The SIL performance parameter for Safety Instrumented Functions relates to a range of average Probability of Failure on Demand (PFDavg) values for low demand mode (or demand mode) applications and a range of Probability of a Dangerous Failure per Hour (PFH) values for high demand mode (or continuous mode) applications.
The reliability calculations that are performed as part of the SIL verification allow end-users to optimize their Safety Instrumented Function design, through technology, redundancy, and/or proof test interval selections. Many SIL certified devices have been designed to achieve a low dangerous undetected failure rate which allows for longer intervals between proof tests while still achieving a low PFDavg. The advantages are obvious, less process interruption, less maintenance work, lower lifecycle costs.
The extension of proof test intervals could however result in a SIF application being classified as a high / continuous demand mode application. This paper will explain the rules for classifying an application as low demand or high / continuous demand. It will explain how exida uses three different modes of operation and the motivation for this. Why does an application that seems to be a low demand application, all of a sudden need to be considered as a high demand application? Finally, the paper will also explain the differences between low demand and high demand applications and the consequences for the SIL verification.