Combustible vapor dispersion

Combustible Vapor Dispersion (CVD) - A mathematical estimation of the probability, location, and distance a release of combustible vapors will exist until dilution will naturally reduce the concentration to below the LEL or no longer considered ignitable (typically defined as 50% of the LEL). 

Combustible Vapor Dispersion (CVD)— A mathematical estimation of the probability, location, and distance of a release of combustible vapors that will exist until dilution naturally reduces the concentration to below the lower explosive limit (LEL), or will no longer be considered ignitable (typically defined as 50% of the LEL). For basic studies, the normal expected wind direction is utilized (based on historical wind rose data). Real-time modeling is sometimes used during incident occurrence to depict area of vapor coverage on plant maps for visual understanding of the affected areas based on wind speeds and direction. 

VDI Part 1 models the dispersion of vapor plumes with output consisting of vapor ctiriccntration as a function of time and downwind distance and denser-than-air vapor releases. VDI Part 2 determines the downwind distance to the lower flammable limit of a combustible vapor. Part 2 may also be used in conjunction with Part 1 to model a toxic gas emission. 

Critical Air Supplies— Air supplies for ventilation of control rooms, prime movers, emergency generators, etc., should be located at the least likely location for the accumulation of combustible vapors or routes of dispersion. 

The ideal solution is to locate the flare in a perpendicular location to the prevailing wind (i.e., crosswind) with adequate spacing from the facility. This should preferably also be at a lower elevation than the rest of the facility. This is in case of release of heavy vapors that have not been adequately combusted in the flare exhaust. Because of the larger spacing requirements for flares (i.e., distance to avoid heat radiation effects and vapor dispersion requirements), they should be one of the first items sited for the design of a new facility. 

All areas that are subject to a possible vapor cloud formation should be provided with maximum ventilation capability Specific examination should be undertaken at all areas where the hazardous area classification is defined as Class 1, Division 1 or Class If Division 2. These are areas where hydrocarbon vapors are expected to be present, so verification that adequate ventilation is provided to aide in the dispersion of combustible vapors or gases is necessary The following practices are preferred ... 

Example The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT equivalent method is used in this example other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any govern-mentaf unit requires a specific type of analysis. 

Are combustible powders or vaporizing liquids processed If these can be released and dispersed as a cloud an explosion can result. For example, there have been severe explosions at bread flower mills. 

Data on dispersion and combustion of aerosol-air clouds are scarce, although Burgoyne (1963) showed that the lower flannmability limits on a weight basis of hydrocarbon aerosol-air mixtures are in the same range as those of gas- or vapor-air mixtures, namely, about 50 g/m. ... 

Generally, at any moment of time the concentration of components within a vapor cloud is highly nonhomogeneous and fluctuates considerably. The degree of homogeneity of a fuel-air mixture largely determines whether the fuel-air mixture is able to maintain a detonative combustion process. This factor is a primary determinant of possible blast effects produced by a vapor cloud explosion upon ignition. It is, therefore, important to understand the basic mechanism of turbulent dispersion. 

In the application of the multienergy concept, a particular vapor cloud explosion hazard is not determined primarily by the fuel-air mixture itself but rather by the environment into which it disperses. The environment constitutes the boundary conditions for the combustion process. If a release of fuel is anticipated somewhere, the explosion hazard assessment can be limited to an investigation of the environment s potential for generating blast. 

The major objective of the experimental program was to obtain data that could be used to assess the accuracy of existing models for vapor cloud dispersion. The combustion experiments were designed to complement this objective by providing answers to the question, What would happen if such a cloud ignited ... 

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. 

The environmental problem of sulfur dioxide emission, as has been pointed out, is very much associated with sulfidic sources of metals, among which a peer example is copper production. In this context, it would be beneficial to describe the past and present approaches to copper smelting. In the past, copper metallurgy was dominated by reverberatory furnaces for smelting sulfidic copper concentrate to matte, followed by the use of Pierce-Smith converters to convert the matte into blister copper. The sulfur dioxide stream from the reverberatory furnaces is continuous but not rich in sulfur dioxide (about 1%) because it contains carbon dioxide and water vapor (products of fuel combustion), nitrogen from the air (used in the combustion of that fuel), and excess air. The gas is quite dilute and unworthy of economical conversion of its sulfur content into sulfuric acid. In the past, the course chosen was to construct stacks to disperse the gas into the atmosphere in order to minimize its adverse effects on the immediate surroundings. However, this is not an en-... 

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. 

The first major hazard in process plant is fire, which is usually regarded as having a disaster potential lower than both explosion and toxic release2. However, fire is still a major hazard and can under the worst conditions approach explosion in its disaster potential. Fire requires a combustible material (gas or vapor, liquid, solid, solid in the form of a dust dispersed in a gas), an oxidant (usually oxygen in air) and usually, but not always, a source of ignition. Consider now the important factors in assessing fire as a hazard. 

The experimentation in the field of gas cloud fires appears to be limited. The unique set of large-scale experiments that involve the release, dispersion, ignition, and combustion of flammable natural gas clouds in the open air is that with the code name Coyote. Coyote series trials conducted by LLNL in 1983 at California s Nevada Test Site, Nevada provided an integrated dataset for use in validation studies [64,65]. The objective of the experiments was to determine the transport and dispersion of vapors from LNG spills, and in addition to investigate the damage potential of vapor cloud fires. Transient simulations... 

If a combustible gas release is not ignited immediately, a vapor plume will form. This will drift and be dispersed by the ambient winds or natural ventilation. If the gas is ignited at this point, but does not explode, it will result in a flash fire, in which the entire gas cloud bums very rapidly. It is unlikely to cause any fatalities, but will damage steel structures. If the gas release has not be isolated during this time, the flash fire will bum back to a jet fire at the source of the release. A flash fire is represented by its limiting envelope, since no damage is caused beyond it. This envelope is usually taken as the LEL of the gas cloud. 

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. 

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