COVER STORY | STEAM TURBINE DEVELOPMENT Due to the requirement for a very light blade with thin
profile sections, a different frequency tuning strategy needed to be developed for this particular design. Many more airfoil design sections were required to enable more local changes, to allow the designer to more easily influence the complex mode shapes of the higher modes and tune their frequencies. The performance of the LP flowpath is highly sensitive to the shape of the airfoils of the last stages. For this reason, an integrated design system was applied that focuses on achieving high aerodynamic performance, whilst also considering the requirements of other design aspects, such as mechanical integrity and ease of manufacturing. The assessment methods for the non-aerodynamic aspects in the design system are of lower fidelity than used in the mechanical integrity and design disciplines, however it enables a more efficient design development because there are fewer design iterations needed to achieve the product requirements. In order to have a sufficiently stiff last stage blade,
Above:
CAD model of the 75in (1.905m) LSB Source: Arabelle Solutions
the aerodynamics layout was adapted to increase airfoil camber. Local changes in the section area, and profile shape to influence the bending and torsional stiffness of the profile over the blade height have been applied to tune the higher modes. In other designs, the geometry has been modified after the 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 also developed. Additionally, FE simulations have been used to anticipate the deflection of the blade during the machining process.
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 (1.7526m) 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 actual 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 behaviour. The first two blade rows have been tested assembled in the rotor and rotated 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.
Above: Schematic of the Arabelle turbine island with the moisture separator reheater (MSR) Source: Arabelle Solutions
28 | October 2024 |
www.neimagazine.com
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 mode shape 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 on-site
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