In engineering, the ability to predict and mitigate failures before they occur is critical. Finite Element Analysis (FEA) has become a powerful tool in diagnosing structural problems and providing insight into potential solutions. A recent project done by Simumech demonstrated just how accurate and valuable FEA can be when applied correctly.
The Challenge: Vibrations Leading to Fatigue Cracks
One of our clients reported unexpected vibrations in their paper pulp storage tank when liquid levels were low. The tank, 60 meters high and 17 meters in diameter, was constructed from duplex stainless steel. The issue was traced to liquid falling from a 900mm-diameter top inlet pipe, striking the surface over 15 meters below. The resulting impact generated sloshing waves, which in turn excited the tank shell and caused unwanted vibrations. These vibrations were strong enough to excite the connected piping, raising concerns over long-term fatigue and potential structural cracking.
Investigating the Problem: High-Resolution Measurements and Simulation
To diagnose the issue, a third-party company conducted vibration measurements using high-definition video capturing. This advanced technique allowed for detailed tracking of the tank’s dynamic response under real operating conditions. The vibration measurements were derived directly from the video footage, leveraging cutting-edge technology to extract precise displacement data.
In parallel, Simumech performed a comprehensive Finite Element Analysis (FEA) study to analyze the vibration modes of the tank shell and assess the structural effects of the fluid impact. The FEA model incorporated:
- Natural frequency analysis to identify resonant modes of the tank and the connected piping.
- Linear static analysis to reinforce the nozzle to shell junction to improve its stiffness and strength.
Key Findings: FEA vs. Real-World Behavior
The results of the FEA study closely matched the video measurements, confirming that the observed vibrations were a result of resonance excited by the sloshing waves. The analysis successfully identified the most critical vibrational modes, correlating them with the video footage.
Implementing Solutions: Addressing the Root Cause
Based on the analysis, multiple mitigation strategies were evaluated, including:
- Reducing the Impact Energy – The inlet pipe was modified to include two inlets: one at the bottom and one at the top, controlled by valves. Since the pump was not strong enough to push liquid exclusively from the bottom, the design ensured that liquid would always pour from just above the liquid level, preventing free-fall impacts.
- Avoiding Low-Level Operation – While modifying the flow rate was considered, it was deemed impractical due to the time it would take to fill the tank. However, operational guidelines were adjusted to minimize operation in the lowest liquid-level phase, as this was when the issue occurred.
Validating the Fix: Post-Implementation Monitoring
After implementing the dual-inlet modification, the tank was monitored under operational conditions. Vibration levels were significantly reduced, and no further excitation of the connected piping was observed, validating the effectiveness of the FEA-driven approach.
Lessons Learned: The Role of Shell Thickness in Vibration
One unexpected insight was the influence of the upper shell thickness. While the upper sections of the tank do not require significant thickness for pressure purposes, their structural flexibility can contribute to increased vibration susceptibility. This highlights the importance of considering global shell dynamics, even in areas where pressure is not a primary concern.
Conclusion: The Power of Advanced Simulation
This project underscores the value of FEA to diagnose and resolve real-world engineering challenges. When used correctly, FEA provides accurate predictions of structural behavior, enabling engineers to implement effective solutions before costly failures occur.
For engineers dealing with similar challenges, this case study reinforces the need to integrate simulation early in the design and troubleshooting process. Whether addressing vibration failures, fatigue issues, or structural resonance, a well-executed FEA approach can significantly enhance reliability and prevent long-term damage.