Abstract
The present paper introduces a new approach of advanced-FTA (a-FTA), which is applicable to sequential failure logic (SFL), to study how to combine risk analysis and allocation of safety-integrity levels (SILs) to safety-related systems taking a batch-process plant as an example. Fault tree diagrams for the hazardous event resulting from the loss of the safety functions are developed taking common cause failures into account. The relationships among failures of the subsystems, demands to activate the safety functions and the hazardous event are studied. It is found that the SFL governs the system failures. The top event frequency is estimated using conventional FTA (c-FTA) and a-FTA. The former might overestimate the top event frequency nearly ten times higher than those obtained with aids of a-FTA. Furthermore, it is demonstrated that the SILs obtained through a-FTA conform to those resulting from the architectural constraints on hardware safety integrity required by IEC 61508.
