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.
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