Heat exchangers are essential in industrial applications for transferring heat between fluids. However, they are susceptible to multiple mechanical failures, being one of them flow-induced vibrations (FIV). This case study examines a specific incident where a heat exchanger experienced multiple failures at the welded connections of tube bundles to the tube header boxes. The primary objective is to investigate the causes of these failures and recommend effective solutions to enhance the heat exchanger’s reliability.
Understanding the Context of Heat Exchanger Failures
The heat exchanger in this case is designed to extract heat from hot flue gas flows. The client reported fatigue failures at the welded connections, raising concerns about the system’s structural integrity. The investigation aimed to determine whether these failures were indeed caused by FIV and identify the underlying mechanisms. A thorough examination of operational conditions, material properties, and design features was necessary for a comprehensive understanding of the failures.
Identifying the Issues Related to Flow-Induced Vibrations
The primary issue involves the welded connections between tube bundles and tube header boxes. The suspected cause of these failures was flow-induced vibrations, which can lead to fatigue over time. The suspected cause of these failures was flow-induced vibrations, which can lead to fatigue over time. The heat exchanger operates at elevated temperatures of 200 °C, a factor that can affect material properties and fatigue resistance.
Several factors complicated the analysis. The high operating temperature alters material properties, making them more prone to fatigue. Additionally, complex flow dynamics within the heat exchanger create challenges in accurately predicting vibrational effects. The existing baffle plate design may not adequately address problematic vibrational modes. Furthermore, potential coupling effects between structural oscillations and flow turbulence add complexity to the analysis.
Methodological Approach to Analyzing the Vibration Problem
A comprehensive methodology was employed to effectively address the flow-induced vibration problem:
- Computational Fluid Dynamics (CFD) simulations were conducted to analyze the flow field around the tube bundles. This approach allowed for monitoring the forces acting on the tube walls, essential for determining excitation frequencies and amplitudes from turbulent flow.
- A modal frequency analysis was performed using a CAESAR II model to calculate the natural frequencies of the tube bundles under various configurations. By comparing these frequencies with the excitation frequencies identified in the CFD simulations, the likelihood of resonance was assessed.
- Supplementary hand calculations were performed to calculate vortex shedding frequencies using the Strouhal number and Reynolds number, providing a theoretical basis for comparison with CFD results.
- A fatigue analysis was conducted according to ASME VIII Div 2, focusing on assessing stress amplitudes at the header box welds under cyclic loading.
Insights from the Analysis of Vibration Dynamics
The analysis revealed several critical insights.
- The CFD simulation results indicated a broadband excitation frequency range of 0.5 to 5 Hz, significantly overlapping with the natural frequencies of the tube bundles. This overlap suggests a high potential for resonance. Downstream tubes experienced amplified vibrational forces due to interference from upstream tubes.
- Specific frequencies of concern were identified at 1.5, 1.7, and 2.2 Hz, where the highest amplitudes were observed. The analysis indicated that cyclic loads were notably larger in the transverse direction, consistent with vortex shedding and Von Karman vortex theory. These findings underscored the need to address the interactions between flow characteristics and structural dynamics.
- The modal frequency analysis confirmed the CFD findings, showing that the modal frequencies of the tube bundles fell within the problematic excitation frequency range. Increasing the modal frequencies to above 10 Hz could significantly reduce the risk of resonance and fluid-elastic instability.
- Additionally, the fatigue analysis calculated the effective total equivalent stress amplitude at the header box welds to be 46.0 MPa, which is 49.2% of the fatigue allowable for an infinite number of cycles at 200 °C. While this value is below the allowable limit, coupling effects between structural oscillations and flow turbulence increase the risk of fatigue failure. The analysis also indicated that the gap velocity in the flow field exceeded the critical velocity for transverse oscillations, further highlighting the risk of fluid-elastic instability.
Recommended Solutions to Mitigate Vibration Issues
Based on the analysis, several solutions were proposed:
- Increasing the modal frequencies of the tube bundles to shift them outside the range of flow excitation frequencies. This can be achieved by anchoring the baffle plates to the side walls of the heat exchanger and enhancing the overall stiffness of the tube bundle structure. These modifications would reduce turbulence-induced vibrations and prevent oscillations and cyclic loading.
- Optimize the baffle plate design. By redesigning the baffle plate configuration to target specific frequency ranges and adjusting the positioning of baffle plates, more effective damping of vibrations can be achieved. This solution would require detailed modeling and potentially prototyping of new designs.
- Conduct a coupled CFD-mechanical analysis, which could provide a more accurate understanding of fluid-structure interactions. This approach would allow for precise predictions of stress amplitudes and fatigue life, facilitating the optimization of design changes before implementation.
- Lastly, exploring alternative tube bundle configurations could fundamentally alter flow characteristics. Investigating different tube arrangements and considering alternative shapes or surface treatments may improve both vibration resistance and heat transfer efficiency.
Assessment of the Proposed Solutions for Vibration Management
The proposed solutions were evaluated based on effectiveness, feasibility, and alignment with project objectives. Considering the options mentioned above the most promising immediate solutions appear to be increasing modal frequencies and optimizing baffle plate design.
These approaches can be implemented relatively quickly and directly address the identified problems. By implementing these strategies, it is possible to enhance the reliability and longevity of heat exchanger systems, improving overall performance and reducing maintenance costs in the long term.
The insights from this case study can serve as a valuable reference for engineers and designers in the field of thermal and mechanical systems.
