Diagnosing and Eliminating Resonance Using ODS, Modal and Finite Element Analysis

Diagnosing and Eliminating Resonance

By Alex Tomsick
Pioneer Engineering was contacted to troubleshoot excessive vibration on a fin fan structure at an oil refinery. The structure supports two belt driven fans. The vibration amplitudes measured on both fans were severe and well above allowable vibration limits for these critical assets. The dominant frequency component in the measured vibration data was at the fan operating speed of 498 cpm. Initial troubleshooting performed by Pioneer Engineering’s analyst indicated the amplitudes were likely related to a natural frequency of the fan structure that was being excited by forced vibration at fan operating speed. This indicated the excessive amplitudes were likely caused by resonance.  

Resonance is an aggravating condition in which inertial and stiffness forces lose the ability to resist vibratory forces leaving only damping forces to control response amplitude. This results in large amplification of vibration response, which occurs with a forcing frequency is coincidental with a natural frequency. 

To confirm the vibration of the fan structure was being aggravated by resonance, Pioneer Engineering performed Operating Deflection Shape (ODS) analysis which identifies the in-situ response frequencies and deflection shapes. Pioneer then performed Modal Analysis to identify the natural frequencies and mode shapes. If the ODS and Modal data show the in-situ response frequency and shape matches the natural frequency and mode shape a resonant condition is confirmed. 

Pioneer Engineering used MEscope with a 16-channel vibration data acquisition system to collect operating deflection shape (ODS) data on the fan structure. Figure 1 represents the operating deflection shape of the fan structure at 483 cpm.

Operating Deflection Shape of the Fan Structure at 483 cpm

Figure 1: Operating Deflection Shape of the Fan Structure at 483 cpm

The animation of the ODS data collected on the fan structure showed the motion of the C-channel matched the expected first bending mode shape. To verify this motion was related to the C-channel’s first bending mode, modal data of the structure was acquired with a large impact hammer.

Modal FRFs Measured on the C-channel of the Fan Structure

Figure 2: Modal FRF's Measured on the C-channel of the Fan Structure

Figure 2 represents the overlaid modal FRFs that were measured on the C-channels. Based on the modal data, it was found that the first lateral natural frequency of the C-channel was at 492 cpm. Given the proximity of this natural frequency to the operating speed of the fan, it was determined that the fan structure was resonant. To eliminate resonance of the fan structure, structural modifications had to be made to shift the natural frequencies away from motor and fan forcing frequencies. The structural modifications needed to shift natural frequencies to provide an adequate separation margin and not result in any new resonant conditions. Pioneer Engineering produced a finite element (FE) model and tested structural modifications with the FE model to verify proposed changes would solve current resonance problems and not produce any new resonant conditions.

Modified Finite Element Model

Figure 3: Modified Finite Element Model

Figure 3 shows the finite element model with the final structural modifications. Multiple different design modifications were evaluated using the finite element model and based on the finite element model results, the final modifications shifted all natural frequencies of the fan structure to acceptable separation margins. Pioneer Engineering recommends that all natural frequencies be separated by at least twenty-five percent from the closest forced vibration frequency to guarantee the natural frequencies will not be excited. These changes shifted the first natural frequency of the C-channels from 540 cpm to 1,290 cpm, which eliminated the resonant condition of the fan structure.  

Do you have a suspected resonant condition? Pioneer Engineering is here to help you solve the problem. To learn more about our mentoring and services, and to set up a no obligation consultatio, contact our sales team.