Safety Instrumented Systems (SIS) have been used for a long time in process plants. As the name suggests these are the controls that operate critical (and expensive) pieces of equipment in a plant. Any deviation in operation would lead to a catastrophic event and having reliable controls is a must. An instrument must have a Safety Integrity Level (SIL) rating to be used in these applications. Most of the time, triple redundancy is the strategy used.
Historically, SIL rated equipment was quite expensive. Because of the design and certification requirements, they were too expensive to be considered for other process controls. People realized that SIL rated products were more reliable, but the opportunity cost was too great.
SIL rated controls are becoming much more common and much of the premium has gone away. With improvements in manufacturing, standard products have a much higher Safety Factor Fraction (SFF). This is the method used to determine the SIL rating. The cost of obtaining a SIL rating can now be spread over an entire product family, rather than just equipment used in SIS applications. This is especially true in process transmitters.
A perfect example of this is the Partial Stroke Test (PST) on a control valve. For a proper SIS installation, a PST must be performed regularly to verify the valve will operate. During this process it was observed that a valve signature could be obtained. The signature charts actuator pressure, set point and position. Generally, a valve signature for standard control valves was a complete cycle (close-open-close). However, a PST would often produce the same information. This lead to a PST being used for non-safety process equipment.
It is important to keep in mind that an SIS installation requires both equipment and methodology to maintain the rating. Only installing SIL rated equipment will not realize the benefits of the SIL rating. Asset management software can provide solutions for maintaining this equipment and should be considered. The upshot is SIL equipment is a modest investment and can provide significant returns.
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