Why putting 2 out of 3 voting instrumented switches as a protection ( for example high pressure at gas plant inlet manifold)?

to ensure the integrity of the instrumentation reading and avoid malfunction on the system.

2 oo 3 instrumentation switches are installed :

-To increase reliability due to confirmed alarm from 2 different switches

-To avoid production lost due to nuisance trip as it would be the case 1 oo 1. 

to ensure spurious trip in the equipment due to spike indication can be avoided.

in case one instrumentation malfunction it will not give impact to the equipment protected unless two of three instrumentations have the same problem.

in this case, main inlet shut off valve will not responding to close while only one intrumentation error.

For its manimal safety for high pressure 

improve integrity and avoid nuisance malfunction resulting in process upset , trips etc. SIS Voting logic should be developed based on the intended function of the safety instrumented systems e.g. if the equipment is safety critical etc.

In let pressure for High pressure in oil and gas slug catcher minimal safety SIL  3 ( PITHHA and PITHHB and PITHHC )  redondant safety in case to discharge to High flare heater .

Can you imagine in case the imoact in case the instrumentation has malfuction and trip and stop the production unnecessary?

To increase reliability and also avoid nuisance trip as would be the case with1 out of1