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Research Papers: Gas Turbines: Structures and Dynamics

Onset of 12X Vibration and Its Prevention

[+] Author and Article Information
John J. Yu

 GE Energy, 1631 Bently Parkway South, Minden, NV 89423john.yu@ge.com

J. Eng. Gas Turbines Power 132(2), 022502 (Nov 04, 2009) (10 pages) doi:10.1115/1.3155794 History: Received March 16, 2009; Revised April 13, 2009; Published November 04, 2009; Online November 04, 2009

This paper discusses real cases on three different machines where subsynchronous vibration occurred suddenly at a frequency exactly equal to one-half (12X) of the operational speed of 3600 rpm. In two cases, vibration amplitude increased from around 2 mils (51μm) pp to over 12 mils (305μm) pp, causing the machine to trip unexpectedly. The 12X vibration was even sustained during shutdown at speed below 3600 rpm after trip. Unlike other reported experimental results, shaft orbit measured by proximity probes was fairly circular and dominantly composed of the 12X component without significant components at multiple orders of the 12X. The onset of the subsynchronous 12X vibration was sometimes triggered by changes in bearing lube oil temperature and supply pressure, which would typically be believed to be caused by oil whirl that could occur at close to the 12X frequency. An upward shift in shaft centerline plot was observed to occur prior to the onset of the subsynchronous vibration. The 12X vibration was successfully prevented by changing bearing clearance or adjustment of bearing oil temperature and supply pressure. Parametric excitation analysis on 12X vibration is also given in this paper. The main purpose of this paper, unlike previously published papers on analytical or experimental results, is to show shaft orbital and centerline plots as well as spectrum data due to the 12X vibration, including effects of operating conditions in real rotating machines.

Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Machine train diagram for Case History 1

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Figure 2

Direct, 12X, and 1X vibration trend plots at Bearing 3Y-probe

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Figure 3

Shaft centerline plot at Bearing 3

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Figure 4

Orbital plots at different stages at Bearing 3

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Figure 5

Waterfall plot measured by Bearing 3Y-probe showing subsynchronous components around 1800 cpm prior to onset of the full-blown 12X vibration

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Figure 6

Spectrum plot measured by Bearing 3Y-probe when full-blown 12X vibration developed (primarily 12X component without significant harmonics)

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Figure 7

Damage on Bearing 3 surface

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Figure 8

Machine train diagram for Case History 2

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Figure 9

12X vibration occurred when power output increased to 3.6 MW

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Figure 10

12X vibration occurred when oil temperature dropped to 115°F

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Figure 11

Changes in shaft centerline at Bearing 2 during oil temperature variation test

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Figure 12

Waterfall plot measured by Bearing 2X-probe showing subsynchronous components around 1800 cpm prior to onset of 12X vibration

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Figure 13

Zoomed spectrum plot with frequency resolution of 1.88 cpm (0.03 Hz) when high vibration excursion occurred

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Figure 14

12X vibration occurred when oil supply pressure increased from 11.5 psi (gauge) to 15 psi (gauge)

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Figure 15

Cascade plot measured by Bearing 2X-probe showing no vibration excursion when speed increased from 3600 rpm to 3940 rpm during overspeed test

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Figure 16

Machine train diagram for Case History 3

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Figure 17

Vibration excursion occurred at Bearing 3Y-probe not due to 1X component

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Figure 18

12X vibration orbits and spectrum at 3600 rpm

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Figure 19

12X vibration orbits and spectrum at 3245 rpm

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Figure 20

Waterfall plot measured by Bearing 3Y-probe showing subsynchronous components around 1800 cpm prior to the full-blown 12X vibration

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Figure 21

Time-dependent stiffness varying within each synchronous 1X vibration cycle

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