Earlier I mentioned Emerson’s Dean Taggart’s work with complex sequences in safety instrumented systems, based on an ongoing oil sands gasification project. John Kingston, from Emerson local business partner Spartan Controls, is presenting on this topic along with Emerson’s Chuck Miller at the upcoming ISA Expo 2007.
I received a copy of the submitted paper that, among other things, explores the separation between basic process control systems (BPCS) and safety instrumented systems (SIS). Historically, the SIS was a separate entity, but with technological advances, this has begun to change. The authors note that the IEC 61508 international safety standard does not provide a definition of separation. It does mention physical separation as a highly effective technique. Given that the standard is much more performance-based than prescriptive-based, they note that there are few statements defining separation.
The paper refers to a few specific clauses in 61508-1 such as 126.96.36.199, where when the control system places a demand on one of the safety-related systems, then it “…shall be separate and independent” from the safety-related systems. In order to satisfy this clause the control system must be proven sufficiently independent from the SIS. Certification agencies like the various TÜV organizations and other third-party testing labs help provide this proof for SIS suppliers per the IEC 61508 performance standards.
61508-1 Clause 188.8.131.52 addresses common cause failures by requiring functional diversity, technology diversity, diverse parts, services, and support, and that the BPCS and SIS not share common operational, maintenance, or test procedures, and that they be physically separated. Safety instrumented systems like DeltaV SIS address these in the authors’ words:
Those factors [governing independence] include diversity, which essentially means that the BPCS and SIS should have different components, operating systems, chip sets, central processing units, etc. When looking at sharing parts, services, and support systems, once must ensure that the BPCS and SIS have different power sources, and that a safety network dedicated to safety related communications is used. They should not share test procedures, which means that if you are testing either the BPCS or the SIS, that those tests should be able to be run completely independently of each other. Finally, physical separation applies to how the architecture of the system is laid out, and how cabinetry is designed; in essence, this is where one would look at separating DCS cabinets from SIS cabinets, and perhaps maintaining the SIS from a different workstation than the one used for the BPCS.
A final clause that is discussed, 61508-2 Clause 184.108.40.206 explores how non-safety functions implemented in an SIS need to be treated as safety-related unless it can be shown it is sufficiently independent (that the failure of any non-safety-related functions does not cause a dangerous failure of the safety-related functions.) This implies that control and safety functions can exist within the same system as long as sufficient care is taken in design and throughout the IEC 61511 safety lifecycle.
The authors summarize the implications of separation well:
Essentially, everything all boils down to good engineering designs and practices. One must consider the standards carefully, and understand the implications before going down a certain path. One cannot simply look at a system and know if it satisfies these requirements, because almost every system has a different level of independence. One must look at the specific details of a system to verify that it satisfies the requirements.
Dean summed up how these applied to the asphaltene gasification project:
The complexity of the process led to a need for integration as well as separation. Integration brings the benefits of integrated development and operating environments, less training cost, simpler architectures, faster and more reliable communications, reduced integration time, better handling of status information, and improved fault handling. The safety requirements of gasification focus on preventing damage to the burner, reactor, and syngas cooler, as well as operator safety. The process itself leads to the need for an intricate startup, as well as multiple methods of shutting down the process depending on the current state. An integrated but separate solution can provide several advantages while still providing the required amount of separation.