Elsevier

Engineering Failure Analysis

Volume 16, Issue 7, October 2009, Pages 2202-2208
Engineering Failure Analysis

Failure analysis of the draft tube connecting bolts of a Francis-type hydroelectric power plant

https://doi.org/10.1016/j.engfailanal.2009.03.003Get rights and content

Abstract

In this paper, the failure of the bolts that fasten the draft tube of a 95-MW Francis turbine is presented. The fracture of the bolts is especially frequent when the machine operates at partial load. Fracture surface analysis and stress measurements under several power levels were done. Stress was also measured, while pressure relief in the spiral case was performed. Finally, stress measurements were conducted while pressured air was injected at the stay vanes and the machine was operating at partial load. With the strain measured, stress on the bolts was calculated.

The fracture surface analysis showed that fatigue is the failure mechanism. Stress measurements revealed that strong vibration and broad stress variation is present when the machine operates at powers below 80 MW. Air injection effectively decreases vibration and stress fluctuation, but pressure relief in the spiral case did not show any beneficial effect.

Introduction

The variable demand on the energy market requires great flexibility in operating hydraulic turbines; therefore, turbines are frequently operated over an extended range of regimes far from the full load and the best efficiency point. Francis turbines operating at partial load present pressure fluctuations that are due to the Von Carman Vortex in the vanes and Vortex Rope in the draft tube [1], [2]. These phenomena generate strong vibrations and noise that may produce failures on the mechanical elements of the machine.

Several methods have been proposed to decrease the vortex and the vibrations in these kinds of turbines. The most effective of them is air injection over the stay vanes [3], or air admission into the draft tube [1]. Blommaert et al. [4] reported improvement in the stability of a 90-kW turbine model using an active control with a rotating valve exciter. Susan-Resiga [5] presented a method to decrease the vortex rope by using a jet issued from the crow tip. The method was evaluated with a numerical model of a turbine where the jet proved effective. Later, Susan-Resiga et al. [6] presented the development of a test rig where an artificial vortex rope was generated. Experimental tests on the rig showed that a jet injected axially at the conical diffuser inlet effectively suppresses the vortex.

This paper shows the failure analysis of the bolts fastening the draft tube of a 95-MW Francis-type hydroelectric power plant. The plant has three units located in line and the draft tube of each unit is made up of two parts: the part near the turbine is formed by two uncovered cones made of low carbon steel wall 18 mm thick and the other part, which is far from the turbine, is embedded in concrete. The two uncovered cones are joined to each other by bolts and in the same manner are joined to an upper and a lower flange embedded in concrete. Between the lower cone and the lower flange, a seal ring and a ring with wedge form in the cross section are assembled to compress the cone against the flanges and to guarantee fixation and sealing. Fig. 1, shows the actual draft tube including sketches of the draft tube assembly with the upper and lower flanges. The failure consists of very frequent fracture of the upper bolts, which increases the time lost in repairs and reduces plant reliability. Failures become more frequent when the plant has to operate at partial load. The analysis was conducted on unit two (where failures are most frequent) located between units one and three.

Visual examination of the system and strain measurements on an upper and a lower bolt during plant operation under several conditions were done. The aim is to understand the bolt failure mechanism and find the operating conditions which cause the failure. We also evaluated the effect on the bolt stress of air injection over the vanes and the pressure relief in the spiral case. The actual bolts are usually preloaded between 70% and 90% of the yield strength. The experimental bolts were preloaded at 80% of the yield strength, which is within that range.

Section snippets

Methodology

Visual examination of fracture surfaces to identify the failure mechanism was performed. Also, strain measurements on an upper and a lower bolt were carried out. The bolts were located at the position where the failures are most frequent. The strain was measured by using an FLA-3-11 TML strain gage on the upper bolt and an FLA-10-11 TML strain gage on the lower bolt. Each strain gage was connected in quarter Wheatstone Bridge to a DC-104R TML dynamic strain recorder and the data was registered

Visual examination

Visual examination clearly revealed that the fracture was by the fatigue mechanism as can be noted in Fig. 3. Although most fractures have occurred at the threaded area of the bolt (Fig. 3a), some of them have occurred near the head of the bolt (Fig. 3b). The crack propagated normally to the main axis of the bolt and started at the root of the thread or at the changing of section between the head and the round section, due to the stress concentration. In general, the fracture surfaces present

Conclusions

Frequent failures of the upper bolts of the draft tube were investigated by visual examination and stress measurements under several working conditions. Fracture surface revealed that the failure mechanism was high-cycle fatigue and that cracks begin at the root of the thread in most cases.

Stress measurements showed that at power below 80 MW, high stress variation is present, producing fatigue failure. The stress condition was worst when units one and three were operating at 95 MW than when those

References (10)

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