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calibrating the graph or LOPA then it is likely that the number of asset based SIF’s would reduce
(because they are not financially justified) and so lessen the overall impact on SIS size.
IEC61511 includes the statement in section 1 (scope) “may be applied in non-safety applications
such as asset protection”. The intent of this statement is not clear but, since the standard is titled
“Functional
Safety
-
Safety
Instrumented Systems for the Process Industry Sector”, one could
conclude that the suggestion is that the
methodology
could be appropriate to determining
performance requirements for instrumented systems for financial protection. It does not follow that
the intent was that such functions should be implemented in the SIS. This point is relevant since it is
common that instrumented functions to protect against financial loss outnumber those protecting
against loss of life by a factor of 2:1. If all these functions are implemented in the SIS then it is
perhaps three times larger and more complex than it needs to be for safety protection alone, and so
consideration needs to be given to separating these functions into a dedicated SIS and (say) a
process shutdown system. Any such requirement needs to be defined and understood by all parties.