Failure modes of shell and tube heat exchanger components and preventive measures

Thread Content

I. Composition and Structure of Shell-and-Tube Heat Exchangers Shell-and-tube heat exchangers are a traditional type of heat exchange equipment that is widely used. Due to its robust structure and the ability to be manufactured from various materials, it has exceptional adaptability. Shell-and-tube heat exchangers are widely used in various industries; in cement production plants, they are often used as oil coolers in the equipment’s light oil stations, as well as for cooling engine oil in vehicles. For a long time, steel shell-and-tube heat exchangers have held a dominant position in terms of production and usage, thanks to their advantages such as robust structure, high reliability, strong adaptability, and wide range of available materials. With the development of technologies such as enhanced heat transfer, shell-and-tube exchangers have seen continuous improvements in both manufacturing techniques and heat transfer performance. However, due to the complexity of the structure and the diversity of operating conditions, local failures in the heat exchanger or even its complete failure can often occur. Working principle of shell and tube heat exchangers: The main components of a shell and tube heat exchanger include the cylinder, end caps, tube bundle, tube sheet, baffle plates, connections, flanges, etc. Under different operating conditions and with various media, various forms of failure can occur. From a structural perspective, the areas prone to failure are the joints between various components, such as the connections between pipes and tube sheets. From a stress perspective, discontinuities in the structure’s surface, especially areas where stresses change abruptly, can lead to failure due to the presence of additional stresses, such as at the welds between the cylinder and the tube sheet. In terms of operating conditions, high temperatures and pressures can cause thermal stresses or additional stresses; corrosive working fluids can also pose a problem; frequent start-up and shutdown cycles can induce fluid-induced vibrations in the heat exchange tubes, all of which can result in failure of the cylinder, the heat exchange tubes, or even the entire system. II. Reasons for shell-and-tube heat exchangers 1. Corrosion and abrasive wear 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, the heat exchanger heads, and the small-diameter nozzles. The main causes of tube bundle corrosion and abrasive failure are: ① fouling corrosion; ② corrosive fluids; ③ local corrosion due to the accumulation of foreign substances on the inner wall of the tubes; ④ crevice corrosion at the tube ends, etc. Preventive measures include: ① Regular cleaning of the tube bundle; ② Proper selection of materials; ③ Addition of corrosion inhibitors to the fluid; ④ Installation of filtering devices and buffering structures at the fluid inlet, etc. 2. Decreased heat transfer capacity: During the operation of the heat exchanger, high hardness of the working medium, or the presence of particles and suspended solids in the fluid, can 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, thereby causing a rapid decline in heat exchange capacity. Prevention methods: ① Thoroughly understand the areas prone to contamination, the substances that cause contamination, and the degree of dirt, and conduct regular inspections; ② When fluids are prone to scaling, equipment or structures that are easy to inspect, disassemble, and clean must be used. Tube bundle leakage occurs when the heat exchange medium is subject to corrosion, stress corrosion, intergranular corrosion, or damage from collisions and wear; microscopic cracks appear on the tubes, and if high tensile stresses or alternating stresses are present, these cracks will expand rapidly, leading to leakage. At this time, blocking the pipe is commonly used on-site as an emergency repair measure. In fact, after the tube is blocked, the increased temperature difference stress accelerates its own stress corrosion; as a result, the tube suffers more severe damage very quickly, leading to the complete failure of the entire tube bundle. Methods to prevent tube leakage should consider aspects such as material selection, corrosion protection, damage prevention, reduction of tensile stress, and vibration prevention. If a leak in the tube bundle is detected during operation, it is preferable to remove and replace the tubes rather than trying to seal the leak. 4. Failure of the connection between the tube bundle and the tube sheet. Depending on the operating conditions of the heat exchanger, the types of joints connecting the tube bundle to the tube sheet can be categorized into three: welding, expansion joining, and a combination of expansion and welding. Different joint types result in various failure modes. For products welded using conventional methods, the welding technique can be changed during major repairs in order to enhance the strength of the welds between the heat exchange tubes and the tube sheet. ①Change from single-sided welding to double-sided welding; ② Replace the parallel welding structure between the tube sheet and the front end of the tube bundle with an angled welding structure in which the tube bundle extends for a certain length. Use specialized tooling to change the position of the tube bundle and tube sheet from horizontal to vertical, and alter the vertical welding pattern between the heat pipes and the front side of the tube sheet to a horizontal welding pattern in order to meet the requirements of angled welding. Improving the welding process through the above two methods can yield good results. The operating environment of the shell in case of cylinder failure is essentially the same as that of the tube bundle; therefore, the failure modes of the shell and the preventive measures can be based on those applicable to the tube bundle. Corrosion is the main mode of failure in shell-and-tube heat exchangers. In addition to the corrosive nature of the acidity or alkalinity of the heat transfer medium itself, the corrosiveness of the working fluid, tensile stresses in the shell or tubes, and gaps between the tubes and the tube sheets can all accelerate corrosion, leading to the failure of the heat exchanger. Preventive measures include: reducing gaps, using corrosion-resistant materials, improving the quality of welds, performing regular cleaning, adding corrosion inhibitors to the fluid, controlling temperature fluctuations in the system, and reducing the use of flange connections. III. Online detection methods for the failure of shell-and-tube heat exchangers 1. Scaling: After the heat exchanger has been in operation for a period of time, if scaling occurs severely on the tube walls, the heat transfer capacity decreases, and the outlet temperature of the heat exchange medium fails to meet the requirements specified in the design parameters. The scale reduces the inner diameter of the tubes, which in turn increases the flow velocity and leads to higher pressure losses. At this point, the degree of fouling can be determined by examining operational records such as flow rate, pressure, and temperature. 2 Corrosion and wear: Heat transfer media, dirt, and other factors can cause corrosion and wear on the inner and outer surfaces of the heat exchanger shell and tubes. A thickness gauge is typically used for the housing to measure and determine from the outside those parts of the housing that have suffered corrosion or thinning. 3 Leakage: The middle part of the pipe ruptures due to reasons such as corrosion or induced vibration; the connection between the pipe and the tube sheet leaks because of factors like corrosion or fatigue damage. Fluid sampling can be used to check for leaks and damage in the tube bundle. As can be seen from the above analysis, the failure of shell-and-tube heat exchangers is related to various factors such as material structure, heat transfer medium, and operating conditions; sometimes it is the result of the combined effect of several such factors. Therefore, various influencing factors must be taken into comprehensive consideration during the material selection, design, manufacturing, assembly, and operation of heat exchangers to prevent problems before they occur.