SIL Levels Apply to Safety Instrumented Functions

I caught up with Riyaz Ali who is in our organization managing Emerson’s Fisher brand of valves and regulators. You may recall Riyaz from some earlier posts on safety valve local control panels, partial-stroke test in safety applications, and testing safety solenoid valves.

Riyaz has been hearing more and more questions from process manufacturers, consultants, integrators, and other automation professionals about the adoption of the IEC 61508 and IEC 61511 international safety standards. These questions tend to get very specific about the safety integrity levels (SIL) for the components within the Safety Instrumented Function (safety loop.) Today all components of the safety instrumented function (SIF) including the logic solver, sensor, and final control element may have microprocessors that can perform self-diagnostics and communicate these diagnostics digitally to the logic solver.

Riyaz wanted to help clarify some questions on SIL ratings and field devices. If a process manufacturer hears that that field device is “SIL 3-rated” in accordance with IEC 61508, this is not the case. Field devices alone are not capable of a particular SIL rating.

These devices may be suitable for use in a SIL 3-rated safety instrumented function. In other words, this SIL rating applies to the entire loop and not the individual components within the loop.

The second key point Riyaz made with me is that a single microprocessor-based device (categorized as Type B in the IEC 61508 part 2, table 3) cannot have suitability for use in a SIL 3 safety instrumented function without additional hardware fault tolerance per these IEC standards.

Obviously, there is quite a bit to these safety standards and their application, and I hope some of these blog posts on the topic of safety help you in your adoption of these standards in your facilities.

Posted Friday, April 27th, 2007 under Safety.

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