Achieving Safety Integrity Level (SIL) targets can be difficult when proof test intervals approach turnaround intervals of five years or more. However, some process units have planned and predictable unplanned shutdowns multiple times a year. During these shutdowns, it may be possible to document that the safety devices functioned properly. This can be incorporated into SIL verification calculations to show that performance targets can now be met without incorporating expensive fault tolerance, online testing schemes, etc. This can result in considerable cost savings for an operating unit.
This paper will discuss various solutions to meet a SIL target, taking credit for planned and unplanned shutdowns to help meet a SIL target, justification for applying diagnostic coverage in SIL verification calculations, summary of determining diagnostic credit, applying diagnostic credit from a shutdown event, and a case study.