POWER September 2011 - 32

TOP PLANTS
the second phase would be conversion to a 2 x 1
arrangement. A supplementary-fired HRSG was
also selected, to provide peaking capability.
City officials determined that S&L's previous
experience related to repowering projects
would help to make the Hopkins repowering
project successful. The city assumed responsibility
for construction management, commissioning,
and start-up, while S&L provided
technical and field support. This teaming arrangement
also contributed significantly to
the project's success.
Modifications Performed to
Repower the Plant
Yee shared three examples of important interface
areas that required modifications for reuse.
Controls. The original Hopkins 2 was designed
with a conventional cascading bypass
system and was equipped with nonmodulating
steam turbine intercept valves. Yee pointed
out that it would have been too expensive
to replace these valves with ones capable of
modulating. Thus, the system could not be
used for turbine start-up in the same manner
as a typical bypass system designed for a new
combined cycle.
" Here's how the repowered Hopkins is
started: The cascading bypass system warms
up steam piping initially, " he said. " Prior to
steam roll, high-pressure and cold-reheat bypass
steam are diverted to the high-pressure
sky vent, and the reheater is evacuated. "
Controls are arranged this way because
1. Steamy side. As part of the repowering
project, a heat recovery steam generator
(HRSG) was added to a new combustion turbine
to produce steam to power the plant's
existing steam turbine, making it a true repowering
project. The plant has provisions to
add a second combustion turbine and HRSG
in the future. Courtesy: City of Tallahassee
the nonmodulating intercept valves could not
control overspeed if reheat pressure were to
increase during the steam turbine roll. Cold
reheat steam is admitted to the reheater during
the turbine roll at a controlled and increasing
rate dictated by turbine acceleration
and speed control. Control system modifications
were made to accommodate the various
start-up modes required.
Condenser. The heat-rejection capability
of the original condenser was evaluated
for use in the repowered configuration. Yee
pointed it was important to " keep in mind
that extractions for the feedwater heaters,
now abandoned in place, had to be capped,
increasing flow to the condenser. " Engineers
said the existing condenser was adequate for
the duty, but that a few modifications were
needed. They included:
■ Redistribution of the heat source to eliminate
hot spots.
■ Rerouting steam turbine low-point drains.
■ Replacing the first feedwater heater in the
condenser neck with the bypass diffuser.
Steam Turbine. The steam turbine was
evaluated to determine what modifications
and/or operating limits were required to ensure
its suitability for the repowered plant
(Figure 1). Overall, 13 areas were studied,
including these four:
■ Verifying the mechanical integrity and
permissible operating limits, considering
all loads.
■ Defining the temperature limits of all turbine
components, including design limitations.
■ Determining the maximum flow through
all turbine sections to the condenser-this
was to be done to verify the unit's ability
to handle the repowered steam flow
with feedwater heater extraction nozzles
capped.
■ Defining the start-up requirements, procedures,
and operational limits of the repowered
turbine, including ramp limits for
cold, warm, and hot starts.
The repowered Hopkins Unit 2 has a
couple of additional unique features, Yee
explained. For example, a bypass stack was
installed ahead of each HRSG to permit
simple cycle operation in the unlikely event
of a steam turbine/generator failure. This bypass
stack does not have a typical damper.
To minimize leakage, a solid steel plate is
used to direct hot gas to the HRSG during
normal operation. When needed in the event
of a steam turbine/generator failure, the plate
can be removed from the stack and placed in
front of the HRSG in the transition duct to
block the gas flow to the HRSG.
32
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Profile of the New Plant
The S&L project team provided their expertise
as far as knowing what controls, systems, and
other equipment to reuse, modify, relocate, or
abandon in place. Because solutions vary from
site to site due to differences in design criteria,
margins, and other factors, it was important
that the engineering firm had the experience to
deal with a wide variety of challenges.
The new and modified equipment installed
at the repowered plant included, but was not
limited to, the following:
■ New HRSGs (Nooter/Eriksen Inc.)
■ New gas turbines (GE Energy)
■ New digital control system (Emerson Process
Management/Ovation)
■ Modified existing condenser (Westinghouse
Electric Corp.)
■ Modified existing steam turbine (Westinghouse
Electric Corp.)
■ New selective catalytic reduction technology
(Vector Systems Inc.)
■ New bypass stack (Braden Manufacturing
LLC)
■ New critical piping/P91 piping (BendTec)
The plant is located on 230 acres of land
seven miles west of Tallahassee. The area,
which is covered with oaks and pines, provides
a haven for a variety of wildlife, including
birds, deer, alligators, and even a
community of beavers.
Protection of the environment is a prime
concern of the city's electric utility and the
plant employees. Chemists in the plant's
laboratory perform more than 40 tests on water
every day, including tests for metal contaminants,
mineral precipitates, and acidity.
A separate environmental monitoring facility
also provides independent oversight and verification
of all environmental safeguards.
Realizing the Vision of a Leaner,
Cleaner Plant
The first phase of the repowered Hopkins
Unit 2 has run well since being declared
commercial on June 1, 2008, according to
City of Tallahassee General Manager Robert
E. McGarrah, Manager of Power Production
Triveni Singh, and Plant Manager Clark
Sheehan. The repowered unit was completed
on time, without any lost-time accidents, and
8% under the $156 million budget.
The repowered unit has met all expectations
regarding efficiency goals and environmental
performance. As a result of the
efficiency gains, the city projects $12 million
to $24 million per year in fuel savings for its
electricity customers, depending on the cost
of natural gas. ■
-Angela Neville, JD, is POWER's
senior editor.
POWER | September 2011
http://www.powermag.com
POWER September 2011

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