Author(s)

S. S. Fattah, F. M. Khoshnaw & R. A. Saeed

Abstract

During periodic inspections of unit 2 in Derbendikhan hydropower station, cavitation damages and fatigue cracks were observed in the runner and draft tubes. Operation data (field data) between 1997–2002 were taken from the station; these data included head and output. The Prototype performance curve was obtained from the model test data and drawn by using the available equations, charts and tables from the literature and catalogs. The results showed that the unit 2 in Derbendikhan hydropower station operated abnormally which led to cavitation damages and fatigue cracks. For simulating the effect of overloading procedure on fatigue cracks in the runner, laboratory measurements were performed using martensitic stainless steel notched specimens. Four modes of overloading procedure are accomplished which were tension (T), compression (C), tension-compression (TC) and compression-tension (CT). For each overloading mode four different load values 150N, 200N, 250N and 300N were applied. The results showed that the beneficial effect (crack retardation) was observed under certain values for each overloading mode. Keywords: cavitation damage, fatigue, hydropower station, overloading. 1 Introduction The major problems in operating the hydraulic turbines are the occurrence of cavitation and fatigue failures. Cavitations are the formation of water vapor and air bubbles in the water due to the reduction of pressure below the vapor pressure corresponding to the temperature of the water. The cavity bubbles collapse close to a solid surface at regions of higher pressure, which generates highly localized stresses. Repetition of this phenomenon for a large number of times causes local

Keywords

cavitation damage, fatigue, hydropower station, overloading.