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A full-scale test bench for the analysis of frictional and modal behavior of nozzle guide vanes (NGV) is an advanced tool enabling high design accuracy for these components equipped with an increasing number of turbo-expanders. This subassembly is a key feature enabling operational flexibility for the expander. During the NGVs design, great attention must be paid to the natural frequencies of its kinematic chain, which are influenced by internal clearances and by friction. In this paper, the results of a testing campaign performed on an NGV assembly employing realistic blade geometry are presented together with details concerning the test bench commissioning and setup procedure. The testing campaign involves three different combinations of blade orientation and preload corresponding to typical design conditions of the expander. In addition, two bushing clearance values corresponding to different worn-out conditions were investigated. The measurements of the global friction coefficient based on actuator force detection are summarized. After that, the reconstruction of mode shapes based on experimental modal analysis is explained. The results highlight the importance of loading conditions on the actual value of friction force and their influence on blade natural frequencies. The testing campaign was used to properly validate finite element models to be used for further investigations.

It is nowadays a common understanding that, due to the uneven availability of renewable energy sources, the operation of traditional power generation plants and especially of combined cycles has to be more flexible and subject to more frequent and, most probably, colder starts than in the past. This phenomenon translates into a negative impact on maintenance intervals and operating costs as resulting by a higher increase rate of equivalent operating hours. In addition, the optimization of start-up time, largely driven by initial components temperature, has become a key performance indicator for the profitability of such plants. The case study presented in this paper deals with a steam turbine in a combined cycle power plant, commissioned on the early 2000s. The turbine is currently operating daily for half of the year and occasionally for the other half, collecting about 150 warm and 30 cold starts per year. The application to the steam turbine of a Thermal Warming System (TWS) is analyzed in detail by assessing casing and rotor temperature distribution, in transient operation. The control algorithms, that allows maximizing the system effectiveness while safeguarding against possible issues due to uneven temperature distributions, are also discussed. The resulting increase in average starting temperature, as evidenced by online rotor temperature calculation, is then considered with respect to its benefits for maintenance optimization and plant profitability.