PSI - Issue 13

974 Evgeniia Georgievskaia / Procedia Structural Integrity 13 (2018) 971–975 E. Georgievskaia / Structural Integrity Procedia 00 (2018) 000 – 000 At the second stage HF-loads, which accompany operation of hydraulic turbine in all operating modes including modes being close to optimum, become determinative. High-frequency loads in hydraulic turbines occur as a result of water flow through the profiles of stator columns, guide vanes and runner blades. Usually in the modes of high power being close to design values amplitude of high-frequency loads is insignificant. For example, in the most stressed area of runner blades (in the absence of cracks) the appropriate dynamic stresses don’t exceed several MPa. But since the material already has macro-cracks with a threshold length, formed at the first stage of operation, even small in amplitude loads affect the rate of crack development. Due to high frequency of influence (tens and hundreds Hz) and high level of local stresses in the area of crack tips high-frequency loads quickly result in structure destruction. It is apparent that such cracks are hazardous and shall be considered during evaluation of reliability and safety of hydraulic turbine operation. Thus, crack length corresponding to instant of qualitative change in crack growth mechanism can be taken as thre shold value separating hazardous and non-hazardous cracks (Fig. 3a): LF curve correspond to crack development only under action of low-frequency loads, and LF+HF curve under action of combination of low- and high-frequency loads.

Fig.3. (a) Criterion of crack hazard; (b) Determination of critical crack sizes

4. Forecasting of cracks development in hydraulic turbines

Considering modern computation and diagnostic capabilities it is reasonable to forecast crack development in elements of hydraulic turbine by calculation and experimental methods, in the base of which principles of l inear fracture mechanics are laid. It is supposed that during operation in elements of hydraulic turbine fatigue damages are accumulated including micro- and macro- cracks of small size, which can’t be detected by standard metal non destructive testing (NDT) methods within the limits of standard diagnostic studies. Considering design features, large overall dimensions of parts and assemblies of hydraulic turbine, limited sensitivity of non-destructive testing methods, complexity of element surface preparation for testing and other factors crack detection at early stages of their development turns to be extremely difficult. As a result equipment with non-detected, but actually existing cracks with length of several millimeters minimum is generally accepted into operation. Local stresses in the area of crack tip are determined based on finite element calculation using 3D-model of hydraulic turbine. The main complexity is reliable determination of value for dynamic load component within the whole range of operating modes that requires development of special engineering methods. Some approaches to this problem solving are given in article of Georgievskaia E. V. (2017. The energy approach…). Detected defects considering their actual geometry or design cracks, which size is determined firstly by sensitivity of used NDT methods and accessibility of testing places are set as initial defects. Size and arrangement of design defects are chosen based on experience in diagnostic of hydraulic turbines considering features of equipment operation at the certain hydroelectric plant. Such approach can be implemented in widely used up-to-date calculation software systems, for example, ANSYS. As a result of calculation stress intensity factor is determined, which together with material specifications defines rate of crack development dℓ/dN , where N is a number of loading cycles. If the range of stress intensity factor (SIF) doesn’t exceed threshold value K th , then crack is not developed. Upon achievement of value K С (fracture toughness) by stress intensity factor destruction occurs. Parameters K th and K С are parameters characterizing crack resistance of