i need to know the signal selection process when 3 transmitters are used for single process measurement. what happens when one of the transmitter fails OR goes beyond acceptable deviation limit? what happens when one more transmitter fails OR the difference between the two becomes more than the acceptable deviation limit?

thanks in advance

 

NOTE: My answer assumes that triple redundant transmitters are only used for critical control loops and protection loops. Also, the transmitters are installed in such a manner that a defective transmitter can be safely replaced while running. Therefore, any failed transmitter should be replaced within a few hours after it is detected, and thus there should almost never be a case of more than one transmitter in a triple redundant set failing at a time!

Now, there are (I retired more than one year ago, so some things may have changed) two schools of thought at GE Energy on what to do in the event of failure of one transmitter. Gas and Steam turbine controls continue to operate on the median selected signal of the three transmitters. The failed device would generally fail high or low, and the median would still be a valid control signal. GE Power plant controls philosophy is to reject the failed transmitter and use the average value of the remaining two transmitters as the control signal. I don't really know which approach, if either, is better.
There is a common approach on failure of a second transmitter - trip or do a controlled shut down.

There are those who say you should continue to operate with only one transmitter. To those I say go back and read my note, above. If you are not going to maintain your equipment, and keep an inventory of critical replacement parts on hand, why bother with all the redundancy in the first place?

 

You use a function called Median Signal Select. This outputs the middle of the three inputs. So if one input goes significantly higher or lower than the other two it is ignored. If used with a deviation alarm, the faulty reading can be investigated and fixed before there is any impact on operation.

If two transmitters fail in the same direction, there is a problem. But common-mode analysis and avoidance is an essential part of any redundancy scheme.