Major Material Advances
Realizing a robust steam turbine design operating at USC steam conditions is all about selecting the right materials of construction. Alstom and its partners completed extensive studies of forgings, castings, and piping on high creep rupture strength, resistance to embrittlement, metallurgical stability, low oxidation velocity, oxidation layer strength, and ease of manufacture. Inside the turbine, advanced materials in the HP and IP rotor, inner casings, valve casings, and inlet blading stages were selected. In particular 9% to12% Cr ferritic steels are used in order to maintain operational flexibility. Table 3 shows the specific materials used in the TPP steam turbine design. The CB2 materials for castings and the FB2 materials for forgings were developed in the COST 522 program.
Table 3. Steam turbine material specifications for AEP’s John W. Turk, Jr. Power Plant. Source: Alstom
Figure 5 shows the progression of material selection from X20 steel used in a conventional subcritical design to the active European development program COST 536, which is developing suitable steam turbine materials for the next generation of USC steam turbines. Materials from the COST 501 program for applications up to 1,130F (610C) have been in operation for almost 10 years. New materials with higher creep strength and higher oxidation resistance are available from the COST 522 program for operation up to 1,165F (630C) and will be applied to many Alstom USC steam turbine projects under construction in Europe (up to 1,148F/620C for reheat) as well as the TPP steam turbine.
5. Steel history. The development history of steel alloys for steam turbine components. Source: Alstom
Turbine Design Features
Constructing rotors from several smaller forged disks allows the use of different materials for each section of the rotor, to match the optimum material for the exact operational conditions with a specific stage on a rotor. The high and intermediate inlet rotor selection for TPP is an FB2 material that has been developed for improved creep properties. Stress levels of welded rotors during thermal transients can be up to 40% lower compared to monoblock rotors operating under the same conditions. Alstom’s welded rotors, therefore, have an additional benefit of allowing faster start-ups and/or lowering the life consumption rate compared to monoblock rotors. Examples of forged disk materials and welding were photographed in the Alstom manufacturing plant (Figure 6).
6. Forged steel. Rotor sections are queued for their next machining operation (top). First-step rotor welding begins (bottom) for the TPP steam turbine in the Alstom factory. Courtesy: Alstom
Radial symmetry is a big concern, in particular at higher temperatures. The HP turbine shrink ring design, utilized by Alstom successfully since the 1960s, eliminates inner casing bolt flanges and therefore maximizes radial symmetry. Lower inner casing stresses reduce creep and distortion, thus extending unit life and outage intervals. Because the inner casing is in symmetrical compression, ovalization, as known from flange designs, does not occur. The benefits of this design are long-term stable clearances and sustained efficiencies. In regard to USC applications, the benefits extend to long-term reliability and excellent operational flexibility. Figure 7 shows an HP inner casing with rotor before and after assembly into the lower outer casing.
7. Close tolerances. A high-pressure inner casing with rotor before (top) and after (bottom) assembly into the lower outer casing. Courtesy: Alstom
The basic double-shell IP turbine design with horizontal split outer and inner casings is common but was adapted to higher-temperature USC conditions through judicious selection of materials. In addition, the IP turbine inner casing was modified with a more-harmonic mass distribution in the inner shell to minimize distortions at elevated temperatures in the inlet section. The benefit is long-term sustained clearances and efficiencies.
Many Efficiency Improvements
The Alstom principle of using separate cylinders for the HP and IP turbines gives the steam path designer full freedom to optimize the number of turbine stages given the long expansion line of an USC unit compared to a subcritical unit. The number of stages in the HP turbine, as well as in the IP turbine, were increased by about 25%, compared to a typical subcritical application.
Other design parameters were considered when maximizing performance of the TPP steam turbine design for USC steam conditions. Full arc inlet scrolls improve efficiency and minimize component thermal fatigue damage. In this design the control valves typically operate wide open with flow control through the boiler feed pump. Overpressure operation provides the turbine flow margin to produce additional electrical output when required.
The TPP steam turbine will also be equipped with Alstom’s latest steam path and sealing technology. Alstom has continuously improved the airfoil design and optimized the complete steam path by reducing gap and leakage flow interactions with the main flow. Brush seals and abrasive coating seals will be considered to further improve efficiency.
The exhaust area plays a critical role in steam cycle efficiency; therefore, optimizing the performance of the last stage blade is critical. The TPP exhaust area of 4 feet x 72.1 feet ideally covers the operating range at the various design ambient conditions.
—Heinrich Klotz (heinrich.klotz@power.alstom.com) is a product specialist for Alstom Turbomachines Group, Alstom Power Systems GmbH, Germany. Ken Davis (kedavis@aep.com) is manager for New Generation Design & Engineering for AEP. Michelle Nowak is sales manager and Eric Pickering (eric.pickering@alstom.power.com) is regional sales development manager for Alstom Power Inc., USA.
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