Process safety is of major importance in chemical industry. Numerous activities have targeted the modification of conventional risk assessment strategies by computer aided approach. In this paper, a software tool for Hazard and Operability (HAZOP) study based on process simulation is presented. Individual components of the proposed software tool are described and the principal methodology of their function is explained. As the simulation engine, commercial process simulator Aspen HYSYS was employed. Proposed tool was applied to a case study of an ammonia synthesis plant based on an existing plant. Hazardous events and operability problems in the syngas purification unit and ammonia synthesis loop have been detected and reported. The steady state multiplicity phenomenon in the ammonia synthesis loop has also been successfully identified. Based on simulation data evaluation performed in the semi-automatic manner by the proposed tool, a HAZOP-like report containing HAZOP deviations and their causes and consequences was generated. (C) 2018 Elsevier Ltd. All rights reserved.