Safety dispersion models

S. R. Hanna and P. J. Drivas, Guidelines for Use ofUapor Cloud Dispersion Models American Institute of Chemical Engineers, Center for Chemical Process Safety, New York, 1987. 

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). 

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... 

Significant modifications were made to the following topics dispersion modeling, source modeling, flammability characterization, explosion venting, fundamentals of electrostatics, and case histories. This new edition also includes selected materials from the latest AICHE Center for Chemical Process Safety (CCPS) books and is now an excellent introduction to the CCPS library. 

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. 

An area which deserves special attention with respect to safety is the storage of liquid ammonia. In contrast to some other liquefied gases (e.g., LPG, LNG), ammonia is toxic and even a short exposure to concentrations of 2500 ppm may be fatal. The explosion hazard from air/ammonia mixtures is rather low, as the flammability limits [1334]-[1338], [1343] of 15-27% are rather narrow. The ignition temperature is 651 °C. Ammonia vapor at the boiling point of-33 °C has vapor density of ca. 70% of that of ambient air. However, ammonia and air, under certain conditions, can form mixtures which are denser than air, because the mixture is at lower temperature due to evaporation of ammonia. On accidental release, the resulting cloud can contain a mist of liquid ammonia, and the density of the cloud may be greater than that of air [1334]-[1344], This behavior has to be taken into account in dispersion models. 

Figure 7.36a and b proves the applicability of shortcut calculations based on the ideal equilibrium model for the estimation of process conditions. The results of rigorous process simulation based on the transport-dispersive model are in very good agreement with the shortcut calculation for isocratic (a) as well as nonisocratic (b) SMB processes. Expectedly safety margins have to be taken into account when the process conditions of an SMB process are estimated by shortcut calculation. The scattering of the numerical data results from an increased grid size for the numerical calculations that has been chosen in order to reduce computer time. The model parameters coincide with the data for the protein separation presented in Section 6.6.2.2.3 the separation quality of the SMB process was set to 99.9% purity. 

The basic analytical models of MACCS2 provide atmospheric transport, dispersion, deposition, and dose calculation for the released inventory. A Gaussian dispersion model based on the Tador-Gur parameterization is used for this SAR to provide consistency with previous safety studies on the HCF. Radioactive isotopes are allowed to decay and build up daughter activity during transport and deposition. The decay is based on an input file of half-lives and decay schemes for hundreds of isotopes. Dose pathways modeled were radioactive plume inhalation and immersion, groundshine from deposited radiation, and resuspension inhalation dose. Groundshine and resuspension inhalation pathways are very minor contributors to dose. 

Another use for computers in safety is modeling. Most common is modeling of processes and the hazards the processes create. Modeling allows people to anticipate what might happen or understand what did happen. The accuracy produced by a model depends on the availability of accurate data, the inclusion of factors that can affect the phenomenon and adequate representation in the mathematical manipulations. There are reasonably successful models in safety for automobile accident reconstruction, for gas dispersion, and for fire behavior in small buildings. Some software allows users to construct a model of a unique... 

The quality of consequence models, especially dense gas dispersion models, started to be assessed around 1980. From that time, experimental data, both at laboratory scale and at field scale, have been gathered. Among the more famous large-scale experiments are those performed by Lawrence Livermore National Laboratory in the LFnited States (with names like Desert Tortoise, Coyote, and Burro) and those by Shell and UK Health and Safety Executive in Europe (Maplin Sands, Thomey Island). These experiments resulted in a considerable improvement of the available models, reducing the range of variation between the predictions of the different models (McQuaid, 1983). 

Hanna, S. R., and R J. Drivas. 1987. Vapor Cloud Dispersion Models, Center for the Chemical Process Safety of the American Institute of Chemical Engineers, New York. 

Chemical emission and dispersion modeling is a quantitative tool that straddles environmental engineering and system safety engineering. Dispersions can happen through the atmosphere, soil, or water. In planning for anergency response for process plants, tanker truck crashes, or rail car accidental chanical dispersions, one of the steps to determine how serious a chanical release would be is to perform a dispersion model of the accident. Many models are currently in use, but an intemationaUy recognized... 

One of the biggest advantages of dispersion modeling is that you do not have to look at unlikely worst-case scenarios but can investigate the gamnt of scenarios. Dispersion modeling also fits neatly into other safety analyses and risk assessments. 

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. 

HSE—Health and Safety Executive. 2010. Review of FLAGS version 9.0—Dispersion modelling capabilities. Available at http //www.hse.gov.uk/research/ rrpdf/rr779.pdf, accessed on January, 2013. 

---