Typical SIL 2 Architecture

ITEM Failure Rate (per hr) (X) % Safe Failure C° (%) PFDavg 1 YEAR TEST INTERVAL (Tl) PFDavg 3 YEAR TEST INTERVAL (Tl) MTTFS (years) TI=3 

The analog signal comparison can be performed between two or more transmitters that are part of the SIF or between a SIF transmitter and a DCS transmitter (measuring the same process variable). In order to take credit for the comparison it needs to be performed in the SIS logic solver. The software in the Safety Instrumented System is safety critical rated. This is not the case for the DCS software. 

The results of the PFDavg calculations for SIL 2 Case 3 indicate that for a one-year test interval the PFDavg is 7.01 E-03. This provides a Risk Reduction of 143 (S1L2). The final elements contribute a significant majority to the PFDavg. 

For a three-year test interval the PFDavg is 1.20E-03. This provides a Risk Reduction of 82 which no longer meets the requirements of S1L2. A spurious or nuisance trip is likely to occur every 11.1 years. 

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