Reducing Vibration Risk For Relocated Centrifuges Via Test & Analysis

A summary discussion about this project is also presented as a blog

Introduction

The wastewater and water industry are becoming well-versed in reducing rotating machinery vibration risk during the plant design or redesign process (both well-before and after the machinery has been purchased).

The focus is typically on pump systems in new or modified plants, as well as blowers on occasion. The goals are two-fold, help ensure the machinery system complies with: 1) any specified vibration acceptance criteria at commissioning, and 2) natural frequency separation margin requirements that help drastically lower vibration risk over the life of the plant.

This case study involves thorough baseline vibration testing and providing recommendations based on dynamic analysis and structural resonance evaluation before relocating the Decanter centrifuges with a different support structure within a Wastewater Treatment Facility (Figures 1 and 2). The work involved comprehensive pre-modification testing, finite element analysis (FEA), and evaluation of existing and proposed centrifuge installations.

Figure 1. One of the two duplicate centrifuges that were being relocated to a significantly modified location in the plant (Figure 2).

Figure 2. Yellow – two relocated and one new centrifuge/ motor/ platform supported by another location within the building. The platforms will be mounted on a new concrete slab poured on an existing floor. The individual support system for each centrifuge is isolated from the platforms.

Objectives

The primary risk reduction project objectives were as follows:

• Assess the vibration levels and identify the existing forcing function or excitation sources of the currently installed centrifuges.
• Identify the structural natural frequencies of the currently installed centrifuges.
• Predict the structural natural frequencies and dynamic displacements when the centrifuges (Figure 3) are relocated on the proposed support structure after “calibrating” the FEA model with the existing support structure.
• Evaluate the potential for structural resonance due to centrifuge operation on a different structure.
• Provide specific recommendations to the 60% plant design plan to mitigate any identified issues.

  Figure 3. Typical Decanter Centrifuge used in waste water treatment plants.

  Key Test Findings:

• The vibration levels of the existing centrifuges were not severe, and set a reasonable post-modification vibration acceptance criteria.
  Isolators were working effectively, as evidenced by lower support structure vibrations compared to machine vibrations.
• Primary vibration excitation sources were identified as the residual imbalance forces of the main motor and centrifuge bowl.
• Vibration data suggested that the proposed steel support structure should be designed to avoid a potential structural resonance up to a bowl maximum running speed of 2800 rpm (46.7 Hz).
• MSI recommended commencing structural modeling (Figure 4) to predict natural frequencies and make design adjustments as necessary.

   

  Key Analysis Findings:

  Natural Frequencies:
• No structural natural frequencies were identified below 25 Hz, which is within the frequency range of maximum dynamic input during run-down/start-up (Figure 5).
• Dominant structural frequencies were above 30 Hz, indicating no resonance near the bowl running speed of 2600-2800 rpm (43.3 Hz to 46.7 Hz).
  
  

  Figure 4. FEA Model of Centrifuge Platform (top and underside views)

   

  

Figure 5. Example of one structural natural frequency modes (29.5 Hz)

• Peak floor slab deflections during start-up/run-down were predicted to be 0.0275 inches (27.5 mils) (Figure 6).
• Dynamic deflections at the top of the subbase were 0.24 inches, with most deflection occurring within the subbase rather than the building structure.
• During normal operation, deflections were significantly smaller, on the order of mils.
  

  Figure 6. Maximum deflection occurred at start-up/run-down and was predicted to be 0.0275 inches (27.5 mils) and 0.24 inches at the floor slab and top of the subbase, respectively. Deflection is significantly lower during normal operation.

  
  Subbase Evaluation:
• Side-to-side swaying natural frequencies of the subbases were predicted to be 34.0 Hz and 36.1 Hz.
• Stiffening of subbases may be required post-commissioning to mitigate any excessive deflections.
  

   

Recommendations

1. Structural Design Adjustments (Figure 7):

• Modify the support structure design to avoid natural frequencies within the operational speed range of the centrifuges.
• Consider diagonal bracing or other stiffening methods for subbases if post-commissioning deflections are unacceptable.

2. Consultation with OEM:

• Engage the original equipment manufacturer (OEM) to confirm the target natural frequencies for subbases and ensure alignment with the FEA predictions.

3. Commissioning and Monitoring:

• Conduct thorough commissioning and monitoring of the centrifuges post-installation to validate the predictions and adjust the structural design as needed.

4. Dynamic Loading Considerations:
   • Confirm that the dynamic loading inputs used in the analysis are conservative to account for worst-case scenarios during start-up and run-down phases.
     

     Figure 7. An example modification recommendation provided by MSI. The example shows one analyzed approach to stiffen the subbase in order to re-locate a natural frequency mode further away from running speed while not introducing another potential problem.

     Result

     MSI’s comprehensive testing and FEA provided valuable insights, allowing for targeted design adjustments that successfully ensured the reliability and stability of the centrifuge installations. By identifying and mitigating potential resonance issues, the project successfully addressed the vibration concerns, paving the way for a stable and efficient centrifuge support system.

     A shorter summary of the above case history is provided in a blog.