POWER August 2011 - 52

PLANT OPERATION & MAINTENANCE
Damage Mechanisms
Load following involves rapid increases and
decreases in process temperatures, which
create significant thermal stress on pressure
boundaries. When plant loads change, the
consequences are numerous: pulverizers or
mills go off and on, furnace temperatures and
heat profiles are altered, pollution control requirements
change, and steam and flue gas
velocities vary. All of these changes can force
1. Cracked header. Cold feedwater introduced
to a hot header caused the crack in this
economizer header. The cold water created a
large through-wall temperature gradient change
in temperature during startup and during offline
top-off opportunities. Courtesy: EPRI
the unit to operate away from the original design
point.
A few important material damage mechanisms
are responsible for the majority of the
financial impacts caused by operating coal
plants in flexible modes. The severity of the
impact of these mechanisms can be mitigated
to a certain extent through improved
plant operation and process controls, but it
is impossible to completely eliminate the reduction
in major component life caused by
flexible operation. Examples of these damage
mechanisms follow.
Thermal Fatigue. This phenomenon
can produce cracking in thick-walled components,
especially castings such as turbine
valves and casings. Also affected are boiler
superheater and reheater headers, where
ligament cracking is commonly seen between
tube stubs. These headers are expensive,
thick-walled vessels operating under
high steam pressure, making this damage of
particular concern to plant owners. Header
cracking (Figure 1) is caused by frequent,
large temperature swings associated with
flexible operation and, in some cases, by
thermal quenching produced either by condensate
formed during idle standby or poorly
controlled attemperator sprays (again associated
with transient operation).
Thermal Expansion. Several systems
2. Leaky tubes. Tube-to-header cracking
caused by thermal transients is shown in this
photo. Variation in tube thermal expansion,
caused by differences in tube length, increases
stress in some tubes. Courtesy: EPRI
in a coal plant consist of components that
undergo high thermal growth relative to surrounding
components. The most important
example of this phenomenon is the large
movement of boiler structures relative to
the cooler support framework. This part of a
plant includes waterwall sections, gas ductwork,
and the ties used to support superheat
and reheat tubing. These support ties are
designed to accommodate growth but are
subject to accelerated life consumption if
the frequency of thermal cycling increases.
Differential expansion also contributes to
tube-to-header cracking in superheaters and
reheaters (Figure 2).
Corrosion-Related Issues. Two-shifting,
3. Waterwall cracks. Waterwall damage
caused by corrosion fatigue is often found in
steam generators. Courtesy: EPRI
or any other operation that challenges the
ability of a plant to maintain water chemistry,
can lead to increased corrosion and accelerated
component failure. Increased levels of
dissolved oxygen in feedwater can be the result
of condenser leaks, aggravated by morefrequent
shutdowns. Other factors affecting
chemistry include the increased need for
makeup water and the interruption in operation
of the condensate polishers and deaerators.
Corrosion and fatigue can combine to
accelerate damage to waterwalls (Figure 3).
Fireside Corrosion. Load cycling and relatively
quick ramp rates under staged conditions
will have a negative impact on both fireside
corrosion and circumferential cracking.
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Rotor Bore Cracking. When subjected
to transients in the temperature of the admitted
steam, the high-pressure and intermediate-pressure
steam turbine rotors can
suffer thermo-mechanical stress excursions,
resulting in low-cycle fatigue damage. This
damage can result either from introducing
hot steam to a relatively cold rotor exterior,
or the opposite. In both scenarios, the
problem arises from the massive rotor forging
and the resulting time required for the
metal temperature difference between the
rotor exterior surface and the inner (bore)
region to equilibrate.
Impacts on Environmental Control
Equipment
Load following and other modes of flexible
operation can affect the performance and
reliability of flue gas desulfurization (FGD)
equipment and selective catalytic reduction
(SCR) systems. The chemical processes involved
in these systems require precise control
of the reaction conditions, which are
influenced by reagent flow, water flow, and
flue gas temperature.
Startups of FGD systems should be minimized
because of the need to purge systems
to avoid slurry solidification, the impact of
fuel oil residues on linings, and their lengthy
warm-up time. Low-load operation of FGD
systems may be difficult to optimally control
if the reagent flow is at a fixed rate.
Operation of large coal-fired plants at low
load can force units with SCR systems to operate
with lower flue gas temperatures. Low
temperatures create operational problems for
SCRs because of the formation of ammonium
bisulfate (ABS), a sticky liquid that can
fill catalyst pores, thus diminishing catalyst
surface area and reducing reactivity.
Strategies for Mitigating Flexible
Operation Damage
A range of strategies will be needed to mitigate
damage to coal units caused by flexible
operation. These strategies should be generally
assessed in terms of benefit-to-cost ratio
when selecting action plans for specific units.
Significant capital investment in improveddesign
boiler components may be warranted
for new, efficient units with control technology
installed.
In older plants, the most cost-effective
strategy from a life-cycle cost perspective
may be to focus on improved operator performance
and selected plant controls upgrades.
This approach could also include installation
of additional process sensors (typically temperature),
strategically located to guide operators
through transients to reduce damaging
over-temperature events. Increased attention
to the location, operation, and capacity of
POWER | August 2011
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POWER August 2011

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