SIL Determination for High Demand Safety Instrumented Functions in the Process Industry
ESC’s Dr Esteban Bernechea presented at the American Society of Safety Professionals – Middle East Chapter (ASSP-MEC) 13th Professional Development Conference and Exhibition in January 2019 in the Kingdom of Bahrain.
Dr Esteban’s presentation was on SIL Determination for High Demand Safety Instrumented Functions in the Process Industry and his full paper is now available for download below.
SIL Determination for High Demand Safety Instrumented Functions the Process Industry by Dr. Esteban Bernechea and Simon Burwood
Functional safety is one of the cornerstones of the safety strategy for Process Safety Management (PSM) program of any process plant. It is therefore of great importance that the lifecycle of Safety Instrumented Functions (SIFs), including specification, design, validation, commissioning and operation & maintenance, is in accordance to the requirements of the recognised functional safety standard: IEC 61508  or its sector-specific derivative.
SIFs are one of the layers of protection which are designed into process plants to prevent the occurrence of major hazard scenarios; this means that the failure or absence of a SIF increases the probability of a hazardous scenario occurring and escalating into a major accident. It is then necessary to estimate the probability of the SIF failing to act on a demand, in order for the users to determine if the target risk levels of their organization or country are being achieved.
Deterministic Safety Integrity Levels (SILs) have been developed to classify the SIFs depending on their probability of failure. However, the way in which the SIL is determined varies depending on how the SIF operates (the frequency with which it is demanded to act). Because most of the SIFs that are found on the process industry operate in low demand mode (based on the analysis of more than 10,000 SIFs by Engineering Safety Consultants Ltd. ), the literature regarding SIL Determination has been historically concerned with this type of function.
This paper demonstrates how SIL Determination can be carried out in order to achieve the requirements of IEC 61511  for SIFs operating in high or continuous demand modes; how causes of demand are to be treated, Independent Protection Layers (IPLs) and dependency issues, as well as risk targets and equations to perform the required calculations are discussed. This will allow readers to have a deeper understanding of the methodology used for performing SIL Determination studies for high / continuous demand mode SIFs.
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