PSI - Issue 68

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

564

6

a

b

Fig. 3. (a) crack growth kinetic diagram; (b) threshold value definition ℓ th for stage 2.

Table 2. Technical characteristics of Francis and Kaplan turbines. Parameter Value

Parameter

Value

Initial defect, mm

10

Start of fast crack growth ℓ th , mm Allowable crack growth ( ℓ cr - ℓ th ), mm

43.4 12.2

Critical crack length ℓ cr , mm Duration of slow crack growth

55.6

Duration of fast crack growth

~5.4·10 8 > 72 000

2.6·10 9 17 300

cycles hours years

cycles hours years

> 11.5

< 2.8

ℓ !" = {ℓ # ,ℓ $ } ℓ # = ℓ(' ! [* ) ℓ ] ℓ $ = ℓ([ ! ]) , [ ! ] = ' ! [* # ]

(1)

(2)

(3)

Definitions: [ n ℓ ] – crack length margin, [ n k ] – material properties margin, K с – material fracture toughness. The data presented shows that for the given scenario of HU operation, stage 1 of slow crack growth will last for more than 11.5 years. At the same time, stage 2 of rapid crack growth is estimated to be no more than 2.8 years. While the standard maintenance interval (with full non-destructive method control available between major repairs) is 5-7 years for large hydraulic units. Thus, ignoring the effect of HF-loads on the HU lifetime estimate may lead to the runner destruction before the scheduled HU maintenance, which is unacceptable. Worth noting: the transition from lifetime in terms of crack length to duration of reliable and safe operation (in hours) directly relates to HU’s working modes. Т res (time) can be significantly different for different HU operation scenarios with the equal length of allowed crack growth. This allows to manage the HU lifetime not only by choosing the next maintenance time, but also by selecting the most lifetime-optimal scenario for the HU operation. In general, after long HU service, the development of a maintenance and repair strategy should rely on: