by Ron Nichols
Abstract: This paper discusses the identification, selection, implementation and management of Non-SIF IPLs through the process lifecycle.
Layers of Protection Analysis (LOPA) in conjunction with the Process Hazard Analysis (PHA) is now a key tool used by the chemical, oil and gas industries to assist companies in identifying, implementing and managing the critical safeguards needed to achieve their risk tolerance targets. The LOPA is used to identify the number of Independent Protection Layers (IPLs) and their integrity needed to reduce the likelihood to an acceptably low frequency that an initiating cause will progress to an undesired consequence.
Since the acceptance of ISA 84.00.01 / ISA/IEC 61511, the life cycle management of safety instrumented systems is now being implemented throughout industry. The required safety integrity level (dependability) for the safety instrumented functions (SIFs) are obtained by closing the LOPA gaps between the existing mitigated event likelihood (MEL) and the company’s target mitigated event likelihood (TMEL). Often a SIF is combined with non-SIF IPLs to achieve the risk reduction gap closure, reducing the SIL requirement assigned to that SIF.
To maintain acceptable risk targets, all IPLs, not just SIFs, must be managed through the lifecycle of the process. This is because many LOPA gaps are closed by only non-SIF IPLs the SIL assignment for many SIFs depend on the use of non-SIF IPLs used in that LOPA.