TNT equivalency model

The TNT equivalent model is often used as a simple method of estimating the mass of TNT per mass unit fuel gas whose detonation results in the same blast wave at the same distance. One kg of TNT translates into an energy of 4520 kJ. The equivalent for hydrogen is 2.22 kg TNT per Nm gas. The weakness of this model is to ignore the pressure-time characteristic differences between a gas cloud and a detonative TNT explosion. In the short range, the model overestimates the pressure. Furthermore the model does not take into account the influence of turbulence and confinement [113]. 

If the consequences of the vapour cloud explosion are to be assessed using the TNT equivalent model this is accounted for, contrary to an explosive, by a yield factor a < 1. Different experience-based values are quoted for the yield factor. The range lies between 0.0002 and 0.2 occasionally even higher values are given [15]. The differences show that the yield factor is a quantity which strongly depends on the (not foreseeable) boundary conditions of release and explosion. For practical calculations a value of a = 0.2 is recommended. It is the upper bound of the indicated region. The duration of the positive phase of a vapoiu cloud explosion (t in Fig. 10.31), which is the only one to be treated with the TNT equivalent model, amounts to 100-300 ms. 

It is obvious that smaller pressure values are obtained than with the TNT equivalent model, if a yield factor of 0.2 is used there. 

Figure 10.38 shows several of the preceding calculations comparing models and different situations of confinement. Whilst the latter cannot be accounted for by the TNT equivalent model, judging the degree of confinement introduces a subjective element into the treatment with the TNO and BST models. The curves show that this judgment can considerably influence the results, particularly in the near field. 

According to [66] the distance-dependent side-on peak overpressure in the far fleld (Sachs scaled distance according to Eq. (10.167) R > 2) can be calculated using the TNT equivalent model (vid. Sect. 10.6.3.1). For the near fleld the following relationship should be used (vid. [2])... 

Applying Eq. (10.176) to the near field, different expansion energies of the BLEVEs only affect the radius of the near field. In order to illustrate the different impacts, the results of the application of the TNT equivalent model (cf. Example 10.25) to the present problem are shown in Figs. 10.39 and 10.40. 

The distance-dependent pressure was calculated using the TNT equivalent model of Sect. 10.6.3.1. A yield factor of 20 % was assumed. The use of curve no. 7 of the multienergy mode, whose application is recommended in [80], does not lead to substantially different results. Pressure and conditional probability of death as functions of the distance on the ground are shown in Fig. 10.46. 

Explosion blast wave largely on the basis of observed explosions, TNT equivalent model (vid. Sect. 10.6.3.1)... 

The TNT equivalency model is based on the assumption of equivalence between the flammable material and TNT, factored by an etq>losion dency term ... 

The problem with the TNT equivalency model is that litde, if any, correlation exists between the quantity of combustion energy involved in a VCE and the equivalent weight of TNT required to model its blast effects. This result is clearly proven by the fact that, for quiescent clouds, both the scale and strength of a blast are unrelated to fiiel quantity present. These faaors are determined primarily by the size and nature of the partially confined and obstructed regions within the cloud. 

The TNT equivalent model requires the specification of the explosion efficiency. The TNO multi-energy method requires the specification of the degree of confinement and the specification of a relative blast strength. 

FIGURE 3.7. Logic diagram for the application of the TNT equivalency model. 

All of the methods (except the TNT equivalency) require an estimate of the vapor concentration— this can be difficult to determine in a congested process area. The TNT equivalency model is easy to use. In the TNT approach a mass of fuel and a corresponding explosion efficiency must be selected. A weakness is the substantial physical difference between TNT detonations and VCE deflagrations. The TNO and Baker-Strehlow methods arc based on interpretations of actual VCE incidents—these models require additional data on the plant geometry to determine the confinement volume. The TNO method requires an estimate of the blast strength while the Baker-Strehlow method requires an estimate of the flame speed. 

The largest potential error with the TNT equivalency model is the choice of an explosion efficiency. One needs to ensute that the yield corresponds with the correct mass of fuel. An efficiency range of 1-10% affects predicted distances to... 

Using the TNT equivalency model, calculate the distance to 5 psi overpressure (equivalent to heavy building damage) of an VCE of 10 short tons of propane. Data ... 

TNT equivalency model An explosion model based on the explosion of a thermodynamically equivalent mass of TNT. 

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. 

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. 

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 ... 

The CDC T-10 model tested in Belgium can treat complete chemical munitions up to 105-mm in diameter. A larger mobile unit (TC-25) was tested extensively at Porton Down, England (Blades et al., 2004) (see Figure 4-1). A still larger unit (TC-60) with an explosive capacity of 60 pounds of TNT-equivalent is now available (Bixler, 2005). It can handle munitions over 200 mm in diameter, according to the manufacturer. Table 4-1 provides the dimensions of the pressure chambers for the three CDC models. 

What, if any, are the scale-up requirements needed to implement the technology None. Available models can destroy projectiles up to 210 mm in diameter. None. DAVINCH has destroyed large Japanese recovered CWM (1 meter long, 0.2 meters diameter, 19 kg mustard agent/ lewisite agent mix). Volume of inner vessel is 30 times that of EDS-2 and explosive containment is 20 times EDS-2. None. May want to increase explosive containment capability beyond 5.1-lb TNT equivalent or increase physical size of detonation chamber beyond 2-meter diameter if need exists for greater capability. None, although there are size limitations on the types of munitions that can be destroyed. 

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