TUNE YOUR LOOP: Advanced Design and Safety Instrumented Level (SIL) Verification

TUNE YOUR LOOP:  Questions to keep you sharp.

Tune Your Loop is a series of posts consisting of questions from ISA Training Courses.

1. What benefit will 1oo2 transmitters provide compared to a switch and transmitter voted 1oo2, and why?

2. Will triplicated switches offer the same level of performance (safe & dangerous) compared to triplicated transmitters, and why?

What are your answers before you read ahead?

1. What benefit will 1oo2 transmitters provide compared to a switch and transmitter voted 1oo2, and why?

Transmitters voted 1oo2 offer a benefit over a switch and a transmitter in that the transmitter analog values can be compared for discrepancies. This provides an additional level of comparison diagnostics beyond the standard level of diagnostics already provided by the transmitter. Comparing a discrete switch signal with an analog transmitter signal offers no such benefit.

Assuming failure rates for both devices are the same, the MTTFspurious for both arrangements will be the same, but the RRF for the voted transmitter arrangement will be considerably higher (due to the higher level of diagnostics).

2. Will triplicated switches offer the same level of performance (safe & dangerous) compared to triplicated transmitters, and why?

A voted transmitter arrangement will offer a high level of comparison diagnostics. Comparing three switches offers no comparison diagnostics at all. (If one switch is stuck, there is still no way to automatically detect the failure.)

Assuming failure rates for both devices are the same, the MTTFspurious for both arrangements will be the same (both are equally fault tolerant), but the RRF for the voted transmitter arrangement will be considerably higher (due to the much higher level of diagnostics).

From the ISA course EC54 Advanced Design and SIL Verification pre-instructional survey.

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