TNT models

UVe are modeled by equivalence of the flammable material to TNT by correlations with observed UVCEs (TNO model), or by computer modeling. Only the simple TNT model is discussed here. 

Previous studies of Vapor Cloud Explosions (VCE) have used a correlation between the mass of a gas in the cloud and equivalent mass of TNT to predict explosion overpressures. This was always thought to give conservative results, but recent research evidence indicates that this approach is not accurate to natural gas and air mixtures. The TNT models do not correlate well in the areas near to the point of ignition, and generally over estimate the level of overpressures in the near field. Experiments on methane explosions in "unconfined" areas have indicated a maximum overpressure of 0.2 bar (2.9 psio). This overpressure then decays with distance Therefore newer computer models have been generated to better simulate the effects... 

Empirical models, which are also called quasi-theoretical. They are based on a limited amount of experimental data and represent the simplest models for treating vapour cloud explosions. The TNT-model (vid. [2]), the TNO multienergy model [15] and the Baker-Strehlow model [50] belong to this group. 

Example 10.24 Effects of an explosion of hexogen according to the TNT model... 

The TNT model postulates a linear relationship between the mass of fuel and that of TNT in order to express the explosion potential of a gas cloud. This implies that the cloud is regarded as a homogeneous unit. This is normally is not true. Rather we have to expect turbulent zones (caused either by the initial momentum or by obstacles such as structural elements of the plant) and quiet zones. Whilst in turbulent zones an explosion may occur, in the quiet zones slow combustion without notable pressure rise is to be expected. 

The TNT model is well established for high explosives, but when applied to flammable vapor clouds it requires the c3q>losion yield, T), determined from past incidents. There are several physical differences between TNT detonations and VCE deflagrations that limit the theoretical validity. The TNO multi-energy method is directly correlated to incidents and has a defined efficiency term, but the user is required to specify a relative blast strength from 1 to 10. The Baker-Strehlow method uses flame speed data correlated with relative reactivity, obstacle density and geometry to replace the relative blast strength in the TNO method. Both methods produce relatively close results in examples worked. 

The virtual distance is then obtained by subtracting the geometrical distance between the centre and the external surface of the vessel (expanding gas initial surface), from the distance calculated by the TNT model to get the P overpressure. The proper distance to be used in the TNT equivalency method (AIChE/CCPS, 2000), is obtained adding the actual geometrical distance to the virtual distance. 

A principal parameter characterizing an explosion is the overpressure. Explosion effect modeling generally is based on TNT explosions to calculate the overpressure as a function of distance. Although the effect of a TNT explosion differs from that of a physical or a chemical explosion (particularly in the near-field), the TNT model is the most popular because a large data base exists for... 

An explosion model is used to predict the overpressure resulting from the explosion of a given mass of material. The overpressure is the pressure wave emanating from a explosion. The pressure wave creates most of the damage. The overpressure is calculated using a TNT equivalency technique. The result is dependent on the mass of material and the distance away from the explosion. Suitable correlations are available (2). A detailed discussion of source and consequence models may be found in References 2, 8, and 9. 

Equation 9.1-24 presents this model, where W is tilt weight of TNT, m,. is the weight of material... 

To allow for spray- and aerosol-formation, the mass of fuel in the cloud is assumed to be twice the theoretical flash of the amount of material released, so long as this quantity does not exceed the total amount of fuel available. Blast effects are modeled by means of TNT blast data according to Marshall (1976), while 1 bar is considered to be upper limit for the in-cloud overpressure (Figure 4.18). Because experience indicates that vapor clouds which are most likely to explode... 

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. 

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. 

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. 

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. 

Thorn K A, KR Kennedy (2002) N NMR investigation of the covalent binding of reduced TNT amines to soil humic acid, model compounds, and lignocellulose. Environ Sci Technol 36 3787-3796. 

The company uses the TNT equivalence method for screening purposes and the Baker-Strehlow methodology to model blast effects for more in-depth studies. The hazard classifications are as follows ... 

All mathematical models require some assumed data on the source of release for a material. These assumptions form the input data which is then easily placed into a mathematical equation. The assumed data is usually the size or rate of mass released, wind direction, etc. They cannot possibly take into account all the variables that might exist at the time of the incident. Unfortunately most of the mathematical equations are also still based on empirical studies, laboratory results or in some cases TNT explosion equivalents. Therefore they still need considerable verification with tests simulations before they can be fully accepted as valid. 

An application well-suited for IMS is the decommissioning and cleanup of sites where extensive manufacturing of explosives has taken place in the last century and where widespread contamination of soils and waters has occurred [74]. Decontamination of model metal scrap artificially contaminated with TNT and of decommissioned mortar rounds stiU containing explosives residue was followed by sampling surfaces with analysis by a portable mobility spectrometer. Mixed anaerobic microbial populations of bioslurries were employed in decontamination of scrap and the mortar rounds, and the IMS analyzer was seen as a sensitive field... 

BUSTER/TNT (Bricogne and Irvin, 1996) is another likelihood based refinement package that excels especially in cases in which the model is still severely incomplete (Blanc et al., 2004 Tronrud et al., 1987). It uses atomic parameters but also has a novel solvent and missing model envelope fimc-tion. The optimization method is a preconditioned conjugate gradient as implemented in the TNT package (Tronrud et al., 1987) that had a faithful audience in the pre-likelihood era. 

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