Flammable materials dispersion

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). 

If a large amount of a volatile flammable material is rapidly dispersed to the atmo vapor cloud forms. If this cloud is ignited before the cloud is diluted below its lower flammability limit, a UVCE occurs which can damage by overpressure or by thermal radiation. Rarely are UVCEs detonations it is believed that obstacles, turbulence, and possibly a critical cloud size are needed to transition from deflagration to detonation. 

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. 

Confined Explosions. In situations where the vapors are confined within a building, vessel, or other such enclosure, flammable materials with flash points below the temperature within the enclosure may have the potential for an explosion. Similarly, in confined situations, combustible materials, regardless of temperature, can pose a potential for explosion if dispersed as an aerosol, mist, or dust. 

There is an upper limit to the mass of material that can contribute to an explosion, regardless of release duration. This upper limit results from dispersion effects, which dilute the flammable material at the cloud boundaries to concentrations below the lower flammable limit. These dispersion effects can apply to all release modes. 

Safer plants are designed so that those incidents, which do occur, do not produce knock-on or domino effects. For example safer plants are provided with fire breaks between sections to restrict the spread of fire, or if flammable materials are handled, the plant is built out-of-door so that leaks can be dispersed by natural ventilation. 

Air intakes to heating and ventilation systems, air compressors for process, instrument and breathing air, and to prime movers for gas compressors, power generation and pumps should be located as far as practical from contamination by dust, toxic and flammable materials release sources. They should not be located in electrically classified areas. If close to possible vapor releases (as confirmed by dispersion analyses( they should be fitted with toxic or combustible gas detection devices to warn of possible air intakes hazards and snutdown and isolate the incoming air ductwork and fans. 

First, there must be a release of a flammable material at suitable conditions of pressure or temperature. These include liquified gases under pressure, ordinary flammable liquids (especially at elevated pressures and/or temperatures), and flammable gasses. When a flammable liquid spills, some or all of it will vaporize and/or form an aerosol. This dispersion is called a vapor cloud. 

Following their release, toxic and/or flammable materials that become airborne are carried by the wind and transported away from the spill site. While being transported downwind, the airborne chemical(s) mix with air and disperse. 

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. 

If the released and dispersed materials are flammable, the dispersion calculation can serve to find out which part of the cloud lies within the limits of explosion and can therefore bum or explode. This is shown in Example 10.17. If the material is toxic it affects the health of people, as described in Sect 2.6. The effects can then be calculated using a probit relation. This is shown in Example 10.18. 

For flammable materials, hazard level end points are generally on the order of several percent. The assessment of flammable hazards typically involves modeling dilutions on the order of 10 1 or 100 1. For the same amount of material released, the hazard zones are therefore much smaller than for toxic clouds and source characteristics will play a more important role, along with atmospheric dispersion. A knowledge of momentum jet and buoyant versus dense gas plume mixing is important to these dispersion calculations. Key variables include site source details such as ... 

Decision about extent of zone, that is, distance from source (in all directions) up to which an explosive atmosphere exists before the material is dispersed. Rate of release (velocity, size, and geometry of sources), ventilation, and relative density of flammable material (mainly gas) are important considerations. 

Combustion of polymeric materials involves a complex process, where both condensed and vapor-phase reactions occur at exposed surfaces that are sources of flame and/or thermal radiation of the most common parameters measuring the flammability of polymeric materials are heat release rate (HRR) and mass loss rate (MLR) from cone calorimetry. Recently, nanocomposites containing nanoparticles have been of great interest in the composite industries. In particular, polymer blends containing clays have not been comprehensively studied for their flammability, in spite of the fact that most plastic products are made out of blends of more than two polymer. Furthermore, because the dispersion of nanoparticles is a key factor in determining the HRR and MLR of nanocomposites [23-26], we investigated correlations between flammability and dispersion in air and under nitrogen, especially for polymer blends. 

In order to determine the location and extent of the hazardous areas it is necessary to establish the sources of release and the likely dispersion of the flammable material from the source in any direction to the point where its concentration is below the LEL. Factors such as the rate of release, concentration, volatility, ventilation, topography and density of the gas/air mixture will need to be taken into account. A contour line joining the points so established, both vertically and horizontally, is the boundary between adjacent zones. 

Baker (1996) provides guidelines to determine the mass of flammable material. For small releases of flammable materials, a typical approach would be to obtain the fuel mass between the flammability limits using a dispersion model. 

Dispersion models estimate the evolution and the features of the cloud, such as concentration, temperature, velocity and dimensions as a function of time and position. In the case of flammable substances, these models facilitate the prediction of an area where a fire or explosion might occur and the quantity of flammable material in that area. 

Only a very thin film of oil is needed to produce an explosion in the pipework of compressed-air systems, even if the oil is not dispersed into the air prior to ignition. Many industrial fires and explosions have occurred in systems utilizing oil-lubricated air compressors. Therefore, adding air to an empty hydrocarbon line, or to purge a hydrocarbon line, is potentially hazardous because trace amounts of flammable materials may be clinging to the pipe walls. On one occasion, an explosion occurred in a cmde-oil pipeline to which air had been added, even though the line had been first emptied by displacement with water between two scraper plugs. 

Dispose of oxygen only in an area free of flames, sparks, and flammable materials where the oxygen can be dispersed rapidly. An empty, open cement parking area is ideal. A fiime hood may be used to disperse oxygen slowly if there are no sparks or flammable materials in the hood. 

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. 

Flammability = 4, ie, very flammable gas, very volatile, and materials that in the form of dusts or mists form explosive mixtures when dispersed in air Health = 2, ie, hazardous to health, but may be entered freely with self-contained breathing apparatus Reactivity = 0, ie, is normally stable when under fire-exposure conditions and is not reactive with water... 

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. 

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