Factors Causing Reduction in the Bearing Capacity of Hydraulic Turbine Structural Elements in the Course of Long-Term Operation
Abstract
Accumulation of fatigue damage is the main mechanism governing the incipience and growth of cracks in the blade system of vertical hydraulic power units both during the running-in period and after long-term operation at the stage of exhausting the equipment service life. However, the causes, location and incipience mechanism, and the crack growth rate at an early stage of operation and beyond the design service life differ significantly from each other. This circumstance does not make it possible to use the experience gained from control of crack formation on the blades of turbine runners, which is successfully applied in the running-in period, to predict the flaw occurrence time and flaw growth rate after a long-term operation. The development of fractures during the running-in period is predominantly due either to the presence of initial flaws and large stress concentrators of geometric or technological origin, or the presence of off-design loads applied on the equipment. During this period, the fracture develops quite quickly and is not repeated after carrying out restorative repair and eliminating its causes. During operation beyond the design service life, fractures develop according to different scenarios: damages accumulate gradually under the effect of relatively low dynamic loads and is accompanied by irreversible changes in the condition of metal. This eventually leads to loss of bearing capacity of a certain hydraulic turbine element. Sometimes, this process can last as long as several decades. The problem of the turbine critical elements’ gradually losing their bearing capacity becomes especially topical on extremely long operation periods when the operating time exceeds 300 000 h, which corresponds to about 40 years of operation. As an illustration, the article outlines the principal factors causing reduction in the bearing capacity of the runner, the main component determining the hydraulic turbine service life, which affects the turbine service life as a whole. All these circumstances, which take into account the individual design and operational features of the hydraulic unit, should be considered in assessing the crack incipience time and their growth rate beyond the design service life.
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