November 2011

Bonus Report: Process Control and Information Systems

Safety instrumented function design reduces nuisance trips

Implementing low-cost best practices can provide peace of mind

Kern, A.G., Tesoro Corp.

A conundrum frequently facing safety system designers and plant managers is whether to use two transmitters in a one-out-of-two configuration (1oo2) or three transmitters in a two-out-of-three configuration (2oo3). While both configurations may satisfy the safety requirements, 2oo3 is traditionally considered the only choice when nuisance trip reduction is also a high priority, despite its higher cost, greater contribution to probability of failure on demand (PFD), and often, a sense of transmitter overkill. In recent years, the concepts of diagnostic coverage, discrepancy alarms and transmitter self-diagnostics have gained acceptance and become proven in use. This trio of concepts gives

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