Research Papers: Gas Turbines: Structures and Dynamics

Spiral Vibration of a Turbogenerator Set: Case History, Stability Analysis, Measurements and Operational Experience

[+] Author and Article Information
Lothar Eckert

 ALSTOM, Zentralstrasse 40, 5242 Birr, Switzerland

Joachim Schmied

 DELTA JS AG, Technoparkstrasse 1, 8005 Zurich, Switzerland

J. Eng. Gas Turbines Power 130(1), 012509 (Jan 11, 2008) (10 pages) doi:10.1115/1.2747645 History: Received April 30, 2007; Revised May 06, 2007; Published January 11, 2008

A hydrogen-cooled turbogenerator operating at 3600 rpm showed the phenomenon of spiral vibration with a forward rotating unbalance vector. The relative shaft vibration measured at the non-driven end-bearing was close to the trip level. Spiral vibration is observed at various types of rotating machinery with both rotation directions of the unbalance vector, i.e., forward and backward. Spiral vibration is caused by a vibration-induced hot spot on the shaft surface generated by friction. The turbogenerator has three bearings: two main bearings and the brush gear bearing. The carbon brushes sliding on the slip rings were identified as the hot spot location. Potential modifications were studied using hot spot stability analyses with a rotor dynamic model of the generator rotor on three journal bearings. The applied method, introduced by Schmied (1987, “Spiral Vibrations of Rotors” Proc. 11th Biennial ASME Design Engineering Div. Conf., Vib. Noise, DE-Vol. 2, Rotating Machinery Dynamics, Boston, MA, ASME H0400B, pp. 449–456), allows the handling of general systems. The hot spot model is based on the theory of Kellenberger (1978, Ingenieur-Archiv, 47, pp. 223–229; 1980, Journal of Mechanical Design, 102, pp. 177–184) using a thermal equation between the shaft’s thermal deflection and the shaft displacement at the hot spot location. Three different relations between the heat input and the shaft vibration were modeled: heat input proportional to the shaft displacement, to the shaft velocity, and to the shaft acceleration. The model, in which the heat input is proportional to the velocity, is the most suitable variant for slip rings. This was confirmed by comparison with the measured vibration behavior. A modification of the shaft line was selected based on the calculation results and was successfully implemented. This generator and other generators with the same modified brush gear unit have been in operation for more than four years.

Copyright © 2008 by ALSTOM
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Figure 1

Measured displacement p-p (peak-to-peak) amplitudes showing spiral vibration: (a) polar plot of bearing pedestal vibration; (b) amplitude versus time plot of relative shaft vibration

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Figure 2

Turbogenerator during running test in factory

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Figure 3

Polar plots of measured vibration at different rotor speeds n and period T for one 360deg turn: (a) relative shaft vibration at NDE-bearing; (b) horizontal DE-bearing pedestal vibration

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Figure 4

Measured relative shaft vibration at NDE-bearing. Rotor speed n=3600rpm.

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Figure 5

Brush gear unit: (a) view on removable brush carriers; (b) brush carrier with three brushes

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Figure 6

Three heat input mechanisms of brushes

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Figure 7

Rotor model of the turbogenerator with slip ring shaft between the NDE- and the end-bearings

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Figure 8

Slip ring shaft (SR) modes at rated speed: (a) horizontal; (b) vertical

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Figure 9

Thermal deflection due to hot spot at slip ring 1: (a) horizontal deflection; (b) vertical deflection

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Figure 10

Stability charts for the three different heat input models: (a) case a: heat input ∼ shaft displacement; (b) case b: heat input ∼ shaft velocity; (c) case c: heat input ∼ shaft acceleration

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Figure 11

Campbell diagram of horizontal and vertical slip ring shaft mode: (a) original design; (b) selected modification: five tilting-pad bearings at NDE- and end-bearings

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Figure 12

Hot spot stability chart before and after modification. Heat input proportional to shaft velocity.

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Figure 13

Polar plot of measured relative shaft vibration at NDE: (a) before modification; (b) after modification

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Figure 14

Typical measured relative shaft vibration at NDE: (a) polar plot; (b) amplitude versus time plot

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Figure 15

Circular orbit of the shaft center at the slip rings

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Figure 16

Circumferential distribution of added and eliminated heating efficiency




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