PSI - Issue 68

Evgeniia Georgievskaia et al. / Procedia Structural Integrity 68 (2025) 559–565 Evgeniia Georgievskaia / Structural Integrity Procedia 00 (2025) 000–000

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2. Purpose and objects of research The main purpose of the study is preventing unexpected (far ahead of the original forecast) destructions of responsible hydraulic unit’s components using the main lifetime-determining component, the hydraulic runner, as an example. This study’s purpose defines destruction as a hydraulic runner’s structural integrity violation resulting in the need to shut down the HU for repairs or replacement. The proposed approach is applicable to hydraulic units of any type and size. Large HUs with unit capacities exceeding 25 MW, having completed their design lifetime, are the best candidates for this method. Most hydraulic units have a design lifetime ranging between 25 and 40 years. Even after 50 years of operation, many HUs are still operating without any major damage, see IHA (2023). Table 1 displays the technical characteristics of the most commonly utilized hydraulic turbines, Francis and Kaplan ones.

Table 1. Technical characteristics of Francis and Kaplan turbines. Turbine type Parameter

Characteristic value

rate, m 3 /с

Kaplan

20÷1 000

head, m

5÷70

power, МW

3÷300

diameter of runner, m

≤ 11

rate, m 3 /с

Francis

2÷1 000 20÷600 10÷1 000

head, m

power, МW

diameter of runner, m

≤ 9

High-capacity hydraulic units have several features related to their large size and long lifetime. It is worth noting that hydraulic units have a high degree of individuality and can operate in all capacity ranges without significant limitations on operating time and mode change frequency. Establishing a reliable statistical base is not possible due to these characteristics. Therefore, statistical methods cannot reliably predict when dangerous defects in HU’s lifetime-determining components will occur. In addition, lifetime-determining components have a long manufacturing cycle (more than 1 year), therefore, in case of a failure the HU will have a long down-time. At the same time, large size of these high-capacity hydraulic units, rare repairs (once in 5-7 years) and the limited availability of some components for metal condition monitoring make it difficult to find dangerous defects at an early stage using traditional non-destructive testing methods. 3. High- and low-frequency load and impact characteristics A wide range of external static and dynamic loads, which are mechanical, hydraulic, and electromagnetic in origin, influence the hydraulic unit during operation. Dynamic loads affect HUs even during stationary operating modes, not to mention transient and non-stationary modes. This is mostly related to hydraulic loads. Amplitude and frequency range of external loads are determined by the particular HU’s design characteristics and operating modes. A wide range of external dynamic loads causes a wide range of internal stress pulsations in all HU’s components. The amplitudes and frequencies of external impacts as well as the dynamic characteristics of HU components are responsible for determining the amplitude and frequency of these pulsations. In case of proximity of any of the HU’s eigenfrequencies to external impact frequencies, resonance phenomena occur, provoking an increase in stress pulsations' amplitudes. This also occurs, in particular, for runners of high-capacity hydraulic units with Francis turbines during the presence of resonant frequencies associated with pressure pulsations in the inter-blade channels as well as pressure pulsations in the water passage due to the rotor-stator interaction (RSI). These pulsations’ frequencies are often close to blade eigenfrequencies considering the water added mass effect.