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.
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