POWER August 2011 - 62

PLANT OPERATION & MAINTENANCE
program must be reformulated to optimize
remaining boiler tube life.
Often, the key to reducing waterwall tube
wastage is simply using a nitrogen blanket
over the internal tube wetted surfaces when
the boiler is removed from service or draining
it. Also consider a nitrogen blanket in the
condensate storage tank to minimize the oxygen
content in the condensate during starts.
In heat recovery steam generators
(HRSGs), the typical cycling problems can
be traced back to superheater and reheater
drains that fail to clear accumulated condensate.
Flow-accelerated corrosion (FAC) in the
low-pressure evaporator and failure of feedwater
heaters from thermal shock and FAC
also are often found (Figures 7 and 8).
Fuel Systems. Coal pulverizers are prone to
fireside explosions, especially when using western
coals. They require careful fuel purging and
the addition of an inert gas blanket (inerting)
when they are cycled off-line. Also, pulverizers
7. Inflexible headers. Reheater outlet
header and tube-to-header weld fatigue damage
at the end of the header is shown. The
high-stress location is the short run of tubing
where the tube cracked, due to low tube flexibility.
The stress is a result of thermal loading
and deflection during startup cycles. Courtesy:
Intertek-Aptech
are prone to much increased mechanical wear
when they are cycled or operated at the low end
of their design minimum flow rates. (For more
information on proper operating and maintenance
of coal pulverizers, see " 'Blueprint' Your
Pulverizer for Improved Performance, " March
2009 and " To Optimize Performance, Begin at
the Pulverizers, " February 2007 in the POWER
archives at www.powermag.com.)
Another unanticipated consequence of cycling
a solid-fuel plant is the additional maintenance
required on the coal silos. When burn
rates are difficult to predict, the holding time of
western coals in silos can extend beyond six or
seven days, the maximum time considered safe.
Longer storage of western coals often causes an
increase in the frequency of hot spots and fires
inside the coal silos. The same safety issues apply
to coal pile management and inventory control,
as burn rates constantly change due to plant
load cycling or low-load following.
Air and Gas Systems. The forced and induced
draft fans on coal-fired units can range
from a few hundred to well over a thousand
horsepower. Frequent starting and stopping of
the fans-and, just as importantly, the motors
that drive the fans-will increase failure rates,
inspection intervals, and motor-fan maintenance.
In some cases, fans have required retrofit
of new drive systems for soft or low-stress
starting. Additionally, air heaters and baghouses
are subject to wet gas corrosion, plugging, and
damage caused by operating below the wet gas
dew point during low-load operations.
Water Systems. It may seem counterin8.
Leaky heaters. This HRSG feedwater
heater tube was one of many failures that resulted
from cold-end corrosion during cycling.
There were numerous leaks at the tube-toheader
connection caused by thermal cycling
damage. Courtesy: Intertek-Aptech
tuitive, but makeup water consumption greatly
increases for a cycling unit because of the
large amounts of water used during startup.
In fact, it's possible that the plant's demineralized
water supply and water storage may
need to increase to support the increased
boiler water usage.
The entire feedwater system is affected by
thermally cycling plants. For example, feedwater
heaters are subject to cooldown and
rapid heating during a hot or warm startup cycle,
and this often leads to early tube failures.
Boiler feed pumps are normally designed for
full-load operation and experience accelerated
wear when operated at low loads. Also, boiler
feed pumps may be required to stop and start
several times for one startup cycle, thus causing
many feedwater transients. In some plants,
a smaller, low-load feedwater pump may be
necessary for a cycling unit to limit damage
to the main feedwater pumps. In others, boiler
feedwater and condensate pumps may require
added recirculation capability, and the economizer
may require recirculation to keep exit
water temperatures in check.
Combustion Turbines. Gas-fired combined
cycles may be quick-acting in comparison
with a coal-fired plant, but they are not
62
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immune to the same thermal stress-related
equipment damage described above. Based
on our experience with combined cycle
plants, we have found that the largest line
item on a plant's operating and maintenance
budget is costs related to cycling.
One reason cycling costs are proportionally
higher in combined cycles is that turbine
inspection and repair intervals are transitioned
to being principally based on numbers of starts
rather than operating hours. Therefore, the cost
to maintain combined cycle availability is high
on a per-unit-of-electricity-produced basis. The
cost of replacement parts found damaged during
overhauls, or " parts fallout " due to cycling, is
often a factor of two or more over the plant life
compared with baseload operation.
Asset Management Principles
Managing power plants to operate at low cost
and without excessive damage is the goal of
every plant owner (see sidebar, p. 66). The
cost of baseload operation is, for example,
two-dimensional-the costs are based on operating
hours at full load. However, cycling a
unit poses a multi-dimensional problem that
is difficult to comprehend or cost estimate.
The challenge expands progressively because
" cycling operation " is a broad concept that
encompasses load-following, low load, hot
starts, warm starts, and cold start of a plant
with different lengths of time between operations;
it is definitely not solely time-based.
Following are some useful suggestions for
how to begin defining the true cost of cycling
your plant. First, examine all the operating
and maintenance cost categories that apply
beyond fuel. We discussed a number of examples
earlier. Equipment suppliers often
provide life-reduction curves based on the
time or number of instances that a steam turbine,
for example, exceeds temperature and
pressure design and ramp rate limits when
started or load cycled. Other areas in your
cycling operation cost accounting include:
■ Increased equipment maintenance costs.
■ Increased forced outages costs.
■ Heat rate losses during startup through
synchronization and loading to zero load.
■ Heat rate losses during operation at loads
lower than full load, including increased
auxiliary power usage.
■ Heat rate losses due to cycling, such as leaky
seals and valves, and worn out or less-efficient
heat transfer equipment during cycling.
■ Additional startup fuel, chemicals, and
staff required.
■ Long-term loss of critical equipment life,
such as the boiler/HRSG components (superheaters,
reheaters, economizers, feedwater
heaters, condenser, and other balance-ofplant
equipment).
POWER | August 2011
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POWER August 2011

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