Analysis of the causes of failure in heat exchanger tube bundles

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I. Analysis of the reasons for explosions in heat exchangers 1. Self-made heat exchangers, which involve arbitrary and substantial changes to their structure and materials; as a result, the quality of fabrication is poor, they do not meet the standards for pressure vessels, and the strength of the equipment is reduced. 2. The welding quality of the heat exchanger is poor; in particular, the weld joints are incompletely welded. No non-destructive testing of the welds or burst tests were conducted. As a result, the weld joints leaked or suffered from fatigue fractures, leading to a large leakage of flammable and explosive fluids and ultimately causing an explosion. 3. Due to corrosion (including stress corrosion and intergranular corrosion), the pressure resistance strength decreases, leading to tube bundle failure or severe leaks; in the presence of an open flame, this can result in an explosion. 4. When conducting a airtightness test on the heat exchanger, oxygen is used to increase pressure or flammable refining gases are employed for leak testing, which can cause physical and chemical explosions. 5. Operating violations and operational errors lead to valve closure, causing overpressure explosions. 6. The failure to discharge waste over a long period leads to an accumulation of flammable and explosive substances such as nitrogen trichloride; coupled with excessively high operating temperatures, this can cause violent explosions in heat exchangers such as those used for liquid chlorine. 7. Peroxide explosion. II. Analysis of the causes of leaks in heat exchangers Most accidents involving combustion, explosion, asphyxiation, poisoning, and burns in heat exchangers are caused by leaks. Leakage of flammable and explosive liquids or gases can lead to fire and explosion when they come into contact with an open flame. Release of toxic gases may cause suffocation and poisoning, while leakage of highly corrosive fluids can result in burns. The areas most prone to leakage are the weld joints, the connections between the head and the tube sheet, the connections between the tube bundle and the tube sheet, and the flange connections. III. Safety accidents caused by leaks in heat exchangers
The main reasons for leaks in the tubes of heat exchangers include corrosion, frequent start-ups and shutdowns, excessive temperature fluctuations, rapid expansion and contraction of the heat exchanger leading to leaks at the tube-to-sheet joints, as well as manufacturing defects in the equipment itself. Due to severe corrosion (such as steam droplets, hydrogen sulfide, carbon dioxide), tube leakage occurs. Due to frequent start-ups and shutdowns, there are excessive temperature fluctuations, causing the equipment to expand or contract rapidly, which leads to leaks in the tube expansions on the tube sheet. Defects in the heat exchanger’s manufacturing lead to leakage at the weld joints. Due to the rise in operating temperature, the bolts elongate and the fastened parts become loose, resulting in flange leakage. Leaks are caused by thermal shocks resulting from loosening at the assembly points of the heat exchanger tube bundle, tube vibration, startup/shutdown and emergency shutdowns, as well as mechanical shocks due to improper operations during routine maintenance. IV. Analysis of the causes of tube bundle failure in heat exchangers The tube bundles in shell-and-tube heat exchangers, synthesis reactors, and waste heat boilers are the weak points and most prone to failure. The main form of tube bundle failure is corrosion cracking. Various issues such as a rapid decline in heat transfer capacity, collision-induced damage, tube rupture, and bundle leakage. Common causes are as follows: Corrosion: Heat exchangers are mostly made of carbon steel. Oxygen-induced polarization corrosion caused by oxygen dissolved in cooling water is extremely severe; consequently, the service life of the tube bundles is often only a few months or one to two years. Additionally, many of the working media are corrosive. For instance, in the cooling water tanks of carbonization towers used in small-scale nitrogen fertilizer production, under the combined effects of corrosion from highly concentrated ammonium carbonate solution and corrosion due to ammonium bicarbonate crystallization, carbon steel cooling tanks sometimes develop leaks after just two or three months of use. Scaling: During the operation of heat exchangers, scaling can occur on both the inner and outer walls of the tube bundle. The thermal resistance of this scale layer is much higher than that of the metal tubes, which leads to a rapid decline in the heat exchange capacity; in severe cases, it can even block the flow channels of the heat exchange medium. Flow-induced vibration: To enhance heat transfer and reduce fouling, it is common practice to increase the shell-side fluid flow velocity. As the flow rate of the fluid in the shell side increases, the likelihood of induced vibrations also **increases**, which leads to vibrations in the tubes within the tube bundle and ultimately results in the destruction of the tube bundle. The common forms of damage are as follows: 1. Collision damage. When the amplitude of the tubes is large enough, it causes the tubes to collide with each other; tubes located on the outer periphery of the tube bundle may also collide with the inner wall of the heat exchanger shell. During the collision, the pipe wall wore down and became thinner, eventually cracking. 2. Cutting of the tube at the baffle: There is a radial gap between the holes in the baffle and the tube; when the amplitude of the tube’s lateral vibration is large, this leads to repeated collisions between the tube wall and the inner surface of the baffle holes. Due to the relatively small thickness of the baffle, the pipe wall comes into contact with it multiple times and frequently, thereby enduring significant impact loads; as a result, localized damage in which the pipe is cut may occur within a short period of time. 3. Failure at the joint between the tube and the tube sheet. This connection structure can be regarded as a fixed-end constraint; when the tube vibrates, it undergoes lateral deflection ； The stress at the joints is the greatest; therefore, it is one of the areas where tube bundle failure is most likely to occur. 4. Failure caused by the propagation of material defects: If the pipe material itself has defects (including those resulting from corrosion and abrasion), then under the alternating stresses induced by vibration, the defect cracks located along the direction of the principal stresses will propagate rapidly, ultimately leading to the failure of the pipe. 5. The tensile stress in a vibrating alternating stress field can also serve as a stress source for stress corrosion. Flow-induced vibration causing pipe failure tends to occur in areas where the deflection is relatively large and the cross-flow velocity on the shell side is high. This area typically includes U-shaped elbows, the inlet and outlet connections on the shell side, the tube sheet area, the gaps around the baffle plates, and the tubes subjected to compressive stress. V. Causes and treatment methods for scale formation in heat exchangers
1. Particulate fouling: Accumulation of solid particles suspended in the fluid on the heat exchange surface. This type of fouling also includes a layer of larger solid particles that settle on the horizontal heat exchange surface due to gravity, namely the so-called sedimentary fouling, as well as the deposition of other colloidal particles. 2. Crystal fouling: Deposits formed when inorganic salts dissolved in the fluid crystallize on the heat exchange surface, usually occurring under conditions of supersaturation or during cooling. Typical fouling includes calcium carbonate, calcium sulfate, and silica scale on the cooling water side. 3. Chemical reaction fouling: Fouling that results from chemical reactions occurring on the heat transfer surface; the material of the heat transfer surface does not participate in these reactions, but it can act as a catalyst for them. 4. Corrosive fouling: Fouling resulting from the corrosion of heat exchange surfaces by corrosive fluids or impurities present in such fluids. Generally, the degree of corrosion depends on the components in the fluid, the temperature, and the pH value of the fluid being processed. 5. Biological fouling: Except for seawater cooling systems, biological fouling generally refers to microbial fouling. It may produce slime, which in turn provides conditions for the growth of biological fouling. This fouling is highly sensitive to temperature; under suitable temperature conditions, it can form a fairly thick layer of buildup. 6. Solidified fouling: Fouling formed when fluid solidifies on subcooled heat exchange surfaces. For example, when water is below the freezing point and freezes into ice on the heat exchange surface. The uniformity of the temperature distribution has a significant impact on this type of fouling. VI. Anti-corrosion measures for heat exchangers: During design, steam should be placed on the tube side to prevent high-speed gases from flowing through the shell side. When there is a medium with high flow rate in the shell side, multiple shell side inlets can be designed to buffer the pressure; in addition, anti-scour plates should be installed to reduce the scouring and corrosion caused by high-speed fluids on the equipment. To prevent the accumulation of residual liquid and deposits, double-sided butt welding and continuous welding should be used as much as possible during welding; lap welding and spot welding should be avoided. In welding processes, based on practical experience, the stress that causes stress corrosion cracking is primarily residual stress, and this residual stress is mainly composed of internal stresses resulting from cold working and welding. Heat treating cold-worked and welded parts helps to eliminate residual stresses, which in turn helps to prevent the occurrence of stress corrosion. Stress relief heat treatment is commonly used to eliminate residual stresses, along with other methods for removing such stresses, such as hydrostatic testing, vibration aging, and hammering. Additionally, nylon straps must be used for lifting the tube bundle to ensure that the metal surface remains smooth and free of scratches, allowing it to be inserted into the shell smoothly. Corrosion-resistant materials such as duplex stainless steel, Hastelloy, titanium, titanium alloys, and copper are used; these materials have strong corrosion resistance, which can increase the service life of heat exchangers. However, they are expensive, resulting in high manufacturing costs and substantial initial investment, factors that are generally difficult for companies to accept, making their widespread use challenging. Electrochemical protection methods can not only prevent stress corrosion cracking, but also halt the growth of cracks even if they occur, provided that the protection parameters are chosen appropriately. Sacrificial anode/cathode protection or surface coating with corrosion-resistant metals can be employed. Cathodic protection: It uses an external direct current power supply to convert the anode on the metal surface into a cathode, thereby providing protection. This method consumes a lot of electricity and is costly, so it is used very rarely. Anodic protection method: The equipment to be protected is connected to an anode powered by an external source, which causes a passivation film to form on the metal surface, thereby providing protection. Carbon steel heat exchangers have a low cost, but poor corrosion resistance. The use of sacrificial anode protection technology can extend the service life of heat exchangers, but the protective effect of this technology is limited to a short length at the inlet of the tubes; cathodic protection is difficult to achieve deep within the tubes. As a result, the application of sacrificial anode protection in heat exchangers is greatly restricted. In corrosive media, the addition of small amounts of certain substances that can **reduce or even stop** the corrosion of metals is known as corrosion inhibitors. Figure 6 shows a comparison before and after the use of a corrosion inhibitor; the addition of such an inhibitor should be done in a way that does not affect the production process or product quality. Stress corrosion can be controlled by removing dissolved oxygen and oxidizing agents from the medium. Reducing the mass concentration of Cl- in the medium, as well as strictly controlling the mass concentration of sulfur in it, are also effective measures for controlling stress corrosion. On the metal surface, a corrosion-resistant coating layer is applied using certain coating methods to prevent direct contact between the metal surface and corrosive agents. This technical approach is the most cost-effective; it was initially used to prevent corrosion in gas media, with the coatings employed being mostly solutions of organic polymer mixtures. Currently, there is a gradual shift towards oil- and solvent-resistant coatings, high-temperature coatings, highly resistant anti-corrosion coatings, and coatings for special environments. When starting up the heat exchanger, first fill the container with the cold fluid and close the inlet, then slowly inject the hot fluid, striving to minimize the difference in thermal expansion between the tube formed by the incoming fluid and the shell. After parking, use dry compressed air to remove all fluid from the heat exchanger, which helps to minimize stress and prevent stress corrosion. While driving, the inlet and outlet valves should remain fully open to prevent a decrease in flow rate, which could lead to impurities in the fluid settling on the surface of the pipes and causing corrosion as a result of scaling.