Management of High-Demand Safety Instrumented Functions

This whitepaper discusses the management of safety instrumented functions (SIFs) classified as high-demand mode, focusing on the guidelines provided by the IEC 61511 standard. It outlines the distinctions between low-demand, high-demand, and continuous modes of operation for SIFs, emphasizing the importance of understanding failure probabilities and system reliability in safety-critical industries. The paper details the choice of reliability measures, specifically the average Probability of Failure on Demand (PFD) and the average Probability of Failure per Hour (PFH), and the implications of selecting one over the other for system design and operational follow-up. Furthermore, it explains the SIL allocation process for determining the maximum tolerated PFD or PFH based on risk acceptance criteria and discusses how demand rates influence SIL requirements. The paper also highlights the impact of these choices on follow-up practices, including testing intervals, and suggests the need for industry guidelines to standardize practices in managing high-demand SIFs.