Evaluating the Effectiveness of Safety Instrumented Systems Using Layer of Protection Analysis and Fault Tree Analysis in the Isomerization Unit of Isfahan Oil Refinery, Iran

Investigating the hazardous factors and points in process industries through risk management and evaluation for preventing accidents is particularly important. Oil and gas refineries have several risks such as fire, explosion, or the release of toxic substances, which might have catastrophic and irremediable consequences. Conducting safety integrity level (SIL) studies in the process industries for assessing safety status and reliability of complex engineering systems is of particular importance. This study aimed to investigate the effectiveness of safety instrumented systems (SIS) in the isomerization unit of the gasoline production complex of Isfahan oil refinery, Iran. In the first part of the study, the semi-quantitative method of layer of protection analysis (LOPA) was used to analyze the layers of protection and determine the required SIL of the system (target SIL). In the second part, using the fault tree analysis (FTA), the existing SIL of the system was verified. Findings: In all selected scenarios, the existing SIL of system is consistent with the required SIL (target SIL). The frequency of final reduced event corresponded to the acceptable risk standard of the refinery as well, which indicated the adequacy of the existing independent protection layers (IPLs). Accurate designing and using the state-of-the-art equipment lead to an acceptable level of safety in this process, and the system is working with a proper reliability