Leakage and Vibration Issues in Heat Exchanger Tube Bundles

 
Heat exchangers are crucial thermal exchange devices in industrial applications, and the health of their tube bundles directly impacts overall system performance. A tube bundle consists of numerous tubes arranged in a specific pattern. Over time, these tubes can suffer from issues such as leakage and vibration due to medium erosion and corrosion. These problems not only reduce heat exchange efficiency but can also cause equipment damage and affect production safety. This article explores leakage detection methods, repair strategies, and the forms of damage caused by heat exchanger tube bundle vibrations, offering solutions to enhance the operational reliability of heat exchangers.

Leakage Detection and Repair of Heat Exchanger Tube Bundles

 
The tube bundle of a heat exchanger is a core component responsible for heat transfer and fluid flow. During prolonged use, the tubes in the bundle may experience leakage due to factors such as medium erosion, corrosion, or fatigue. To ensure the normal operation and extend the lifespan of the heat exchanger, timely leakage detection and effective repair methods are crucial. The following sections detail the methods for detecting and repairing leaks in the tube bundle to accurately identify issues and implement appropriate solutions.

1. Leakage Detection

 
Tube bundles in heat exchangers are prone to becoming "high-risk areas" for leakage over extended periods of operation. To accurately locate leakage points, sampling ports are typically installed at the low-pressure outlet of the cooling water, and the samples are periodically analyzed. Detection of cooling medium components in the cooling water indicates the presence of leaks within the tube bundle. Subsequently, pressure testing (hydrostatic testing) is used to meticulously check each tube and determine the exact location of the leakage. These methods ensure rapid identification and resolution of leaks, preventing further system damage and efficiency loss.

2. Repair Methods

 
The choice of repair methods depends on the severity and number of leaks. The following are common repair methods, each providing adaptive solutions for different leakage conditions and tube bundle statuses.

Few Tube Leaks

 
For cases with a limited number of leaks and minimal impact on heat exchange efficiency, plugging methods are commonly used. This involves inserting conical metal plugs at both ends of the leaking tubes and welding them in place. The plugs usually have a cone angle of 3-5°, with the larger end diameter slightly larger than the expanded internal diameter of the tube, ensuring effective sealing. This method effectively addresses minor leakage issues without significantly impacting system operation.

Multiple Tube Leaks

 
When there are numerous leaking tubes, excessive plugging can severely impact the heat exchanger's operational efficiency (generally, plugging should not exceed 10% of the total number of tubes). In such cases, replacing the damaged tubes is a more appropriate choice. For thin-walled non-ferrous metal tubes, drilling or reaming techniques are used for removal, while thicker-walled tubes are cut using oxy-acetylene flames. During tube removal, it is essential to ensure that both ends of the tube are completely removed while keeping the tube sheet hole intact. After removing the defective tube, a new tube is installed into the tube sheet hole. The new tube's size and material should match those of the original tube. During installation, proper alignment and tight connection with the tube sheet must be ensured. The connection can be made using either expansion or welding: expansion involves using an expander to roll and expand the tube end to fit the tube sheet hole tightly, while welding involves using welding techniques to secure the tube to the tube sheet. The appropriate connection method should be selected based on specific conditions.

Tube Bundle Vibration and Damage

 
To enhance heat transfer and reduce fouling layers, the flow rate of the shell-side fluid is often increased. However, increased flow rates can induce vibrations, leading to tube bundle vibrations and potential damage. Common forms of damage include:

1. Tube Collisions

 
When the amplitude of tube vibrations is large enough, it can cause collisions between tubes, especially with those on the periphery potentially colliding with the heat exchanger shell's inner wall. During collisions, the tube walls can become worn and thinned, eventually leading to cracks. Such damage not only affects the heat exchanger's functionality but can also lead to severe leakage issues.

2. Tube Wall and Baffle Hole Collisions

 
Radial gaps between the tube and baffle holes can result in repeated collisions between the tube wall and the inner surface of the baffle holes during significant lateral vibrations. Due to the thin thickness of the baffles, frequent contact with the tube wall can subject the tube to substantial impact loads, potentially causing localized damage in a short period.

3. Stress Concentration at Connections

 
In fixed-end constrained connections, lateral bending of the tube can lead to maximum stress at the connection point, making it one of the most vulnerable areas for tube bundle failure. Stress concentration can cause material damage or failure at the connection point, affecting the stability of the entire heat exchanger.

4. Material Defects and Alternating Stress

 
Defects in the tube material (such as those caused by corrosion or abrasion) can lead to rapid crack propagation under alternating stresses induced by vibrations, ultimately causing tube failure. Damage from flow-induced vibrations typically occurs in areas with significant deflection and high shell-side flow rates, including U-bends, shell-side inlet and outlet connection areas, tube sheets, baffle cutout areas, and tubes under compression stress.

Conclusion

 
Leakage and vibration issues in heat exchanger tube bundles are critical factors affecting equipment performance and operational safety. By promptly detecting leaks, employing appropriate repair methods, and addressing various forms of damage caused by tube bundle vibrations, the operational efficiency and lifespan of heat exchangers can be significantly improved. Regular maintenance and inspection, combined with reasonable design improvements, are key to ensuring long-term stable operation of heat exchangers. Understanding these issues and taking preventive measures will help maintain equipment functionality and ensure smooth production processes.