In refineries, thermal imaging has been used for many years to monitor the interior temperatures of furnaces, particularly the furnace wall-tubes, in the presence of combustion gas flames. The temperature range in these processes varies from 400 to 1200°C. Flame combustion byproducts contain gases of H2O, N2, CO2, NO and small residues of ashes and other particles that emit thermal radiation toward wall tubes resulting in heating of the tubes. Typically, a mid-infrared (MWIR) instrument is used, equipped with a narrow band-pass filter centered at 3.90μm. In this band there is a void in the emission spectrum of these gases making them transparent, and an instrument operating only in this band can provide very high quality thermal images of the furnace interior.
Operating temperatures at other points in petrochemical-related processes, closer to ambient temperature, can also be very critical. For example, a 10°C temperature difference from desired temperature at the coil output of a heat exchanger of a large ethylene plant can result in substantial revenue loss per year. Monitoring of these conditions is usually accomplished using a long wave infrared (LWIR) imaging radiometer operating in the 8-14μm spectral bands.
This paper will review the evolution of techniques for furnace wall-tube monitoring, discuss current techniques and conclude with the description of a modern dual-band approach. In this approach a single, portable uncooled thermal imager is deployed in a refinery to monitor both the status of high temperature elements such as wall tubes and the operating condition of the furnace and its ancillary equipment. Case histories with thermographic illustrations will be presented.