Research Papers: Gas Turbines: Structures and Dynamics

Operational Modal Analysis of Torsional Modes in Rotating Machinery

[+] Author and Article Information
Eoin Peter Carden

Lloyd’s Register Consulting,
P.O. Box 47230,
Stockholm 100 74, Sweden
e-mail: Peter.Carden@lr.org

Mattias Lindblad

Lloyd’s Register Consulting,
P.O. Box 47230,
Stockholm 100 74, Sweden
e-mail: Mattias.Lindblad@lr.org

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 9, 2014; final manuscript received July 10, 2014; published online September 4, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(2), 022501 (Sep 04, 2014) (7 pages) Paper No: GTP-14-1353; doi: 10.1115/1.4028210 History: Received July 09, 2014; Revised July 10, 2014

Traditional experimental modal testing techniques rely on controlled and measured excitation together with measured responses in order to identify the mode shape, natural frequency, and damping factor of each mode. Applying a controlled and measured excitation to a rotor train when in operation is logistically difficult and especially challenging in the field. Operational modal analysis (OMA) identifies the modal parameters of a system from measurement of response due to some (unknown) excitation. OMA has proven successful over the past several decades on nonrotating structures but has relatively rarely been applied to rotating machinery. Case studies are presented demonstrating the use of OMA in identifying torsional modes on an electric motor driven reciprocating compressor, on a diesel engine driven fire water pump, and on a marine propulsion system. In contrast to lateral modes, torsional modes of rotor trains are typically not speed dependent. However, phenomena exist whereby the torsional modes may be different at stand still, off-load and at different loads. The case studies provide examples of such phenomena and also of significant differences between predicted and measured behavior which suggests that improvements in industrial practice would be beneficial. Such improvements should be based on reconciliation of measured and predicted behavior and OMA offers a valuable tool to facilitate this. OMA provides a significant benefit in investigating and understanding torsional behavior in operation.

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Fig. 1

Case study 1—an example stability diagram from OMA using the two dynamic torque channels. The circles illustrate the stable modes or harmonics.

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Fig. 2

Case study 1—contour plot of one torque channel on the rotor during coast down

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Fig. 3

Case study 1—zoom in between 35.5 Hz and 36.5 Hz of the contour plot shown in Fig. 2

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Fig. 4

Case study 3—the drive shaft with the strain gauge installation, between the bearing and the coupling

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Fig. 5

Case study 3—spectrogram of the torsional strain data measured during a load sweep test. Note: the arrows in the top of the figure illustrate where the torsional natural frequencies are indicated and the arrows at the bottom show the orders from the shaft (xS) and from the gear ratio (xG).



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