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hello everyone...

I am using triconex ESD system in my plant. In some trip logics they have used 2oo3 voting system as per our requirement. In our older plants we were able to apply mos on all 3 transmitters at time. But in this new one, they have provided a provision in MOS logic itself that only one transmitter can be override by applying MOS. They told me that it is the requirement of SIL system as plant is of SIL 3. please help me in understanding this why this is SIL requirement and how it is effecting SIL level. why only one MOS bypass is permitted?

The SIL rating of a loop determines the input architecture and the final element architecture for the given loop. The input architecture for a SIL 3 rated loop will require a minimum of unrevealed failure robust for the initiator (1002 configuration) or unrevealed failure robust and revealed failure robust initiator architecture (2003 configuration). For the 1002 configuration only one initiator at a time is allowed to be overidden by an MOS. For the 2003 configuration NO initiator is allowed to be overidden by a MOS. Overriding of any initiator of a 2003 configuration is un-necessary as any ONE initiator (transmitter) can be worked on without causing a trip (ESD). Under this condition the given loop of the 2003 will automatically be reduced to a 1002 configuration. In both of the above cases plant integrity is maintained as normally double jeopardy ( a second initiator failing whilst another one is being maintained) is not allowed for in the original design. The choice between 1002 or 2003 configuration is determined by the loop trip test intervals which you want to do under your normal maintenance requirements.

What is unrevealed failure robustness and revealed failure robustness?

Unrevealed failure means that say for example a pressure transmitter used for a high pressure trip fails and gives zero mA output. Then a high pressure trip can never occur as the transmitter will always read zero until it is discovered that it has failed.

Revealed failure means that should the same transmitter fail such that it gives maximum output (20 mA) then a process trip will occur due to the trip setting being exceeded. So the fault is self revealing.

Regular trip testing will reveal an unrevealed failure.

Unrevealed failure robust means that you install 2 transmitters in 1002 configuration. If one transmitter fails unrevealed then the other transmitter will probably function and cause the trip to operate should the pressure exceed the trip value.

For revealed failure robustness 3 transmitters can be installed in 2003 configuration. If a transmitter fails then it requires only one of the remaining two to see a trip situation and take appropriate action.

Double jeopardy is not normally considered i.e. 2 transmitters failing at the same time.

1002 configurations will tend to cause more nuisance trips. 2003 configuration whilst being more expensive is not prone to nuisance trips like the 1002 configuration.