The main regularities of the formation of shock air waves and air compression waves during combustion or detonation of fuel-air clouds are briefly considered in the article. Specific features are discussed related to the mathematical model of fuel-air mixture explosion, which is used in the Methodology for determining the calculated values of fire risk at the production facilities. The considered Methodology uses the same mathematical model as the corresponding Safety Guide on the assessment of the emergency explosion consequences approved by Rostechnadzor with some differences that are discussed in the article.
The approach of the Methodology is based on the consideration of one-dimensional pressure waves that are formed in the atmosphere during detonation or turbulent combustion of spherical uniformly mixed clouds. For such processes the approximate dependences of pressure on the distance are built. The validity, accuracy, as well as the field of application of such dependencies are considered in the work.
The conclusion is made about sufficient accuracy of the used dependencies and their good compliance to the current analogues and experiments, the provisions establishing the field of application of the extrapolation formulas for deflagration combustion are missing in the Methodology text. Due to this reason when calculating the situations are possible that at the same distance from the place of explosion of the clouds of fuel-air mixtures the pressure on the front of the detonation wave is significantly less (up to 7 times) than at the front of the deflagration waves. For deflagration combustion rates considered in the Methodology (up to 500 m/ s), this result is incorrect.
The reasons for this discrepancy are shown in the article.
In Russia the Methodology is approved as the only document on which basis the fire risk indices are calculated, which in turn are one of the main criteria for making managerial decisions in the design and construction of industrial and social objects. It is shown that considered in the article specific features of the numerical explosion models of fuel - air mixtures cloud lead not always to the justified increase of the conservatism of the obtained results. It is noted that the indicated conservatism can be deleted by introducing appropriate limitations on the field of application of the corresponding approximations.
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