I am instrumentation engineer new at SIS system understanding and there some complexity with voting arrangements
my question is how voting logic related to SIL ?

 

Two out of three voting (2oo3) is a typical setup for instrumentation or control system where very high integrity is required - so as part of the design of any system we try to determine whether such systems can be fault tolerant or determine circumstances where it can fail to danger {Or consider, an ESD system fails safe but fails to operate on demand}.

As you assert SIS is a complex area of industry, with different industries having differing ideas on SIS.
AND there is always the human element which at times is hard to predict.



Further reading:
https://en.wikipedia.org/wiki/Safety_instrumented_system

https://en.wikipedia.org/wiki/Safety_integrity_level
"According to IEC 61508, the SIL concept must be related to the dangerous failure rate of a system, not just its failure rate or the failure rate of a component part, such as the software. Definition of the dangerous failure modes by safety analysis is intrinsic to the proper determination of the failure rate"

https://control.com/forums/threads/2oo3d-voting.42416/

 

I am instrumentation engineer new at SIS system understanding and there some complexity with voting arrangements
my question is how voting logic related to SIL ?

During a LOPA or risk assessment there may be a over pressure or overfill scenario which is above the companies tolerable risk. To reduce the risk the team will implement a SIF with a SIL requirement. If the SIL is high a 1oo1 transmitter may not meet the requirements, but a 1oo2 may. Note- there are many cases in which 2oo1 voting may be used for high availability, and not to meet a SIL.