F o r m o r e i n f o r m a t i o n o r a q u o t e , c a l l S t r u c t u r a l I n t e g r i t y A s s o c i a t e s , I n c . a t 8 7 7 - 4 S I - P O W E R
30 District Energy / First Quarter 2011
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. Plan-
ning a program of risk-based inspec-
tions 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 inter-
nally 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 inspec-
tion planning. In lieu of drawings, a visu-
al 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 “nondestruc-
tive 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-
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, radiog-
raphy 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 indi-
cate 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 indus-
try and professional organizations in
the area of risk-based inspections of
equipment and piping. Some notable
examples of guidelines they have pro-
duced 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
Co
urt
esy
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sso
ciat
es.
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.
Co
urt
esy
Stru
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ral I
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sso
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es.
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.
F o r m o r e i n f o r m a t i o n o r a q u o t e , c a l l S t r u c t u r a l I n t e g r i t y A s s o c i a t e s , I n c . a t 8 7 7 - 4 S I - P O W E R
District Energy / First Quarter 2011 31
look at specific equipment and lets own-
ers calculate a risk factor that can be
applied to their equipment as a ranking
input to an inspection program. ASME’s
standard takes a broad approach and
recommends reviewing both quantita-
tive and qualitative factors as a basis
for an inspection program.
There are multiple interrelated
factors that affect the overall risk
of an individual piping component.
These include
• fluid service,
• component location and
• potential damage mechanisms.
Multiple interrelated factors
affect the overall risk of an
individual piping component,
including fluid service,
location and potential damage
mechanisms.
The type of fluid being conveyed in
the piping system is a critical factor in
prioritizing piping inspections. The flu-
ids utilized by an energy supplier range
from natural gas and fuel oil to steam
and chilled water. The relative hazard
posed by each of these fluids needs
to be considered together with other
facility-specific factors to determine the
risk of an individual piping system com-
ponent. For example, chilled water is
generally treated as a low-hazard fluid;
but a chilled-water line rupture in a tun-
nel distribution system can potentially
expose personnel to an inundation risk
– thus elevating the hazard rating of the
piping component in the affected areas.
The location of the piping drives its
relative risk factor. Components located
in vaults or manholes, or near a build-
ing or tunnel egress, all garner a higher
risk factor, but for different reasons.
Vaults or manholes are typically permit-
confined spaces with single-point egress.
Failure of a piping component while these
spaces are occupied by personnel would
obviously have major consequences, thus
elevating the relative risk factor.
Buildings or tunnels have multiple
egress points, which would seem to
suggest they are safer; but piping com-
ponents located near any routine-use
egress point have a high ‘man-pass’
frequency and, consequently, a higher
relative risk.
Other location-specific risk factors
for piping components include their
proximity to vehicular traffic, whether
they are buried or above ground, and
their relative accessibility. It is self-
evident, for example, that an exposed,
unprotected piping system is at greater
risk of a catastrophic impact by a vehicle
than piping components that are remote
to roadways. The same is true for pip-
ing located above ground versus com-
ponents below ground. An inaccessible
component, however, can create risks
that are easy to underestimate and
that can even obscure multiple other
attendant risks. For example, a piping
component installed in a location with
inadequate clearances to walls, floors
or other obstructions may not allow for
proper fitup and welding; and the initial
construction inspection of this com-
ponent’s welded joint could have been
subpar or even overlooked. Because of
this piping component’s inaccessibility,
it has an increased risk of failure and
would therefore rank high in priority in
a risk-based inspection program.
In addition to fluid service and
component location, the potential dam-
age mechanisms that may affect piping
systems also need close scrutiny. One
common issue faced by energy suppliers
is the myriad causes of piping corrosion,
Engineer Solutionor Modify Operating
Procedures
Can FailurePotential BeDetected?
ComponentFailure
Document andAnalyze Failure
Piping Bill ofMaterial
Document
Identify SystemRisk Factors
RankRisk Factors
Risk Rank PipingComponents
Defect Found?
Perform SystemAs-Built Walkdowns
Determine Scope ofNDE Inspections
Inspect HighestRanking Components
Analyze andDetermine Cause of
Defect
YES
NO
REFINE
NO
YES, REFINE
Figure 1. Example of a Risk-Based Piping Inspection Program.
F o r m o r e i n f o r m a t i o n o r a q u o t e , c a l l S t r u c t u r a l I n t e g r i t y A s s o c i a t e s , I n c . a t 8 7 7 - 4 S I - P O W E R
Joe Maciejczyk, PE, is an asso-ciate with Structural Integrity Associates in the firm’s Annapolis, Md., office. A registered profes-sional engineer, Maciejczyk has more than 25 years of experience
in the design, fabrication, inspection, testing and operation of boilers, pressure vessels and piping systems. His experience spans multiple industries including utility power plants, district heating and chilled-water facilities, as well as cogenera-tion and industrial chemical processing facilities. Maciejczyk holds a bachelor of science degree in mechanical engineering from Louisiana State University and a master of business administra-tion degree from the University of Pittsburgh. He can be contacted at [email protected].
The potential for expansion joint failure is one of the risk factors to be considered in planning piping system inspections. Although expansion joints are designed to last 12 years or more in service, premature failures occur; and when they do, a systematic program approach to replacing them is needed. Start treating your expansion joints like an engineered asset versus a commodity. Make sure you have a data sheet for each joint containing all its design and fabrica-tion information; the Expansion Joint Manufacturer’s Association (EJMA), which publishes expansion joint standards, has such data sheets available for your use in documenting the parameters under which your expansion joints are operating. Then review each joint failure and make sure your findings are captured on the data sheet. Procure replacement joints from an EJMA member, which will ensure they adhere to industry quality and engineering requirements. New joint designs are continually introduced into the marketplace. If you are looking to adapt a new design, make sure the supplier of the new joint design can meet or exceed the data sheet requirements. If there is any doubt,
utilize the services of the vendor and a mechanical engineer to properly advise you on the purchase. Often misunderstood in the procurement of a new expansion joint is the joint’s design flexibility. Joints can be designed and built to expand, contract, translate, limit or allow movement in all planes and angles. All this information is an output from a piping stress analysis and needs to be recorded on the joint’s data sheet. If your improved vigilance on joint procurement is not solving your expansion joint problems, reviewing and updating the system stress analysis may be appropriate. Missing or broken hangers or supports, changes in operating conditions (pressure, temperature and cycles) can all have negative effects on expansion joints. In addition, changes or inadequacies in system water chemistry can also have adverse effects on your joints’ metallurgy, leading to premature failures. Make sure your expansion joint program investigates and resolves metallurgical issues. A program approach, and attention to detail with your expansion joints, pays dividends with improved joint life, reduced maintenance and reduced costs.
F o r m o r e i n f o r m a t i o n o r a q u o t e , c a l l S t r u c t u r a l I n t e g r i t y A s s o c i a t e s , I n c . a t 8 7 7 - 4 S I - P O W E R
Reprinted with Permission from Combined Cycle Journal
