Background and objective: To control and prevention of accidents, attention to root causes of accident occurrence is very important. Safety risk of process units located in metropolis, must be always under control and in accordance with risk acceptance criteria of the community. In this regard, the purpose of this study is identification of functional failures, root cause analysis and incident outcomes arising from gas release in the Town Border Stations (TBS).

  Materials and method: Using at the same time of both methods the Functional Hazard Analysis (FuHA) and Failure Mode & Effect Analysis (FMEA), identification of the failure locations and qualitative risk analysis were carried out. For identification and analysis of the causes accident, Bow tie analysis method was also used.

  Results: Occurrence probability of identified top events was 0.71 and its failure rate was 1.24 per year. Unsafe behavioral (FP=0.36, &lambda=0.446) and mechanical causes (FP= 0.133, &lambda= 0.142) had highest and lowest the contribution in the top event occurrence. Vapor cloud explosion (VCE) had the highest probability (0.261) and failure rate (0.243) among the all identified incidents outcomes.

  Conclusion: Prevention of the root causes and attention to the human factors have the considerable contribution in accident control in the process units. In the combined approach used in present study, if are considered to be, the barriers role against of the root causes and incident outcomes occurrence, it could be an appropriate approach for identification of root causes and control of the hazards process.