During outages, tube skin thermocouples should be tested
for accuracy and potential failure. Tube skin thermocouples have a reputation
for being inaccurate and failing prematurely. Inspection and testing during
outages is an important step to improve reliability. Thermocouple leads are
often the root failure by being exposed to flame and radiation, or lifting off
the tube surface. Attachment welds should be inspected with dye penetrant for
cracks that can cause the thermocouple to read firebox temperature. The
sheathing protecting the thermocouple leads should be inspected for any
breaches and kinks.
Some thermocouples will have a temperature drift due to
long term exposure or temperature cycling. Procedures can be developed to
determine if the tube temperature is accurately measured by heating an area
adjacent to the tube and monitoring the temperature rise. A calibrated contact
pyrometer can measure temperature at that point and be used for comparison to
the thermocouple.
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