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.