Safety instrumented function (SIF) — emergency shutdown
A reliability block diagram of an IEC 61511 emergency-shutdown loop — 2-of-3 voting pressure transmitters, a redundant logic solver, and 1-out-of-2 shutdown valves. The classic sensor → logic → final-element SIF architecture, with the engine computing the loop reliability.
For the functional-safety engineer verifying a SIL target
What this shows
The canonical safety-instrumented-function architecture from IEC 61511, laid out as a series of three subsystems: sensing, logic, and final elements. Sensing uses 2-out-of-3 voting transmitters (tolerates one failure and rejects one spurious trip); the logic solver is a redundant pair; the final elements are two shutdown valves in a 1-out-of-2 (parallel) arrangement so either can close the line.
The reliability is computed across the loop. The engine evaluates the 2oo3 sensor group exactly, the two parallel pairs as 1 − (1−R)², and the series chain — giving a loop reliability of ≈ 0.9972. Because every subsystem is redundant, there's no single point of failure, and the importance ranking points at the valves (the lowest-reliability stage) as the place a SIL upgrade buys the most. The success-space companion to the fault tree a safety case also needs.