HIGHEST OUTPUT OF GEARED INDUSTRIAL STEAM TURBINE:
PRODUCT LINE SST-400 - EXPERIENCE, FUTURE TRENDS AND
APPLICATION IN COMBINED CYCLES
Milan Korista, Siemens Industrial Turbomachinery, Czech Republic
Lenka Kuchtova, Siemens Industrial Turbomachinery, Czech Republic
Don Haswell, Siemens Industrial Turbomachinery, Czech Republic
Jaroslav Lahoda, Siemens AG, Germany
A typical feature of modern generator-drive steam turbines for industrial applications in the
power range up to 60 MW is the use of a gearbox between the steam turbine and the
generator. The technical concept of Siemens’ SST-400 turbine, which is a modern and at the
same time well-proven product, is environmentally-friendly due to the high efficiency, and
the decreased materials and fabrication energy demand. In this paper the SST-400 is presented
by references from Thailand, Malaysia and Turkey.
Relevant market drivers can be summarized as follows:
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emissions reduction
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fuel consumption reduction
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combined heat and power production
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rising mean value of the power output of industrial generator-drive applications
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optimized investments
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environmentally friendly life cycle
The sustained demand to reduce investment cost of steam turbines for industrial applications
over the last 20 years has led to introduction of the concept of a gearbox between the steam
turbine and the generator for generator-drive steam turbines in the power range up to 60 MW.
The decision to use a gearbox is driven by technical and economical requirements. Increased
speed of rotation decreases the geometrical dimensions of the flow path with immediate effect
on the mass of the core turbine. On the other hand it is necessary to take into account two very
important side effects. Mechanical efficiency of the gearbox and changed slenderness ratio of
the stator and rotor blades of the flow path bring new influences on energy conversion of the
turbine and heat rate of the cycle, which are, at the end, the most important factors evaluated
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by investors. Gearbox efficiency is less than one and mechanical loss dissipated into heat
there must be taken away by a sufficient amount of lubricating oil. This loss can be totally
compensated by the positive effect of the increased blade-slenderness ratio and can even bring
certain additional efficiency gain. The chain of cause and effect behind this is the
redistribution of the components of the losses in the blade path. Increased slenderness ratio of
the blades decreases the role of the peripheral loss on the overall cascade efficiency. To reach
a really positive effect by increased speed it is necessary to attend to design of stator and rotor
blade shrouds and tips. The blade technology of Siemens’ SST-400 product line utilizes well
proven reaction profiles and the shrouds are designed to decrease as much as possible the
peripheral and other losses which might otherwise cause a negative effect when the speed of
the core turbine is increased by use of a gearbox. There is another positive economic side
effect of the geared concept of the generator-drive steam turbine which comes from the
possibility to reduce the rotation speed of the generator to half synchronous speed and thus
use a four-pole generator, which is cheaper than a two-pole machine.
Figure no.1 – Arrangement of the Siemens’ SST-400 condensing steam turbine with axial
exhaust (condenser is not shown)
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Figure no.2 – Assembled Siemens’ SST-400 condensing steam turbine with axial exhaust
The above concept is implemented in the Siemens product line SST-400 which covers
different sizes of condensing, heating as well as backpressure turbines. A strong feature of
this machine is the possibility to install single or double extraction devices. Different
orientations of the live steam inlet pipes, control valve arrangements and orientations of the
radial exhaust nozzles are available. The most modern solution for the exhaust nozzle of
condensing turbines, i.e. axial exhaust, is implemented for all sizes of the last-stage blade
groups and is starting to be the standard solution in these applications. To meet customers’
technical and delivery requirements in time, sets of standardized components were created,
such as control wheel with nozzle chest, groups of last-stage blades for condensing turbines,
extraction devices etc. and software tools supporting effective calculation and design process.
These measures show how Siemens has focused on industrial steam turbine applications, and
built its technological portfolio to supply modern but well-proven solutions in generator
drives.
Current market demand puts a certain priority on environmentally-friendly manufacturing
methods as evinced in Siemens’modern production methods and worldwide manufacturing
and supply chain. The SST-400 product line, which is the subject of this paper, is fabricated
in Europe: the manufacturing methods comply with ISO 14001 and are subject to ISO 9001
verification. The newest version can be equipped with a remote monitoring system which
gives the customer the additional benefit of reduced maintenance costs.
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Figure no.3 – Parts of the blade path of the Siemens’ SST-400 condensing steam turbine
Siemens’ SST-400 product line brings relatively high flexibility to design and build generator
drives for different industrial applications. In this paper we would like to focus on the highly
efficient energy conversion which can be attained in small combined-cycle plants, for which
the SST-400 turbine represents the highest-output geared-turbine solution. It is known from
experience that scaling the size or power output of the technological unit brings a cost
response which is not a smooth curve. The reasons are forced by changes of the technological
components. For example, the possibility to use a four-pole generator instead of two-pole.
The SST-400 product line offers the highest-output geared steam turbine where all
components which are pushing the technical constraints are the subject of a continuous
development process. Our long term experience with key turbine components brings the
customer a guarantee of money well invested. Moreover, references are available for all key
components such as:
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reliable control wheel with multi-nozzle group control,
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highly efficient HP reaction blading with enhanced shrouding,
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internally controlled extraction devices based on double-seated-valve technology for
HP extractions up to 45 bar,
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internally controlled extraction devices based on adaptive-stage technology for LP
extractions,
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groups of last-stage blades for condensing turbines with either water-cooled condenser
or air-cooled condenser,
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highly efficient groups of last-stage blades for turbines used in district heating plants,
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reliable gearbox concept based especially on long-term experience with the
manufacturer and supplier FLENDER AG, which is also a part of Siemens AG.
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Current market trends and requirements relevant for the selected topic can be summarized as
follows:
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The need to reduce emissions combined with the desire for efficiency improvement
driven by the demand to reduce fuel consumption. This can be achieved only by a
highly efficient turbine with high flexibility of the operation modes. Highly efficient
and well proven reaction blades create a reasonable presumption to reach the goal.
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Cogeneration production (combined heat and power production) to achieve maximum
utilization of fuel. Very often this requirement is coupled with a request for
independent power and heat control which is solved by controlled-extraction turbines.
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The rising mean value of the power output of industrial generator drive applications.
This point creates the potential for economically effective solutions provided the size
of the turbine is manageable within the geared concept.
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Continuous demands to optimize performance ratio of investments which is a basic
economic criterion.
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Environmentally-friendly life cycle including fabrication, erection, commissioning,
operation, maintenance and its decommissioning at the end of the turbine lifetime.
The SST-400 product line was designed and developed to meet the above-mentioned trends
by the use of standardized sets of components with high flexibility for adaptation to projectspecific requirements, e.g. bleeds, extractions, second admission typical for double-pressure
HRSG’s of combined cycles, etc. The technical concept allows for the use of a four-pole
generator up to the maximum available size. The concept of a four-pole generator and an
appropriate gearbox for the highest transmitted power outputs, together with the welloptimized standard components, forms the basis for a “kitset” design. The degree of
standardization and the effective production methods ensure economically effective solutions.
The blade path uses reaction blading, which, from the efficiency point of view, is less
sensitive to operation off the optimum or guaranteed load point. That brings a significant
advantage and the benefit of decreased fuel consumption for plants, the operation of which is
dependent on the available energy supply or the consumption of the generated electrical
energy, especially, for example, in captive power plants. The adopted reaction blading
combined with the up–to-date technology of the last-stage blade groups for condensing
applications is well-proven and represents the materialized outcome of Siemens continuous
research and development activity. The turboset concept allows for either a radial exhaust, or
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a modern axial exhaust layout. The latter brings the customer the benefit of cost reduction in
respect of the turbine room civil works. The SST-400 with an axial exhaust represents the
state-of-the-art simplified layout of the complete turboset for installation at ground floor level.
The turbine is mounted on a skid, the generator is anchored to the concrete foundation and the
gearbox in between is mounted on an oil tank integrating the active parts of the oil system
such as pumps, throttle valves to control oil pressure, and emergency and safety valves of the
oil circuit. Integration of these components of the oil system into one central package
minimizes the risk of oil leakage. The core steam turbine SST-400 is pushing the limits of
four-pole generator and gearbox technology development. Siemens AG is working on the
coordinated development of all these three major components under one brand, with the
potential for synergetic effects.
This turbine is highly flexible in operation: it has already proven successful in combinedcycle power-plant (CCPP) applications, not only together with Siemens gas turbines but also
with gas turbines of other brands. A number of machines have also been sold to the utility,
IPP as well as captive power plants. Our references are located in Thailand, Malaysia and
Turkey.
Figure no.4 – “kitset” design of the Siemens’ SST-400 condensing steam turbine
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