An Emerson Process Experts blog reader saw the post, Thermocouples or Temperature Transmitters in Safety Systems? and emailed me this question:
Is multi-point TC assembly available in SIL rating? We are installing 24 ten-points TC assemblies in 24 tubes in a reactor (total no of tubes in reactor is in the thousands). Process group says that these temperature measurements may have to be classified as SIL rated.
This is a high pressure (1650 psig) and high temperature reactor.
I appreciate the guidance and wisdom of this community on how to best respond.
Temperature transmitters are SIL certified as stand-alone devices, with differing failure rates and SFF values for different sensor configurations. Temperature sensors of different types have differing failure rates and modes, which also vary with application and environmental stress factors. Methods are provided in the FMEDA report to combine transmitter and sensor failure rates, and account for the transmitter’s sensor diagnostic capabilities.
In the case of thermocouples the "unsafe" failure modes include a short or drift which is about 5% of the failures historically. They are assigned a Failures in Time (FITs) where 1 FIT = 1 x 10-9 failures per hour, which is part of the combined transmitter/sensor calculation to determine the likelihood of an unsafe failure not being detected. A multi-point sensor is just a collection of single sensors so the frequency of failure would be additive if they were all involved in the SIS loop. However the sensor itself does not have a SIL rating. There are some multipoint transmitters that are a good fit for interfacing to multipoint sensors, however at this time I don't know of any that have a SIL rating. A SIL rating is only available in single/dual input transmitters today.