Vapor dispersion

Plate Layouts Cross-flow plates, whether bubble-cap, sieve, or valve, are similar in layout (Fig. 14-28). Possible zones on each plate are Active vapor-dispersion Peripheral stiffening and support Disengaging Distributing Downcomer... 

The peripheral stiffening zone (tray ring) is generally 25 to 50 mm (1 to 2 in) wide and occupies 2 to 5 percent of the cross section, the fraction decreasing with increase in plate diameter. Peripheiy waste (Fig. 14-28) occurs primarily with bubble-cap trays and results from the inabihty to fit the cap layout to the circular form of the plate. Valves and perforations can be located close to the wall and little dead area results. Typical values of the fraction of the total cross-sectional area available for vapor dispersion and contact with the liquid for cross-flow plates with a chord weir equal to 75 percent of the column diameter are given in Table 14-6. 

The plate thickness of bubble-cap and sieve plates is generally estabhshed by mechanical design factors and has little effect on pressure drop. For a sieve plate, however, the plate is an integral component of the vapor-dispersion system, and its thickness is important. 

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). 

The problem with Eq, (26-60) is that the eddy diffusivity changes with position, time, wind velocity, and prevailing atmospheric conditions, to name a few, and must be specified prior to a solution to the equation, This approach, while important theoretically, does not provide a practical framework for the solution of vapor dispersion problems,... 

Early in the morning of November 19, 1984, large quantities of LPG leaked from a pipeline or tank. The heavy LPG vapors dispersed over the 1-m-high dike (3 ft) wall into the surroundings. The vapor cloud had reached a visible height of about 2 m (6 ft) when it was ignited at a flare pit. 

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. 

Explosions from vapors occur due to the rapid transfer of heat from one molecule to the next molecule. Additionally, vapors can only ignite when present in certain concentration ranges - known as their lower and upper explosive limits. Also, vapors disperse due to diffusion and convection therefore, if a vapor cloud is released and is not ignited, the hazard is soon gone. 

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). 

Atmospheric vapor releases or liquid spills within a petroleum or related facilities commonly occur every day. They are a major source of the origin of catastrophic incidents. In order to provide an inherently safer facility the common release of process vapors to atmosphere or liquids to grade within the facility should be prevented or eliminated wherever practical. Not only does this improve the safety of a facility it also decreases the amount of fugitive emissions or liquids that occur therefore decreasing any potential harm to the environment. Containment of waste gases and liquids, human surveillance, increased testing, inspection and maintenance, gas detection and adequate vapor dispersion features are all measures to lesson the probability of an incident occurring. 

Activate fixed fire extinguishing systems or vapor dispersion mechanisms (i.e., water sprays). 

Onshore Process Area Liquid Spill Gas Release NFPA30 1. Hydrants 2. Monitors 3. Hose Reel 4. Vapor Dispersion Deluge Spray 1. NFPA 24 2. NFPA 24 3. NFPA 24 4. NFPA 15... 

Sprinkler - Water deflector spray nozzle devices used to provide distribution of water at specific characteristic patterns and densities for purposes of cooling exposures, suppression of fires and vapor dispersions. 

Vapor Dispersion Vapors from certain materials can be dispersed or moved using water spray or air movement. Reducing the concentration of the material may bring the material into its flammable range. 

Since 1978, large-scale LNG spill tests have been conducted by a joint team from Lawrence Livermore National Laboratory (LLNL) and the Naval Weapons Center (NWC) (Koopman et al., 1981). The test site was located at NWC, China Lake, California. The program, sponsored primarily by the Department of Energy, had as its principal objective the acquisition of data to aid in modeling both vapor dispersion and thermal radiation effects (from LNG vapor cloud fires). 

The vapor dispersion process is described mathemahcally as a vapor flux (7) through any plane at a height z, and is expressed by... 

Vapor Dispersion The movement of vapor clouds in air due to turbulence, gravity spreading, and mixing. 

LNG Vapor Dispersion Prediction with the DEGADIS Dense Gas Dispersion... 

LNG Vapor Dispersion Prediction with the DEGADIS Dense Gas Dispersion Model, Report GRI 0242, Gas Research Institute, Chicago, 111. 

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. 

In vapor dispersion analysis, gases and two-phase liquid-vapor mixtures are divided into three general classes ... 

If the material released to the atmosphere is not ignited, the spill can be accompanied by flash vaporization, liquid entrainment, and/or liquid accumulation (with pool formation and evaporation), and associated vapor dispersion. Absence of an immediate ignition source allows a vapor cloud to form as the vapors disperse downwind. A portion of this vapor cloud may be flammable, and if the gas has any toxic components, it can also pose a toxic hazard. The downwind extent of the flammable hazard depends on the size of the release, the upper and lower flammability limits of the material, and the air entrainment rate. 

If aerosolization is a major contributor to a hazard, a portion of the airborne liquid can be removed by placing an angled plate over the release point. The released material strikes the plate, which disperses much of the momentum and enhances coagulation of liquid droplets. In this way a portion of the liquid will drop out of the cloud. However, the loss of momentum may result in less efficient vapor dispersion and, hence, a larger hazard area. 

Attempts can be made to minimize the impact area of hazardous vapor releases. This can be accomplished by taking steps to enhance near-field vapor dispersion and thus increase the dilution rate to break down aerosols, or to move the cloud away from sensitive areas. The techniques listed below have all been used ... 

Dilwali, K. M and K. S. Mudan. 1987. Dike Design Alternatives to Reduce Toxic Vapor Dispersion Hazards. Paper presented at the AIChE Vapor Cloud Conference Proceedings, Boston, MA. New York American Institute of Chemical Engineers. 

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