POWER September 2010 - 18
Ten Years of Experience
with FAC in HRSGs
We first reported on combined-cycle plant
reliability concerns due to erosive wear and
flow accelerated corrosion (FAC) in heatrecovery
steam generator (HRSG) pressure
parts at the 1999 EPRI Maintenance Conference.
More than 10 years later, these
damage mechanisms remain significant
contributors to forced outages, pressure
part repairs, and major component replacement.
The highest risk components are, with
few exceptions, located within the boiler
casing in modern HRSGs and, compounding
the problem, are often inaccessible.
FAC damage can rapidly progress when
less-than-ideal water chemistry conditions
exist in conjunction with two-shift cycling
operations. The damage is exacerbated by
certain mechanical design configurations
and choice of materials. Many combinedcycle
plants have experienced detectable
wall thinning of susceptible components
before 25,000 hours of operation. Figures
1 through 5 are examples of the many failure
modes caused by FAC in HRSGs.
Natural Gas Plants Proliferate
FAC became an issue in the U.S. in 1986
after a pipe rupture in the Surry Nuclear
1. High local turbulence at lowpressure
(LP) evaporator lower
headers. At a large horizontal gas path
HRSG, extensive flow-accelerated corrosion
(FAC) damage was found at riser piping to the
lower headers from the downcomer manifolds
and at the tube-to-header weld above these
risers (purple area). The plant had high duct firing
capability, and steam production was very
high. The damage occurred only in the leading
LP evaporator panels with the highest heat input
(flow rates). FAC damage was not found
at the header at the lower bends, a short distance
away. Source: Tetra Engineering Group
Power Plant resulted in six fatalities. Subsequent
nuclear and conventional boiler
piping failures focused industry attention
on FAC and its causes. Although there
were combined-cycle power plants using
HRSGs at the time, they were few in number
and relatively small in size. FAC did
appear in these early cogeneration plants,
principally because:
■ Export steam required additional
makeup of demineralized water and/or
cleanup of returned condensate.
■ Export steam water chemistry requirements
impaired HRSG water chemistry.
■ Favorable rates and high steam demand
required high plant capacity factors.
■ Many units operated under older feedwater
and boiler water chemistry guidelines
(pH between 8.8 and 9.2 with low
dissolved oxygen) derived from mixed
metallurgy system and industrial boiler
guidelines.
The deregulation of the natural gas
industry, beginning in the early 1990s,
encouraged construction of larger combined-cycle
plants. Deregulation of electricity
markets and low natural gas prices
spurred a " dash for gas " during the 1990s.
However, rising natural gas prices and expiring
long-term power purchase agreements
pushed many plants into peaking
or intermediate-load service. Operators of
these new units also experienced FAC, but
under different circumstances:
■ New chemistry guidelines based on allferrous
metallurgy.
■ More resistant materials in high-risk areas
of tube and pipe.
■ Some units had high capacity factors,
but many went to two-shift cycling
when fuel prices rose and excess capacity
was built in some regions.
■ Export steam became less common, and
plant construction shifted to refineries
and other large steam users.
FAC Problems in Early Plants
Now that many early plants have more
than 10 years of service, we are able to
critique these HRSG designs. In these
early-model HRSGs, the primary areas of
FAC damage are either internal (tubes and
headers) or in boiler connecting piping
(evaporator risers and short feedwater
piping segments). The FAC damage was
often extensive, requiring major replacement
and repair of pressure parts. Also,
there is a large two-phase steam/water
exposure in HRSG low- and intermediatepressure
components. This contrasts with
conventional boiler experience, where
FAC is largely confined to feedwater lines
that have piping and feedwater heater
components.
2. FAC damage found in lower LP header area. Readings of the average and
minimum tube thickness were taken with nondestructive testing tools at another plant. The data
show the observed wear in one-half of the LP evaporator (two modules wide). The peak wear is
directly above the riser piping and decreases as you move away. Not shown in the figure is that
the location of peak wear on the tube was away from the source of fluid flow. This indicated that
the local turbulence was caused by the flow impacting on the side of the tube/header orifice.
Source: Tetra Engineering Group
Minimum tube thickness reading
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
37
42
47
52
57
Tube number
18
www.powermag.com
POWER | September 2010
62
67
72
Average tube thickness reading
Thickness (mm)
http://www.powermag.com
POWER September 2010
Table of Contents for the Digital Edition of POWER September 2010
Contents
POWER September 2010 - Cover1
POWER September 2010 - Cover2
POWER September 2010 - Contents
POWER September 2010 - 2
POWER September 2010 - 3
POWER September 2010 - 4
POWER September 2010 - 5
POWER September 2010 - 6
POWER September 2010 - 7
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