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Experimental Modal Analysis


Location

Florence - test bench
 
Problem
 
High vibration levels were recorded on front journal bearing (in correspondence of plinth No. 1) of the steam turbine
 
 
Figure. 1                              
 
 
Overview
 
High vibration levels were recorded on the probe SV-2H on the steam turbine front journal bearing, when rotational speed approached the maximum value (1xRev component at about 176 Hz).
Aim of the test was to determine if any resonance frequency of the basement could be excited by the steam turbine at maximum speed so that it could justify the high vibration level recorded.
Eventually, some modifications of the structure could be suggested by the experimental results
Figure. 2 Measurement points                      
 
 
Activities
 
SINT Technology suggested to carry out an experimental modal analysis (impulse method) in order to verify the structural response of the system and, if necessary, to make some modifications on the structure in order to move up or down the resonant frequency .
 

Experimental analysis

 
 Experimental_analysis_2
Figure. 2 Measurement points                             
 
 
Results
 
 
 
 Experimental_analysis_3
 
Figure. 3 Synthesized FRF                             
 
 Experimental_analysis_4
 
Figure. 4 Stabilization diagram                             
 
 Experimental_analysis_5
 
Figure. 5 Mode deflection shape                             
 
The experimental modal analysis carried out on the plinth No.1 showed a natural frequencies at 172Hz, that is a value near to the 1xRev component with the steam turbine running at maximum speed.
 
Suggested modification
 
 
Figure. 6 Implemented modifications                             
 
 
Figure. 7 Implemented modifications                             
 
Conclusion
 
The experimental modal analysis carried out on the plinth No.1 showed a natural frequencies at about 172Hz, that is a value near to the 1xRev component with the steam turbine running at maximum speed.
Considering the corresponding modal shape, some modifications were implemented in order to move the frequency (increasing the stiffness).
After these modifications, the amplitude of the response of the structure at that frequency widely decreased as shown in Figure 8; therefore the modifications have been effective for moving the resonance away from that condition.
 
 
Figure. 8 Response before and after modification                             
 
Advantages
 
  • Quick problems troubleshooting thanks to real time data elaboration
  • Quick Modal parameter estimation based
  • Wide range of validation tools such as modal assurance criterion, modal phase collinearity, mean phase deviation, etc
  • Real time deflection shape