by Keith Brumbaugh P.E., CFSE & Dave Grattan P.E., CFSE
Many operating units have a common reliability factor which is being overlooked or ignored during the design, engineering, and operation of high integrity Safety Instrumented Functions (SIFs). That is the Human Reliability Factor. In industry, there is an over focus on hardware reliability to the n’th decimal point when evaluating high integrity SIFs (such as SIL 3), all to the detriment of the human factors that could also affect the Independent Protection Layer (IPL). Most major accident hazards arise from human failure, not failure of hardware. If all that were needed to prevent process safety incidents is to improve hardware reliability of IPLs to some threshold, the frequency of near miss and actual incidents should have tailed off long ago - but it hasn’t. Evaluating the human impact on a Safety Instrumented Function requires performing a Human Factors Analysis. Human performance does not conform to standard methods of statistical uncertainty, but Human Reliability as a science has established quantitative limits of human performance. How do these limits affect what we can reasonably achieve with our high integrity SIFs? What is the uncertainty impacts introduced to our IPLs if we ignore these realities?
This paper will examine how we can incorporate quantitative Human Factors into a SIL analysis. Representative operating units at various stages of maturity in human factors analysis and the IEC/ ISA 61511 Safety Lifecycle will be examined. The authors will also share a checklist of the human factor considerations that should be taken into account when designing a SIF or writing a Functional Test Plan.