POWER June 2011 - 58

COMBUSTION TURBINES
3. Alstom GT24/26. The sequential combustion system is a unique feature of the
GT24/26. Compressed air is heated in the first combustion chamber (EnVironmental or EV combustor)
by adding about 50% of the total fuel (at baseload). The pressure is halved after the
combustion gases expand through the single-stage high-pressure turbine. The remaining fuel
is added into the second combustion chamber (Sequential EnVironmental or SEV combustor),
where the combustion gas is heated a second time to the maximum turbine inlet temperature.
The gas then expands through a four-stage low-pressure turbine. Courtesy: Alstom
and economic factors that are usually led
by the projected price of natural gas. The
conventional wisdom was once to place
aero-derivative units in remotely located
applications (including offshore) and to
place heavy frame industrial units in easily
accessible baseload applications. However,
about 10 years ago in the U.S., the
combined cycle, based on the aero-derivative
turbine, became very popular given
both the speed at which a plant could be
constructed and its superior efficiency.
Both CT types are available in differ4.
Cold-end-drive option. The typical single-shaft combustion turbine has the compressor
and turbine sections on opposite ends of the shaft. The driveshaft in this figures extends from
the compressor section, hence it is a cold-end-drive engine. If the driveshaft extended from the
turbine end, it would be a hot-end-drive engine. Source: WorleyParsons Services Pty Ltd.
Combustion
chamber
ent configurations. In the hot-end-drive
configuration the output shaft is at the
turbine or exhaust end, where the higher
temperatures can affect bearing life and
make servicing of those bearings more difficult.
In the cold-end-drive configuration,
the output shaft extends out the front of
the compressor, allowing the exhaust gas
flow to be axial in some models (Figure 4).
With a cold-end-drive turbine, the driven
equipment is also relatively cooler and
easier to service. However, the compressor
inlet and ducting must be configured to accommodate
the output shaft and the driven
equipment. In sum, the CT configuration
is usually selected, when possible, by application
and site configuration.
Single-spool, integral output shaft CTs
Compressor
Generator
Atmosphere
air intake
Exhaust to
atmospheric
5. Two-shaft option. The typical two-shaft or split-shaft combustion turbine configuration places
the compressor and gas generator turbine on a common shaft. The hot exhaust gases " aerodynamically "
connect the power or free turbine to produce shaft power for the generator or other driven load.
The advantage of this configuration is that the output shaft speed can be controlled to that required
by the generator to eliminate a gearbox. Aero-derivative configurations use multiple gas generator
sections on concentric shafts to improve efficiency. Source: WorleyParsons Services Pty Ltd.
Gas generator section
Combustion
chamber
Fuel
in
Starter
Gas
Compressor
Shaft
generator
turbine
Hot gas
flow
Air intake
58
Exhaust
www.powermag.com
Power
turbine
Shaft
Generator
Power turbine section
Shaft
Turbine
(in both hot-end and cold-end-drive configurations)
are used primarily to drive electric
generators. However, the high torque
required to start pumps and compressors
under full pressure results in high turbine
temperature during the start-up cycle when
internal cooling airflow is low or nonexistent.
The solution has been to connect a
single (or two or three) spool compressor/
turbine section (also called a gas generator)
that produces hot gases to a separate
power or free turbine. The gas generator is
not physically connected to the power turbine
shaft but is coupled aerodynamically
(Figure 5). Also, the power turbine can be
designed to operate at the same speed as
the generator, not the gas generator, and
often eliminates the need for a gearbox to
match speeds. (Typical gearbox losses are
2% to 4% of power.)
Keep It Clean
It's often said that the key to achieving maximum
CT life is keeping the combustion air,
fuel, and lubricating oil clean. There are few
areas in the plant where an owner can more
easily improve long-term performance, and
therefore profitability, of the plant than in
these three systems.
Clean Air. The inlet and exhaust systems
should be selected for the minimum practical
pressure drop because those losses are
POWER | June 2011
http://www.powermag.com
POWER June 2011

Table of Contents for the Digital Edition of POWER June 2011
