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Page | 015 7.7 Single and Multi-shaft Design 163
• No cooling steam required
• Reduced starting power
• Reduced emissions
• Standardized design
• Simplified torsion analysis
B. Increased flexibility
• Simplified commissioning
• Steam turbine trips do not stop power generation
• Maintenance of gas turbine (GT) is possible during steam generation
(SG) cooling
C. Optimized shutdown
• Shutdown steam turbine (ST) at reduced gas turbine (GT) power
Multi-shaft combined cycle systems have one or more gas turbine generators and
HRSGs that supply steam through a common heater to a separate single steam
turbine generator unit. Both configurations perform their specific functions, but the
single-shaft configuration excels in the baseload and midrange power generation
applications.
The multi-shaft combined cycle system configuration is most frequently applied
in phased installations in which the gas turbines are installed and operated prior to
the steam cycle installation and where it is desired to operate the gas turbines
independent of the steam system. The multi-shaft configuration was applied most
widely in the early history of heat recovery combined cycles primarily because it
was the least departure from the familiar conventional steam power plants. The
single-shaft combined cycle system has emerged as the preferred configuration for
single phase applications in which the gas turbine and steam turbine installation and
commercial operation are concurrent.
Multi-shaft systems have one or more gas turbine-generators and HRSGs that
supply steam through a common header to a separate single steam turbine-
generator. In terms of overall investment, a multi-shaft system is about 5% higher
in costs.
The primary disadvantage of multiple-stage combined cycle power plant is the
number of steam turbines, condensers and condensate systems, and perhaps the
cooling towers and circulating water systems required by the bottoming cycle.
A Heat Recovery Steam Generator (HRSG) is a heat exchanger or series of heat
exchangers that recovers heat from a hot gas stream and uses that heat to produce
steam for driving steam turbines or as process steam in industrial facilities or as
steam for district heating.
An HRSG is an important part of a combined cycle power plant (CCPP) or a
cogeneration power plant [4]. In both of those types of power plants, the HRSG uses
the hot flue gas at approximately 500–650 C from a gas turbine to produce high-
pressure steam. The steam produced by an HRSG in a gas turbine combined cycle
power plant is used solely for generating electrical power. However, the steam |
