POWER February 2013 - 19

ASSET MANAGEMENT
1. Trouble spots. Areas of the steam cycle affected by layup and startup practices. Source: EPRI
Corrosion
Impurity ingress
High carry-over due to
drum level control and
suspended solids
HP turbine
- System filled
with aerated
water
- pH and redox
changed lead
to dissolution,
precipitation,
and
deposition of
oxides
High Fe, Cu into boiler
during startup
- Cu alloy and c-steel oxidation during layup
- Oxide transport and deposition during startup
Damage Mechanisms from Improper Layup Practices
Inadequate layup practices allow metal to
remain in contact with oxygen and water
for extended periods of time. The presence
of salts, particularly chloride salts, can accelerate
the corrosive effects of water and
oxygen on metal surfaces.
Corrosion fatigue is a major failure mechanism
in boiler tubes. The " fatigue " portion
of the corrosion fatigue mechanism depends
on mechanical and thermal stresses developed
during the cycle. At a greater than 2%
strain, the protective magnetite oxide of the
stressed component cracks, exposing unprotected
surfaces to the boiler water. Pitting,
the " corrosion " portion of the mechanism,
can significantly increase the propagation rate
through the material (Figure 2). Pitting corrosion
is the most prevalent layup-initiated corrosion
mechanism. Other corrosion mechanisms,
such as stress corrosion cracking and various
under-deposit corrosion mechanisms, are initiated
or exacerbated by pitting corrosion.
Poor layup practices also affect steam
turbines. Left open to the atmosphere and
sitting over a full and warm hotwell, the
steam turbine is bathed in warm and humid
conditions that are conducive to pitting,
particularly on the low-pressure turbine
blade/disk surfaces in the phase transition
zone, which is a precursor to corrosion fatigue
on the turbine (Figure 3). In cases
where the turbine has been contaminated
(for example, following a condenser tube
leak), corrosion can be rapid and severe.
The corrosion potential and damage risk
during a unit outage are nearly independent
February 2013 | POWER
Attemperation
Boiler
HP heaters
Deaerator: good
deaeration at all loads
LP heaters
Condensate polisher
Deposition
LP
IP
turbine
Condenser
turbine
Turbine:
- Pitting during layup
- Washing of deposits
NH3 + O2 + CO2
- Poor deaeration
- Corrosion of Cu alloys
Makeup
Fe fouling and rapid
exhaustion by CO2
components or systems should be incorporated
into a comprehensive layup procedure for
the unit (see sidebar " Damage Mechanisms
from Improper Layup Practices " ).
The shutdown and layup periods should be
viewed as a continuation of the good water
chemistry practices used during operation.
The primary purpose of the cycle chemistry
is to provide protective oxide surfaces on all
components throughout the steam and water
circuits in order to minimize corrosion
and reduce concentrating and performancerobbing
deposits on heat transfer and aerodynamic
surfaces. The primary purpose of
the shutdown and layup practice should be to
preserve those protective oxide surfaces and
prevent damaging corrosion (Figure 1).
Based on EPRI research and personal
experience, three guiding principles should
govern all layup decisions and practices:
of the outage duration. Mitigation of corrosion
and preservation of the asset need to be
implemented during the unit shutdown process.
Two key points should be emphasized:
■ Thermal cyclic stresses and rapid startup/
shutdown operations mean that shortterm
outages can actually be more detrimental
than longer, planned outages
with controlled shutdowns.
■ Corrosion activity is most aggressive in
the minutes and hours following shutdown
when the moist, aerated environment
is at its highest temperature. A rule
of thumb is that the rate of corrosion
doubles for each 10 degrees Celsius of
temperature elevation.
Frequent, short-term outages (from unit
cycling) are more problematic and damaging
than traditional, but infrequent, long-term
outages, because the percentage of operating
life and annual hours that the components are
strained or imperfectly protected is increased
by nearly an order of magnitude. The more
time cumulatively spent in these operating
conditions and the higher the frequency of cycling,
the greater the chance of damage.
As a simple example, consider the number
of times you can bend a paper clip before it
breaks. Regardless of how slowly you repeat
the cycle, the paper clip will eventually
fail. But more rapid and frequent cycling
decreases the time before it breaks. Unlike
a paper clip, steam-generating equipment
protection must also consider correct water
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3. Pitiful problem. Deposits that contain
chloride aggressively pit the turbine
when it is allowed to sit in moist conditions.
These pits become sites for corrosion fatigue.
Courtesy: EPRI
2. Permanent damage. This image
shows oxygen pitting in a boiler drum that
was given inadequate layup protection.
Courtesy: EPRI
chemistry (corrosive damage) and operating
temperatures (thermal stresses).
19

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POWER February 2013

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