Burning cloud

Fireball A burning fuel-air cloud whose energy is emitted primarily in the form of radiant heat. The inner core of the cloud consists almost completely of fuel, whereas the outer layer (where ignition first occurs) consists of a flammable fuel-air mixture. As the buoyancy forces of hot gases increase, the burning cloud tends to rise, expand, and assume a spherical shape. 

If ignition of fuel-rich mixture occurs, the release will bum as a fireball. Burning will occur primarily in the outer layer of the fuel-rich cloud. As the buoyancy of the hot gases increases, the burning cloud rises, expands, and assumes a spherical shape. Damage is again caused by direct flame contact and radiant heat. 

Experimental and simulated burning cloud snapshot for Coyote trial 3 (1 m height, 103 s). (From Rigas, F. and Sklavounos, S., Chem. Eng. Sci., 61,1444, 2006. With permission from Elsevier.)... 

In the last section, our QS criteria for group combustion are apphed to the case of a dodecane cloud burning in air. Results obtained indicate that virtually all QS burning clouds of practical interest would bum as a total group. 

Flammability Flammability hazard is concerned with the ease with which materials can be ignited and continue to bum. A major consideration is the rate of burning. Clouds of fine combustible dusts, for example, bum so rapidly that they have the force and effect of an explosion. There are various criteria which have been developed to identify flammable materials. Flash points, fire points, and autoignition temperature are 3 common measures of flammability. Flame propagation and the explosive or flammable range are measures commonly used for gases, vapors and air-suspended fine combustible dusts. PSTofbric materials are included in this class. 

The primitive earth long remained covered in darkness, wrapped in dense burning clouds into which water vapor poured continuously from volcanic emissions. When temperatures finally cooled sufficiently, the clouds began to melt into rain. At first, falling on incandescent rock, the rain evaporated, but the evaporation... 

If the values of peak pressure calculated exceed the burst pressure of the vessel, then the consequences of the resulting explosion should be determined. As in Sections 3.3 and 3.4, the resulting effects are a shock wave, fragments, and a burning cloud. Although the pressure at which the vessel may burst may be well below the maximum pressure that could have developed, it is frequently conservatively assiuned that the stored energy released as a shock wave is based on the maximum pressure that could have developed. 

Obstacles, Process Equipment, and Piping—Structural steel and other obstacles create turbulence in the burning cloud, which increases the overpressure. The profile, size, and location of obstacles will all influence the amount of overpressure developed. 

Aniline.—Burns with a very smoky flame, clouds of soot being produced. Typical of many aromatic substances. i,2 Dibromoethane.—Does not burn until vapour becomes hot and then burns with a slightly smoky flame. Typical of substances rich in halogens such as cldoroform, chloral hydrate, and carbon, tetrachloride. (Note, however, that iodoform evolves copious fumes of iodine when heated in this way.)... 

Zinc is a bluish-white, lustrous metal. It is brittle at ordinary temperatures but malleable at 100 to ISOoC. It is a fair conductor of electricity, and burns in air at high red heat with evolution of white clouds of the oxide. 

Isophthahc acid dust forms explosive mixtures with air at certain concentrations. These concentrations and other information on burning and explosiveness of isophthahc acid dust clouds are given in Table 27 (40,41). Fires can be extinguished with dry chemical, carbon dioxide, water or water fog, or foam. 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Fire Hazards - Flash Point Not pertinent (combustible solid) Flammable Limits in Air (%) Not pertinent Fire Extinguishing Agents Water, dry chemical, carbon dioxide Fire Extinguishing Agents Not to be Used Not pertinent Special Hazards cf Combustion Products Not pertinent Behavior in Fire Not pertinent Ignition Temperature (deg. F) 842 (dust cloud) Electrical Hazard Not pertinent Burning Rate Not pertinent. 

In some cases, however, only the part of the cloud which is within the flammable range is considered to burn. This may be a factor of 10 less than the total cloud. For further discussion of explosion efficiency see CCPS (1989) or Feeds (1986),... 

On many occasions employees have entered a cloud of flammable gas or vapor to isolate a leak. In the incident described in Section 7.2.1 (d), this was done to avoid shutting down the plant. More often, it has been done because there was no other way of stopping the leak. The persons concerned would have been badly burned if the leak had ignited while they w ere inside the cloud. 

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

Fay, J. A., and D. H. Lewis, Jr. 1977. Unsteady burning of unconfined fuel vapor clouds. 16th Symposium (International) on Combustion, pp. 1397-1405. Pittsburgh, PA The Combustion Institute. 

In-cloud overpressure is dependent on outflow velocity, orifice diameter, and the fuel s laminar burning velocity. 

Experimental research has shown that a vapor cloud explosion can be described as a process of combustion-driven expansion flow with the turbulent structure of the flow acting as a positive feedback mechanism. Combustion, turbulence, and gas dynamics in this complicated process are closely interrelated. Computational research has explored the theoretical relations among burning speed, flame speed, combustion rates, geometry, and gas dynamics in gas explosions. 

This concept can be generalized for more arbitrarily shaped clouds, provided that a reasonable estimate can be made of combustion process development in terms of burning velocity and flame surface area. According to Strehlow (1981), a conservative estimate of source strength is made by... 

These results were analytically reproduced by Taylor (1985), who employed a velocity potential function for a convected monopole. This concept makes it possible to model an elongated vapor cloud explosion by one single volume source which is convected along the main axis at burning velocity, and whose strength varies proportionally to the local cross-sectional cloud area. 

Rosenblatt, M., and P. J. Hassig. 1986. Numerical simulation of the combustion of an unconfined LNG vapor cloud at a high constant burning velocity. Combust. Science and Tech. 45 245-259. 

The literature provides little information on the effects of thermal radiation from flash fires, probably because thermal radiation hazards from burning vapor clouds are considered less significant than possible blast effects. Furthermore, flash combustion of a vapor cloud normally lasts no more than a few tens of seconds. Therefore, the total intercepted radiation by an object near a flash fire is substantially lower than in case of a pool fire. 

In order to compute the thermal radiation effects produced by a burning vapor cloud, it is necessary to know the flame s temperature, size, and dynamics during its propagation through the cloud. Thermal radiation intercepted by an object in the vicinity is determined by the emissive power of the flame (determined by the flame temperature), the flame s emissivity, the view factor, and an atmospheric-attenuation factor. The fundamentals of heat-radiation modeling are described in Section 3.5. 

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