Plan for Safety System Success

The First Step in Achieving--or Restoring--the Performance of Your Plant's Safety Systems Begins With a Cold-Eyed Assessment of Their Current Capabilities. Only Then Can You Begin to Develop a Plan to Bring Them Back Up to Speed

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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

Separate Systems
Two Suppliers, Separate Systems

A system architecture based on completely separate basic process control systems (BPCS) and safety instrumented systems (SIS) from different suppliers, typically with a limited, OPC- or Modbus-based link between the two systems, was once the preferred way to incorporate safety systems into the overall plant automation scheme. Physical separation and different development teams helped to minimize common cause and systemic failures, but a custom interface between the two systems introduces an additional set of development and maintenance concerns. Further, different engineering tools and HMI methodologies increase complexity and training requirements as well as limiting operational visibility and synergy between the two systems.

Single Supplier
Single Supplier, Different Systems, High-level Integration

Another typical architecture is the result of an overall automation system delivery from a single supplier but with different BPCS and safety systems. Both systems are based on in-house products, but have been developed separately (or added to the product portfolio through acquisition) without any significant commonalities. The potential for common cause and systemic failures are addressed as with systems from different suppliers, but a common HMI and more rigorous connectivity will likely help operators be more effective and reduce interface maintenance costs. Engineering tools are likely to remains separate, however, allowing for little improvement with regard to training needs or productivity.

Single Supplier, Similar systems
Single Supplier, Similar Systems, Interfaced

Another system architecture option features similar systems from the same supplier, deployed as two separate systems for BPCS and safety functions. This approach requires that safeguards be in place on the supplier side to ensure that sources of potential common mode and systemic failures are identified, engineered out of the system design or otherwise addressed and managed. And while a similar set of engineering tools and operational displays will boost familiarity and reduce training costs, two separate systems—and all the management discipline involved with them—will need to be separately maintained, resulting in less than optimal engineering productivity. Also, since the two systems share a common heritage, the integrity of the communications link between the two systems should not be a concern.

Single Supplier, Integrated system
Single Supplier with Integrated System

The final architecture option is a fully integrated BPCS and safety system, designed from the ground up to simultaneously satisfy the requirements of both realms. This option is based on, in principle, common hardware and software but using diverse technology and implemented as one system. In addition to sharing the advantages of separate but similar BPCS and safety systems (with similar qualifications), the integrated solution can further leverage the commonalities between the two systems. Common engineering tools and HMI reduce engineering times as well as contribute to more effective operations while maintaining functional independence. This approach allows information management, asset management and production management to be operated across the entire automation platform.

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