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I. Introduction to Tubular Heat Exchangers: Over the years, among the various faults that occur in the water supply heat exchangers in our factory, tube system leaks account for the largest proportion. In a surface-type regenerative heat exchanger, the pressure on the water side is higher than that on the steam side; once a leak occurs in the piping system, feedwater will flow into the shell, resulting in water filling the steam side. Water may flow back into the turbine along the extraction pipes, causing deformation of the turbine cylinders, changes in the expansion difference, vibration of the unit, and even blade breakage. Such accidents, in which the entire plant comes to a stop and water enters the turbine due to a leak in the heat exchanger, have occurred many times at the plant. Therefore, it is very important to analyze the causes of heat exchanger leaks and identify countermeasures to minimize them as much as possible. II. Analysis of leakage causes The internal pipe system leakage in shell-and-tube heat exchangers is mainly divided into tube-related leakage and port leakage. 1 Causes of pipe port leakage 1.1 Excessive thermal stress: During operation of shell-and-tube heat exchangers, the different temperatures of the cold and hot fluids result in differences in temperature between the shell and the tube walls. This difference causes the shell and the tubes to expand at different rates; when the temperature difference between them is large, it may bend the tubes, loosen them from the flange, or even damage the entire heat exchanger. To address this, it is necessary to take into account the effects of thermal expansion structurally and employ various compensation methods. During the start-up and shutdown of the heat exchanger, the rate of temperature rise and drop exceeds the specified limits, which subjects the tubes and tube sheets of the high-pressure heater to significant thermal stresses. This can lead to damage at the welds or expansion joints that connect the tubes to the tube sheets, resulting in leaks at those connections. When there are rapid changes in load during peak demand periods, or when the main unit or the heat exchanger stops operating suddenly due to a fault, if steam supply is stopped too quickly on the steam side, or if water continues to flow into the system on the water side after the steam supply is interrupted, the thin walls of the tubes contract rapidly while the thicker tube sheets contract more slowly. This often results in damage to the welds or expansion joints between the tubes and the tube sheets. This is why the permitted temperature drop rate is only 1.7°C/min to 2.0°C/min, which is stricter than the permitted temperature rise rate of 2°C/min to 5°C/min. 1.2 Tube sheet deformation mainly refers to the deformation that occurs during the processing of the tube sheet, as well as the deformation that arises during manufacturing. Since the tubes are connected to the tube sheet, any deformation of the tube sheet can cause leakage at the ends of the tubes. The water side of the high-pressure tube sheet has high pressure and low temperature, while the steam side has low pressure and high temperature; especially in cases with an internal drain cooling section, the temperature difference is even greater. If the thickness of the tube sheet is insufficient, the tube sheet will deform to some extent. The center of the tube sheet bulges toward the steam side, where the pressure is lower and the temperature is higher. On the water side, the tube sheet develops a central depression. When the load on the host changes, the pressure and temperature on the steam addition side change accordingly. Especially when the peak shaving amplitude is large, the rate of peak shaving is too fast, or there are sudden changes in load, the pressure on the water side can experience significant fluctuations when fixed-speed feedwater pumps are used; these fluctuations may even exceed the rated pressure of the high-temperature heater feedwater. Such changes can cause the tube sheet to deform, leading to leaks at the pipe ends or permanent deformation of the tube sheet. If there is internal leakage in the feed valve of the superheater, shutting down the superheater while the main engine is running will cause the water side of the superheater to heat up, resulting in a rise in pressure at constant volume. If there is no safety valve on the water side or if the safety valve fails, the pressure can rise very high, which may also cause the tube sheet to deform. 1.3 Improper pipe plugging technique: Conical plugs are commonly used for welding to plug the pipe. Apply moderate force when inserting the conical plug ； The impact force is too great, causing deformation of the tube holes and affecting the joints between adjacent tubes and the tube sheet, which can lead to damage and new leaks. During welding, factors such as inadequate preheating, as well as improper weld position and size, can cause damage at the joints between adjacent tubes and the tube sheet. Other pipe plugging methods, such as expansion plugging and explosive plugging, can also cause leakage at adjacent pipe openings if the process is not carried out properly. Therefore, a strict tube plugging process should be followed. 2 Reasons for leakage in the pipes themselves 2.1 Erosion by scouring: One reason is that when the flow velocity of steam is high and the steam stream contains large water droplets, the outer wall of the pipe is subjected to erosion by the two-phase flow of steam and water, which causes it to thin out, leading to perforation or rupture due to the pressure of the feed water. The main reason for the formation of vapor-liquid two-phase flow inside the heat exchanger is that the superheat of the steam in the cooling section and at its outlet does not reach the required level as specified in the design ； Secondly, factors such as an excessively low level of water in the drain of the heat exchanger, or no water at all, or a drain temperature that is much higher than the designed value, or high flow resistance in the drain, or a sudden drop in the extraction pressure, can cause the drain fluid to flash vaporize. As a result, the drain fluid enters the next stage of the heat exchanger containing steam, which then erodes the tubes of the heat exchanger and causes damage ； Third, when a pipe in the high-pressure heater is damaged and leaks, high-pressure feed water rushes out at great speed from the leak site, thereby eroding and damaging adjacent pipes or partitions. Another reason is direct exposure to steam or hydrophobic substances. Due to inappropriate material and fixing method of the impact shield. It breaks or falls off during operation, losing its protection against erosion ； The area of the impact shield is not large enough; water droplets, carried by fast-moving air currents, strike the tube bundle outside the impact shield ； The distance between the shell and the tube bundle is too small, resulting in a high vapor flow velocity at the inlet. Stress corrosion cracking refers to the cracking of metals or alloys caused by the combined effect of tensile stress and a specific corrosive environment. It is characterized by the fact that most of the surface remains undamaged, with only some fine cracks penetrating into the interior of the metal or alloy. Stress corrosion cracking can occur within the usual design stress ranges, hence its consequences are severe. Important factors that cause stress corrosion cracking include temperature, solution composition, the composition of the metal or alloy, stress, and the metal structure. 2.2 Tube vibration: In situations such as excessively low feed water temperature or overloading of the unit, when the steam flow rate and velocity between the tubes of the heat exchanger exceed the design values by a significant margin, the tubular bundle, which has a certain degree of elasticity, will vibrate under the influence of the perturbing forces from the fluid on the shell side. When the frequency of these disturbing forces matches the natural vibration frequency of the tubular bundle or a multiple thereof, resonance occurs, resulting in an increase in the amplitude of vibration. This repeated stress on the joints between the tubes and the tube sheet can lead to damage to the tubular bundle. The mechanisms behind tubular bundle vibration-induced damage include: ① Vibration causes the stress on the tubes or at the joints between them and the tube sheet to exceed the material’s fatigue limit, leading to fatigue fracture of the tubes ； ②The vibrating tube rubs against the metal of the support partition within the tube holes of that partition, causing the tube wall to thin out and eventually leading to rupture ； ③When the vibration amplitude is large, the pipes adjacent to each other in the middle of the span will rub against one another, causing the pipes to wear out or suffer fatigue fractures. 2.3 Erosion at the water inlet end of the tubes Erosive damage at the inlet tube end occurs only in carbon steel heat exchangers; it is a damage process resulting from both erosion and corrosion. The mechanism involves the oxide layer formed on the tube wall surface being destroyed and carried away by the highly turbulent water flow, leading to a continuous loss of metal material. This ultimately leads to the damage of the pipe. Sometimes, the area of damage can extend to the pipe end welds or even the tube sheet: erosion is likely to occur when the feedwater pH is low (below 9.6), the oxygen content is high (above 7 μg/L), the temperature is low (below 260°C), and turbulence is high. 2.4 Corrosion: When the tubes in low-pressure heat exchangers are made of copper, these copper tubes in low-temperature heaters often have to be replaced due to severe leakage. The corrosion rate of copper is lowest at a pH value of 8.5 to 8.8. Carbon steel, on the other hand, requires a pH value of not less than 9.5. The excessively high pH value of the boiler feed water caused corrosion of the copper pipes. The main factors affecting the corrosion of carbon steel tube bundles include oxygen content and feedwater pH: when the dissolved oxygen in the feedwater is too high or the pH value is too low, it can cause corrosion of the inner walls of the high-temperature water tubes; therefore, the concentration of dissolved oxygen in the feedwater should not exceed 7 pg/L, with the pH value maintained between 9.3 and 9.6. If oxygen is present on the shell side, it will cause oxygen corrosion of the outer wall of the tube bundle. Copper deposition: It can cause pitting corrosion, resulting in pitting pits. Temperature affects the formation of the Fe3O4 oxide film on the surface of carbon steel: it is generally believed that above 260°C, the Fe3O4 oxide film is relatively stable. Below this temperature, the degree of protection provided by the Fe3O4 oxide film depends on the pH value of the feed water and other environmental factors. It is safe when the pH value is greater than 9.6. 2.5 Poor material quality and manufacturing processes: Poor quality of the tube material, uneven wall thickness, defects in the tubes prior to assembly, excessive expansion at the flared ends, and pull marks on the outside of the tubes can all lead to significant damage to the tubes when the heat exchanger is subjected to abnormal operating conditions. III. Countermeasures 1. Measures to take after a leak occurs A leak causes the feedwater pressure to drop, resulting in a reduction in the amount of feedwater supplied to the boiler. Therefore, when a leak is detected in the heat exchanger tubes, the heat exchanger should be shut down immediately to reduce the number of damaged tubes and mitigate the extent of the damage. When the unit is shut down, it is necessary to check for leaks in the high-pressure heater and find ways to eliminate them. In the case of port leakage, the existing weld metal should be scraped off and then rewelded, along with appropriate heat treatment to eliminate thermal stress. For leaks in the tubes themselves, it is necessary first to determine the type and location of the leakage in the tube bundle, and then select an appropriate method to seal both ends of the tube. Regardless of the tube plugging process used, to ensure the quality of plugging, the ends of the tubes to be plugged must be properly treated, so that the tube sheets and tube holes are smooth and clean, providing a good contact surface with the plugs. In the case of cracks or erosion at the junction of the tube and the tube sheet, it is necessary to remove the original tube material and weld metal from the ends so that the plug can make tight contact with the tube sheet. 2 Preventive Measures 2.1 Measures to Prevent Port Leakage The heat exchanger should be equipped with tube sheets of sufficient thickness; in addition to proper tube hole processing, cladding, tube expansion joining, and welding techniques, during operation the rate of temperature increase and decrease when starting up or shutting down the heat exchanger must not exceed specified limits. A safety valve is required on the water side to prevent overpressure, and correct methods for blocking tubes must be employed during maintenance. 2.2 Measures to prevent leakage from the pipe itself (1) Measures to prevent erosion by scouring: Limiting the flow rate of steam or drain water on the shell side, and preventing flashing in the drain cooling section ； The steam at the outlet of the steam cooling section must have sufficient remaining superheat ； The impact shield must be securely fixed, have sufficient area, and be made of high-quality material ； Maintain the normal water level on the shell side; operation at low water levels or without water is prohibited. (2) Measures to prevent tube vibration: Install a steam-side safety valve on the steam side of the high-pressure heater ； Limit the flow rate of steam or hydrophobic substances on the shell side ； The spacing between the tubes must be sufficient; this not only reduces the flow velocity on the shell side but also decreases the likelihood of damage due to collisions and friction between the tubes by limiting the length of the free section of the tube bundle. (3) Erosion prevention measures at the water inlet end of the pipe: The flow velocity of the fluid within the pipe affects not only the value of the convective heat transfer coefficient but also the fouling thermal resistance, thereby influencing the overall heat transfer coefficient. Especially for fluids containing particles that tend to settle, such as sediment, too low flow rates can even lead to blockages in the pipelines, severely affecting the operation of the equipment. However, increasing the flow rate will cause a significant increase in pressure loss. Therefore, it is very important to choose an appropriate flow rate. Limiting the feedwater flow rate, shutting down a row of heat exchangers, or having a large number of blocked tubes in the heat exchangers will all result in a significant increase in the flow velocity inside the tubes; in such cases, some of the feedwater should be directed through a bypass to the boiler, or the load on the unit should be reduced ； Keep the oxygen content in the feed water at 7 μg/L or less, and maintain the pH value of the feed water between 9.2 and 9.6. (4) Corrosion prevention measures: Stress elimination. Stress can arise from various sources, such as applied stress, residual stress, welding stress, and stress generated by corrosion products. When selecting materials, making the unit a copper-free system is beneficial for both the corrosion protection of the entire unit and the control of steam quality ； A proper venting system is necessary; it is generally not recommended to use a series connection of pipes in stages, in order to prevent non-condensable gases from accumulating in heat exchangers with lower pressure ； Ensure the proper operation of the air release system; at startup, air must be removed from both the water side and the steam side, and the quality of the feedwater must be satisfactory ； Good anti-corrosion measures should be in place at the time of manufacture to prevent corrosion during storage and transportation. For carbon steel tube heat exchangers, nitrogen filling is typically used on both the steam side and the water side as an anti-corrosion method ； When the heat exchanger is out of use, anti-corrosion measures such as filling it with water, steam, or nitrogen are typically employed depending on the duration of inactivity; additionally, the pH value of the deoxygenated water on the water side is adjusted appropriately to provide protection. (5) Preventive measures against pipe leakage caused by poor materials or manufacturing processes: The pipe wall thickness should be at least 2.0 mm to enhance its resistance to erosion. Before assembly, each pipe must undergo inspections such as flaw detection and hydrostatic testing ； The tube bundle shall be heat-treated and free of visible defects ； The tube holes in the tube sheet should maintain a certain level of roughness, tolerance, and concentricity, and the chamfers or rounded edges of these holes should be smooth without any burrs. (6) Preventive pipe plugging: Perform preventive pipe plugging. It is recommended to create bypass holes of appropriate size on the tube sheet while blocking part of the tubes, in order to reduce the flow rate of the feed water and alleviate corrosion. This method has been used in various power plants both domestically and internationally, and it has proven to be able to appropriately extend the lifespan of heat exchangers and reduce the frequency of leaks. (7) Selection of flow pattern In heat exchangers, to determine which fluid should flow through the tube side and which through the shell side, the following points can be considered as general guidelines: a) Materials that are dirty or prone to forming scale should flow through the side that is easier to clean. For straight tube bundles, the aforementioned materials should generally flow inside the tubes; however, when the bundle can be removed for cleaning, they can also flow outside the tubes. b) Fluids that require an increased flow rate to raise their convective heat transfer coefficient should flow inside the tubes, as the cross-sectional area inside the tubes is usually smaller than that outside, and it is easier to use multiple tube passes to increase the flow rate. c) Corrosive materials should be conveyed through pipes; this allows the casing to be made of ordinary materials, with only the pipes, tube sheets, and end caps requiring corrosion-resistant materials. d) Materials with high pressure flow inside the pipes, so that the outer shell does not have to withstand high pressure. e) Materials with very high or very low temperatures should be conveyed through pipes to reduce heat loss. Of course, for better heat dissipation, the hot materials can also be routed through the shell side. f) Steam is generally introduced into the shell side, as this facilitates the removal of condensate, and the steam is relatively clean; moreover, its convective heat transfer coefficient has little dependence on flow velocity. g) Fluids with high viscosity generally flow through the shell side space; since the flow cross-section and direction keep changing while flowing in the shell side equipped with baffles, choking can occur at low Re numbers (Re greater than 100), which helps to increase the convective heat transfer coefficient of the fluid outside the tubes. It is impossible for all of the above points to be satisfied simultaneously, and they may sometimes be contradictory to one another; therefore, it is necessary to consider the specific circumstances, focus on the key aspects, and make appropriate decisions.