Explosion/blast

There are few problems of praetleal interest that ean be adequately approximated by one-dimensional simulations. As an example of sueh, eertain explosive blast problems are eoneerned with shoek attenuation and residual material stresses in nominally homogeneous media, and these ean be modeled as one-dimensional spherieally symmetrie problems. Simulations of planar impaet experiments, designed to produee uniaxial strain loading eonditions on a material sample, are also appropriately modeled with one-dimensional analysis teehniques. In faet, the prineipal use of one-dimensional eodes for the eomputational analyst is in the simulation of planar Impaet experiments for... 

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. 

In the experiments described in Section 4.1, no explosive blast-generating combustion was observed if initially quiescent and fully unconhned fuel-air mixtures were ignited by low-energy ignition sources. Experimental data also indicate that turbulence is the governing factor in blast generation and that it may intensify combustion to the level that will result in an explosion. 

Methods for vapor cloud explosion blast prediction based on TNT equivalency are widely used. Over the years, many authors, companies, and authorities have developed their own procedures and recommendations with respect to issues surrounding such predictions. Some of the differences in these procedures include the following ... 

The TNT blast data used A substantial scatter in the experimental data on high-explosive blast can be observed which is due to differences in experimental setup. Although often referenced differently, most recommendations can be tracked back to ground burst data developed by Kingery and Pannill (1964). 

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. 

These TNT equivalencies should be used in combination with high-explosive blast data by Baker (1973). Instead of graphical representation, Prugh (1987) recommends the use of simple equations which relate basic blast parameters to distance from the explosion center. These expressions can be readily implemented in a spreadsheet on a personal computer. 

On the basis of an extended experimental program described in Section 4.1.3, Harris and Wickens (1989) concluded that overpressure effects produced by vapor cloud explosions are largely determined by the combustion which develops only in the congested/obstructed areas in the cloud. For natural gas, these conclusions were used to develop an improved TNT-equivalency method for the prediction of vapor cloud explosion blast. This approach is no longer based on the entire mass of flammable material released, but on the mass of material that can be contained in stoichiometric proportions in any severely congested region of the cloud. 

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 ... 

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. 

TNT blast is, however, a poor model for a gas explosion blast. In particular, the shape and positive-phase duration of blast waves induced by gas explosions are poorly represented by TNT blast. Nevertheless, TNT-equivalency methods are satisfactory, so long as far-field damage potential is the major concern. 

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. 

Van den Berg, A. C. 1985. The Multi-Energy method—A framework for vajxir cloud explosion blast prediction. J. of Haz. Mat. 12 1-10. 

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. 

TNT Equivalence. Explosion strength is often expressed as equivalent mass of TNT in order to permit estimates of possible explosion damage. For BLEVEs and pressure vessel bursts, using this equivalence is unnecessary because the methods mentioned above give explosion blast parameters which relate directly to the amount of possible damage potential. However, the concept of TNT equivalence is still useful because it appeals to those who seldom deal with blast parameters. For reasons explained in Section 4.3.1, BLEVEs or pressure vessel bursts catuiot readily be compared to explosions of TNT (or other high explosives). Only the main points are repeated here. 

In the first approach, a vapor cloud s potential explosive power is proportionally related to the total quantity of fuel present in the cloud, whether or not it is within flammable limits. This approach is the basis of conventional TNT-equivalency methods, in which the explosive power of a vapor cloud is expressed as an energetically equivalent charge of TNT located in the cloud s center. The value of the proportionality factor, that is, TNT equivalency, is deduced from damage patterns observed in a large number of vapor cloud explosion incidents. Consequently, vapor cloud explosion-blast hazard assessment on the basis of TNT equivalency may have limited utility. 

The second approach, the multienergy method (Van den Berg 1985) reflects current consensus that turbulence is the major cause of explosive, blast-generating... 

If, on the other hand, blast modeling is a starting point for structural analysis, the TNT-blast model is less satisfactory because TNT blast and gas explosion blast differ substantially. Whereas a TNT charge produces a shock wave of very high amplitude and short duration, a gas explosion produces a blast wave, sometimes shockless, of lower amplitude and longer duration. In structural analysis, wave shape and positive-phase duration are important parameters these can be more effectively predicted by techniques such as the multienergy method. 

The blast originating from a hemispherical fuel-air charge is more like a gas explosion blast in wave amplitude, shape, and duration. Unlike TNT blast, blast effects from gas explosions are not determined by a charge weight or size only. In addition, an initial blast strength of the blast must be specified. The initial strength of a gas-explosion blast is variable and depends on intensity of the combustion process in the gas explosion in question. 

Condensed phase liigh explosives Blasting Miliuiry... 

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