(215b) User Approval of Safety Instrumented System Devices

While some manufacturers are now providing devices that are "certified for use" in safety instrumented systems (SIS), these devices are relatively limited in their range of application and technologies.

Many are not field proven and some are not demonstrating the robustness necessary to survive a chemical plant environment. It is also questionable how relevant these certifications are to the real world. Within an IEC 61508 analysis, an unreal operating environment is assumed: one that does not include the process impact, ambient conditions, stress, manufacturing defects, software errors, installations issues, electrical disturbances, instrument air or other supply system quality problems, etc. Programmable electronic systems are not immune, with some certifications assuming diagnostics coverage factors that are simply not achievable using current practices.

In general, manufacturer failure rate data is 3 to 10 times better for programmable devices than the actual performance observed in the process industry. For mechanical devices, the ratio is 30 to 100 times better. Consequently, owner/operators in the process sector must rely heavily upon prior use experience for selection of devices for SIS. This paper suggests a user approval process for these devices. User approval is a simple process that supports design, engineering, configuration management, and management of change. This paper provides a work process that builds upon the good engineering practices documented in ANSI/ISA 84.00.01-2004 and ANSI/ISA TR84.00.04.