TNO multi-energy method

The TNO method identifies the confined volumes in a process, assigns a relative degree of confinement, and then determines the contribution to the overpressure from this confined volume (TNO is the Netherlands Organization for Applied Scientific Research). Semi-empirical curves are used to determine the overpressure. 

The basis for this model is that the energy of explosion depends highly on the level of congestion and depends less on the fuel in the cloud. 

The procedure for using the multi-energy model for a VCE is as follows 17 

Perform a dispersion model to determine the extent of the cloud. In general, this is done by assuming that equipment and buildings are not present, because of the limitations of dispersion modeling in congested areas. 

Conduct a field inspection to identify the congested areas. Normally, heavy vapors tend to move downhill. 

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The major problem with the application of the TNO multi-energy method is that the user must decide on the selection of a severity factor, based on the degree of confinement. Little guidance is provided for partial confinement geometries. Furthermore, it is not clear how the results from each blast strength should be combined. 

Consider the explosion of a propane-air vapor cloud confined beneath a storage tank. The tank is supported 1 m off the ground by concrete piles. The concentration of vapor in the cloud is assumed to be at stoichiometric concentrations. Assume a cloud volume of 2094 m3, confined below the tank, representing the volume underneath the tank. Determine the overpressure from this vapor cloud explosion at a distance of 100 m from the blast using the TNO multi-energy method. 

The heat of combustion of a stoichiometric hydrocarbon-air mixture is approximately 3.5 MJ/m3, and by multiplying by the confined volume, the resulting total energy is (2094 m3)(3.5 MJ/m3) = 7329 MJ. To apply the TNO multi-energy method, a blast strength of 7 is chosen. The Sachs-scaled energy is determined using Equation 6-25. The result is... 

This method is a modification of the original work by Strchlow et al. (1979), with added elements of the TNO multi-energy method. A complete description of the procedure is provided by Baker et al. (1994). 

Strehlow s spherical model was chosen because a curve is selected based on flame speed, which affords the opportunity to use empirical data in the selection. The procedures from the TNO multi-energy method were adopted for determination of the energy term. Specifically, confinement is the basis of the determination of the size of the flammable vapor cloud that contributes to the generation of the blast overpressure, and multiple blast sources can emanate firom a single release. 

The procedure for implementing the Bakcr-Strehlow method is similar to the TNO Multi-Energy method, with the exception that steps 4 and 5 are replaced by Table 3.3 and Figures 3.5 and 3.6. 

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 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 3.10. Spreadsheet output for Bcample 21a TNO multi-energy method. 

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. 

Calculation of blast overpressure parameters There are three major methods in use today. One is the TNT Equivalency Methbd which gives inaccurate results for vapor cloud explosions. The other two methods are the Strehlow Curves from Baker 1983 and the Multi-Energy Method from TNO 1985. Both provide a family of curves based on flame speed or explosion strength. These curves are used to select dimensionless parameters which are then unsealed to determine the actual overpressures. 

TNO 1985, "The Multi-Energy Method - A Framework for Vapor Cloud Blast Prediction", A. C. Van Den Berg, Journal of Hazardous Materials, Vol. 12, No. 1, Elsevier Science Publications, Amsterdam, The Netherlands, September, 1985, pp 1-10... 

The Multi-Energy method developed by TNO and coded in REAGAS [117] considers a vapor cloud explosion to be composed of a number of sub-explosions. Portions of the gas cloud which are confined up to a certain extent, are correlated with an initial... 

The TNT equivalence, TNO multi-energy and Baker-Strehlow methods require the mass of flammable material in the vapor cloud, and the lower heat of combustion for the vapor. 

The TNT, TNO multi-energy and Bakcr-Strehlow methods are simplified approaches. A further simplification would be to use the initial vapor cloud mass as input without applying a dispersion model, but this mi t overestimate cloud size after it drifts to an ignition source. 

The complete results of the procedure, as a function of distance, are shown in Figure 3.11. For this example problem the TNO multi-energy and the Baker-Strehlow methods produce similar results. Based on the uncertainty inherent in these models, the results are essentially identical. 

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