The International Standards for Functional Safety (IEC 61508 and IEC 61511) are well recognised and have been adopted globally in many of the industrialised countries during the past 10 years or so. Conformance with these standards involves determination of the requirements for instrumented risk reduction measures, described in terms of a safety integrity level (SIL). During this period within the process sector, Layer of Protection Analysis (LOPA) has become the most widely used approach for the determination of the required risk reduction and the appropriate safety integrity level (SIL) for the safety instrumented functions. One significant area for consideration is the application of this technique to certain hazardous events. Experience has identified that there is a type of hazardous scenario that occurs within the process sector that is not well recognised by practitioners, and is therefore not duly handled by the standard LOPA approach. This is when the particular scenario places a high demand rate on the required safety instrumented function. This paper will describe how to recognise a high demand rate scenario. It will discuss what the standards have to say about high demand rates. It will then demonstrate how to assess this type of situation and provide a case study example to illustrate how to determine the necessary integrity level. It will conclude by explaining why it is important to treat high demand rate situations in this way and the resulting benefit of a lower but sufficient required integrity level.
