November 2009

Special Report: Plant Safety and Environment

Avoid confusion when performing safety integrity levels

Here's how to differentiate safety instrumented fuction demand modes

Cheddie, H., Cteris Consulting Inc.; Khalil, Y. A., Zakum Development Company

  Safety integrity levels (SIL) are allocated to safety instrumented functions (SIFs) to establish how the said functions have to be realized, operated, maintained and modified. It is a lifecycle parameter. Two modes of operation have been defined for SIFs, i.e., low demand mode of operation and high demand or continuous mode of operation. In the low demand mode each SIL relates to an average probability of failure on demand (PFDavg) range, while in the high or the continuous demand mode a SIL relates to the probability of dangerous failure per hour (PFH) range. The relation between a SIL and PFDavg and their correspondent risk reduction factor is displayed in Table 1 for low demand

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