POWER August 2011 - 54

PLANT OPERATION & MAINTENANCE
drains is another cost-effective operation and
maintenance strategy.
EPRI has undertaken a number of research
efforts to better understand possible mitigation
strategies.
Efficiency Improvements. One option
for mitigating the effects of flexible operation
is for plants to implement system modifications
that recover plant efficiency lost
to continuous cycling operation. However,
many plants today do not have sufficient
capital available to undertake major system
modifications. To address this challenge, a
recent EPRI study identified modification
measures that have the potential to achieve
the most impact for a reasonable investment.
The options involve modifications to
equipment and operating procedures that
will be cost-effective for reducing heat rate
under cycling operating conditions.
E.ON's Strategies for Managing Cycling
-A Toolbox Approach
The Germany-based utility holding company
E.ON has taken a toolbox approach
to improving the cycling performance of
its coal and combined cycle plants, using
the following tools.
Flexible Operation Studies. These
studies reduce component damage through
procedure optimization and design modification.
Included in the studies are an initial
appraisal of plant-specific risk areas,
installation of additional instrumentation,
flexible operation trials, assessment of
thermal transients, modifications to operating
procedures and design to address
issues identified, repeat trials to confirm
success, and detailed stress analysis to inform
strategy going forward.
Examples of steps taken include optimization
of overfire air to prevent excessive
platen superheater temperatures, use of
progressive drainage to minimize thermal
transients, and optimization of attemperator
spray usage to avoid quenching.
Studies conducted in the mid-1990s
achieved substantial reductions in thermal
transients, reduced hot start times from
90 to 60 minutes, and produced typical
savings of £800 ($1,292) per start in fuel,
auxiliary power, and water costs.
Operator Coaching. Simplified damage
algorithms for creep and fatigue are also
developed for operator coaching. Plant
data for critical components is screened to
identify and understand the most damaging
operational conditions. Operators can
then seek to minimize the extent of such
conditions during future unit starts.
Inspections and Component Assessment.
Targeted inspections based on component
risk, defect history, and operating
regime are conducted during outages. Defect
assessments are made as appropriate.
For high-cost strategic components and/
54
or those whose failure has severe consequences-such
as headers, pipework,
steam chests, and rotors-detailed safety
cases based on finite element modeling
are sometimes required. For severe defects,
these safety cases may stipulate an allowable
number of starts before component
re-inspection is required. Structures and
support systems, which are also affected
by flexible operation, are also examined
and managed proactively.
Maintenance Strategies. Maintenance
strategies are developed about every three
to four years to allow future budgeting and
phasing of component replacement (as
informed by inspection and experience).
These strategies are forward-looking to
anticipate requirements of expected operating
regimes in terms of hours and starts,
and are sometimes scenario-based. They
include a detailed review of site-specific
defect/failure histories to date.
Design Modifications. Modifications
are proposed to the design of replacement
components to " design out " damage
mechanisms, if possible (for example, selective
use of P91 metal in place of 2¼
Cr for thick-sectioned high-temperature
headers, and increased flexibility of header
antler arrangements).
Damage Estimation. Estimates can be
made of damage costs per start to inform
the plant's trading position based on increased
routine maintenance costs, damage
to major components, and estimated
cost of consumables per start.
New-Build Design. Lessons learned are
incorporated into specifications for newbuild
plants.
Adapted from material provided by Dan
Blood, team leader within the Plant Integrity
Department of E.ON New Build &
Technology.
www.powermag.com
For each option, preliminary assessments
were made of the potential benefits (in terms
of electrical output and heat rate recovery)
and possible reliability risks. Examples include
sliding pressure operation, variablespeed
drives for main cycle and auxiliary
equipment, and boiler draft control schemes
and operating philosophy.
Cycle Chemistry Guidelines for Transient
Operations. An area of particular
concern for plants under cycling duty is following
appropriate cycle chemistry guideline
limits during plant startup, shutdown, and
layup. Proper protection of the entire steam
circuit (boiler, piping, feedwater, and turbine)
is critical during these modes of flexible
operation.
With this in mind, EPRI worked with
the 40 members of the EPRI Cycle Chemistry
Technical Advisors Group to develop
cycle chemistry guidelines for all transient
operations and shutdowns. The guidelines
include specific procedures and advice
for cycling, shutdown, startup, and
layup for each boiler and feedwater treatment;
they also cover all major water- and
steam-touched surfaces. Correct layup
procedures, combined with appropriate
chemical treatment during shutdown and
startup, will significantly reduce corrosion
and deposits in the steam cycle equipment,
including the boiler, steam-touched tubing,
and the turbine.
Two-Shift Operating Practice. Changes
in operational practices can be an effective
strategy for mitigating cycling damage.
Several years ago, EPRI studied worldwide
experience with two-shift operation (usually
weekday starts with overnight shutdowns)
and found that economic two-shifting can
be achieved with due care and application of
sound engineering and operational practice.
Among the findings:
■ Many utilities have performed trials on
two-shift operation to reduce startup and
shutdown time. Generally, startup times
can be nearly halved from original baseload
procedures so that large machines can
be synchronized within 35 to 50 minutes
of inserting the first burners-depending
on unit size and configuration-and full
load can be achieved in similar times. A
target time to full load on a 500-MW machine
is approximately 60 minutes.
■ Thermal transients-both in the form
of quenching and high rates of temperature
rise-can be avoided by carefully
managing the unit when off-load
and by adding engineered systems to
alleviate the potential problems. In the
United Kingdom, natural-circulation
drum boilers have been fitted with offPOWER
| August 2011
http://www.powermag.com
POWER August 2011

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