Challenge: Measure inlet and outlet temperature of a process heater using existing three-wire RTDs and provide a 4-20mA process variable signal, representing the differential temperature across the heater, to a local alarm trip for heater protection and share that differential temperature signal with the Basic Process Control System (BPCS). The measurements are in a hazardous area and plant standards require the use of a certified IS system.
Solution: The THZ3-DIN Dual Input Temperature Transmitter with the Associated IS (AIS) option and SPA2 Alarm Trip with self diagnostics capability, and local display/alarm status indicators.
Result: Successful measurements to the BPCS of inlet and outlet temperature, temperature differential, sensor diagnostics, transmitter diagnostics using HART to the BPCS.
Summary: The customer needed to measure inlet and outlet temperature of a process heater using existing three-wire RTDs and provide a 4-20mA process variable signal, representing the differential temperature across the heater, to the BPCS controller. The measurements are in a hazardous area and plant standards require a certified IS system. Utilizing the associated IS and dual RTD input capability of the THZ3 temperature transmitter, the heater’s differential temperature 4-20mA signal was wired in series to a local alarm trip and host BPCS.
The process side alarm trip provided visual and audible heater failure warning and indication while the host BPCS maintained primary control and master alarming capability. Additionally, the HART diagnostics from the THZ3 allowed the BPCS to constantly monitor key parameters such as RTD drift, broken sensors and critical transmitter diagnostics.
Providing a local process side alarm trip substantially saved wiring and operation costs since new signal wires (RTD and alarm) didn’t have to be pulled to support the newly desired local alarms, two IS barriers were not needed for the RTD wires back at the BPCS location, sensor diagnostics continuously monitored by the BPCS and local differential temperature calculation and indication can now be done in the field.