Reliability programs have evolved from inspection during unit maintenance outages to integrity management programs encompassing on-stream tube life monitoring, continual heater and boiler efficiency analyses and detailed and varied inspections during maintenance opportunities. In the simplest programs, heater and boiler reliability focus on preventing failures of the pressure boundary. The strategy is to prevent leaks and ruptures of the tubes and in the case of boilers, the drums too. Further refinements to these programs consider the operations of the heater and boiler and associated hardware to ensure they are operating optimally and at design levels. Oftentimes, poor performance or failure of associated hardware can lead to deterioration of the tubes.
Tube failures result from progressive deterioration from a variety of deterioration mechanisms. Therefore, one needs to understand the active and potential mechanisms in a particular heater and boiler in order to prevent them from causing a failure. For example, in elevated temperature services like boiler and heater tubes, creep and stress rupture are potential deterioration mechanisms. The tube operating variables which affect tube creep life/stress rupture life include: the base metal creep/stress rupture properties, tube metal temperatures, applied stress from internal operating pressure and from mechanical loading (i.e., from supports or lack of supports), and time operating at each unique combination of stress and metal temperature.
Tube reliability not only requires an understanding of the mechanisms by which the tubes can fail, but also requires data on how the previous operating history has impacted tube life, predictions of deterioration rate, how the future operation will impact tube life, and finally, monitoring of operations and deterioration to ensure the analyses and predictions are accurate and appropriate. Historically, inspection data gathered during outages assessed the immediate condition of the tubes with varying degrees of accuracy or success. Typical inspections included a visual examination for bulges in tubes and thickness measurements of accessible tubes. Now, inspections can also include detailed strapping/gauging for bulges, internal ultrasonic (UT) inspection pigs to gather detailed tube wall thickness and diameter maps (including the difficult to inspect heater convective tubes), on-stream infrared tube temperature measurements, destructive testing to identify specific types of deterioration or to determine actual remaining creep life, to name a few.
Components of a typical tube reliability program for individual heaters and boilers include:
a. List of active and potential deterioration mechanisms.
b. Inspection techniques to identify whether the potential deterioration mechanisms are active.
c. Review of historical heater and boiler operations and maintenance repairs records to identify active or previously active deterioration mechanisms.
d. Assessment of previous operations and repairs impact on tube remaining life.
e. Defined tasks or procedures, if practical, to minimize the likelihood of potential damaging mechanisms.
f. Rate of deterioration of tubes for active deterioration mechanisms.
g. Method or technique to assess the impact of process changes or heater and boiler operations on rate of deterioration.
h. Assessment of remaining tube life for each mechanism considering previous operations and repairs, current condition and the rate of deterioration.
i. Defined integrity operating windows in which the tube life and rate of deterioration projections remain valid.
j. On-stream monitoring tasks to ensure operating conditions remain within the boundaries and procedure to address or assess the impact on tube life of out-of-bounds operations.
k. Inspection plan and monitoring/assessment of other hardware and equipment that impact the deterioration of the tubes like burners, hangers and supports, and thermocouples.