Use of combustible gas detectors in Safety Instrumented Systems - A practical application case study

It was determined that it was difficult to detect the absence of hexane in the dryer vessel as a prerequisite for discharging the final product to the packaging room using conventional process variables like level and pressure due to the complexity of the process. It was further determined that the best way to detect the release hazard was to use combustible gas detectors. An analysis was conducted which led to the design being considered a Safety Instrumented System and not a Fire and Gas System. Further risk assessments were conducted including a Fault Tree Analysis, Event Tree Analysis, and Probability of Failure on Demand (PFD) calculations for the combustible sensors as well as PFD average calculations for the Safety Instrumented Function (SIF) to confirm that the design of the SIF met the required risk reduction targets.