Gas cloud

Vapor cloud explosions can result if clouds of flammable vapor in air are formed. It is important to understand how hquids and gases flow through holes in equipment and how resulting vapor or gas clouds are dispersed in air. 

The larger circle is the area that could be exposed to a specified atmospheric concentration of monomethylamine stored as an anhydrous liquid. The smaller circle is the area that could be exposed to a specified atmo heric concentration of monomethylamine stored as an aqueous sohition. The elliptical figures represent a gas cloud caused by an east-southeast wind. 

Uses mathematical models to assess gas cloud movements. Uses gas detectors and weather sensors to alert user of release, and provides plume dispersion, effects, and response information. 

The movement of gases and vapors is more difficult to visualize than that of particulates. However, most gases and vapors have strong absorption peaks in the infrared band. If a flat screen, heated to some 15 C or more above ambient temperature, is positioned on one side of a source with an infrared camera and filter on the other side, then the gas cloud will absorb a certain amount of infrared. Although the basic method is simple, special equipment (camera and filters) is required. 

W.M. ter Ktiile, B. Knoll, P.Ci.M. Hesselink. Measurement and imaging of gases m indusrrni environments with the infrared gas cloud scanner. Appl. Occup. fnviron. Hyg. 8 1), 199,, ... 

A leak had occurred in the pipeline between the towns of Ufa and Asma at a point 800 m (0.5 mi) away from the Trans-Siberian double railway track. The area was a wooded valley. Throughout the area, there had been a strong smell of gas a few hours before the blast. The gas cloud was reported to have drifted for a distance of 8 km (5 mi). 

Figure 4.5. Flame velocity, peak overpressure, and overpressure duration in gas cloud explosions following vessels bursts (Giesbrecht et al. 1981).

Harrison, A. J., and J. A. Eyre. 1986. Vapor cloud explosions—The effect of obstacles and jet ignition on the combustion of gas clouds, 5th Int. Symp. Proc. Loss Prevention and Safety Promotion in the Process Industries. Cannes, France. 38 1, 38 13. 

Mackenzie, J., and D. Martin. 1982. GASEXl—A general one-dimensional code for gas cloud explosions. UK Atomic Energy Authority, Safety and Reliability Directorate, Report No. SRD R251. 

In Figure 6.16, the region originally occupied by the gas cloud is shaded, and the position and shape of the shock wave and the contact surface at different times following the explosion are shown as solid and dashed curves. The shape of the shock wave is almost elliptical, with ellipticity decaying to sphericity as the shock gradually degenerates into an acoustic wave. 

Analysis had shown that the fuel behaved like ethylene-air mixture and the cloud could be so large that it could fill the whole calculation domain up to about 20 m high. The ethylene-air gas cloud was assumed to be a homogeneous stoichiometric mixture with the shape of a box. The following two cloud assumptions were chosen ... 

In a flash fire, a gas cloud or plume fonns and moves in a downwind direction. Subject to contact witli an ignition source, a wall of flame will flash back to the vapor source, sometimes witli explosive force. The beliavior of flash fire flames is not well documented. It is generally assumed tliat a flash fire will spread tliroughout tlie vapor cloud emitted and tliat it can be calculated by gas outflow dispersion. It is generally assumed tliat tlie fire is lethal to anyone within the contours of tlie cloud who is not wearing special protective gear. ... 

Scbwaden, m. noxious vapor or exhalation, damp specif., choke damp gas cloud smoke cloud swath. — feurlger —, fire damp. 

If a significant volume of gas (caused by a leak, for example) is exposed to an ignition source and this gas is mixed with air in proportions that are close to stoichiometric, the gas cloud can cause a lot of damage when it gives rise to a detonation. The accident at Flixborough is one example. The lower explosive limit of hydrocarbons is extremely low. If the carbon chain length exceeds 8, the autoinflammation temperature of a linear hydrocarbon is close to 200°C. All these parameters decrease with pressure. The table below shows to which extent pressure influences the AIT of ethylene ... 

In specific applications, it is critically important to know which isomer is produced in a particular situation in order to ascertain its further reactivity. Indeed, further reactivity, in the form of rate coefficients and product ion distributions, both identifies which reactions generate the same isomeric forms and gives information to enable the isomeric forms to be identified (often by determining the energetics and comparing them with theoretical calculations). One such application is to molecular synthesis in interstellar gas clouds. In the synthesis of the >115 molecules (mainly neutral -85%) detected in these clouds,14 a major production route is via the radiatively stabilized analog of the collisional association discussed above,15 viz. ... 

This would reveal possible routes to the C2H5CNH+ that is believed to be produced in the analogous radiative association reactions that occur in interstellar gas clouds. 

There has been a consistent motivation for the work presented in this chapter the application to molecular synthesis in interstellar gas clouds (see, for example, Herbst,22 this volume). The species in these regions are detected spectroscopically and are thus automatically isomerically identified. The routes to the observed neutral species consistently involve ion-molecule reactions followed by dissociative electron-ion recombination.18 The first step in this process is to determine whether an isomeric ion can be formed which is likely to recombine to an observed neutral species. The foregoing discussion has shown that whether this occurs depends on the detailed nature of the potential surface. Certainly, this only occurs in some of the cases studied. Much more understanding will be required before the needs of this application are fulfilled. 

Rotational spectra provide measurement of the moments of inertia of a chemical species. Bond angles and bond lengths can be derived by making isotopic substitutions and measuring the resulting changes in the moments of inertia. A major drawback of rotational spectroscopies is the limited information contained in a measurement of the moment of inertia. Consequently, while quite precise, it is generally limited to smaller molecules. It is the chief technique used to identify molecules in outer space, such as the components of interstellar gas clouds. 

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... 

The Sun formed some 4.5 Gyr ago (Gyr is a Gigayear or 109 years) from its own gas cloud called the solar nebula, which consisted of mainly hydrogen but also all of the heavier elements that are observed in the spectrum of the Sun. Similarly, the elemental abundance on the Earth and all of the planets was defined by the composition of the solar nebula and so was ultimately responsible for the molecular inventory necessary for life. The solar system formed from a slowly rotating nebula that contracted around the proto-sun, forming the system of planets called the solar system. Astronomers have recently discovered solar systems around... 

Doppler broadening allows for observation that some molecules in a gas cloud may be moving towards the observer and some away from the observer in a line of sight. 

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