Abstract: A temperature monitoring system includes an access port configured to mount to an inspection window of a process heater that includes heating tubulars mounted in an inner volume of the process heater, the access port including at least one visual access portion configured to provide visual access to at least a portion of the heating tubulars; a flange assembly including a bore that extends from a first side configured to mount to the access port to a second side; and a thermal imaging system configured to mount to the second side of the flange assembly, the thermal imaging system including at least one thermal image capture device including a field of view through the bore of the flange of assembly and the at least one visual access portion and into the inner volume of the process heater to capture at least one image of radiation emitted from the heating tubulars.