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					  USC Steam Turbine technology
    for maximum efficiency and
         operational flexibility


         Dr. Rainer Quinkertz
              Andreas Ulma
            Edwin Gobrecht
          Michael Wechsung


               Siemens AG


POWER-GEN Asia 2008 – Kuala Lumpur, Malaysia
October 21-23, 2008




                         Copyright © Siemens AG 2008. All rights reserved.1
                                   Copyright © Siemens AG 2008. All rights reserved.
Abstract
State of the art ultra supercritical (USC) steam parameters provide highest efficiencies (i.e.
lowest fuel costs and emissions like CO2) for coal fired steam power plants. At the same time
these plants also face the requirements of deregulated electricity markets with an increasing
portion of renewable energy. Thus the units have to be capable of flexible load operation and
frequency support. The overload admission has proven to be very effective for flexible
turbine operation at an excellent heat rate. Both main turbine components and controls have
been optimized to minimize thermal stresses, leading to an improved start-up performance.
Despite increased steam parameters and number of load cycles and starts maintenance
intervals have not been shortened. As a result overall life cycle costs are minimized.
Siemens has more than 10 years of experience with USC steam turbines and continues to
optimize associated designs and technologies.
This paper presents the current Siemens steam turbine technology and products for USC
applications and references for operating units.




                                                           Copyright © Siemens AG 2008. All rights reserved.   2
Introduction
According to common forecasts, the worldwide demand for power will increase significantly
over the next two decades, and the current power plant capacity will need to double by the
year 2030. A considerable share of this huge increase in demand has to be covered by coal-
fired units. To save primary energy resources i.e. to reduce fuel consumption, and to reduce
emissions, maximum power plant efficiency is a crucial parameter for power plants.
Therefore steam parameters will have to be maximized to an economically reasonable extent.
State of the art temperatures and pressures for ultra supercritical (USC) applications are
600°C at 270 bar for the main steam and 610°C at 60 bar for the reheat steam.
Furthermore, the share of renewable capacities will increase further to fulfill global emission
reduction targets. As a result electric grids will face a growing share of volatile capacities.
This again requires additional flexibility of the conventional units and, in addition to
combined cycle units, coal-fired units will also operate in the mid merit market. Thus these
units will experience an increased number of starts and load changes, including rapid load
changes, especially within small electric grids.


Siemens USC steam turbines
50 Hz full speed tandem compound turbo-sets for USC steam power plants (SPP) are
available for gross power outputs from 600 – 1200 MW per unit.
The steam turbine set-up for ultra-supercritical applications depends on the unit rating, the
number of reheats selected, and the site backpressure characteristics. A typical turbo-set
comprises three separate turbine modules operating at different pressure and temperature
levels. These modules are the high pressure turbine (HP), intermediate pressure turbine (IP)
and, depending on the cooling water conditions, one, two or three low pressure turbines (LP).
The generator is directly coupled to the last LP turbine. Figure 1 shows an SST-6000 series
Siemens steam turbine with two LP turbines. A comprehensive description of the specific
turbine features of the whole turbo-set is given in [1].




                                                            Copyright © Siemens AG 2008. All rights reserved.   3
HP turbine




                     IP turbine                   LP turbines


Figure 1: SST-6000 series turbo-set


Design features for optimum turbine efficiency
Siemens steam turbines contribute to overall plant efficiency requirements by providing
highest internal efficiencies. These efficiencies have been proven for decades in various
acceptance tests and continue to be a major evaluation criterion for new developments.
One feature is that clearances between rotating and stationary parts of the turbine are very
small due to minimum relative expansion and thermal deformation during transient operation.
These minimum clearances are achieved by a rigorous symmetrical design, which avoids
local material build-ups (HP barrel type) on the one hand, and ensures uniform thermal
loading on the other hand.
Steam leakage through these clearances along the blade path as well as at turbine pistons and
sealing areas is further minimized by using advanced sealing technologies such as abradable
coatings and brush seals. More details are given in [2].
Within the blade path, 3DV™ reaction blading with minimum secondary losses and variable
reaction across the blade path minimizes flow losses. [3] describes this approach in detail.


References and operating experience
Siemens first supercritical unit was built in 1956 and had a main steam temperature of 650°C.
However power output was limited. Table 1 shows a reference list of large scale USC steam
turbines manufactured by Siemens. Obviously steam parameters have increased slightly over
the last years, but power output capacity has increased considerably compared to the first
large scale USC application in Isogo, Japan. Whereas the Chinese units favor higher


                                                           Copyright © Siemens AG 2008. All rights reserved.   4
electrical output, European customers consider 800 MW an optimum unit size. This is
basically due to maximum dimension of the air pre-heater for a one-line configuration of the
pre-heater piping. But increasing power output also reduces operational flexibility because
wall thicknesses of components determine start-up times and permissible number and rate of
load cycles. [4]


 Plant              Country        Power output      Main steam               Reheat Commercial
                                                                              steam         operation
 Isogo              Japan          1 x 600 MW        251 bar / 600°C          610°C         2001
 Yuhuan             China          4 x 1000 MW       262 bar / 600°C          600°C         2007
 Wai Gao Qiao 3 China              2 x 1000 MW       270 bar / 600°C          600°C         2009
 Westfalen          Germany        2 x 800 MW        275 bar / 600°C          610°C         2011
 Eemshafen          Netherlands    2 x 800 MW        275 bar / 600°C          610°C         2012
 Lünen              Germany        1 x 800 MW        270 bar / 600°C          610°C         2012
 Mainz              Germany        1 x 800 MW        273 bar / 600°C          610°C         2013
Table 1: References of Siemens USC steam turbines


Case Study: First Large Scale USC Operation Experiences at Isogo Power Station,
Japan
The order Isogo was placed at the end of the 90s when Siemens was a sub supplier for the
contract owner FUJI ELECTRIC SYSTEMS. FUJI is a licensee for conventional Siemens
steam turbines and a long-term business partner. The Japanese power plant, which is located
in the Bay of Tokyo, has a very compact design due to the restricted area available and is
equipped with separate HP and IP turbines and a double flow 12.5 m2 LP turbine.
In addition to the parameters given in Table 1, the condenser has once through, sea water
cooling and a pressure of 0.0507 bar (1.500 inch Hg). The boiler is hard coal fired. Figure 2
depicts the inside of the turbine building, and Figure 3 shows the cross section of the turbine.




                                                           Copyright © Siemens AG 2008. All rights reserved.   5
Figure 2: Turbine building at Isogo




Figure 3: Cross section of Isogo turbine


The Japanese industrial ministry requires that periodical comprehensive inspections are
carried out on the steam turbine. A complete disassembly and detailed inspection of all
turbine components was therefore arranged with the customer, initially after two years of
operation and then every four years. Consequently, the turbine and the valves were subject to
this strict control from March to June, 2008.
This presented a good opportunity for the manufacturer to inspect the highly stressed
components. At the time of inspection the turbine had a base load operation of approximately
48000 equivalent operation hours (EOH).


The design of the HP and IP turbines and of the valves is basically the same as the previous
supercritical design. Design concept changes were consciously avoided to minimize the
operational risk and to achieve good plant availability and to use existing calculations and


                                                          Copyright © Siemens AG 2008. All rights reserved.   6
manufacturing procedures. However materials for cast and forged parts exposed to USC
operating parameters have been upgraded and coatings have been added.




Figure 4: Isogo HP rotor


In particular 10% chrome steel material was used. This material was developed from the
well-known P91 tube material as a result of COST investigations and is characterized by up
to 30% higher creep rupture strength in comparison with 12% chrome materials used
previously. Figure 4 shows the disassembled HP rotor.


According to material creep laws, components which are hot and under tension are subject to
inelastic changes, particularly in the first operational phase. To validate design calculations, a
validation program was initiated which determined and evaluated changes in component
geometry at exposed areas over time. The creep behavior of shaft and casing components, as
well as of HP main steam valve breechlock nuts of the HP casing and valve casings was
recorded.


The results were consistently positive and showed lower plastic deformation than expected.
This proves the conservative approach of the component design. The maximum creep of the
HP barrel casing, for example, is less than 0.2% in the area of steam admission. This is an
excellent value for 48000 operating hours in comparison with the generally permitted limit of
1% material expansion. This information is extremely valuable for the optimization of new
components and allows a better assessment of the design limits.




                                                            Copyright © Siemens AG 2008. All rights reserved.   7
Further investigations concern the oxidation and scaling behavior of materials used in real
operation. The knowledge gained here was especially valuable and relevant for the anti-
friction property of seal rings and sliding pads. Guide ways, threads and moving parts in the
steam valves are partly coated. Chrome carbide coatings of valve cones, for example, show
no relevant wear or damage, which verifies the suitability of these coatings also for USC
plants. Chrome carbide coatings have been used for SIEMENS steam valves as standard
protection for approximately 10 years and are, in addition to stelliting, a proven design, see
Figure 5 IP control valve cone.




Figure 5: IP control valve cone


A special feature of the USC design is the choice of blade material. Nimonic 80 A has been
used for the first stages of the HP- and IP- turbines. Experiences made at the beginning of the
90s with large steam turbines were drawn upon as well as experiences of the Siemens gas
turbine design for high temperatures. Nimonic can show an unfavorable friction behavior
with austenitic and martensitic material, which is used, e.g. for seal strips. This has been
considered when designing the stages.


The blades at Isogo were also inspected during the March to June outage. As expected, the
condition of the blades was excellent. Surfaces of airfoils, integrated shrouds and pre-stress
of bladings were without findings. The design of the integrated shrouds as well as the
calculated radial clearance could be confirmed as suitable choices here. See Figures 6 and 7
for first Nimonic stages.




                                                           Copyright © Siemens AG 2008. All rights reserved.   8
Figure 6: HP Nimonic stage1                  Figure 7: IP Nimonic stage1


The shaft seals of a steam turbine shall seal efficiently to reduce steam losses. Furthermore,
the sealing needs to resist high temperature, and static and dynamic forces. The HP turbine at
Isogo, for example, is fully equipped with seal strips made from 13% chrome material, which
is martensitic. The inspection confirms the suitability of this material also for temperatures up
to approx. 580° C (1076° F), which dominate at the HP balancing piston entrance where the
highest load is located, see Figure 8 showing the HP thrust balancing piston.




Figure 8: HP rotor thrust balancing piston


The inspection of the relevant components shows obviously the good suitability of the USC
design concept. The results show that the steam turbine is in a very good condition after
almost 48000 EOH.




                                                           Copyright © Siemens AG 2008. All rights reserved.   9
Turbine HP stage bypass for frequency control and power increase
Generally there are many different methods for controlling frequency and increasing power
both within the balance of plant and the steam turbine. Whereas on the water steam side e.g.
condensate throttling or preheater bypass address reserve requirements within minutes, valve
throttling on the turbine side provides power increase within seconds.
The HP stage bypass is a design feature applied to the main steam piping, which routes a
second main steam line onto the HP turbine. Stage bypass steam enters the turbine via at least
two inlet belts in the HP blade path section. Figure 9 shows the piping arrangement as well as
details of the turbine design.


                                                                                                    Main
                                                                                                    Steam
                                                                                                    lines
                  HP turbine


                 main steam
                   valve




                                                       Stage
 bypass                                                bypass
 stage           main steam                            chambers
 control valve

Figure 9: Stage bypass routing from main steam valve combination to HP turbine and cross
sectional view of HP turbine with HP stage bypass chamber


The HP stage bypass is the most efficient solution for rapid load increase because it creates
two load conditions at which there are minimal throttling losses. The first condition occurs
when the valves of the main steam inlet are wide open and the stage bypass is closed. Usually
this is the 100% load point. The second case with minimum throttling losses is with all valves
wide open (VWO) which represents the maximum load point, usually 105%. Consequently,
bypass governing achieves a better part load performance than throttling of the valves at the
100% load point. Manufacturing this design is also more economical than building a nozzle
governed machine with similar part load characteristic.
Design of the HP stage bypass and stationary performance is described in detail in [5, 6].




                                                          Copyright © Siemens AG 2008. All rights reserved.   10
Figure 10 illustrates the transient performance of the HP stage bypass. Like throttling of the
main steam valves, the stage bypass is able to provide a most rapid load increase of about 1%
per sec. The maximum load enhancement in both cases is about 50% of the design value i.e.
with full additional firing load of the boiler.
The diagram also shows that a combination of different measures may be required to meet
challenging grid code requirements of islands e.g. UK with 10% load increase within 10 sec.




Figure 10: Dynamic performance of different load increase measures


Advances in start-up time
Beside load changes, start-up performance has become an important attribute of SPP.
Siemens has proven steam turbine cycling capabilities with different approaches. The design
supports quick and uniform heat-up of thick-walled components. Turbine controls provide
three different start-up modes i.e. speeds. The resulting material fatigue of each cycle is
calculated online. This enables the operator to balance power revenue against lifetime
consumption. Details are given in [7, 8].
As a third approach for fast start-up, Siemens improved the overall start-up process of the
total plant. The new concept was first introduced to combined cycle power plants (CCPP).
This enabled a parallel start of gas turbine (GT)/ heat recovery steam generator (HRSG) and
steam turbine and is described in [9]. In principle, it can also be performed for SPP, provided
that boiler and controls have appropriate capabilities.




                                                           Copyright © Siemens AG 2008. All rights reserved.   11
As a traditional requirement for steam turbine start-up, the steam temperature has to be higher
than the metal temperature. This reduces thermal stresses, especially for hot starts and
restarts.
Different developments have made it more and more difficult or rather time-consuming to
meet this requirement. Firstly, increasing rated steam temperatures from 540°C to 600°C
enlarge the difference between metal temperature and steam temperature after short shut-
down periods (<8 hrs). Then, reduced bypass sizes limit the start-up load, and heating up the
steam physically takes longer at low load because heat transfer is also low. Finally, cost
reduction measures have lead to reduced boiler performances.
Originally, thick walled boiler headers had the main influence on hot start times in steam
power plants because the headers cool down faster than the steam turbine.
With the new boiler design the “hot” turbine became a disadvantage for the overall plant hot
start performance.
For hot start-up after boiler ignition, the plant is kept in bypass operation without producing
power until boiler outlet temperatures meet steam turbine starting conditions (480°C to
500°C). For the customer this is a waste of fuel and the time to synchronization is too long
under deregulated market conditions. Shortening these waiting times increases the steam
turbine dispatch rate.
Thus the focus is on allowing “cold” steam to enter the steam turbine at an earliest possible
point. This enables the turbine to start while the boiler is ramping-up in load without any
additional hold.
As a result, the start-up time is shortened, which leads to an earlier dispatch of the turbine.
Savings are at least the amount of fuel energy which is not dumped into the condenser. The
new start-up method does not reduce turbine efficiency by increasing axial or radial
clearances. Life consumption for the new method is a little higher but still allows more than
6000 hot starts of the steam turbine.




                                                            Copyright © Siemens AG 2008. All rights reserved.   12
                                                                   Typical Hot Start-up Curve
                 300                      600                                                                                                               120
                                                    boiler waiting for ST to start


                 250                      500                                                                                                               100




                                                                                                                                                                Load / Flow / Speed [%]
                 200                      400                                                                                                               80
                       Temperature [°C]
Pressure [bar]




                                                          "new"       "old"
                 150                      300            turbine     turbine                                                                                60
                                                                                                                                Main Steam Temperature
                                                           start      start
                                                                                                                                Main Steam Pressure
                                                                                                                                Main Steam Flow
                 100                      200                                                                                                               40
                                                                                                                                Load
                                                                                                                                Turbine Speed


                  50                      100                                                                                                               20




                                           0                                                                                                                0
                                                0   20              40               60                80                100             120             140
                                                                                          Time [min]


Figure 11: Start-up time reduction with new concept


The steam turbine start-up procedure used to be based on static performance curves of the
boiler and did not take into account any possible ramp rates. Differences between boiler start-
up pressure and pressures inside the turbine (i.e. throttling) were not taken into account.
During opening of the turbine valves and picking–up of load, this throttling is reduced more
and more. Hot piping from boiler to turbine will heat-up the “cold” steam before it reaches
the steam turbine. Change from pure static to more dynamic orientation of the start-up
process is the major achievement of the new procedure.
Typically the hot start-up time can be reduced by 15 min if boiler and steam turbine start
simultaneously (see Figure 11).




                                                                                                            Copyright © Siemens AG 2008. All rights reserved.   13
       temperature differences during cool down and heat up
                                                              30

                                                                    g
                                                                        LCF
                                                                                                                                                          heat up
                                                                        design limit
                                                              20




                                                              10


                                                                                             start-up 1               start-up 2         calculated

                                                               0

                                                                                                                                                          cool down

                                                              -10




                                                              -20

                                                                        LCF
                                                                    g design limit
                                                              -30
                                                                 -100                  -80                -60   -40                -20                0      [K]      20

                                                                                       mid wall temperature minus main steam temperature




Figure 12: Calculated and measured cooling of HP shaft during parallel start-up


Figure 12 shows results of tests carried out at a German SPP which prove the Siemens
approach to allow turbine start-up with steam temperature lower than the turbine shaft
temperature. The boiler at this SPP starts with a ramp-rate of 3% per min. For example, with
main steam 50 K below metal temperature, temperature difference for cool down is only 16
K. This is well below the design for low cycle fatigue which allows 27 K.


Conclusion
Growing worldwide power demand, limited resources for all fossil fuels, CO2 reduction
targets and growing shares of renewable energy set the scene for current and future plants.
Coal will definitely continue to be an important part of the energy mix.
Siemens contributes to these market requirements by offering USC steam turbines for highest
steam parameters i.e. maximum plant efficiency. Besides featuring minimum clearances for
maximum inner turbine efficiency, these turbines are also very flexible to operate. Both short
start-up times and quick load changes are possible. For temporary power increase the stage
bypass is the optimum solution for efficiency and dynamic performance.



                                                                                                                               Copyright © Siemens AG 2008. All rights reserved.   14
Siemens large scale USC steam turbines have been in operation for nearly 10 years now. The
design principles were proven during recent inspections at the Isogo plant in Japan where
blades, casings and rotors exposed to high steam parameters showed far less creep effects,
oxidation and abrasion than expected after 48.000 EOH.


References
[1]     Wichtmann A., Deckers M., Ulm W.
Ultra-supercritical steam turbine turbosets – Best efficiency solution for conventional steam
power plants, International Conference on Electrical Engineering, Kunming, China, July
2005.
[2]     Neef M., Sürken N., Sulda E., Walkenhorst J.
Design Features and Performance Details of Brush Seals for Turbine Applications, ASME
Turbo Expo, Barcelona, 2006
[3]     Deckers M., Pfitzinger E.-W., Ulm W.,
Advanced HP&IP Blading Technologies for the Design of Highly Efficient Steam Turbines,
Thermal Turbine, 2004
[4]     Quinkertz R., Then O., Gerber R.
High efficient and most flexible 800 MW Ultra Supercritical Steam Power Plants -
A common approach of RWE Power AG and Siemens AG, CoalGen Europe, Warsaw, 2008
[5]     Deidewig F., Wechsung M.
Thermodynamic Aspects of Designing the new Siemens High Pressure Turbine with
Overload Valve for Supercritical Applications, ASME Power, Atlanta, 2006
[6]     Wichtmann A., Wechsung M., Rosenkranz J., Wiesenmüller W., Tomschi U.
Flexible Load Operation and Frequency Support for Steam Turbine Power Plants. PowerGen
Europe, Madrid, 2007
[7]     Almstedt H., Gobrecht E., Thiemann T., Wallis A., Wechsung M.
Siemens 600 - 1200 MW Steam Turbine Series for Flexible Load Operation. PowerGen
Europe, Madrid, 2007
[8]     Quinkertz R., Gobrecht E.
State of the Art Steam Turbine Automation for Optimum Transient Operation Performance,
ASME Power, San Antonio, Texas, 2007
[9]     Emberger H., Schmid E., Gobrecht E.
Fast Cycling Capability for New Plants and Upgrade Opportunities, PowerGen Asia,
Singapur, 2005


                                                          Copyright © Siemens AG 2008. All rights reserved.   15
Copyright © Siemens AG 2008. All rights reserved.   16
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                                                           Copyright © Siemens AG 2008. All rights reserved.   17

				
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