PSI - Issue 48

Evgeniia Georgievskaia / Procedia Structural Integrity 48 (2023) 310–317 Georgievskaia / Structural Integrity Procedia 00 (2019) 000 – 000

311

2

Nomenclature HU

hydraulic units

LSS lower supporting spider VMC visual and measuring control PFD

dye-penetrant flaw detection (colour flaw detection)

MCF monitoring using magnetic coercive force UFD ultrasonic flaw detection Нс coercive force Ht

permissible value typical for present metal grade

1. Introduction Hydraulic units (HU) are energy equipment that can successfully operate for several decades. During this period, HUs can be modernized and reconstructed repeatedly. HU modernization often does not affect the whole unit, but includes only a runner replacement, enhancing the efficiency. At the same time, unit supporting structures (supporting spiders, generator rotor’s metal structures, generator shafts and turbines, stay vanes, spiral case, runner chamber, bearing housings) often remain unchanged throughout their lifetime in assumption of having no significant impact on the durability and reliability of the equipment. In fact, when replacing the runner with a new one it is not always possible to obtain the expected benefits from modernized equipment. This is most commonly due to intense crack formation in HU supporting structure elements. Aging of HU supporting structures increasingly highlight lifetime problems. The lifetime exhaustion criteria are emergence and development of fatigue cracks during HU’s late effective lifetime. This occurs due to material degradation under normal operating loads. Such cracks are often widespread, prone to reappear, reduce inter maintenance periods, increase the HU repair and maintenance cost, reduces the economic effect of earlier HU modernization. Numerous examples of such dangerous cracks in HU elements can be found in the literature, for example, Vasiliev et al. (2017), Strelnikova et al. (2017), Dorji and Ghomashchi (2014), Momčilović et al. (2012), Makhutov et al. (2020), Maricic, Haber and Pejovic (2007), Arsić et al. (2019), Sedmak et al. (2016), Neto et al. (2016), Aronson, Zabelkin and Pylev (1986), Georgievskaia (2021). However, systematic crack formation factor investigation and effective HU supporting structure crack formation intensity reduction have received little attention so far. Using the example of a high-power HU supporting spider, this article:  presents effective methods for identifying cracks in HU supporting structure welds in long-term operation  identifies the main reasons for crack formation  proposes possible ways of crack formation intensity reduction to maintain high effectiveness of modernized equipment and to ensure continued reliable and safe HU operation for an extended period. 2. Object of research The object of research is a lower supporting spider (LSS) of a vertical hydraulic unit, equipped with a Kaplan turbine. Unit capacity is 70 MW, nominal rotation frequency is 62,5 rpm. The hydroelectric power plant has seven structurally identical units. LSS is a prefabricated and welded radial structure, consisting of a massive central part and 12 durable support spokes, reinforced by vertical stiffeners (Fig. 1). Spokes rest upon a reinforced-concrete foundation and attach to it with studs. The central part and spokes connect at the top with butt beams whereas at the bottom they connect with several studs, which connect the lower disk of the central part with the spokes. This connection type ensures the possibility of structure disassembly. Inside LSS a lower generator guide bearing housing is installed, which receives unit’s vibration loads. Through the thrust bearing the LSS receives the weight load from all the HU’s rotating parts and water’s axial retroaction. The total load transferred through the trust be aring towards the LSS is more than 750 tons. When assessing LSS support capacity during the design stage, only the axial load from the weight of the rotating HU nodes and from the water flow impact tend to be taken into account, assuming that the load has only an axial component and distributes evenly over the spokes of the spider.