4 - Reliability and maintainability of safety instrumented system

Abstract

Safety instrumented system is the independent layer of protection. Safety instrumented systems have been used for many years to detect hazardous events, and to perform required safety instrumented functions (SIFs) in the process industries to maintain or bring the process back to a safe state. If instrumentation is to be effectively used for SIFs, it is essential that this instrumentation achieves certain minimum standards and performance levels. Safety instrumented systems are used in all process industries. It also requires a process hazard and risk assessment to be carried out to enable the specification for SISs to be derived. Other safety systems are only considered so that their contribution can be taken into account when considering the performance requirements for the SISs. The SIS includes all components and subsystems necessary to carry out the SIF, from sensor(s) to final element(s). To achieve the required function, reliability and maintainability is very important. The aim of this chapter is to design a reliable system and perform regular maintenance to sustain the achieved reliability. To achieve reliability, we have to calculate the average probability of failure on demand and while doing so we take into consideration the PFD value of all the components used in the safety instrumented function (SIF), the allowed spurious trip in a year and also the proof-test interval for testing the individual SIF in a SIS system. The failure of the SIS to achieve the desired function could result in huge consequences for the safety of the monitored system and also for the production availability due to spurious trips. The SIS system in a chemical plant is used to automatically stop the final element (valve or pump) and get the process under control. Fault-tree analysis (FTA) is widely used for identifying the root causes of undesired failures in a system. The traditional static fault trees with AND, OR gates cannot capture the dynamic behavior of system failure, such as sequence-dependent events, spares, and dynamic-redundancy management. In order to overcome this difficulty, the concept of dynamic Fault tree is introduced by adding sequential notion to the traditional FT approach. System failures can then depend on component failure order as well as combination. We have applied the dynamic fault tree concept in this chapter. In the traditional fault tree for the SIS system, we usually assume that exact failure probabilities of events are collected. We mean to say that during design or development stage we do the FTA and during that time we may add new component, which may not have failure date and there may be environment impact on this component during operation. In this chapter, we find the critical components in the SIS system based on FTA and determine the weak paths in the SIS system, where action should be taken to reduce failure.