Appendix B
Introduction to reliability analysis
In a risk analysis, we are concerned about the reliability of various systems, especially barrier and safety systems, and also equipment associated with production, for example, pumps and compressors in a processing plant. By reliability, we mean the ability of the system to function as planned. We express this ability using probabilities and expected values. A separate discipline, reliability analysis, has evolved for studying the reliability of such systems. In this appendix, we briefly summarise some important principles and methods used within this discipline. The reader is referred to Aven and Jensen (1999, 2013) for a more detailed coverage and relevant literature.
B.1 Reliability of systems composed of components
A system, for example, a gas detection system, is composed of 
components (detectors) connected in parallel, that is, the system functions so long as one of the components functions; see Figure B.1. The existence of a connection between 
and 
means that the system functions. Let 
represent the probability that component ...
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