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Research Papers: Gas Turbines: Structures and Dynamics

Mechanical Investigation of a Failed Lock-Pin

[+] Author and Article Information
E. Poursaeidi1

Department of Mechanical Engineering, Zanjan University, Zanjan, Iran; Research and Lab Division, Iran Power Plant Repair Co., P.O. Box 31585-381, Karaj, Iranepsaeidi@znu.ac.ir

A. A. Pirmohammadi

Department of Mechanical Engineering, Zanjan University, Zanjan, Iran

M. R. Mohammadi Arhani

Research and Lab Division, Iran Power Plant Repair Co., P.O. Box 31585-381, Karaj, Iran

1

Corresponding author.

J. Eng. Gas Turbines Power 131(4), 042501 (Apr 09, 2009) (8 pages) doi:10.1115/1.3077660 History: Received December 14, 2007; Revised November 11, 2008; Published April 09, 2009

This paper presents the outcomes of computational mechanics applied in the root-cause investigation on hot section failure of a 25 MW gas turbo generator in the domestic power plant after 2228 start-stops and 52,586 h operation. The failure includes the complete damage of the first and the second stage of nozzles, blades, seals, shroud segments, and also a peripheral damage on the disk of first stage. Several reported cases from the different power plants with similar events evidenced that the failure is a serious common type in the mentioned gas turbine engine. A previous study on complete metallurgical analysis of disk, moving blades, and lock-pins, was done by Poursaeidi and Mohammadi (2008, “Failure Analysis of Lock-Pin in a Gas Turbine Engine,” Eng. Fail. Anal., 15(7), pp. 847–855), which concluded that the mechanical specification of applied materials had been satisfied. Nevertheless, some problems were found in the fractographic results of lock-pins: the typical fatigue fracture surfaces in the neck of failed lock-pins and frankly localized pitting signs near the head of lock-pin. The lock-pins are kinds of small devices that lock the buckets after inserting them into disk grooves. In this work, a 3D finite element model (FEM) of a blade, a disk, and a lock-pin are made and analyzed by the ANSYS software. The results of the FEM showed a reasonable agreement between the analysis and position of fracture on lock-pins. Also, the results showed that the second vibrational mode of the bucket is a possible cause of failure because in this mode the peak stress occurs on the head of the lock-pin. However, inadequate design and long time service reduced the performance of lock-pins for sustaining a severe hot condition in the first stage of the turbine section.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) View of disk grooves after plastically deformations in the failed gas turbo generator. (b) A typical failed lock-pin. The knocked edges are visible on the remaining head.

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Figure 2

(a) Stereomicroscope view shows the fracture surface of a failed lock-pin (10×). (b) The schematic feature in accordance to view (a) presents distinct rapture areas.

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Figure 3

The sketch view of turbine’s hot section with temperatures of some special points (presented by the manufacturer)

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Figure 4

(a) Scheme of loads applied to blades. (b) Aerodynamic load distribution graphs along the blade length.

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Figure 5

The preprocessing of FE model. (a) Meshing feature of three components separately. (b) Assembled model of the disk section, blade, and lock-pin after applying boundary conditions.

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Figure 6

Modal analysis. (a) The first frequency mode is bending type, 1514.82 Hz. (b) The second frequency mode is bending type in the axial direction, 2403.13 Hz. (c) The third frequency mode is torsion type, 3628.38 Hz.

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Figure 7

The frequency response graphs show the three critical points between 100 Hz and 4500 Hz. Graph (a) shows the acceleration of blade root in the axial direction. Graph (b) shows the values of tensile stress along the lock-pin.

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Figure 8

Harmonic analysis. (a) First bending mode (peripheral direction), (b) second bending mode (axial direction), and (c) third torsional mode.

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Figure 9

The magnified views of the second natural mode in Fig. 8

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Figure 10

(a) The view of a damageable lock-pin with week profile at the ends. (b) A lock-pin with the advanced head profiles.

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