| Turbine technology
aerodynamic design was complete, through changes to the airfoil surface, to achieve the required frequency layout. In the Hinkley Point C design process, this has been avoided largely due to the application of the integrated design system. This enabled a much higher aerodynamic performance than would have been possible with a less integrated design process.
An important aspect of the last stage blade development was the understanding of the influence on the frequency behaviour of geometric variations generated during the manufacturing process. This was carefully assessed using finite element (FE) simulations. The geometry was modified through a morphing process. A specific strategy for machining was developed. Additionally, FE simulations have been used to anticipate the deflection of the blade during the machining.
Adaptations to the manufacturing process
Usually, large last stage turbine blades are milled in a horizontal position, with each end fixed. The machining of relatively big and slender blades poses some challenges due to the flexibility of the airfoil in the region far from the support where the blade is clamped. In this configuration, the mid span region of the blade is where the blade experiences the maximum deflection due to the force of the milling tool. The longer the blade, the larger the deflection for a comparable machining tool force. Experience in machining blades of smaller size, for example the 69in last stage blade, highlighted the need to define a dedicated machining strategy to ensure a better control of the geometrical deviation on the airfoil for even longer blades. This included collaborating with the milling machine manufacturer to improve the milling machine for this type of blade design.
To address the relatively large variation in geometry in important regions of the airfoil, which led to a significant and undesirable change of the blade natural frequencies, two strategies were used. The first was the application of a blade milling machine equipped with an additional support in the middle of the airfoil. The second was the introduction of a specific feedback process to ensure quality control during manufacturing. All the blades have been automatically measured in many control sections distributed along the airfoil with a co-ordinate measuring machine (CMM). The aggregate results of these measurements were used to improve the design definition, to reduce the sensitivity of the design to manufacturing variations. The first blades machined, before starting the series production, have been assessed in respect to the natural frequencies considering the average deviation measured in each
section. The contribution of each individual section was calculated with a finite element analysis and the frequency impact estimated by superposition. The specification of where to correct the milling program was provided, identifying the region where to aim for a thicker or thinner profile to ensure that the machined geometry was less sensitive in terms of the resultant blade frequency. After a number of iterations, once a satisfactory manufactured geometry was obtained, during the series production the machined geometry was constantly monitored to ensure an acceptable natural frequency behavior. The first two blade rows have been tested assembled in the rotor and rotating in the spin pit. In the tests, strain gauges were applied to the blade surface and the blades are excited through the use of an air jet impinging on the blades to measure the natural frequencies. The correct frequency behaviour of the manufactured blading was confirmed.
Validating for operation Due to their large size, and low natural (structural) frequency, last stage steam turbine blades can be subject to aeromechanical effects. This is where the unsteady flow interacts with the vibration modeshape to increase the blade vibration amplitude, sometimes to unacceptable amplitudes. There are two main categories of aeromechanical interaction, one self-excited where the vibration itself creates an unsteady force in the flow which is known as flutter, and a forced response type excitation where the unsteadiness is generated by the flow conditions only. Forced response may be due to stochastic unsteadiness such as flow turbulence, or at off design conditions there is also a rotating stall type phenomenon, where the blade is stalled, and rotating stall cells produce an unsteady force and consequently blade vibration. To validate for these phenomena, a test turbine or onsite tests are required. For the new 75in last stages of the HInkley Point LP module, a scale turbine model was manufactured, and a thorough testing campaign was undertaken across the whole operating map. Even at conditions well beyond typical applications, the measured amplitudes were significantly below the dynamic stress limits imposed by the design rules. The tests were used to confirm the applicability of the last stage blade protection diagram, which is used by the operator to ensure that the last stage blade is operating in the correct regime.
Summary: a new blade for a new generation
Efficient cost-effective turbines for very large nuclear applications require very large last stage blades. For the Hinkley Point development, the size of the last stage blade,
Schematic of Arabelle turbine island. MSR = moisture separator reheater (source Aarabelle Solutions)
and the requirement for a relatively low blade weight posed a great challenge to almost all aspects of the design and development process. Unlike typical design projects, these constraints led to the development of new development processes and tools. In summary, the following aspects played a key role in the development process: Improvement of the design system: Enabled the aero designer to implement the frequency and stress layout strategy from the mechanical integrity designer, with less iteration and enabling the achievement of higher performance. The consistency in geometrical modelling between the mechanical design, aerodynamics and mechanical integrity analysis avoided errors and the requirement for additional design iterations.
Frequency layout process: The complex tuning process of the modeshapes of the last stage blade was facilitated by changes to the design system, as well as the consistency of the geometry in the different disciplines and analyses.
Design for manufacturing: Detailed understanding through measurement feedback of the geometrical variations in the manufacturing of the blade airfoil allowed adaptation of the manufacturing process, the milling machine, the tolerance scheme, and the geometry itself to achieve the requirements in terms of modeshape frequency layout and stresses. Validation: Scaled turbine testing over the whole operating map of the turbine ensured that the last stage blade can be operated even in off design conditions, as no aeromechanical issues were identified. The development and application of the design tools and new manufacturing processes led to the creation of a high-performance last stage which could be manufactured at moderate cost. The increase in exhaust area of the last stage compared to the previous 69in design has enabled a significant increase in the power plant output of the Arabelle steam turbine train, with the first units currently being deployed at the Hinkley Point C power plant, UK.
For further information, see Sergey Kostyuchenko, Ivan McBean, Julien Lemaire, Pierre-Alain Masserey, Christophe Berquier, Christian Casu, Steam turbine low pressure 75in last stage blade for nuclear half speed applications, Proceedings of ASME Turbo Expo 2024, 24-28 June 2024, London, United Kingdom, GT2024-127874
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