Vapor cloud explosion models

TABLE 3.5. Confinement Considerations for the Baker-Strehlow Vapor Cloud Explosion Model (Baker, 1996)... 

Example 21b Baker-Strehkw Vapor Cloud Explosion Model Input Data ... 

FIGURE 3.12. Spreadsheet output for Example 21 b Baker-Strehlow vapor cloud explosion model. 

Van den Berg, A. C., C. J. M. van Wingerden, J. P. Zeeuwen, and H. J. Pasman (1987). Current Research atTNO on Vapor Cloud Explosion Modeling. Conference on Vapor Cloud Modeling. Nov. 2-4, Cambridge, MA. New York American Instimte of Chemical Engineers. 

Gas dispersion models provided the toxic effects of chemical releases, fire, or unconfined vapor cloud explosion. 

Chapters 7, 8, and 9 demonstrate the consequence modeling techniques for vapor cloud explosions, BLEVEs, and flash fires, respectively, by presenting sample problems. These problems contain sufficient detail to allow an engineer to use the methods presented to evaluate specific hazards. 

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. 

Making a detailed estimate of the full loading of an object by a blast wave is only possible by use of multidimensional gas-dynamic codes such as BLAST (Van den Berg 1990). However, if the problem is sufficiently simplified, analytic methods may do as well. For such methods, it is sufficient to describe the blast wave somewhere in the field in terms of the side-on peak overpressure and the positive-phase duration. Blast models used for vapor cloud explosion blast modeling (Section 4.3) give the distribution of these blast parameters in the explosion s vicinity. 

A similar acoustic technique was applied by Pickles and Bittleston (1983) to investigate blast produced by an elongated, or cigar-shaped, cloud. The cloud was modeled as an ellipsoid with an aspect ratio of 10. The explosion was simulated by a continuous distribution of volume sources along the main axis with a strength proportional to the local cross-sectional area of the ellipsoid. The blast produced by such a vapor cloud explosion was shown to be highly directional along the main axis. 

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. 

Flgura 4.13. Contributing acoustic signals superimposed on distributed-voiume source model for a pancake-shaped vapor cloud explosion. 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

Given a certain release of a given fuel, the procedure of vapor cloud explosion blast modeling according to HSE can be subdivided into a number of successive steps ... 

One of the complicating factors in the use of a TNT-blast model for vapor cloud explosion blast modeling is the effect of distance on the TNT equivalency observed in actual incidents. Properly speaking, TNT blast characteristics do not correspond with gas explosion blast. That is, far-field gas explosion blast effects must be represented by much heavier TNT charges than intermediate distances. 

Vapor cloud explosion blast models presented so far have not addressed a major feature of gas explosions, namely, variability in blast strength. Furthermore, TNT blast characteristics do not correspond well to those of gas-explosion blasts, as evidenced by the influence of distance on TNT equivalency observed in vapor cloud explosion blasts. 

This approach makes it possible to model a vapor cloud explosion blast by consideration of the two major characteristics of such a blast. These are, first, its scale, as determined by the amount of combustion energy involved and, second, its initial strength, as determined by combustion rate in the explosion process. 

Blast effects can be represented by a number of blast models. Generally, blast effects from vapor cloud explosions are directional. Such effects, however, cannot be modeled without conducting detailed numerical simulations of phenomena. If simplifying assumptions are made, that is, the idealized, symmetrical representation of blast effects, the computational burden is eased. An idealized gas-explosion blast model was generated by computation results are represented in Figure 4.24. Steady flame-speed gas explosions were numerically simulated with the BLAST-code (Van den Berg 1980), and their blast effects were calculated. 

The procedure for employing the multienergy concept to model vapor cloud explosion blast can be divided into the following steps ... 

Explosively Dispersed Vapor Cloud Explosions (Giesbrecht et al. 1981). The Giesbrecht et al. (1981) model is based on a series of small-scale experiments in which vessels of various sizes (0.226-10001) containing propylene were ruptured. (See Section 4.1.2, especially Figure 4.5.) Flame speed, maximum overpressure, and positive-phase duration observed in explosively dispersed clouds are represented as a function of fuel mass. 

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