TNT equivalent method

Example The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT equivalent method is used in this example other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any govern-mentaf unit requires a specific type of analysis. 

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. 

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. 

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 two approaches lead to completely different procedures for vapor cloud explosion hazard assessment. If conventional TNT-equivalency methods are applied, explosive potential is primarily determined by the amount of fuel present in a cloud, whether or not within flammability limits. The cloud center is the potential blast center and is determined by cloud drift. 

Conventional TNT-equivalency methods state a proportional relationship between the total quantity of flammable material released or present in the cloud (whether or not mixed within flammability limits) and an equivalent weight of TNT expressing the cloud s explosive power. The value of the proportionality factor—called TNT equivalency, yield factor, or efficiency factor—is directly deduced from damage patterns observed in a large number of major vapor cloud explosion incidents. Over the years, many authorities and companies have developed their own practices for estimating the quantity of flammable material in a cloud, as well as for prescribing values for equivalency, or yield factor. Hence, a survey of the literature reveals a variety of methods. 

To demonstrate the general procedure in applying TNT-equivalency methods in this work, one of the many methods, namely, that recommended by the UK Health Safety Executive (HSE 1979 HSE 1986), is followed. Note that this is only one of many variations on the basic TNT-equivalency method see Chapter 4 for a review of others. 

In each case, two different methods were used in arriving at estimates the HSE TNT-equivalency method and the multienergy method. The results, in the form of side-on blast peak overpressures for various distances from blast centers, are listed in Table 7.10. In addition, some peak overpressures estimated by Sadee et al. (1976/ 1977) from Flixborough-incident damage patterns are included. The photographs in Figures 7.6a and 7.6b illustrate the practical effects of such overpressures. 

The two methods gave considerably different results when applied to the liquid hydrocarbon storage site case study. The TNT-equivalency method systematically... 

TABLE 7.10. Results of TNT-Equivalency Method and Multienergy Method Applied to Two Case Studies... 

Blast overpressures calculated by the TNT-equivalency method are in reasonable agreement with the overpressures deduced from observed damage (Sadee et al. 1976/1977). This is to be expected, because the Flixborough incident is one of the major vapor cloud explosion incidents on which the TNT-equivalency value of... 

The total inventory of flammable material that could be released was determined, and the TNT equivalence method (from Reference 5) was applied. Using this information, an incident side-on overpressure of 3 psi at 150 ft (0.21 bar at 45 m) was calculated. On this basis, it was determined that the building could sustain the maximum anticipated blast overpressure, and no further evaluation was needed. 

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 ... 

In TNT equivalence methods, the equivalent weight of TNT representing the VCE, WTlNfT, is expressed as ... 

The TNT equivalency method also uses an overpressure curve that applies to point source detonations of TNT. Vapor cloud explosions (VCEs) are explosions that occur because of the release of flammable vapor over a large volume and are most commonly deflagrations. In addition, the method is unable to consider the effects of flame speed acceleration resulting from confinement. As a result, the overpressure curve for TNT tends to overpredict the overpressure near the VCE and to underpredict at distances away from the VCE. 

The advantage to the TNT equivalency method is that it is easy to apply because the calculations are simple. 

The procedure to estimate the damage associated with an explosion using the TNT equivalency method is as follows ... 

Another popular method to estimate overpressures is the Baker-Strehlow method. This method is based on a flame speed, which is selected based on three factors (1) the reactivity of the released material, (2) the flame expansion characteristics of the process unit (which relates to confinement and spatial configuration), and (3) the obstacle density within the process unit. A set of semi-empirical curves is used to determine the overpressure. A complete description of the procedure is provided by Baker et al.18 The TNO multi-energy and Baker-Strehlow methods are essentially equivalent, although the TNO method tends to predict a higher pressure in the near field and the Baker-Strehlow method tends to predict a higher pressure in the far field. Both methods require more information and detailed calculations than the TNT equivalency method. 

TNT Equivalency Method, overpressure calculations 3-1 1 TNT equivalent B-3 Transformation factors, SDOF 6-8—6-13... 

Use TNT equivalent method (Zheng et al. 2004, Joann 1995) to calculate the shockwave overpressure of vapor cloud explosion, and estimate TNT equivalent according to the following formula ... 

The relationship shown in Figure 7.42 is the basis for assessing the behavior of pressure waves using the TNT equivalent method. The effect of an explosion of any explosive substance or the bursting of a gas-filled pressure vessel is estimated on the basis of the effect of an explosion of an equivalent TNT mass. For flammable gases and liquids the equivalent TNT mass is determined from Table 7.27. For gas-filled pressure vessels, the stored energy is first computed from the vessel s pressure and volume ... 

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