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Research Papers: Gas Turbines: Structures and Dynamics

Dynamics of Slant Cracked Rotor for a Steam Turbine Generator System

[+] Author and Article Information
Chao Liu

State Key Laboratory of Control and Simulation
of Power System and Generation Equipments,
Department of Thermal Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: chaoliu08@gmail.com

Dongxiang Jiang

State Key Laboratory of Control and Simulation
of Power System and Generation Equipments,
Department of Thermal Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: jiangdx@tsinghua.edu.cn

1Corresponding author.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received December 9, 2015; final manuscript received September 26, 2016; published online January 24, 2017. Assoc. Editor: Rakesh K. Bhargava.

J. Eng. Gas Turbines Power 139(6), 062502 (Jan 24, 2017) (7 pages) Paper No: GTP-15-1565; doi: 10.1115/1.4035323 History: Received December 09, 2015; Revised September 26, 2016

Root causes of several recent crack failures in turbine units are attributed to oscillation and interaction between generator of turbine unit and devices on the grid, where torsional vibration of the rotor bearing system is observed and identified as an important cause. Exploring vibrational (lateral, torsional, and axial) features in the cracked rotor system with torsional excitation (TE) present can provide a novel view in crack detection and isolation. This work presents dynamic analysis of a cracked rotor system in a steam turbine unit (a typical rotor system with multiple rotors, multiple supports, and oscillating loads), and the vibrational features of the cracked rotor system with comparisons to typical features in monitored vibration data. The results show that coupled vibration in both lateral and torsional components is an effective indicator for cracks in the presence of torsional excitation. Also, vibration characteristics evaluated in different locations of the rotor system are beneficial for fault detection and isolation.

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Figures

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Fig. 1

Model of slant crack in rotor. Crack area is marked as gray, with crack direction at 45 deg to the axial direction, and crack ratios as 0.2 and 0.4 (crack depth/diameter, 0.2 is shown here).

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Fig. 2

Profile of rotor bearing system in steam-turbine generator

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Fig. 3

Natural frequencies in torsional modes of normal and cracked rotor systems, obtained with finite element model

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Fig. 4

Modal shapes of torsional modes in the rotor bearing system, obtained with finite element model

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Fig. 5

Vibration in perpendicular direction close to bearing 7 with torsional excitation, obtained with finite element model. Ranges in y-axis are amplified to show the local sidebands. (a) Normal rotor system, (b) cracked rotor system (0.2), and (c) cracked rotor system (0.4).

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Fig. 6

Vibration collected in practical steam turbine unit (in perpendicular direction, from field data, TE—torsional excitation): (a) vibration (no TE), (b) vibration (TE), (c) spectrum (no TE), and (d) spectrum (TE)

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Fig. 7

Orbits of rotor system in normal and cracked conditions with torsional excitation (TE), obtained with finite element model: (a) normal, no TE, (b) normal, TE, (c) cracked 0.2, no TE, (d) cracked 0.2, TE, (e) cracked 0.4, no TE, and (f) cracked 0.4, TE

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Fig. 8

Orbits at bearing 7 in cracked rotor system of a steam turbine unit (from field data): (a) no torsional excitation and (b) with torsional excitation

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Fig. 9

Axial vibration of rotor system with torsional excitation, obtained with finite element model: (a) normal rotor and (b) cracked rotor (0.2)

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