Spurious activation analysis of safety-instrumented systems

Safety-instrumented systems are used in industries to prevent the development of a process upset into an accident. For most processes, the desired response in the case of a process upset is to shutdown the process, and most safety-instrumented systems are designed so that this state is achieved in response to also specific item failures or loss of power. The side-effect of such fail-safe design may be that the safety-instrumented system is prone to spurious activation, meaning that the normal operation of the process may be interrupted in an untimely manner. In the design of a safety-instrumented system, it is therefore important to quantify the rate of spurious activation and to check the need for additional measures to ensure a stable as well as safe operation of the process. Unfortunately, weaknesses have been identified in formulas for spurious trip rate, and the aim of this paper is to present a further development of currently available analytical formulas. The paper builds the new formulas on a thorough discussion of the concepts of spurious activation, failure classification, and failure propagation in a safety-instrumented system. The proposed formulas are compared with existing ones for selected architectures, and some conclusions are drawn.