POWER April 2017 - 31

OPERATIONS & MAINTENANCE
lead to cracking of the various components,
which may at times be self-relieving. While
the cracking may be initiated by fatigue, the
cycling will tend to propagate the crack.
Solid Particle Erosion (SPE). Low-load
operations, especially in the power blocks of
multiple units such as 2x1 or 3x1 that operate
in 1x1, keep the control valves pinched
back in order to maintain minimum pressure
on the HRSG. Because this action will cause
the steam to accelerate through the valve, any
solids that might be entrained in the system
are then jetted through the valve opening,
causing SPE on the valve stems. While repairable,
this damage is costly and time consuming
to fix.
Control Valve Testing/Sticking. A number
of factors can cause control valves to stick
or fail during a periodic valve test. It can be
electrical (for example, the control signal initiating
the test or providing test feedback can
be interrupted), or it can be hydraulic (such
as the solenoid valve experiencing a fault
such as varnishing or a clogged filter), or the
failure can be mechanical, because the valve
itself is actually stuck, and fails to travel
when tested. If this happens, proper precautions
must be taken to safely shut down the
STG without tripping the unit to prevent an
overspeed event. Mechanical sticking of the
control valve is most likely due to excessive
build-up of blue blush within the valve itself.
Prudent maintenance practices dictate that
these valves be periodically disassembled,
cleaned, and the wear parts replaced in order
for them to provide adequate STG protection.
Non-OEM Parts. While many sites have
had very good luck with non-OEM parts, a
few reports were made of incidents involving
the use of such parts to repair control valves.
While the incidents were not catastrophic in
nature, they did require additional forced outages
to accommodate the repairs.
Plant Startup and Shutdown
With regards to CCGT plant startup and shutdown
processes, a significant variation exists
across the industry, and developing a best
practices procedure would be beneficial.
Fast Start. An Italian CCGT facility
profiled in the EPRI study utilized sparging
steam to maintain pressure on all three HRSG
drums, as well as steam seals, and established
a very impressive record for startup and shutdown
efficiency. This site could be utilized as
a template for best practices going forward.
Bypass System Shutdown. A facility
located in the Midwest has modified its
procedures during shutdown to prevent its
bypass system from opening in an effort
to minimize wear-and-tear on the bypass
valves. This site has estimated that the revisions
to its startup procedures have realized
April 2017 | POWER
net savings of $20,000 to $40,000 per year in
valve maintenance.
Spreadsheet Startup. Two sites utilized
some form of Excel spreadsheet to facilitate
startup. Use of the spreadsheet at one site
resulted in a site-specific, case-specific, and
ambient-specific startup process, while the
other appeared to optimize the startup to
minimize fuel consumption. Development
of a similar type process for best practices
might be beneficial.
Siemens X-Crit. A site visit to a plant that
utilizes Siemens equipment provided an opportunity
to witness the Siemens startup optimization
system, which continually checks
permissives every step of the way, including
what the control room operator referred to as
" x-crits, " or rotor-specific variables. Incorporating
similar permissive checks, something
that considers the OEM stress analyzer, into
a spreadsheet type program would be useful.
Site Visits
In the EPRI study, based on the results of the
initial interviews, four sites were selected for
follow-up on-site visits. These sites were chosen
because the plants were doing something
unique, were deemed to be industry leading,
or had experienced unique problems. Site
visits lasted one to two days and, although
this was not always possible, were meant to
witness a startup and shutdown cycle. During
the site visits, additional time was spent
evaluating unique practices and how their
processes and procedures were implemented
and used.
Some unique approaches were observed
during site visits.
Thermal Blankets. A plant in the Southwest
Power Pool region modified its STG to
include heating blankets on the high-pressure
(HP)/intermediate-pressure shell to maintain
temperatures, helping the plant to all but
eliminate cold and warm restarts. These thermal
blankets are set to maintain temperatures
between 550F and 650F, depending upon the
location. The plant is now able to initiate a
warm or a cold start, in a 1x1 configuration,
and have the plant to minimum stable load
(with the gas turbine in Mode 6Q, which
means that it meets all environmental regulatory
requirements) within 90 minutes.
Spreadsheet. A plant dispatched into the
Midcontinent Independent System Operator
utilizes a unique spreadsheet to optimize
startup sequencing with the goal of minimizing
gas consumption during startup.
5. Start-up sequencing. This combined
cycle plant in Italy has developed an optimized
startup sequence that enables a hot
restart on an 850-MW F-class unit in just 45
minutes. Courtesy: EPRI
which steps need to be performed and time
stamps each step. This then becomes an ambient-specific,
configuration-specific, startup
check-sheet.
Optimized Startup Sequencing. A
unit in Italy has optimized startup sequencing
and is able to perform a hot restart on an
850-MW F-class unit in 45 minutes (Figure
5). According to the site, hot starts (STG HP
inlet temperature >345C) are about 40 minutes,
including a 5-minute purge timer; warm
starts (STG HP inlet temperature 220C to
345C) require 1 hour; and cold starts (STG
inlet <220C) require approximately 2 hours.
Blades. A 2x1 facility based on F-class
technology has had significant issues with
the L-0 STG blades caused by water erosion
and impingement. This facility also has
3x1x1 7C/E units that have been in operation
for about 40 years that have seen severe cyclic
duty, and has a 1x1 7EA unit that has
also had the L-0 STG blades removed as the
result of issues with resonance and tendon
failures.
Ongoing Evolutions
The
spreadsheet accepts operator inputs on ambient
conditions and calculates unit capacities
based on 1x1 and 2x1 operation with and
without duct burners and chillers. The operator
can then enter projected startup time
requirements, and the spreadsheet identifies
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A number of issues in the steam turbine
and generator suggest further examination
to improve cycling response is warranted.
The duty cycle of these units will continue
to change, and as was seen during the EPRI
study, some of the units that are cycling now
may in fact move towards a more base-loaded
profile, and some that are baseload units now
might become peaking units in a few years.
Whatever the case, these steam turbines
will continue to see the stresses of cycling
operations, and prudent operations and maintenance
personnel will have to be in a position
to manage this stress. ■
-Merrill Quintrell (mquintrell@epri.com)
is a principal technical leader with the
operations management and technology
program in EPRI's generation sector.
31
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POWER April 2017

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