Failure modes of heat exchangers

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The main components of a shell-and-tube heat exchanger include: the shell, head, tube bundle, tube sheet, baffle plates, nozzles, flanges, and expansion joints. The primary failure modes include failure of the tube bundle, failure in the connection between the tubes and the tube sheet, and failure of the shell. I. Tube bundle failure 1. Tube bundle vibration failure To enhance heat transfer in the shell side and reduce scaling, it is common to increase the flow rate of the fluid in the shell side. However, an increase in the flow rate of the shell-side fluid often leads to induced vibrations in the tube bundle, resulting in tube collisions and damage, tube severing at the baffle plates, and fatigue failure. 2. Tube bundle corrosion and abrasive failure: Most failures of heat exchangers are caused by corrosion. The most common sites of corrosion are the heat exchange tubes, followed by the tube sheets, heat exchanger heads, and small-diameter nozzles. The main causes of tube bundle corrosion and erosion failure include: fouling corrosion, medium corrosion, erosion due to high flow velocities inside the tubes, electrocorrosion, and crevice corrosion at the tube ends. 3. Reduced heat transfer capacity: During the operation of the heat exchanger, high hardness of the working medium, the presence of particles or suspended solids in the fluid, as well as algae, bacteria, and sediment in the cooling water, can all lead to severe scaling on the inner and outer walls of the tube bundle. As the scale layer thickens, the heat transfer resistance increases rapidly; in severe cases, the scale can block the flow channels of the working medium, resulting in a swift decline in heat exchange capacity. 4. Tube bundle leakage: In cases of corrosion of the heat exchange medium, stress corrosion, intergranular corrosion, or due to collisions and wear, microscopic cracks may form on the tubes. If high tensile stresses or alternating stresses are present, these cracks will expand rapidly, leading to leakage. II. Failure of the connection between tubes and tube sheets. Depending on the operating conditions of the heat exchanger, the types of connections between tubes and tube sheets can be welding, expansion bonding, or a combination of both expansion bonding and welding. Different joint types result in various failure modes. 1. The main problems that occur during welding are: the formation of welding defects such as burn-through or incomplete penetration ； Thermal stress generated at the weld joint leads to stress corrosion ； A gap between the tube and the tube sheet holes can cause crevice corrosion ； When welding stainless steel, changes in the microstructure in the heat-affected zone lead to a sharp decline in corrosion resistance. 2. For expansion jointed connections, residual stresses exist during the expansion process, and the joints may fail due to stress corrosion under temperature and environmental conditions that are favorable for such corrosion ； "The tensile pull-out strength of crimped joints is relatively low; especially when the operating temperature is above 300 degrees, the creep of the material causes the extrusion residual stresses to gradually disappear, making it difficult to ensure the reliability of such joints. 3. The combination of welding and expansion bonding takes advantage of the strengths of each method, offering benefits such as resistance to repeated thermal shocks and thermal corrosion, improved fatigue resistance of the joint, and elimination of interstitial corrosion. However, using expansion welding in combination requires high operational standards, and it is generally employed in situations with relatively harsh operating conditions. III. Cylinder failure: The working environment of the shell and the tubes is essentially the same; therefore, the failure modes of the shell include corrosion (including medium-induced corrosion and stress corrosion, etc.), leakage, explosion, etc.