Essentials of Safety Instrumented Systems
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Throughout the global process and energy industries, the safety instrumented system (SIS) plays an essential role in protecting workers and equipment as well as nearby communities and the environment from harm. Much has changed in the several decades since the first programmable systems for safety protection were developed and deployed, and today the discipline continues to evolve and advance in terms of both methodology and technology.
The key reference methodology that has emerged for managing safety instrumented systems over their entire lifecycle—from risk assessment through design, operations and maintenance—are the IEC’s 61508 and 61511 international standards. The standards originally were developed by industry for industry as technical standards.
But in some arenas, compliance with the standards already carries the force of law. And even in areas where they are not legislated, the standards’ growing acceptance as descriptors of best practices means that non-compliance may have very real liability implications if something does go wrong.
The standards themselves are purposely performance-based: they allow engineers the flexibility to meet industry and society’s safety expectations in more than one way. Indeed, from a technology perspective, today’s digital SIS options increasingly leverage integration and diagnostics to boost safety, availability and productivity even while reducing cost and complexity for end users. But the extent to which safety and control should be integrated or remain separate without compromising safety remains a subject of heated discussion.
For their part, many independent consultants take the side of the standards and the math: integration doesn’t necessarily compromise safety protections, but suppliers and their users need to adequately demonstrate that is indeed the case.
Further complicating the SIS landscape is the fact that many of industry’s installations
predate current standards, and verifying that older systems perform—and continue to perform—to standard is a significant undertaking. Indeed, many first and second generation installations are at or beyond the end of their serviceable lives and need to be migrated to more current technology.
Bottom line, the engineering of safety instrumented systems remains a complex and
subtle task. And once commissioned, both proactive work processes and ongoing corporate commitment are needed to assure that SIS protections do not degrade over time.