most likely from either thermal fatigue,
an external phenomenon, or corrosion
fatigue, an internal actor. Undetected
cracking that is allowed to grow unchecked
can result in forced shutdowns and safety
concerns for operating personnel. Planning a program of risk-based inspections to prevent such issues is fairly
straightforward for boilers, given that
boilers tend to be quite standard in
design (as compared with the unique
nature of most piping installations). The
areas presenting the greatest potential
for problems typically include welds,
boiler tubing and exhaust ducting, and
internal boiler water wall tubing.
Thermal fatigue cracking is most
likely to occur at or near welds. Inspecting
every weld in a boiler may be possible, but
it is impractical in terms of time and cost.
Thermal transients in boilers will flex
components that are the farthest away
from rigid anchors. This flexing causes
fatigue cracks to form and grow. Suspect
areas include welded rigid anchors and
boiler-tube-to-header connections.
Corrosion fatigue cracking, an internally generated attack of cold-side water
wall tubing, most often shows up as a
pinhole leak at the base of a weld.
Nondestructive examination of
the main anchors is the basis
of a sound boiler inspection
program.
Review of the boiler construction
drawings can locate the main anchors
and supports in the system for inspection planning. In lieu of drawings, a visual inspection of the boiler internals can
also locate these anchors. Nondestructive
examination (NDE) of these anchors to
verify their integrity and look at the high-flex connections associated with them
forms the basis of a sound inspection
program. NDE, also called “
nondestructive testing,” refers to the analysis of
system components or materials without
causing damage, using any of a number
of methods such as magnetic particle
testing, radiography, dye checks or vari-
Typically, boiler tubes are inspected using a single-point probe to take readings at stochastically
selected points in an attempt to locate tube wall
loss. Thanks to recent advances in ultrasonic NDE
technology, however, a guided wave inspection,
shown here, can scan an entire tube from one
location to find areas of wall loss or leaks.
The focused-beam ultrasonic NDE technique
shown here targets the inside of the boiler
water tube to look for under-deposit corrosion.
Corrosion deposits can form in boilers that
are cycling and have water chemistry control
difficulties. These deposits can lead to through-wall oxygen pitting.
ous forms of ultrasonic testing (linear
phased array, time of flight diffraction,
A and B scans, etc.).
The type of NDE typically done
for welds in boilers to detect external
cracking is magnetic particle testing.
Components to be examined are placed
under a magnetic field by a handheld
electromagnetic yoke, and magnetic
field-sensitive particles are sprayed or
dusted on the area to be inspected. The
particles align with any defect on the
metal’s surface. Internal tubing issues
are found through such NDE techniques
as fiber optic visual inspection, radiography or linear phased array ultrasonic
scanning. Depending on the particular
defect, it may either be removed by weld
repair, tracked for followup or analyzed
more rigorously if it appears to indicate a larger, more systemic problem.
Technologies for more in-depth analysis
include field metallography and finite
element modeling with software such
as ANSYS.
Cyclic boiler operation can create
other issues that, if unchecked, can also
result in forced outages and loss of
revenue. Some of these include external
corrosion of boiler tubing and exhaust
ducting, and internal boiler water wall
tubing failure. Cyclic operation can put
boiler components in the dew point
temperature range that will promote the
condensation of acids, which can rapidly
destroy tubing exterior or ducting, and
other components. Loss of cycle chem-
istry during downtime, startups or load
changes can lead to deposit formations
in water walls, which can initiate under-
deposit corrosion and oxygen pitting.
Review of operational logs and targeted
followup inspections can locate these
issues and result in procedural refine-
ments to mitigate their occurrence.
Piping Inspections
Over the past 20 years, significant
advancement has been made by industry and professional organizations in
the area of risk-based inspections of
equipment and piping. Some notable
examples of guidelines they have produced include the American Petroleum
Institute’s standard API 581 (Risk-Based
Inspection) and the American Society of
Mechanical Engineer’s standard PCC- 3
(Inspection Planning Using Risk-Based
Methods). API 581 takes a quantitative
