General Instrumentation Queries

Experts can anyone shred some light on my below queries.

  1. Why F&G loops are not SIL rated?
  2. Why 3 wire gas detectors are still being used in the oil & gas industry, when 2 wire GDs are available in the market?
  3. Why are gas detectors not IS loops?

Check below link for Question 2 -


is F&G system be a SIF or SIS?

A Fire and Gas (F&G) system that automatically initiates process actions to prevent or mitigate a hazardous event and subsequently takes the process to a safe state can be considered a Safety Instrumented Function / Safety Instrumented System.

However, it is absolutely critical in a F&G system to ensure optimal sensor placement. If there is incorrect placement of the gas / flame detectors and hazardous gases and flames are not adequately detected, then the SIF / SIS will not be effective.

Correct sensor placement is more important than deciding whether a F&G SIF / SIS should be SIL 2 or SIL 3.

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Appreciated if you guys are more specific to the query raised.