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GAS TURBINES
SINGLE-SHAFT COMBINED CYCLE DESIGNS
SAVINGS RANGE FROM CAPITAL COSTS TO REDUCED FUEL CONSUMPTION
HAMP THORNTON AND THORSTEN WOLF
I
n a multi-shaft design, both prime movers
(gas turbine and steam turbine) have their
own independent generator. A single-shaft
turboset, on the other hand, has a gas turbine and steam turbine on one shaft line,
both driving one common generator.
The advantage of a single-shaft turboset
is that only one generator needs to be connected to the grid. That means one set of bus
ducts, generator, generator breaker, generator step-up transformer and switch yard are
needed which equates to capital cost savings
of 3% to 5% for the plant.
In addition, there is a small gain in plant
efficiency as one large generator has a slightly better efficiency than two smaller ones.
This has an impact on fuel costs over the life
of the plant.
For a plant with two gas turbines, two
single-shafts are an alternative to the traditional 2x1 configuration. With one steam turbine for each gas turbine, the
single-shafts have better
efficiency when only one
gas turbine is in operation.
The single-shafts also
offer better efficiency
than a 2x1 when changing
from one to two gas turbine operation. With
each heat recovery steam
generator (HRSG) producing steam for its own steam
turbine, there is no need to
bring up the second HRSG in
bypass operation to match steam
temperature and pressure before blending
with the steam going to the turbine from the
first HRSG.
The newest Siemens single-shaft design
is based on the SGT-8000H gas turbine
(Figure). It can provide a combined cycle
efficiency of around 61%, as confirmed at
the Cengiz Enerji Samsun plant in Turkey.
This machine is air cooled, uses a thermally
flexible rotor with a central tie bolt and serrated, interlocking discs, can-annular combustors and a 4-stage turbine. It operates at
conventional temperatures (similar to the Gclass) and uses no single crystal alloys.
The SGT-8000H includes Siemens
Hydraulic Clearance Optimization (HCO)
technology, which enables high efficiency
without high temperatures or pressure
ratios. The rationale is that tight, minimized clearances are an efficiency driver.
HCO is applied in an engine with a con22 Turbomachinery International • March/April 2015
ical flow path. The rotor is shifted slightly
aft, opening up the clearance around the
blades. After the casing has reached steady
state temperature, the blades are moved
upstream, resulting in tight clearances.
This feature enables the engine to have
no hot restart restriction. Without this, fast
start and a hot restart transient can result in
blade rubs, or the need for larger clearances.
This and other features allow the engine to
have a 30MW/min ramp rate without performance penalty.
The steam turbine is coupled to the
generator with a self-shifting synchronizing clutch enabling the gas turbine and
steam turbine to start independently. The
clutch is an integral part of the shaft
line and is supplied
with oil from
the lube oil
system of
the turboset.
Figure: SGT-8000H gas turbine
The self-shifting mechanism has no
external drive or friction device and requires
little maintenance. Its engaging and disengaging mechanism uses helical splines to
engage a sliding element of the coupling
when the steam turbine shaft has reached the
speed of the generator. When the steam turbine valves are closed, the speed of the steam
turbine shafts falls off and disengages the
sliding element via the helical splines.
This coupling is a proven element in
Siemens turbosets, where it has been used
since 1995 in units up to 580MW. Enabled
by Siemens Flex-Plant combined cycle
design with a high pressure (HP), intermediate pressure (IP) and low pressure (LP)
bypass system for the full steam flow, the
gas turbine can be ramped to base load first,
allowing the steam turbine to be loaded
when steam has the matching temperature.
The SCC6-8000H single-shaft uses an
SST6-5000 steam turbine with a combined
HP and IP casing and a dual-flow LP casing.
Having the hot steam from HP and IP in one
casing, and the expansion of the cooler LP
steam to condenser pressure in a separate
casing, improves unit cycling. The HP and
IP turbine retains heat and slows cooling
during standstill periods.
Panda Patriot and Liberty
As the first customer for the SCC6-8000H
single-shaft, Panda Power Funds is building
two combined cycle power plants in
Pennsylvania. The first plant consisting of
two SCC6-8000H single-shaft units (the
Panda Liberty Generating Station in
Bradford county, northern Pennsylvania
with a rating of 829 MW) should
start commercial operation in early
2016. The second plant will be the
Panda Patriot Generating Station in
Lycoming County.
Due to proximity to the
Marcellus Shale gas formation,
and overall combined cycle
efficiency, both plants are
expected to operate at
base load. Instead of
river or cooling tower
water, they will use
air to cool the condenser, reducing water consumption to less than 5% of a
wet-cooled facility.
The LP steam turbine has been customized to deliver high efficiency on
cold days. The plant is capable of producing maximum power when temperatures reach 95°F (35°C) and power
demand is highest.
With its double flow LP using a 28
in. (710mm) blade, the steam turbine
can operate with full load conditions at a
backpressure up to 12 in. Hg (400mbar)
and still have enough margin against
fluctuations in backpressure (e.g.,
caused by gusts at the air cooled condenser structure).
Authors
Hamp Thornton is Engineering Manager at Panda
Power Funds, Dallas. Panda Power Funds is a private
equity firm that can develop, acquire, construct,
finance and operate large-scale, natural gas-fueled
power plants. The Fund currently has two combinedcycle plants in operation and four plants under construction in Texas, Pennsylvania and Virginia.
Thorsten Wolf is a Product Manager at Siemens
Power and Gas Division. Siemens offers both single-shaft and multi-shaft designs. For more information, visit www.siemens.com
www.turbomachinerymag.com