Chapter VII - Safety Instrumentation Functions and System (Including Fire and Gas System)

Abstract

Knowledge of safety instrumented systems (SISs) based on IEC 61508, discussed in the previous chapter, has been elaborated here for distinction among the terms SIS, safety instrument function (SIF), and safety integrity level (SIL), used interchangeably. Competency structure of SIS calls for a detailed discussion of reliability theory and relationship between the same and SIS. Bathtub curve and Weibull distributions are applied for SIS reliability calculations. There are various types of failure; random, systematic, and common cause failures in various instrumentation systems need special attention. Also, discussions on special requirements for sensors, logic solvers, and final elements are covered including short discussions on safe programmable logic controller. Redundancy and voting logic play an important role for naturally same needs to be treated. SIS is different for machinery system process plants and nuclear plants; accordingly these are treated separately. Fire and gas system (FGS) is an important issue in all plants so detailed discussions including special attention on philosophy document, detection technology, and area coverage have been included. Perspective and performance-based regulation for FGS needs special attention as opinion is divided. So, pros and cons of both have been discussed.