Failure investigation of an auxiliary steam turbine E. Poursaeidi a,b, * , M.R. Mohammadi Arhani b a Department of Mechanical Engineering, Zanjan University, Zanjan, Iran b Research & Lab Division, Iran Power Plant Repairs Co., P.O. Box 31585-1137, Karaj, Iran article info Article history: Received 6 December 2009 Accepted 22 March 2010 Available online 25 March 2010 Keywords: Vibration FEM analysis Experimental analysis Boiler feed pump turbine Lacing wire abstract This paper presents the results of failure investigation of an auxiliary steam turbine in a power plant. Fractures were occurred at the lacing wires in the L1 blade cascade. The fail- ure was occurred in repaired stages of blades after 47 days of an overhaul operation period. Visual inspection showed some regular fractures in the improper brazed joints and dimen- sional analysis showed that the lacing wire holes in the blades of the L1 stage are smaller than the originals. Fractographic investigation of fractured surface showed that the lacing wires had been exposed to a fatigue stress phenomenon. Finite element analysis showed that there is a high stress critical point near the brazing regions in comparison with original elements. Vibration analysis was performed experimentally and computationally to find the probable intersection points between the excitation harmonics and natural frequencies of blade cascade. Experimental test results verified the FEM analyses with good agree- ments. Obtained results from harmonic response analysis showed an approximate reso- nant condition of L1 blades during the operation of boiler feed pump turbine. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Steam turbine blades are critical components in auxiliary and main power equipments in nuclear and fossil power plants. They convert the linear motion of high temperature and high-pressure steam flowing down a pressure gradient into a rotary motion of the turbine shaft. As the steam enters turbine from the boiler, it passes through different stages such as high pres- sure (HP), intermediate pressure (IP) and low-pressure (LP) zones [1]. Often in large thermal power plants a steam line is branched from the IP turbine to drive an auxiliary small turbine (boiler feed water pump turbine, BFPT), which can be cat- egorized as LP turbines. Due to more expansion in LP turbine, their exhaust blades are the longest in height. According to some designing considerations, such as strength of materials, vibration theory, and turbo machinery, for the best energy extraction from the superheated steam, designing of the blades is encountered with some dimensional restrictions. Usually the moving blades can sustain some huge static loads like centrifugal forces. However, the existence of frequency forces will cause the appearance of alternating stresses so fatigue phenomena, and then finally rupture of applied materials. In the other hand, since blade failures are predominantly vibration related, blade vibration studies have acquired consider- able importance. A good deal of work on the determination of natural frequencies and mode shapes has been contributed by several research workers [2]. Steam turbines employ thousands of blade ranges from a few centimeters in height in the HP turbines to almost 1-m long blades in the last stage of LP turbines. As a result, blade natural frequencies in a machine can be as low as 100 Hz in the last stages of LP rotors and over a few thousand Hz in the first stages of HP rotors. By considering the per-revolution or the nozzle passing harmonics, one stage or more may be operated near the critical condition [3]. Theoret- ically, a single blade performs as same as a cantilever beam. Hence, it can vibrate in its modal shapes with multiple resonant 1350-6307/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.03.006 * Corresponding author at: Department of Mechanical Engineering, Zanjan University, Zanjan, Iran. Tel.: +98 912 2133496; fax: +98 261 6605929. E-mail addresses: epsaeidi@gmail.com, epsaeidi@znu.ac.ir (E. Poursaeidi). Engineering Failure Analysis 17 (2010) 1328–1336 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal