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TECHNICAL BRIEFS

Vibration Fatigue of Centrifugal Fan Impeller Due to Structural-Acoustic Coupling and Its Prevention: A Case Study

[+] Author and Article Information
Frantisek L. Eisinger, Robert E. Sullivan

 Foster Wheeler North America, Inc., Perryville Corporate Park, Clinton, NJ 08809

J. Pressure Vessel Technol 129(4), 771-774 (Oct 25, 2006) (4 pages) doi:10.1115/1.2767371 History: Received April 20, 2006; Revised October 25, 2006

Fatigue cracks from high cycle vibratory loading have developed in a centrifugal fan impeller after a relatively short period of operation. Based on vibration tests of the impeller and acoustic analysis of the internal spaces of the fan, structural-acoustic coupling with matching frequencies involving two-nodal diameter modes (structural and acoustic) was identified as the cause of the vibration problem. This paper gives a description of the problem and identifies available remedies for its elimination. Special attention is given in Case 1 to the acoustics of the system, where changes to the fan casing utilizing a decoupling acoustic chamber attached to the casing was investigated, and in Case 2, where changes needed to the structural characteristics of the impeller by utilizing a modified impeller with a substantially increased natural frequency (by 44%) of the two-nodal diameter vibratory mode shape were studied. Experimental tests confirmed that the structural-acoustic coupling and vibration were significantly reduced in Case 1 and were fully eliminated in Case 2.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Arrangement of centrifugal fan

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Figure 2

Experimentally determined impeller out-of-plane (bending) structural two-nodal diameter mode shape at frequency of 120Hz

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Figure 3

Predicted acoustic frequency of the two-nodal diameter acoustic mode shape of fan casing internal spaces (gas at 163°C), mode p,q,r=2,0,1 at frequency of 114.4Hz

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Figure 4

Predicted acoustic frequency of the two-nodal diameter acoustic mode shape of fan casing internal spaces (gas at 163°C), mode p,q,r=2,0,1 at frequency of 124.6Hz

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Figure 5

Fan casing modified by acoustic decoupling chamber attached to casing

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Figure 6

Maximum (peak) vibratory stress amplitude in fan impeller at toe of welds based on strain gage measurements: (1) impeller stresses before fan modification reaching fatigue limit and (2) impeller stresses in fan with modified casing utilizing acoustic decoupling chamber

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Figure 7

New upgraded structurally modified impeller by reinforcement of side plates

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Figure 8

Experimentally determined out-of-plane (bending) structural two-nodal diameter vibratory mode shape of the new modified impeller at frequency of 173Hz

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