A Consistent Analytical Method to Assess Reliability of Redundant Safety Instrumented Systems

The significance of determining Safety Integrity Level (SIL) has grown substantially in numerous industry sectors since the publication of international standard for functional safety (IEC61508). To detect SIL using IEC61508, the demand mode must first be recognised. However, there is not any 100% accurate method to identify the demand mode. Design engineers must assume the demand mode based on their assessment of the likely number of failures each year in the system under consideration, referred to as demand rate. This signifies that a defective demand mode assumption leads to an incorrect SIL determination. In addition, the formulas provided by the IEC standards for the SIL determination did not include the demand duration. The majority of prior publications in the literature relied on SIL determination using IEC61508. As a result, the literature suffers from the constraints of the IEC61508 standard, which include ignoring demand duration and uncertainty in assuming the system's demand mode for determining SIL. This paper addresses the aforementioned issues in literature through proposing a scenario based-formula to calculate Hazardous Event Frequency (HEF) using Markov process for safety system with dual redundancy. The HEF calculated can then be utilized to select SIL while considering the demand duration and obviating the requirement to use the SIS demand mode. The results demonstrated that the proposed method provides higher accuracy in calculating the HEF value compared to the existing traditional formulas.