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A goal in the design of a safety instrumented system is to achieve a high level of risk reduction (a measure of safety). Safety PLCs have been specially designed using fail-safe circuitry, diagnostics and fault tolerance to increase system risk reduction factor. While this specially designed equipment has achieved its goal; it must be remembered that the system includes sensors and final elements (actuators/valves). Sensors can fail dangerously and can significantly lower the risk reduction factor but this must be carefully evaluated. To evaluate such usage, failure rate and failure mode information must be obtained for the transmitter. Depending on the diagnostic capability of the transmitter, it may be used in fault tolerant configurations to achieve higher risk reduction. This paper describes the information that must be obtained and methods used to evaluate smart transmitters in safety instrumented system applications.
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