ESD transmitters serving critical shutdown level in Safety Instrumented System are recommended to have voting logic configuration. Voting logic is applied to minimize the occurrence of complete loss of production caused by single transmitter fault or spurious trip shutdown. The voting configuration can be 2oo3 or 1oo2D based on SIL assessment and verification.
There are some consideration when applying voting logic in the process:
- Transmitters not to have common tapping to the process line/equipment.
- The transmitters forming the same voting logic shall not be assigned on the same I/O module of Safety Instrumented System.
- Each instrument cable is routed diversely.
- Transmitters are set with the same calibration range.
It is also recommended to have transmitters from different manufacturer to avoid manufacturing defect causing common mode failure. Also on this voting logic configuration, SIS needs to compare the transmitters value and initiate alarm on Human Machine Interface (HMI) for any deviations on measurement among the transmitters.
On a 2oo3 voting transmitter, any single abnormal process variable detection by one transmitter will initiate an alarm. The shutdown will be initiated if two transmitters reach the trip set point.
Here are some application examples where the ESD transmitter with voting logic are used, typically it is part of higher level in shutdown hierarchy level:
- Instrument air header
- Export Pipeline
- KO Drum