When designing a SIL 3 SIS (Safety Instrumented System), many people make the assumption that all they need to do is design the system including all components that can be used in applications up to SIL 3. This is a general misunderstanding that I have run across several times. HFT (Hardware Fault Tolerance) must be adhered to as well. For a SIL 3 design, an HFT = 1 must be followed for final control elements. This means there must be at least 1 level of redundancy to ensure the system can be brought to its safe state. Concerning final control elements such as process valves, this means that there must be 2 process valves in series (1oo2) to ensure that either one of the process valves will shut off the flow of media when asked to do so.
Bill Reeson
Global Project Pursuit & Strategic Account Manager | ASCO
Emerson
T +1 973 966 2097 | M +1 973 495 8561
Bill.Reeson@Emerson.com
Good post Bill. You make a very good point when considering component selection from a Hardware Failure Tolerance HFT perspective. Have encountered this many a times myself. I would like to add that if we look at the entire safety function, I think it is important to note that there are 3 major considerations: