Transmitter Failures (4-20 mA)
In the following table we see an example of the types of transmitter failures and which we should consider when verifying the Safety Instrumented Function (SIF). The rates in the example correspond to the Rosemount model 3051 pressure transmitter.
|Type of failure
|Example of Failure Rates per hour (FITS)
|Fail Safe Undetected (SU)
|These failures cause process shutdown.
|Internal failures detected by transmitter diagnostics. They can be Safe or Dangerous depending on each case.
|Internal failures not detected by transmitter diagnostics that cause the signal to go out of range above 20 mA. They can be safe or dangerous depending on each case.
|Internal failures not detected by transmitter diagnostics that cause the signal to go out of range below 4 mA. They can be safe or dangerous depending on each case.
|Fail Dangerous Undetected (DU)
|“Dangerous Undetected” Failures that affect the probability of failure, and therefore the SIL achieved (example: the 4-20 mA signal is frozen).
|Failures that do not directly impact safety, but affect the diagnostics of other failures. They are not considered to calculate the probability of failure of the SIF.
|Failures that have no effect on the SIF.
|Faults that cause internal or environmental leakage of process fluids. They are not considered part of the safety of the SIF.
The transmitter failures “Fail High” and “Fail Low” can be detected by the PLC if the diagnostic software is implemented in the program.
“Fail Detected” failures are detected by transmitter diagnostics. If it is set to HIGH (over-range) position the diagnostics will carry the signal above 20 mA, and if the position is LOW (under-range) the signal will be less than 4 mA.
Therefore, the type of failure of the SIF (SD / SU / DD / DU) will depend on the following:
- How we have configured the transmitter (HIGH / LOW).
- If the process is shut-down by High or Low level (High / Low Trip).
- If these diagnostics have been implemented in the Logic Solver (PLC).
In this other post we delve deeper into this topic with some example.