Vapor Cloud Explosion Blast Modeling

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). 

Over the years, this concept was refined in several ways. A scale dependency was modeled by the introduction of scale-dependent quenching of combustion. The first stage of the process was simulated by quasi-laminar flame propagation. In addition, three-dimensional versions of the code were developed (Hjertager 1985 Bakke 1986 Bakke and Hjertager 1987). Satisfactory agreement with experimental data was obtained. 

Appendix F is a case study by Hjertager et al. illustrating the above method. Such numerical methods will become more widely used in the long term. These techniques will probably remain research tools, rather than routine evaluation methods, until such time as available computing power and algorithm efficiency greatly increase. 

The concept of numerical simulation of turbulent premixed combustion in gas explosion has also been adopted by others  

The long list of vapor cloud explosion incidents indicates that the presence of a quantity of fuel constitutes a potential explosion hazard. If a quantity of flammable material is released, it will mix with air, and a flammable vapor cloud may result. If 

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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. 

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. 

A more deterministic estimate of a vapor cloud s blast-damage potential is possible only if the actual conditions within the cloud are considered. This is the starting point in the multienergy concept for vapor cloud explosion blast modeling (Van den Berg 1985). Harris and Wickens (1989) make use of this concept by suggesting that blast effects be modeled by applying a 20% TNT equivalency only to that portion of the vapor cloud which is partially confined and/or obstructed. 

Van den Berg, A. C., C. J. M. van Wingerden, and H. G. The. 1991. Vapor cloud explosion blast modeling. International Conference and Workshop on Modeling and Mitigation the Consequences of Accidental Releases of Hazardous Materials, May 21-24, 1991. New Orleans, USA. proceedings, pp. 543-562. 

Van den Berg, A. C., and A. Lannoy. 1993. Methods for Vapor Cloud Explosion Blast Modelling, Journal of Hazardous Materials, vol. 34, pp. 151-171. 

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. 

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

If, on the other hand, a vapor cloud s explosive potential is the starting point for, say, advanced design of blast-resistant structures, TNT blast may be a less than satisfactory model. In such cases, the blast wave s shape and positive-phase duration must be considered important parameters, so the use of a more realistic blast model may be required. A fuel-air charge blast model developed through the multienergy concept, as suggested by Van den Berg (1985), results in a more realistic representation of a vapor cloud explosion blast. 

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. 

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. 

TNT-equi valency methods express explosive potential of a vapor cloud in terms of a charge of TNT. TNT-blast characteristics are well known fiom empirical data both in the form of blast parameters (side-on peak overpressure and positive-phase duration) and of corresponding damage potential. Because the value of TNT-equiva-lency used for blast modeling is directly related to damage patterns observed in major vapor cloud explosion incidents, the TNT-blast model is attractive if overall damage potential of a vapor cloud is the only concern. 

Figure 6-24 Sachs-scaled overpressure and Sachs-scaled positive-phase duration for the TNO multi-energy blast model. Source Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs (New York American Institute of Chemical Engineers, 1994) used by permission.

These blast loadings have been widely used in the past for blast resistant design throughout the industry. However, many owners have developed specific blast loading criteria more in line with their specific circumstances. With advances in the modeling of vapor cloud explosions (Baker 1983, CCPS Explosion Guidelines), the trend is toward the use of VCE based blast loads. 

Words and phrases including process safety, vapor cloud explosions, HAZOPs, layers of protection, shelter-in-place, safety instramented systems, did not exist in nty early years. We initially looked at the technical reasons for failures. Other words and phrases like blast modeling, failure modes, gas dispersion were no doubt within specialist s language, but were not in the vocabulary of most of us at manufacturing sites. The approaches to process safety continued to evolve with more clarity and focus. 

For these and other purposes, blast-modeling methods are needed in order to quantify the potential explosive power of the fuel present in a particular setting. The potential explosive power of a vapor cloud can be expressed as an equivalent explosive charge whose blast characteristics, that is, the distribution of the blast-wave properties in the environment of the charge, are known. 

If, on the other hand, the multienergy model is employed, the total quantity of fuel present in a cloud is of minor importance. Instead, the environment is investigated with respect to potential blast-generative capabilities. Fuel-air jets and partially confined and/or obstructed areas are identified as sources of strong blast. The explosive power of a vapor cloud is determined primarily by the energy of fuel present in these blast-generating areas. 

The EPA guideline for evaluation of the far-field (distant) explosion hazards is based on a TNT [TriNitro Toluene] model. Evaluation of the near-field (within the vapor cloud, or near the explosion center) requires departure from the blast-pressure and blast-impulse curves for TNT or modification of the distance/quantity relationship [22]. 

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