USC Steam Turbine technology for maximum efficiency and operational flexibility. Dr. Rainer Quinkertz Andreas Ulma Edwin Gobrecht Michael Wechsung

Transcription

1 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 All rights reserved. 1 Copyright Siemens AG All rights reserved.

2 Abstract State of the art ultra supercritical (USC) steam parameters provide highest efficiencies (i.e. lowest fuel costs and emissions like CO 2 ) 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 All rights reserved. 2

3 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 A considerable share of this huge increase in demand has to be covered by coalfired 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 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 All rights reserved. 3

4 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 All rights reserved. 4

5 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 steam Commercial 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 m 2 LP turbine. In addition to the parameters given in Table 1, the condenser has once through, sea water cooling and a pressure of 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 All rights reserved. 5

6 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, 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 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 All rights reserved. 6

7 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 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 All rights reserved. 7

8 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 antifriction 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 All rights reserved. 8

9 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 EOH. Copyright Siemens AG All rights reserved. 9

10 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. HP turbine Main Steam lines main steam valve bypass stage control valve main steam Stage bypass chambers 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 All rights reserved. 10

11 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 All rights reserved. 11

12 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 shutdown 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 All rights reserved. 12

13 Typical Hot Start-up Curve boiler waiting for ST to start Pressure [bar] Temperature [ C] "new" turbine start "old" turbine start Main Steam Temperature Main Steam Pressure Main Steam Flow Load Load / Flow / Speed [%] Turbine Speed 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 startup 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 All rights reserved. 13

14 temperature differences during cool down and heat up g LCF design limit g LCF design limit start-up 1 start-up 2 calculated mid wall temperature minus main steam temperature heat up cool down [K] 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, CO 2 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 All rights reserved. 14

15 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 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 [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 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 All rights reserved. 15

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