Chapter 4 - SIF, SIS and SIL with SIL calculation

Abstract

The chapter starts with conceptual discussions on Safety instrumentation function (SIF) and Safety instrumentation system (SIS) with Safety integrity level (SIL) requirements. From safety functions, SIF discussions turn to the requirements for SIL, including various SIL classifications and performance-level discussions. As a part of basic discussions, safety barriers and SIS including SIF protection, SIF control functions, and SIS basic plant control system relations have been covered. SIS discussions include design feature, integrity, diagnostics features, along with coverage of systematic failures/common cause failures. Failure rate, reliability, availability, maintainability, and dependability are the main focus of discussions. As a part of reliability discussions, mean time between failure and mean time to repair issues also have been covered. Various components of reliability, reliability block diagram, and analysis concept are included in the chapter. As a part of discussions on SIS planning and design, SIS at different levels, such as field, logic solver, and actuator levels including integrated approaches have been covered. As a part of discussions on SIL, Probability of failure on demand (PFD), and probability of failure per hour with comparisons, focus has been put on safe failure fraction, hardware failure tolerance, and risk-reduction factor in relation to PFD. Different SIL types, along with risk reductions pertinent to different configurations for various PFDs have also been included. Detailed discussions on SIL calculations and SIL determination methods, including the direct calculation method and determination of the same with a risk matrix and risk graph, have been provided.