Safety integrity level (SIL) versus full quantitative risk value

Abstract

Safety management of technical equipment is not possible without risk assessment. Therefore, many standards are available for risk assessment, e.g. ISO 13824:2009 General principles on risk assessment of systems involving structures or ISO/IEC 31010:2009 Risk management – Risk Assessment Techniques. In different industrial sectors risk assessment is fundamental step to determine required safety integrity level (SIL), eventually performance level (PL), which guarantees risk linked to some equipment on acceptable level. Standards applied for risk management based on SIL in different industrial sectors differ in methods used for risk evaluation and SIL determination. IEC 61508-5 accepts the use of qualitative, semi-quantitative or quantitative approach for risk evaluation and SIL determination. The standard uses hazardous event severity matrix as an example of qualitative approach for SIL determination, the standard furthermore uses layer of protection analysis (LOPA) as an example of semi-quantitative approach. The standard also uses Risk graph method as an example of both qualitative and semi-quantitative approach. IEC 62061 only presents one semi-quantitative approach for risk evaluation and SIL determination based on combination of probability and severity of consequences. This approach is different from the approach presented in IEC 61508-5. Similarly ISO 13849-1 recommends the use of qualitative method combining probability and severity of consequences for risk evaluation and PL determination, however, distinctly from IEC 61508-5 as well as IEC 62061. All these standards evaluate risk in the first step and in the second step they set safety systems reliability requirements, which should lower risk onto an acceptable level. The elemental question is, how exactly these standards evaluate risk in their methods. Another question is what acceptable level of risk is implicitly hidden in their requirements for choice of SIL and PL. This paper addresses these questions.