A more detailed discussion about this project is also presented as a case history.
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.
More recently, an Architecture Engineering firm wanted to use the same basic risk reduction process for a wastewater treatment plant centrifuge related project. Significant plant structural and piping modifications were required and the risk of introducing high-profile vibration issues was high.
Plant modification plan: The plant's existing units, consisting of two large Decanter Centrifuges, were to be relocated, and a third new centrifuge was being added. A completely new support structure was being designed creating an opportunity for harmful resonance issues (Figure 1).
The risk reduction project objectives included:
- Assessing current vibration levels via specialized testing
- Identifying excitation sources
- Predicting the structural natural frequencies and dynamic displacements for the new baseline centrifuge support structure via Finite Element Analysis (FEA)
- Evaluating via FEA support structure modifications for potential issues
The key deliverable was recommended specific modifications to the plant redesign 60% plan to address any identified potential vibration issues
Figure 1. 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.
Testing and Analysis: Initial pre-modification vibration testing revealed that the vibration levels of the existing centrifuges were acceptable and set a reasonable acceptance criteria for the centrifuges after the plant modifications were complete. The main sources of vibration as identified was residual imbalance forces from the main motor and centrifuge bowl. Data showed that that the proposed steel support structure needed to avoid structural resonance at speeds up to 2800 rpm (46.7 Hz) by a recommended 15% separation margin.
Further evaluation using Finite Element Analysis (FEA) models of the centrifuge platform was conducted. Key findings indicated that no structural natural frequencies were below 25 Hz, and dominant structural frequencies were slightly above 30 Hz, well clear of the centrifuge's operating speed range of 2600-2800 rpm (43.3 Hz to 46.7 Hz). Predicted peak floor slab deflections during start-up and run-down were minimal, and dynamic deflections at the top of the subbase were within acceptable limits.
Recommendation summary: Based on the analysis, several recommendations were made to ensure the new support structure would perform effectively. These included modifying the support structure design to avoid natural frequencies within the centrifuge's operational speed range and utilizing one of the recommended stiffening methods for the subbases (Figure 2). Collaboration with the original equipment manufacturer (OEM) was advised throughout the process. Thorough commissioning and post-installation monitoring were recommended to validate the predictions and adjust the structural design if needed.
Figure 2. 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.
In addition, please learn more about our vibration analysis software, VibVue.®
More details are provided in a case history.
