Perfecting Proof Test Effectiveness; How More Extensive Testing Can Improve Safety and Reduce Cost!

We all know that a period functional test of the components in Safety Instrumented Functions is necessary to ensure the appropriate safety integrity is maintained. These tests on some of the most reliable equipment in a plant are occasionally disruptive and intrusive in the process. However, in general the necessity of the tests is acknowledged. The second edition of the IEC 61511 functional safety standard for the process industry sector identifies the need to take the proof test effectiveness into consideration. This proof test effectiveness indicates the percentage of undetected failures revealed during a proof test. This parameter can have quite an impact on the overall achieved safety integrity. At the same time the parameter shows that more comprehensive tests will positively influence the achieved safety integrity. This paper will discuss the concept of proof test effectiveness and through a practical example, using a floating ball valve, show how more extensive tests will improve safety and at the same time can reduce cost, e.g. maintenance cost due to lower proof test frequency requirements as well as equipment cost due to lower level of redundancy required.