Safety Instrumented Function-Focused Approach

by | Jul 8, 2010 | Safety, Services, Consulting & Training | 0 comments

Update and Bump: Here’s information on Russell’s and Andy’s UK Safety Seminar Series.

Emerson process safety experts Russell Cockman and Andy Crosland recently presented at the Institution of Chemical Engineers (IChemE) North West Members Group in the United Kingdom. The topic of their presentation was SIL Determination: an Integrator’s Perspectives.

IEC 61511 Process Safety Lifecycle Diagram
From the IEC 61511 safety lifecycle, Russell and Andy noted that the typical scope for an SIS integrator is the implementation phase–performing the design and engineering of the safety instrumented system (SIS). This phase will ideally occur after the safety requirement specifications (SRS) have been established in the analysis phase of the IEC 61511 safety lifecycle.

Per IEC 61511, the SRS provides requirements for the required safety instrumented functions (SIFs) and their associated safety integrity. At the most basic level, the SRS provides a description of all of the safety instrumented functions that are necessary to achieve the required functional safety. IEC 61511-1, Clause 10.3 provides a list of information that should be provided by an SRS.

Russell and Andy pointed out that one of the biggest challenges for managing the safety lifecycle is that the IEC 61511 lifecycle is seemingly idealistic when it comes to the sequential progress of the safety lifecycle phases. In reality, most projects are managed in such a way that the SIS implementation phase overlaps with the analysis phase, in order to optimize the project schedule. The hazard and risk assessment, layer of protection analysis (LOPA), and safety requirements specification activities often occur in parallel of the SIS Design & Engineering activity. Sometimes, project schedules force progress in such a way that safety loops are designed before the SIL is even specified. This leads to increased cost and complexity because designs are go overboard.

Additionally, without a structured process for identifying safety functions, anything safety related ends up going into the safety system. This leads into poor separation of the safety and control systems, since control functions will end up being implemented in the safety instrumented system instead of the BPCS where they actually belong. The SIS becomes overloaded with non-SIS functions and overblown SIL ratings to compensate for poor specifications.

They stress that the process engineer should compile the SRS since the SIS cannot be properly validated without it. The SRS should be the primary source of requirements for all design and selection information, and the SRS is what the SIS is validated against before any hazards are introduced to the process. Key for a successful and efficient process is to get the SRS completed early in the process. Also by taking a SIF-focused approach, it narrows the focus on just the safety related parts, becomes a single information source, confirms design documentation current revisions, and drives SIF design and verification.

They conclude their presentation by pointing out that there are major benefits to be gained by taking this kind of approach to safety lifecycle management. Because the SIS will be designed to deliver the functional safety that is required by the process, the resulting design will be much less conservative, less costly, and less complicated. This can reduce CAPEX cost by 20% and the reduced complexity will drive OPEX down by 25% because maintaining the SIS will require less work. Beyond cost, another important benefit is easier standards compliance. The simplest solution is typically the safest solution, and you can’t do SIL verification or SIS validation without specifying the requirements for SIFs. It just makes good sense to manage projects this way for improved safety.

I’ll save their experiences and guidance on SIS validation for a future post.

Russell and Andy are doing a series of Safety Instrumented Seminars in the UK over the next few weeks. If you’re in the UK and involved in process safety, you might want to catch one of their sessions and hear their recommendations and experiences first hand. I’ll update this post with a link to the schedule once it’s published.Here is the schedule for their UK Safety Seminar Series.


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