peter J. Drivas, Guidelines for Use of Vapor Dispersion Models, 2d ed. (New York American Institute of Chemical Engineers, 1996), pp. 24-32. 

Among the models required for hazard assessments, vapor dispersion models are perhaps the most complex. This is primarily because of the varied nature of both the release scenarios and the chemicals that may enter into the environment. 

When chlorine is released into the air from a container, liquefied chlorine expands 470 times to the gaseous phase. It is quickly dispersed and at a faster rate by air motion if a strong wind is blowing. Chlorine is heavier than air and can accumulate in low-lying regions. Under wet conditions it is quickly hydrolyzed, or, in bright sunlight, it is photolyzed to form chlorine radicals. Some heavy vapor dispersion models for chlorine have been developed and found deficient. Some models are deficient because methods used to estimate the dispersion of heavy gas clouds apply to the case of dispersion over flat, uninterrupted terrains. It has always been assumed that the concentration-time profile follows the top-hat behavior. 

III-l] HAVENS, J.A., A Description and Assessment of the SIGMET Liquefied Natural Gas Vapor Dispersion Model, US Coast Guard Rep. CG-M-3-79 (1979). 

Coldrick, S., Lea, C.I, Ivings, M.I, 2009. Validation database for evaluating vapor dispersion model for safety analysis of LNG facilities—Review. The fire protection research foundation. Health and Safety Laboratory—HSE. 

This recommended practice is intended to apply to faciUties that (/) handle or store flammable or explosive substances in such a manner that a release of ca 5 t of gas or vapor could occur in a few minutes and (2) handle toxic substances. The threshold quantity for the toxic materials would be determined using engineering judgment and dispersion modeling, based on a potential for serious danger as a result of exposures of <1 h. 

Vapor Cloud Source Dispersion Models (Workbook of Test Cases)... 

FIG. 26-53 Effect of iuitial acceleratiou andhiioyaucy ou the release of gases. (Adapted from S. R. Hanna and F. J. TDtivas, Giiideliues for Use of Vapor Cloud Dispersion Models, 1987. Used hy permission of the American Institute of Chemical Engineers, Center for Chemical Fr ocess Safety. )... 

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). 

Hanna, S. R., and Dtivas, P. J., "Guidelines for Use of Vapor Cloud Dispersion Models."... 

Gas dispersion models provided the toxic effects of chemical releases, fire, or unconfined vapor cloud explosion. 

The atmospheric dispersion model for dense ammonia vapor evolves a slice of the plume, from the source to receptor (Kaizer, 1989 ... 

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 Vapor Cloud Dispersion Models, DIERS/ AIChE, 1987. 

Fumes and vapors discharged to the environment via a chimney form a plume, which is approximately cone shaped. Mathematical modeling of dispersal rates is possible. The Gaussian dispersion model is commonly used to calculate the concentration of pollutants at coordinate positions X, Y and Z. (The coordinates are measured from the plume centerline.) The equation used is ... 

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... 

It was concluded, based on dispersion modeling [the vapor cloud was predicted to travel only 30 ft (9 m) to its lower flammable limit] and the lack of confinement within the cloud, that the risk of a VCE was low. Since the building was designed to be fire resistant, no further study was required. 

In some cases, detailed dispersion modeling tools may not be available or their use is not warranted. To calculate the size of the flammable portion in the vapor cloud, other less precise, though sufficiently conservative, methods are available. Reference 5 cites a number of company, insurance, and governmental practices for estimating quantities of materials that could become involved in an explosion or fire. Some conservative approaches for determining the quantities of materials released include ... 

Two types of neutrally buoyant vapor cloud dispersion models are commonly used the plume and the puff models. The plume model describes the steady-state concentration of material released from a continuous source. The puff model describes the temporal concentration of material from a single release of a fixed amount of material. The distinction between the two... 

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