Toxic materials dispersion

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. 

Eye. Adverse effects may be produced by splashes of Hquids or soflds, and by materials dispersed in the atmosphere. The eye is particularly sensitive to peripheral sensory irritants in the atmosphere. Toxic effects that may be induced include transient acute inflammation, persistent damage, and, occasionally, sensitivity reactions. ToxicologicaHy significant amounts of material may be absorbed by the periocular blood vessels in cases of splash contamination of the eye with materials of high acute toxicity (58). 

Batch equipment located indoors. A release of flammable/toxic material tends to disperse slower than if the release is outdoors. May lead to large concentration buildup and result in operator exposure. Confined flammable releases are also more likely to result in explosion with larger overpressures. 

Many sophisticated models and correlations have been developed for consequence analysis. Millions of dollars have been spent researching the effects of exposure to toxic materials on the health of animals the effects are extrapolated to predict effects on human health. A considerable empirical database exists on the effects of fires and explosions on structures and equipment. And large, sophisticated experiments are sometimes performed to validate computer algorithms for predicting the atmospheric dispersion of toxic materials. All of these resources can be used to help predict the consequences of accidents. But, you should only perform those consequence analysis steps needed to provide the information required for decision making. 

The chemical and physical phenomena involved in chemical process accidents is very complex. The preceding provides the elements of some of the simpler analytic methods, but a PSA analyst should only have to know general principles and use the work of experts contained in computer codes. There are four types of phenomenology of concern 1) release of dispersible toxic material, 21 dispersion of the material, 3) fires, and 4) explosions. A general reference to such codes is not in the open literature, although some codes are mentioned in CCPS (1989) they are not generally available to the public. 

The U.S. Environmental Protection Agency concentrates on modeling the dispersion of toxic material in the environment and the health effects thereof rather than accident prevention, The... 

Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release the airborne toxic material is carried away by the wind in a characteristic plume, as shown in Figure 5-1, or a puff, as shown in Figure 5-2. The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air. 

A wide variety of parameters affect atmospheric dispersion of toxic materials ... 

An indication of the hazard associated with the use of a toxic material in the form of a vapour or dispersed dust is given by a limit value. The threshold limit value (TLV, expressed as p.p.m. or mg m-3) represents a level under which it is believed nearly all workers may be repeatedly exposed to on a day-to-day basis without adverse effect. These values are up-dated annually and recommended by the American Conference of Governmental Industrial Hygienists (ACGIH).12 Since 1984, in the UK, the Health and Safety Executive (HSE) has adopted two types of limits, but only for those compounds which are available and used in the UK.13 These are the recommended limit (RL, as p.p.m. or mg m 3) which represents good practice and realistic levels for the degree of exposure, and the control limit (CL, as p.p.m. or mg m 3) which is applied to the relatively smaller number of substances having unusually serious toxic effects. 

Steam curtains are best used for diluting heavier-than-air releases of flammable vapors, not toxic materials. For flammable materials the level of dilution with air that has to be obtained is the lower explosive limit toxic materials could require dilution to <100 ppm range. Moreover, while steam curtains can provide the thermal effects that will help disperse flammable material, they hinder the absorption effects needed for toxic materials, especially materials that are water-soluble. 

An air curtain works somewhat like a water or steam curtain, but offers no potential to absorb a toxic material or increase dispersion from thermal effects. An air curtain merely provides air movement that promotes air dilution. 

If a toxic material is dispersed into the air, the engineer/scientist must know how high its concentration can be without causing danger to people. The Occupational Safety and Health Administration (OSHA) has set concentration levels for many substances. This concentration level is called the permissible exposure limit (PEL). The PEL is synonymous in most application with the TLV-TWA (threshold limit value-time-weighted average). 

Recently there has been an increasing interest of modelling the dispersion of toxic material in local-scale urban areas. Types and simplicities of the models for urban emergency preparedness, first of all, depend on two main issues (i) scales of the considered processes, and (ii) aim of the modelling (as a risk assessment or operational forecasting tool). 

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. 

The dispersion analyses that yielded the EPA tables were based on an averaging time of 6 s as compared with the 10 min and 60 min used for the toxic gas and vapor dispersion analyses. As with toxic materials, the neutral density equations would be used for listed gases and vapors that have a molecular weight <28 or where the product of vapor pressure and molecular weight is <500 millimeters of mercury (mm Hg). In contrast, the dense vapor equations would be used for listed... 

Air pollution occurs when the concentration of natural and/or man-made substances in the atmosphere becomes excessive and the air becomes toxic. Emissions from transportation, industry, and agriculture are man-made sources of air pollution. Primary pollutants are gases, liquids, and particulates dispersed into the atmosphere through either man-made or natural processes. In the United States, the primary pollutants are carbon monoxide, sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs), and particulate matter (soot, dust, etc.). Secondary pollutants are derived from primary pollutants that undergo a chemical reaction and become a different type of toxic material. In the United States, secondary pollutants are ozone, photochemical smog, and acid rain. 

These sequences are in general represented by event trees, also called accident sequence diagrams. Several examples of event trees are shown in Figs. 10.2, 10.3, 10.4 and 10.5. Releases of materials under pressure can additionally be accompanied by missile flight of fragments of the pressure boundary (e.g. vessels or pipework). If a gas is not flammable but toxic atmospheric dispersion follows its release. This is also true in cases of delayed ignition. 

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