Can Functional Testing Be Replaced by Inspection After Demands?

Safety instrumented systems are used to reduce the risk of major accidents in the process industry. To claim a certain risk reduction from such systems, it is necessary to perform reliability analyses, within internationally accepted frameworks, such as the IEC 61511. The main factors that influence the reliability are the failure rates of the components, the level of redundancy of hardware and software, and the frequency of periodic functional tests. The need for regular periodic functional testing has always been a challenge for the industry, as the tests often mandate a shorter or longer stop of the production. For those safety instrumented systems that must respond to rather frequent demands, the industry has started to ask if it is possible to replace functional testing with thorough inspection after demands, and how information from responses to demands and the following inspection may be credited in the reliability analyses. In this paper, the authors discuss the main characteristics of such a ?testing strategy?, and highlight some of the cautions, challenges, and conditions of use