Typical on-stream inspection programs
incorporate visual examination of the firebox, external visual examination of
casing and components, infrared examination of tubes and heater casing and
stack, and monitoring of tubeskin thermocouples. Tell-tale holes is another
practice, although in limited use, for identifying unexpected or accelerated
corrosion of tubes onstream. Other analyzers and instrumentation monitor heater
and boiler operation and these are important parts of an overall reliability
program but these are typically monitored for heater and boiler performance.
Tubeskin thermocouples can be an important
component in a reliability program, especially for on-stream inspection.
Thermocouples measure the temperature of tube walls in-service. They serve a
couple purposes. First, the thermocouples can alert to abnormal operation if
temperatures dramatically change. Second, they provide a means to calculate and
monitor remaining tube creep life. Strategic placement of the thermocouples is
necessary so that the entire firebox can be reasonably monitored.
Malfunctioning burners or unbalanced firing of burners can create local hot
zones in the firebox and lead to premature failures. In addition, tubes that
historically operate hot due to their placement in the coil may need a
thermocouple, especially if it represents the most severe service of the tubes.
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