Using fault trees to analyze safety-instrumented systems

Safety-instrumented systems are protection functions frequently seen in automotive, chemical processing, and oil and gas refining systems. These functions are designed to engage in case a hazardous condition arises and mitigate any potentially catastrophic consequences. Because of the potential for loss of life or other safety-related risks related to these systems, safety-instrumented systems usually have a very strict reliability requirement. Fault Tree analysis is a method of analyzing a system to determine its availability and identify weak points. This method uses a qualitative and quantitative approach that graphically shows how component failures logically combine to create system failures, and quantifies the system failure probability using failure rate data from component failures. Due to its powerful and flexible nature, Fault Tree analysis is an ideal method for analyzing safety-instrumented systems to determine if they are meeting their reliability goals, to find weak points in the design, or for focusing maintenance efforts. Fault Trees may also be used to determine the spurious trip rate of the safety system, that is, how frequently the safety system will engage unnecessarily. This paper will provide a guide to using Fault Tree analysis software for these purposes.