When Heat Exchangers Falter:

Heat exchangers are constantly subjected to dynamic thermal environments. During operation, startup, and shutdown, the materials within the heat exchanger experience continuous temperature fluctuations. These temperature differences cause the material to repeatedly expand and contract. Over time, this cyclical thermal stress can lead to the formation and propagation of microscopic cracks, a phenomenon known as thermal fatigue. These cracks are particularly prevalent in areas with significant temperature gradients or constraints, such as U-bends or where tubes are welded to tube sheets. Eventually, these cracks can grow into larger fissures, compromising the tube’s integrity and leading to leaks.

Identifying thermal fatigue early is crucial to prevent catastrophic failure. Visual inspection is a primary method, looking for visible cracks or discoloration, especially at stress concentration points. Dye penetrant inspection (DPI) or magnetic particle testing (MPT) can reveal surface cracks that are not readily visible to the naked eye. For sub-surface crack detection, ultrasonic testing (UT) can be employed. Furthermore, monitoring operational parameters like temperature cycling rates can help predict potential areas of concern.

Tube leaks, a common and problematic issue, often manifest as a result of various underlying causes. These can include the formation of cracks or holes in the tube surface due to deposit build-up, which creates localized corrosive environments. Similarly, general corrosion, erosion from high fluid velocities and abrasive particles, or even improper cleaning methods can thin the tube wall, making it susceptible to leaks. Once a leak forms, it can significantly impact heat exchanger efficiency as fluids bypass the intended heat transfer path. More critically, if fluids from different streams mix, it can lead to dangerous reactions or contamination, posing a significant safety risk.

Beyond operational factors, manufacturing and installation errors also contribute significantly to tube leaks. Under-rolling during fabrication occurs when the tube is not expanded sufficiently into the tube sheet hole. This creates a potential leak path between the tube’s outer diameter (OD) and the tube sheet hole’s inner diameter (ID). Conversely, over-rolling happens when the tube is expanded beyond the required wall reduction. This can severely damage the tube sheet or adjacent tubes. When a tube sheet “ligament” (the material between adjacent tube holes) is over-rolled, it can deform the ligaments of surrounding joints, inducing stress and causing them to leak. Such widespread deformation drastically increases the entire tube sheet’s susceptibility to leaks, leading to higher repair costs and persistent efficiency drops.

Several methods are used to pinpoint tube leaks. Pressure or vacuum testing is an easy hand held method that can be used to identify a drop in pressure or leak in a tube. Helium leak detection is a highly sensitive method where helium gas is introduced to one side, and a detector on the other side identifies escaping helium. Lastly, hydro testing is a common method used after fabrication where a vessel is filled with water under pressure and monitored for any leaking joints.

Mechanical failure in heat exchanger tubes is a broad category driven by factors such as vibration, improper installation, and operational stress.

Excessive vibration is a pervasive culprit. Flow-induced vibration, stemming from the interaction between fluid flow and tubes, can lead to tube wear and fatigue failure. Tube wear occurs when tubes repeatedly rub against support structures like baffles or even against adjacent tubes. This constant friction gradually erodes the tube material, leading to thinning and eventual perforation.

Fatigue failure, on the other hand, results from the continuous cyclic stress imposed by vibration. Even if individual stress levels are below the material’s yield strength, prolonged exposure can initiate and propagate fatigue cracks, particularly at stress concentration points like U-bends or areas with sharp geometric changes. If these cracks are not identified and addressed early, they can rapidly grow, culminating in catastrophic tube ruptures.

Beyond vibration, improper design or installation can create inherent weaknesses. Incorrect material selection, inadequate support structures, or tube-to-tubesheet joint issues during fabrication can all lead to “weak points” susceptible to premature wear or even sudden, catastrophic failure under normal operating conditions.

Visual inspection can reveal signs of rubbing or deformation. Eddy current testing (ECT) is highly effective for detecting fatigue cracks, thinning, and pitting in non-ferromagnetic tubes. Remote visual inspection (RVI) using borescopes allows for internal examination of tubes. Vibration analysis and modal analysis can identify resonant frequencies and predict potential vibration issues. For larger-scale issues, hydrostatic testing can reveal gross mechanical failures by subjecting the entire vessel to high pressure.

By understanding these distinct failure mechanisms and employing appropriate detection methods, industries can significantly improve the reliability and lifespan of their critical heat exchange equipment, safeguarding both efficiency and safety.

For more information on tube failures due to corrosion and erosion, check out another article in the series: https://www.elliott-tool.com/common-types-of-tube-failure-fouling-corrosion/