Since first introducing titanium steam turbine blade technology in 1999, Siemens has proven and gone on to further develop the concept in a number of test locations. The latest advancements aim to provide even greater benefits.
The Russian System Operator (SO) is by far the largest energy company in the world. Not only is the geographical area it controls the largest country in the world, also one single system operator controls this territory. Although the existing Energy Management Systems (EMS) are still functioning within operational limits, the organisation decided to replace the central and territorial systems with modern state-of-the-art systems.
Following Siemens product strategy of scaling LP-exhaust areas, the company developed a new member of the 50 Hz standard stage groups. The new 16m2 blade path for steam turbine applications will be running with an original row in the company’s spin bunker soon, undergoing the last steps of the product development programme.
The LP blade path design covers the market requirements for increasing the exhaust areas to reduce exhaust losses by given volumetric flow numbers. Using these large exhaust areas could reduce the number of flows and therefore the number of casings. The new 16m2 stage group therefore completes the 50 Hz family with 5m2, 6.3m2, 8m2, 10m2 and 12.5m2 exhaust areas.
Figure 1. Last stage titanium blade of 16m2 blade path
The outstanding design feature of this 16m2 blade path is the last stage titanium blade. Centrifugal load is the main factor in limiting the maximum size of the last stage LP blade. This is where titanium technology comes into its own as it has half of the mass density as the conventionally used steel and nearly the same yield strength. TiAl6V4 is a common titanium alloy that is used for turbine blades and in the aerospace industry.
The development of this blade and the blade path itself covers the customers’ requirements for:
- Low life-cycle turbine costs
- High performance aerodynamic design for the highest turbine efficiency
- Reliability and robust mechanical design for a wide range of applications
- Flexibility for the operational behaviour
- Risk mitigation through various test programmes.
Figure 2. Risk mitigation by extended test programmes
The new titanium blade represents the second generation of Siemens titanium blade design. The knowledge and field experiences from the 60 Hz 10.3m2 (42”) L-0R was merged with modern 3D calculating methods as well as wide steel experience to release this blade. As a result of aero-dynamical and mechanical design, the blade height exceeds 1400mm (56”). The absolute geometrical dimensions challenge all parts of the supply chain and manufacturing process. With experiences from the 10.3m2 blade it was possible to set up the manufacturing processes with qualified vendors in short times. Time consuming and costly test series were reduced to a minimum.
Figure 3. 3D profile shaping of stator blades
The high performance 3D design uses airfoils optimized with a 3D-Navier-stokes solver. The mechanical design and the frequency tuning was done with 3D-finite element codes. Further approaches of design were new analyzing methods for aero-elastic effects using combined methods of flow analysis and structural mechanics. Multiple design iterations between aerodynamic design, structural analysis and frequency tuning have been conducted to come up with an overall optimum design for this blade. The further increase of the exhaust area leads to smaller hub-to-tip ratios. This emphasizes the effect of 3D aero-dynamical flow effects. One well-known measure to improve the reaction at the hub of the last stage is the circumferential lean of the stator blade. In addition to that and to improve the inlet conditions to the last rotating blade, the stator blades also will be swept in the axial direction.
The aerodynamic design of the rotating blade itself also required the extension of existing design rules. Most noteworthy is the fact that the rotating blade reaches such a high velocity at the tip that the relative inlet flow Mach number of the tip exceeds the sonic limit, thus demanding an advanced fully supersonic tip section design. The well proven backward curved tip section design was kept for the transonic blade sections. For the supersonic blade sections a divergent tip section was designed. The result of the supersonic profile development was a low energy loss coefficient throughout the application range. This was validated by a special cascade test rig.
Further features of the blade are an interlocked shroud design and a mid-span snubber. As the blade twists due to centrifugal force, the shroud and snubbers of adjacent blades come into contact and build a continuously coupled system. The tuning of this coupled system and the investigation of different mode shapes and the nodal diameters is a complex approach, which will be validated by model turbine and test wheel application. The shroud of the blade was designed and built with combined free forms for optimized mass distribution, reduction of sealing losses and erosion protection of the adjacent blade contact surface. The geometry of the contact surface is a good balance between contact stresses and frictional damping. The contact surfaces of snubber and shroud use coated layers of tungsten carbide to protect those areas against fretting.
Figure 4. Model turbine view on joint
In addition to the coating, the contact surfaces as well as the complete blade surface will be peened to obtain compressive residual stresses and therefore extend lifetime. In addition, the steeple will be strengthened through pressure rolling, which will increase the resistance against stress corrosion cracking and fatigue.
The erosion protection of the stage will be done by standard elements dependent on the level of wetness of the turbine with moisture removal slots in the hollow vane or heated hollow vanes. With regard to this it shall be pointed out that titanium has got a higher resistance against water droplet erosion than any unprotected blade made from steel.
The design programme was and will be supported by a detailed engineering test series. As mentioned above, one of the first test programmes was the validation of the supersonic profile sections. A cascade test rig was built in 2003 and several flow variations were carried out. A further major part of the validation programme was the model turbine where the original 16m2 blade path was scaled down by a factor of five using the similarity of aerodynamics and mechanics. The model turbine was installed at Stuttgart University and has been running under steam conditions for more than 18 months.
The model turbine features outstanding measurement equipment. The rotating blades were installed with strain gauges. The LA-0R blade furthermore is observed with the non-touching BESSY-system. The rotor of the turbine is divided so that the last stage can run in a stand-alone operation. The couplings are also equipped with strain gauge instrumentation for a detailed power measurement. Beside the strain gauge technique the stator blades are endowed with airfoil holes for pressure measurement. Also, all flow surfaces behind the moving blades can be measured with a traversing system. The experiences made with the model turbine were fed back into the design process. In addition to aerodynamic investigations and vibration investigations the complete operational behaviour, e.g. at windage conditions, was investigated. Therefore a second turbine was as a gear turbine coupled while the valves for the model turbine were closed.
Figure 5. Model turbine exhaust
The final step of the validation and test programme for 16m2 L-0R is the full-scale spin bunker test. A complete row of blades will be assembled in a test rotor. Multiple airfoils, roots and steeples will be equipped with strain gauge applications for detailed vibration analysis. Due to the size of the blade and the design features, all manufacturing and assembly processes will be adopted and optimized with the test wheel. With the test wheel the impact of tolerances on the design can be investigated.
As a result of customers’ need for high efficiency power plant solutions the LP steam portfolio will be enlarged continuously. To cover these requirements Siemens PG has developed the world’s largest 50 Hz LP blade path and last stage rotating blade. This new blade represents the second generation of titanium blades and is the result of the merger of field experience with an ambitious development programme.