POWER August 2011 - 58

PLANT OPERATION & MAINTENANCE
Make Your Plant Ready for
Cycling Operations
Cycling your steam power plant is inevitable, so now is the time to learn how to
minimize equipment damage and assess the true costs of cycling. Whether
cycling is required by the grid operator because of renewable integration or
other factors, you must be proactive about updating operating processes and
upgrade equipment so the transition to cycling operation goes smoothly.
By Steven A. Lefton and Douglas Hilleman, PE, Intertek-Aptech
F
ew conventional steam plants were designed
to follow load, cycle from minimum
to full load every day, or shut down
and start up daily, as so many plants are called
upon to do these days. The challenge for owners
of plants required to operate in this way is to
fully understand the effects on plant and component
life expectancy, and the actual costs, of
these new operating profiles. If the actual costs
are unknown, making a profit becomes a matter
of luck rather than good management.
In a competitive electricity market, not
knowing your true generating costs could
place your plant or your company in economic
jeopardy. For example, if the actual cost of cycling
a unit is not included in your bid price,
and the plant must cycle, not only are you not
being compensated for unknown damage to
plant equipment, but you also are not being
compensated for future maintenance and unplanned
repair outages. Those costs often far
exceed the short-term profits gained by submitting
an artificially low market bid price.
Furthermore, the bill for the true cost of
operation will arrive after the fact, often
years later, in the form of lower capacity factors,
less generation, and higher production
costs. If, at that time you update production
costs to reflect your new reality, your plant
may no longer be competitive, resulting in
even worse economics.
The same scenario applies in a regulated
environment: Future higher-than-expected
operating and maintenance costs have to be
justified to the public utilities regulators and
may not be recoverable.
The following discussion applies equally
to solid fuel-fired steam plants and natural
gas-fired combined cycle plants.
Cycling for Dollars
Cycling your plant without understanding
the costs or applying care and expert guidance
when cycling often leads to significant
damage, frequent forced outages, and loss of
power generation. Bad long-term decisions
58
1. Common problems in cycling plants. A survey of 215 steam plants found many
common equipment problems. Source: Intertek-Aptech
600
500
400
300
200
100
535
225
170
140
140
125
75
75
50
may be driven by short-term or uninformed
decisions, making your plant unreliable during
high-profit periods. The fatigue damage added
to an older baseload power plant causes creep
fatigue interaction damage, rapid increase in
boiler tube failures, and many other component
failures, including turbine generator and
balance-of-plant early creep fatigue failures.
In effect, excessive cycling will either decrease
the life expectancy of your equipment, or the
costs to maintain the equipment will rise,
sometimes significantly. Also remember that
poorly written startup and shutdown operating
instructions or a failure to follow standard
processes can often contribute to increased cycling
damage and resultant costs.
In our experience, about 60% to 80% of
all power plant failures are related to cycling
operations. If plant management understands
the costs and has a process that provides actual
operating costs, it is able to take proactive operational
measures rather than deal with unexpected
costs and poor performance years after
damage to plant systems is done (Figure 1).
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Locating Failures
Failures in coal plant equipment caused by frequent
cycling are not isolated events but often
occur at unexpected times throughout a plant.
Many of the failure modes and locations are not
unexpected, but others may be surprising even
to an experienced plant operator. Prevention of
these failures or managing failure rates is key
to success. One approach to reducing cycling
damage is to modify the unit's low-load limit to
prevent cycling damage. By reducing the lowload
limit on a 750-MW supercritical gas-fired
unit from 150 MW to 28 MW at night, it was
possible to make the daily cycling load profitable,
but it was no small task.
The following subsections are provided to
give you a sense of the magnitude of systems
and components within a typical coal-fired
plant designed for baseload operation that
are affected by cycling and load-following
service. Although they are not listed separately,
controls are also affected; some plants
must upgrade old analog controls to digital
controls in order to improve response to rePOWER
| August 2011
Thermal stress
Corrosion
Axial displacement
Water induction
Vibration wear
Water and particle wear
Centrifugal stress
Windage heating
Rapid cooldown
Number of actions taken
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POWER August 2011

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