Infrared scanning provides a means to check the accuracy of tube skin thermocouples. Furthermore, infrared thermal scanning of tubes helps fill the gaps created with the tubeskin thermocouples. Infrared scanning inspection can determine local tube metal temperatures in the areas not covered by the tube skin thermocouples. Generally, tube ruptures occur in very local areas of overheat. Infrared scanning has proven effective in identifying localized “hot spots” before they cause a failure. A periodic scan of heaters is common practice. The inspection intervals should be shorter for furnaces with coking tendencies (such as crude, vacuum, heavy oil hydroprocessing and coker units), furnaces susceptible to fouling (such as dry point fouling in naptha hydrotreating unit), and steam methane reforming furnaces. Longer intervals may be used for furnaces in non-coking and non-fouling susceptible services. For furnaces that have frequent decoking activities (such as ethylene/olefin cracking units), a case-by-case evaluation is needed to determine how often infrared scanning should be performed to complement routine monitoring that typically uses a handheld pyrometer.
Personnel performing Infrared (IR) surveillance of heaters should be knowledgeable and appropriately trained and qualified per the owner users requirements for IR scanning (e.g. ASNT SNT-TC-1A, PCN Condition Monitoring or owner user standard/practice). In addition, scanning personnel should be aware of the factors that may impact IR survey results such as flame environment, emissivity, infrared radiation and characteristics of different materials, infrared camera functions, and the limitations and accuracy of the method. When performing IR scanning, inspection personnel should review prior IR survey results, current heater operating status, Integrity Operating Window temperature limits, and any new operations or maintenance issues to ensure that all areas of concern are inspected.
An external IR scan of the heater should include an assessment of heater casing and stack for refractory damage. Internal IR scans of the firebox through each sight port should include an assessment of:
a) viewable (i.e. radiant or convection) heater tubes for overall temperatures and hot spots,
b) tube skin thermocouples,
c) tube supports and refractory for spalled refractory or broken tube supports, and
d) burner tiles and fuel gas tips for damage or plugging.
IR surveys should be conducted on a scheduled interval based on Integrity Operating Windows, API 530/Omega design metal temperature(DMT) limits, unusual/poor operation or control, steam air decoking or when deemed necessary. External and internal IR scan results that indicate significant temperature differences from previous or anticipate IR survey results should be evaluated. The owner user should specify guidelines for acceptable temperature limits for each heater. Results outside of owner user guidelines should be reported immediately. Inspection reports should include documentation of IR camera settings used for the survey, drawings of heater tube locations and results, and images of all significant findings.
Accuracy of infrared scanning can be influenced by the skill of the operator, the angle of incidence, the nature of the combustion products, flame patterns, and scale on the tubes. The infrared operator ideally would be certified in infrared technology and have experience scanning heaters and boilers. The presence of flames can mask the tubes if the operator must scan through the flames. Another significant limitation is scale on the tubes. The temperature of scale on the tubes tends to be hotter than the tube since it may not be tightly bonded to the tube. During outages it can be beneficial to remove scale in areas to allow the operator to scan a “scale-free” area and compare to other scaled areas. This could allow better interpretation of results. Grit blasting stainless steel tubes has also been shown to improve the accuracy of infrared by mottling the surface enough to reduce reflectivity which artificially causes a higher temperature measurement.
External casing can be inspected on-stream both visually and using infrared. Visual examination can identify areas of distortion and holes. These can indicate hot areas of lost refractory and promote continued deterioration. A periodic infrared scan of the case is more effective than visual examination in identifying “hot spots”, holes, and cracks. Regular inspection of the firebox is critical for reliability. Inspection can identify poor flame pattern of improperly firing burners, fuel rich operation as evidenced by afterburning, and changes in appearance of tubes, supports, refractory, etc. These inspections help identify changes early. Any changes or problems can be addressed or analyzed to prevent further damage and deterioration from occurring.
Header boxes should be visually examined for evidence of process leaks. These could indicate a leaking plug header for those heaters with fittings or a leaking instrument connection like a a thermowell. If there is evidence of process leakage, understanding the cause should be investigated.
Tube skin thermocouples and infrared scanning have some limitations in reading temperatures accurately. The particular type of thermocouple should have a mid-range rating for the expected tube metal temperature for improved accuracy. Thermocouple wires can have the potential to drift with time at temperature and so they require recalibration or periodic replacement. Another significant problem is the attachment of the thermocouple to the tube. If it is poorly attached, the thermocouple can separate from the tube and begin reading firebox temperatures.