Integrity and life estimation of turbine runner cover in a hydro power plant

This paper presents integrity and life estimation of turbine runner cover in a vertical pipe turbines, Kaplan 200 MW nominal output power, produced in Russia, and built in six hydro-generation units of hydroelectric power plant „Đerdap 1” in Serbia. Fatigue and corrosion-fatigue interaction have been taken into account using experimentally obtained material properties, as well as analytical and numerical calculations of stress state, to estimate appropriate safety factors. Fatigue crack growth rate, da/dN, was also calculated, indicated that internal defects of circular or elliptical shape, found out by ultrasonic testing, do not affect reliable operation of runner cover.


INTRODUCTION
esigning, construction and running hydro power plants is a complex task.Due to limited possibilities of periodic inspections, hydro turbines and their components are designed for at least 40 years of operation.Therefore, extensive researches and testing of hydro power plant equipment have been carried out all over the world.Turbine and hydromechanical equipment testing has been introduced recently in Serbia as well.Special attention has been given to the problem of reduced material plasticity, as explained below.Along with experimental tests carried out on the cover, made of cast steel 20GSL (GOST), tests were performed in order to determine mechanical properties and fracture mechanics parameters.Test results indicated large dispersion of material plasticity, i.e. elongation (A5) and contraction (Z), [1,2].Namely two specimens met the demands of the standard (A5 = D 23% and 27%), while two other had significantly lower values of elongation (A5 = 8% and 9%).Obtained values of fracture mechanics parameters (critical stress intensity factor -KIc, critical fatigue crack length -ac, fatigue threshold -ΔK th , coefficient in Paris equation -C, exponent in Paris equation -m p , fatigue crack growth rate -da/dN) are presented in Tab. 1, as shown also in [1,2].

STRESS STATE
urbine runner cover at the hydro power plant ''Djerdap 1'' is presented in Fig. 1.
Loading is defined for normal operation conditions, comprising combined action of pressure, axial force and torsion moment, as explained in [1].Finite element mesh for axisymmertic model is shown in Fig. 2. The results, indicating max.stress 81.1 MPa, is shown in Fig. 3.

FATIGUE AND FATIGUE-CORROSION ESTIMATION
atigue and fatigue-corrosion interaction are the most important aspects of turbine cover life and integrity estimation.Here, this combined effect of fatigue and corrosion is estimated by using the equation for safety factor, [3]: where σ -1 is the fatigue strength, obtained by standard testing, in conditions simulating the real, i.e. in water, ψ σ coefficient of loading asymmetry, equal to ratio of maximum fatigue strength to the maximum static strength (470 MPa in this case); σm mean cycling stress; σмо residual stress -in this case σmо = 60 МPа, [1]; σа amplitude of cycling stress.Fatigue strength depends on number of cycles and can be evaluated by using logarithm curve, obtained from Weller's curve: In the case of new material:  Having in mind coefficient of sensitivity to cycle asymmetry: 23.9 0.05 470 a safety coefficient for corrosion-fatigue strength is then: 23.9 0.05 (81.1 60) 8.34 2.02 Influence of internal defects on fatigue of cast steel 20GSL is an important issue and should be carefully evaluated.Toward this aim the mechanisms of microcrack initiation and conditions of propagation of microcracks to macrocracks have been established, [2,3].Based on test results establishment of the following empirical relation has been established: where: σ -1 -lower limit of fatigue strength dispersion; β -material coefficient, dmax -maximum size of the defect in cast material.
For the cast steels with defects (material with reduced plasticity), the following equation for fatigue strength holds: Based on this equation, one can conclude that defect sizes up 0.5 mm do not influence fatigue strength, whereas the maximum acceptable defect size (d = 1.5 mm) reduces fatigue strength σ-1 from 118.5 MPa to 91.15 MPa.Anyhow, the corrosion-fatigue safety factor is still satisfactory, S σ = 1.63.

ESTIMATION OF SERVICE LIFE OF THE RUNNER COVER THROUGH THE USE OF FRACTURE MECHANICS
stimation of runner cover service life through the use of fracture mechanics has been carried out according to methodology presented in paper [3].In the area of stable crack growth, Paris' equation describes the behaviour of the material with sufficient accuracy: where ∆K is the stress intensity factor range, defined by ∆K=M∆σ√a, M=1.21πQ, defect shape parameter.In the case of an internal defect, measuring 6 mm in diameter, detected by ultrasonic inspection, Q=1.65, [4].If the critical crack length is calculated for σmax=81.1+60=141.1 MPa, m m M number of cycles until reaching the critical size of the internal defect within the turbine runner cover, made of cast steel 20GSL with reduced plasticity (mp=3.15,cp=5.7•10 -1 ), can be calculated by the integration of Paris' equation:

CONCLUSIONS
esults of fatigue strength tests carried out on large specimens, as well as obtained values of fracture mechanics parameters, enabled the estimation of service life of turbine runner cover with reduced plasticity.Defects up to 0.5 mm have no influence on fatigue strength, whereas defects with acceptable value (1.5 mm) reduce fatigue strength from 118.5 to 91.15 MPa, still enabling reliable operation of the cove.E R Finally, internal defect with size of 6 mm, as recorded by NDT, does not jeopardize integrity of turbine cover, since predicted fatigue life is more than 783 years.

Figure 1 :
Figure 1: Vertical Kaplan turbine, with nominal output power of 200 MW.

Table 1 :
Fracture mechanics parameters at 23°C, for the stress intensity factor range ΔK = 10 MPam.