DEFINING THE MAXIMAL UNDESIRABLE EVENT IN AMMONIA PRODUCING PLANT RISK ANALYSIS

Abstract

there is a known major consequence from possible causes (incidents). In a simple process, it is not difficult to select the maximal undesirable event (MUE), but for a complicated process involving multiphase equilibrium systems and numerous interacting plant units, it is an independent task to determine the MUE. The form of accident (explosion, fire, poisoning, and so on) and the scale are dependent on the plant state, hazardous-component distributions, and process parameters, i.e., on a set of in general stochastic factors that cannot be characterized unambiguously. It is not always possible to determine the MUE from statistical data on characteristic accidents for the sector, since accidents are quite rare, and are usually such that the statistics are not representative. The main criterion for an MUE, which corresponds to an accident in the chemical process, is provided by the form and content of the hazardous substance in some part of the equipment [2]. It is proposed to use the following to determine the distribution of hazardous substances over the equipment: • calculations on the material balance for cases in which the events that initiate accidents do not cause changes in the working parameters; • the determination of characteristics that displace the equilibrium towards the initial substances when there are deviations in working conditions. Basic Hazards in Ammonia Plant. Undesirable events in a plant for making ammonia differ one from another in origin, mechanism, effects on staff, and those on the equipment and environment, in addition to differences in the possible scale of losses. Identifying mechanisms on the basis of cause-effect logic is based on a detailed description of the process [3]. Research on ammonia production shows as follows: • the raw materials, intermediates, and final products are chemically stable substances and are not subject to spontaneous exothermic changes in composition even at very high temperatures and pressures; • neutral (noncombustible) gases such as nitrogen, carbon dioxide, and air are mainly at high pressures in large volumes. An explosion in equipment containing these gases involves the formation and dispersal of fragments. However, in world practice of operating ammonia plants, there have been no cases where the pressure in the equipment has risen above the yield point of the steel from which it is made [4]; • there are two groups of equipment in making ammonia (Table 1); • failure of or damage to the equipment leads to unplanned shutdown by the automatic systems (group 1); and • equipment failure leads to some form of accident (group 2). Chemical and Petroleum Engineering, Vol. 39, Nos. 3–4, 2003