Introduction. The authors have classified numerous publications, addressing the assignment of explosion and fire safety categories to premises, buildings and outdoor facilities, into the three groups: 1) sources of information that are in effect (including in-house and region-wide documents), sources that were in effect; 2) manuals and guidelines on category assignment; 3) publications that confirm (refute) or clarify some provisions, specified in regulatory sources. This article can be included into the third group of publications.Goal. Analysis of different methods, used to identify the value of Z factor; identification of strengths and weaknesses of each method, development of recommendations on the application of these methods.Objectives. The objective is to identify the substance-related factor contributing to explosions, use particular cases to demonstrate the efficiency of this or other identification method.Results and discussion. The analysis of Z factor identification methods, describing the contribution of vapours of highly flammable liquids to an explosion, has proven that three types of procedures can be used to find the Z factor value:the method of tables (that uses the maximal possible tabular value of Z = 1; for gases and aerosols Z = 0.5; for vapours of highly flammable liquids Z = 0.3);the computational method based on a pattern of three-dimensional gas and vapour spreading on the premises; however, this method, if applied, may involve a high probability of errors due to numerous conditions limiting its applicability; hence, the unexplainable value of Z may exceed 1. Besides, the computational method is extremely laborious. Its application requires the clarification of conditions for its use;the graphical method (based on the dependency graph of Z on the X parameter). This method is the simplest and the most reliable one. When the graphical method is used to find the value of Z, the excess oxidant ratio must be taken as being equal to one, and the Х parameter must be calculated according to the following formula: Х = 0.99 Рs.v/Сst.c.Conclusions. The graphical method, used to find the value of Z, is simple and reliable. When the Х parameter is identified, the excess air ratio is used: φ = 1.9, which leads to the underestimation of Z, the vapour-related factor contributing to explosions. To prevent the unreasonable underestimation of Z, the excess air ratio must be disregarded or taken as being equal to 0.99.
