Today, the bid specifications for more and more new plants include not only compliance with the IEC 61508/61511 standards but also "integrated safety" as a base requirement. While at first blush this contradicts long industry practice of ensuring diversity by physically separating safety systems from basic process control systems, new technology together with users' desire to reduce costs and improve productivity are fueling an industry-wide movement to integrated systems.
Integration, or at least "interfacing," of safety instrumented systems with basic process control systems is in fact not a new practice. Indeed, the IEC standards' non-prescriptive language doesn't rule out even the physical integration of control and safety in the same box or on the same network. Rather, the standards assert that functional safety cannot be compromised by a failure or by maintenance activities associated with the basic process control system.
Diagnostics technology, meanwhile, has advanced in its ability to intercept dangerous faults, and some of today's integrated safety alternatives feature embedded diversity in hardware and software that reach all the way back to separate development teams. As a result, some of today's integrated safety system options can meet demanding SIL 3 application requirements even without the use of hardware redundancy.
"Process safety systems suppliers continue to cost reduce their hardware offerings and integrate their safety solutions with basic process control systems," wrote Barry Young, principal analyst for the ARC Advisory Group in a recent report on the global safety systems market. "Suppliers offering a truly integrated offering of process and safety are saving end users substantial project costs in engineering and lifecycle expense," he said.
With current technology a range of separate, interfaced or integrated solutions are possible among process control system and safety system suppliers (see "Integrated Safety: The Four Architecture Options" below). The most highly evolved option—an integrated platform from a single supplier that is designed from the ground up to perform both safety and control functions—is typified by the ABB System 800xA process automation platform.
Integrated Safety: The Four Architecture Options