After reading about the “10 Truths of Safety Instrumented Systems” in a ControlGlobal.com email (Control Magazine’s online website), and getting a copy, I’d spoken to our of our safety experts, Andrew Dennant, and was working up an analysis of the 10 truths…
Andrew and I will add just one point to the discussion underway.
When thinking about availability, consider the entire safety loop, not just the logic solvers, since the majority of failures occur outside the logic solver. An analysis of the data sources like the Offshore Reliability Database (OREDA) and exida’s Safety Equipment Reliability Handbook shows that up to 92% of hardware failures happen in the field and only 8% happen in the logic solver. Studies by the Health and Safety Executive in the UK show that less than 15% of all failures are hardware-related. Doing the math, 15% x 8% = 1.2%, puts us in the range of failures that are caused by the logic solver.
The key to high process availability is having a smart SIS which diagnoses the complete safety function including the logic solver, sensor, and final control element, correctly engineered in accordance with globally-agreed best practice safety standards, aka IEC 61511.
UPDATE: The figures cited in the Health and Safety Executive study are from the publication, Out of Control: Why control systems go wrong and how to prevent failure (2nd edition), orderable from the HSE site.