Primer on Safety Instrumented Systems and Process Safety

by | Jul 6, 2009 | Safety, Services, Consulting & Training

Jim Cahill

Chief Blogger, Editor

The Flow Control magazine website has a great Safety Instrumented Systems Primer interview with Emerson’s Mike Boudreaux. You may recall Mike and his views on process safety and safety instrumented systems in earlier posts.

If you’re not already steeped in the language of process safety with things like safety integrity levels (SIL), safety instrumented functions (SIF), IEC 61511, etc., the questions and answers help provide a good primer. I’ll share just a few snippets from the Q&A, but you’ll want to read the entire interview.

Mike addresses the question on how safety instrumented systems (SIS) have come to be:

Much of the focus has been to reduce process risk through inherently safe design and independent layers of protection (IPL). Safety instrumented systems are one of the many layers of protection that are used to deliver increased process safety.

Further on this point, he describes why an SIS is important:

When a process cannot practically be designed to be inherently safe, an SIS can be used to reduce risks to an acceptable level. An SIS can be designed to deliver a specified safety integrity level (SIL) of risk reduction. IEC 61508 defines SIL 1 through SIL 4, with each SIL designating a relative level of risk reduction provided by a safety instrumented function (SIF) by an additional order of magnitude.

For those new to the world of process safety, Mike also shares his view on common pitfalls in process safety SIS design and implementation:

When developing a safety requirements specification (SRS), process manufacturers sometimes go overboard and make the SRS too complex to be practical, or they go in the opposite direction and don’t provide a consistent set of documentation… the SRS should provide a functional description and the integrity requirements for each SIF. The SRS is the document against which all of the safety lifecycle activities are verified and validated. As such, it is important that this documentation be simple to use and maintain.

The other major pitfall is the complexity of SIF design and SIL verification. Mike offers:

Knowing which devices to use, selecting the appropriate hardware fault tolerance, correctly applying prior-use data, and designing the most economical SIF to minimize capital and operating costs while maximizing availability, can be a difficult task. End-users should make sure the people performing this work are competent in the area of process safety systems design and, more specifically, SIF design and SIL verification.

On advancements in SIS design strategy, Mike describes how technology is playing a key role:

Improved device diagnostics is being driven by technology advancements in microprocessors and device design. Diagnostics reduces the dangerous undetected failure rates for devices. Automated online proof testing and device diagnostics will deliver safer systems, because failures will be detected whenever they occur.

I hope you’ll get as much from the interview as I did.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.