The prediction of the aeromechanical behavior of low-pressure blades represents one of the main challenges in the Steam Turbine Industry. The evaluation of forced response and damping is critical for the reliability of new designs and usually requires expensive validation campaigns such as Wheel Box Tests (WBT). A WBT consists of one or more blade rows assembled on a rotor and spun at the desired rotating speed in a vacuum cell, with synchronous excitation provided by various sources. The WBT provides accurate information about the blade modes frequency, the alternating response level, and allows the evaluation of the mechanical damping. Given the large effort in terms of costs and time associated to the experimental activity, the possibility to rely on the output of a numerical code either during the first steps of a new design or to investigate the effect of minor changes to a current design would be extremely beneficial to the development of future products. In order to compute the non-linear forced response of shrouded blades of steam turbines, custom numerical solvers must be developed, since commercial finite element (FE) solvers do not perform this kind of analysis in the frequency domain. In this paper, the forced response of a blade with shrouds of a low pressure steam turbine is computed and numerical results are compared with the experimental Wheel Box Tests performed at GE Oil & Gas. The calculations require a three-step procedure: in the first step, a non-linear static analysis is performed in ANSYS® in order to compute the actual contact area on the shroud surface and the distribution of static normal loads, then a reduced order model of the blade is generated in ANSYS® taking into account the stiffening effect on the blade of the pre-stress due to the centrifugal force, finally the reduced model is imported in a numerical code and the non-linear forced response of the blade is computed. The numerical code solves the balance equations of the system in the frequency domain, by means of the Harmonic Balance Method, imposing cyclic symmetry boundary conditions of the system. An interpolation procedure is implemented in order to manage the non-perfectly matching meshes of the shroud contact surfaces, while the tangential and normal contact stiffness is computed with a numerical model based on the contact mechanics principles. The numerical and the experimental results around some of the critical resonances of the system are compared in order to assess the reliability and accuracy of the numerical tool for its future implementation in the mechanical design practice of the blades.

To enhance the Steam Turbine product line rotating speed and efficiency, GE Company has developed a new generation of high rotating speed Steam Turbine Low Pressure (LP) Sections named HS family (see also Cosi at al., [1.]). The master component of the family, and its smallest size, is the 4-stage LP section HS8, capable of variable speed operation up to 11250 rpm. The aeromechanical validation of HS8 was carried out in two steps: a full-scale rotating test in a vacuum chamber (so called wheel Box Test, WBT) and a full-size test vehicle campaign in steam (Low Pressure Development Turbine, LPDT). During both tests the 4-stage rotors were equipped with a reliable system of strain gages and thermocouples. Aim of the present paper is to present an overview of the experimental results and post-processed data from both tests. Measured blades modes frequencies, responses and quality factors from both WBT and LPDT are described and compared, and the behavior of these parameters at different mass-flows and backpressures is explored. Then, interesting results from the comparison of damping (or Q factors), in WBT and LPDT test are presented. Finally, a methodology for nodal diameter configuration identification is described. To the best of authors’ knowledge the present paper is the deepest investigation about damping in WBT and prototypical test for steam turbine last stage blades.

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The continuous increase of variable renewable energy and fuel cost requires steam turbine power plants to operate with high flexibility. Furthermore, the reduction in electricity price is forcing many existing and new district heating power plants to further optimize the heat production to maintain a sustainable business. This situation leads to low pressure steam turbines running at very low volume flow for an extended time. In this work, a case study of an existing 30 MWel district heating power plant located in Europe is presented. The customer request was the removal of the steam turbine last two stages along with the condenser to maximize steam delivery for district heating operations. However, based on the experience gained by GE on low load during the last years, the same heat production has been guaranteed without any significant impact on the existing unit, excluding any major modification of the plant layout such as last stage blading and condenser removal. Making use of the latest low flow modeling, the minimum cooling flow through the low-pressure turbine has been reduced by more than 90% compared to the existing unit. Optimization of the hood spray system and logic will reduce trailing edge erosion during low load operation leading to a significant extension in the last stage blade lifetime. These modifications, commercialized by GE as the Advanced Low Load Package (ALLP), provide a cheap, flexible and effective solution for the customer. With today’s knowledge, GE has the capability to guarantee low load operation minimizing the mass flow through the low-pressure turbine to the minimum required for safe operation. As a benefit to the customer, this option allows a gain in operational income of about 1.5 M€ per year.

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