Voices: Rezabek

When Safety Instrumented Systems Don't Communicate Well with the Rest of the Plant

Troublesome gaps in communication between smart transmitters and older systems can make diagnosing faults a tricky problem.

By John Rezabek
Jan 2, 2015

One of three "voting" temperature transmitters connected to the plant's SIS (Safety Instrumented System) indicated a reading 30 degrees less than its two redundant comrades. Voters A, B and C are connected to independent RTD temperature sensors in separate wells to minimize common mode failures, and the three thermowells are within a couple feet of one another in the same process stream. They're also wired to three separate analog input cards, each in a separate chassis of the shutdown system. It's common for voting transmitters in a SIL- (safety integrity level) rated interlock to have a "deviation alarm," just in case no one notices when one of three measurements that should be identical is way off. In our case, 30 degrees was 3% of full scale and not quite enough to trip the "deviation" alarm. But people noticed "one of these things is not like the others," and we were compelled to investigate. Curiously, we acted on our first suspicion―a failing RTD―and wrote a work order for scaffolding to be built so the sensors could be tested and replaced if necessary.

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