TABLE OF CONTENTS "When purchasing real estate, the three most important selection criteria are location, location, and location. When purchasing a Safety Instrumented System, the three most important selection criteria are diagnostics, diagnostics, and diagnostics."
Field devices include sensors, sensing lines, final control elements, field wiring, and other devices connected to the input/output terminals of the logic system. These devices are often the most critical and, probably, the most misunderstood and misapplied elements of safety systems. The emphasis paid toward field devices in the design and application of safety systems is disproportionally low compared to the potential impact that these devices can have on the overall system performance. It's estimated that approximately 90% of safety system problems can be attributed to field devices (see the analysis in Section 9.2).
Relays, solid state, and general-purpose programmable logic controllers (PLCs) have been used as safety system logic solvers. PLC's have the advantage of being software programmable, but they have very different dangerous failure mode splits compared to relays; therefore, their safety performance is very different. (These were the subjects of Chapters 7 and 8.) Many people (and naturally vendors) tended to focus on the difference between logic solver technologies. Since general-purpose and "safety" PLCs are now available with lower undetected dangerous failure rates and improved reliability, the focus is turning toward the field devices.
There are many publications available describing different types of field devices for various applications. The main objective of this chapter is to cover issues that need to be addressed...
