Burnout Function in Temperature Transmitter is a very important feature that saves the plant and equipment. In this article, we will discuss the burnout function feature of the temperature transmitter.
The temperature transmitter receives the input signal from sensors like RTD and Thermocouple. The transmitter converts the voltage signal into a 4-20 mA current signal for transmission to PLC or SCADA.
The transmitter can also convert the sensor signal into an FF signal or any kind of signal depending on the signal requirement at the processor.
We do the various parameter setting in the transmitter for better process control and troubleshooting purpose. The various parameters are Device tag, primary variable, secondary variable, Tertiary variable, range of all variables, LRV, URV, Engineering units, Damping, Transfer function (Linear/Square root), Display setting, etc.
One very important parameter ” Burnout Function” in the temperature transmitter is also programmed or configured.
Burnout Function in Temperature Transmitter
A transmitter converts the sensor voltage into current or other types of signal. What happens if RTD opens or exhibits high resistance? Do you get the exact temperature reading? In this case- you will not get the actual temperature reading. If there is a process interlocking with high temperature, the plant trips & causes production loss. But, in this condition. the plant should not trip if there is no process fault.
In the second case where RTD resistance short circuits. At zero resistance, RTD reads -242.021 °C temperature. Think, during this scenario, if process temperature actually rises, the PLC will not output a temperature high signal and the plant will remain operative. This may cause untoward incidents like fire and blast.
The solution to the above problems lies with the configuration of the Burnout function in the temperature transmitter.
The Burnout function facilitates the control room operator to know the reason for high or low temperature. Whether it is because of the process temperature high or it is because of sensor failures.
When a transmitter detects sensor failure ( sensor open or short circuit), it outputs the current as per the pre-configured value. At the transmitter end, we can configure the low value, high value, or other defined values as per requirement.
How does a transmitter detect sensor failure?
According to “NAMUR NE 43″ the transmitter outputs a sensor fault when the loop current is below 3.6 mA or above 21 mA. The transmitter monitors the current for at least 4 seconds before outputting the sensor failure fault.
The setting of high and low current in the event of sensor fault depends on the type of process and before setting one needs to check the process requirement.
After setting of burnout function in the temperature transmitter, the Instrument technician/engineer must note down the setting in the logbook or at the transmitter. It should be kept in mind that before carrying out any activity on a temperature transmitter that has a burnout feature must be bypassed from the PLC or DCS.