Baker-Strehlow method

Application of the Baker-Strehlow method for evaluating blast effects from a vapor cloud explosion involves defining the energy of the explosion, calculating the scaled distance (/ ), then graphically reading the dimensionless peak pressure (Ps) and dimensionless specific impulse (i ). Equations (4.41) and (4.42) provide the means to calculate incident pressure and impulse based on the dimensionless terms. 

The energy term E must be defined to calculate energy-scaled standoff R. The energy term represents the sensible heat that is released by that portion of the cloud contributing to the blast wave. Any of the accepted methods of calculating vapor cloud explosive energy are applicable to the Baker-Strehlow method. These methods include ... 

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. 

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 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 Baker-Strehlow method requires a specification of the chemical reactivity, the obstacle density and the geometry. 

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 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. 

Various techniques determining loading from explosions (TNT equivalent, multienergy methods, Baker-Strehlow method and computational fluid dynamics) are available, mainly developed for hazard studies for chemical plants [24]. In the case of solid detonation, the TNT equivalent technique is the most widely used approach. In the case of a gas or vapour cloud, the elevation of the explosion and the reaction characteristics may suggest other approaches. 

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 ... 

Baker and his colleagues (1983) compared the Strehlow et al. (1979) curves to experimental data, then applied them in research programs, accident investigations, and predictive studies. They developed the methods for use of Strehlow s curves. 

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. 

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