Shell-and-tube heat exchangers are currently the most widely used type of heat exchange equipment. Compared with other types of indirect heat exchangers, they provide a much larger heat transfer area per unit volume and have better heat transfer efficiency. Due to their compact and robust structure and the ability to be manufactured using various materials, they are highly adaptable and widely used, especially in large-scale installations and high-temperature, high-pressure applications. Among various feedwater heat exchanger failures, pipe system leaks account for the largest proportion. In surface-type regenerative heat exchangers, the water-side pressure is greater than the steam-side pressure. Once the pipe system leaks, feedwater will rush into the shell, causing steam-side flooding. Water may then backflow into the turbine along the extraction steam pipes, causing turbine cylinder deformation, differential expansion changes, unit vibration, and even blade breakage. Numerous incidents of entire unit shutdowns and turbine flooding caused by heat exchanger leaks have occurred in the plant. Therefore, analyzing the causes of heat exchanger leaks and finding countermeasures to minimize leakage is crucial.
First, Analysis of Leakage Causes in Shell-and-Tube Heat Exchangers. Leakage in the internal piping of shell-and-tube heat exchangers is mainly divided into leakage of the heat exchange steel tubes themselves and leakage at the tube ends.
1. Causes of Leakage at the Tube Ends
1.1. Excessive Thermal Stress: During operation, the temperature difference between the cold and hot fluids in a shell-and-tube heat exchanger causes a temperature difference between the shell and the tube walls. This difference leads to different thermal expansion between the shell and the heat exchange steel tubes. When the temperature difference is large, it may cause the heat exchange steel tubes to bend, loosen from the tube sheet, or even destroy the entire heat exchanger. Therefore, the effect of thermal expansion must be considered in the structure, and various compensation methods must be adopted. During heat exchanger start-up and shutdown, if the temperature rise and fall rates exceed the specified limits, the heat exchanger tubes and tube sheet will be subjected to significant thermal stress. This can damage the welds or expansion joints connecting the heat exchanger tubes and tube sheet, leading to port leaks. During peak load changes or sudden shutdowns due to main unit or heat exchanger failures, if steam supply to the steam side stops too quickly, or if feedwater continues to enter the water side after steam supply stops, the thin walls of the heat exchanger tubes shrink rapidly, while the thick tube sheet shrinks slowly. This often results in damage to the welds or expansion joints between the heat exchanger tubes and the tube sheet. This is why the specified allowable temperature fall rate is only 1.7℃/min – 2.0℃/min, which is stricter than the allowable temperature rise rate of 2℃/min – 5℃/min.
1.2. Tube Sheet Deformation: This mainly refers to deformation during tube sheet processing and the deformation generated during processing. Since the heat exchanger tubes are connected to the tube sheet, tube sheet deformation can cause leaks at the ends of the heat exchanger tubes. The high-pressure heater tube sheet exhibits high pressure and low temperature on the water side, while the steam side has low pressure and high temperature, especially in models with built-in condensate cooling sections, where the temperature difference is even greater. If the tube sheet thickness is insufficient, it will deform to some extent. The center of the tube sheet will bulge towards the steam side, where the pressure is low and the temperature is high. On the water side, the center of the tube sheet will be concave. When the main unit load changes, the pressure and temperature on the steam side of the heater will change accordingly. Especially during periods of large peak load adjustment, rapid peak load adjustment, or sudden load changes, the water side pressure will also change significantly under the condition of using a constant-speed feedwater pump, potentially exceeding the rated pressure of the high-pressure heater feedwater. These changes can cause tube sheet deformation, leading to leaks at the heat exchanger tube ports or permanent deformation of the tube sheet. If there is an internal leak in the high-pressure heater’s steam inlet valve, after the high-pressure heater is shut down during main unit operation, the water side of the high-pressure heater will be heated and pressurized to constant volume. If there is no safety valve on the water side or the safety valve malfunctions, the pressure may rise very high, also causing tube sheet deformation.
1.3. Improper tube plugging process: Conical plugs are commonly used for tube plugging. When inserting the conical plug, the force should be moderate; excessive hammering force can deform the tube hole, affecting the connection between adjacent heat exchanger steel tubes and the tube sheet, causing damage and new leaks. During welding, improper preheating, weld position, and dimensions can damage the connection between adjacent heat exchanger steel tubes and the tube sheet. Other plugging methods, such as expansion plugging or explosion plugging, can also cause leaks at adjacent tube openings if the process is not handled properly. Therefore, strict plugging procedures should be followed.
2. Causes of Leakage in the Heat Exchanger Steel Tubes Themselves.
2.1. Erosion: One cause is that when the steam flow velocity is high, and the steam contains large water droplets, the outer wall of the heat exchanger steel tube is eroded by the two-phase flow of steam and water, thinning it and causing perforation or bulging due to feedwater pressure. The main reasons for the generation of two-phase steam-water flow inside the heat exchanger are:
1) The steam inside and at the outlet of the superheated steam cooling section does not reach the design required superheat.
2) Factors such as excessively low or no condensate level, condensate temperature far exceeding the design value, high condensate flow resistance, or a sudden drop in extraction steam pressure cause condensate flashing, resulting in condensate carrying steam when entering the next stage heat exchanger, scouring and damaging the heat exchanger tubes;
3) When a heat exchanger steel tube inside the high-pressure heater is damaged and leaks, high-pressure feedwater rushes out from the leak at extremely high speed, scouring and damaging adjacent heat exchanger steel tubes or baffles. Another reason is the direct impact of steam or condensate. This is due to unreasonable anti-scouring plate material and fixing method, causing breakage or detachment during operation, losing its anti-scouring protection function; insufficient anti-scouring plate area, allowing water droplets to move with the high-speed airflow and impact the tube bundle outside the anti-scouring plate; and too small a distance between the shell and the tube bundle, resulting in a very high steam velocity at the inlet. Stress corrosion cracking refers to the cracking of metals or alloys caused by the combined action of tensile stress and a specific corrosive medium. Its characteristic feature is that most of the surface remains undamaged, with only a portion of fine cracks penetrating the interior of the metal or alloy. Stress corrosion cracking can occur within the commonly used design stress range; its consequences are severe. Important factors contributing to stress corrosion cracking include temperature, solution composition, the composition of the metal or alloy, stress, and the metal’s structure.
2.2. Vibration of Heat Exchanger Steel Tubes: When the feedwater temperature is too low, or the unit is overloaded, and the steam flow rate and velocity between the heat exchanger steel tubes exceed the design value significantly, the elastic tube bundle will vibrate under the influence of the shell-side fluid disturbance. When the frequency of the excitation force matches the natural vibration frequency of the tube bundle or a multiple thereof, tube bundle resonance will occur, greatly increasing the amplitude. This leads to repeated forces at the connection between the heat exchanger steel tubes and the tube sheet, causing tube bundle damage. The mechanisms of tube bundle vibration damage generally include:
① Vibration causes the stress in the heat exchanger steel tubes or the connection between the heat exchanger steel tubes and the tube sheet to exceed the fatigue endurance limit of the material, resulting in fatigue fracture of the heat exchanger steel tubes.
② Vibrating heat exchanger steel tubes rub against the metal of the partition plate in the tube holes of the supporting partition plate, thinning the tube wall and eventually leading to rupture;
③ When the vibration amplitude is large, adjacent heat exchanger steel tubes at the middle position of the span will rub against each other, causing wear or fatigue fracture of the heat exchanger steel tubes.
2.3. Erosion at the Inlet End of Heat Exchanger Steel Tubes: Erosion damage at the inlet tube end only occurs in carbon steel heat exchangers. It is a damage process involving both erosion and corrosion. The mechanism is that the oxide film formed on the tube wall metal surface is destroyed and carried away by the highly turbulent feedwater, resulting in continuous loss of metal material. This ultimately leads to the failure of the heat exchanger steel tube. Sometimes the damage can extend to the tube end weld and even the tube sheet. Erosion is more likely to occur when the feedwater pH is low (less than 9.6), oxygen content is high (greater than 7 μg/L), temperature is low (less than 260℃), and turbulence is high.
2.4. Corrosion: When the tubes of the low-pressure heat exchanger are made of copper, the copper tubes in the low-pressure heater often have to be replaced due to severe leakage. The corrosion rate of copper is lowest at a pH of 8.5~8.8, while carbon steel requires a pH of not less than 9.5. Excessively high pH in the boiler feedwater leads to corrosion of the copper tubes. The main factors affecting the corrosion of carbon heat exchanger steel tube bundles are: oxygen content and feedwater pH value. Excessive dissolved oxygen or low pH in the feedwater will corrode the inner wall of the high-pressure heater steel tubes. Therefore, the dissolved oxygen concentration in the feedwater should not exceed 7 pg/L, and the pH value should be maintained between 9.3 and 9.6. If oxygen is present on the shell side, it will cause oxygen corrosion on the outer wall of the tube bundle. Copper deposition: This can cause pitting corrosion, forming pits. Temperature affects the formation of the Fe3O4 oxide film on the carbon steel surface: It is generally believed that the Fe3O4 oxide film is relatively stable above 260℃. Below this temperature, the protective effect of the Fe3O4 oxide film depends on the pH value of the feedwater and other environmental factors. A pH value greater than 9.6 is considered safe.
2.5. Poor Material and Workmanship: Inferior material and uneven wall thickness in the heat exchanger steel pipes, defects before assembly, excessive expansion at the expansion joints, and tensile damage on the outer surface of the pipes can all lead to significant damage to the steel pipes when the heat exchanger encounters abnormal operating conditions.
Second, Countermeasures for Leakage in Shell-and-Tube Heat Exchangers
1. Post-Leakage Handling Measures: Leakage causes a decrease in feedwater pressure, reducing the amount of feedwater delivered to the boiler. Therefore, upon discovering a leak in the heat exchanger piping system, the heat exchanger should be shut down immediately to minimize the number and severity of damage to the steel pipes. When the unit is shut down, the high-pressure heater should be checked for leaks, and solutions should be sought to eliminate them. For port leaks, the original weld metal should be scraped off before re-welding, and appropriate heat treatment should be performed to relieve thermal stress. For leaks in the heat exchanger steel pipes themselves, the form and location of the leak should be identified first, and a suitable plugging process should be selected to seal both ports of the heat exchanger steel pipes. Regardless of the tube plugging process used, to ensure the quality of the plugging, the ends of the plugged tubes must be properly treated to ensure the tube sheet and tube holes are round, clean, and have a good contact surface with the plug. If there are cracks or erosion at the connection between the heat exchanger tube and the tube sheet, the original heat exchanger tube material and weld metal at the end must be removed to ensure tight contact between the plug and the tube sheet.
2. Preventive Measures
2.1. Port Leakage Prevention Measures: In addition to using sufficiently thick tube sheets and employing good tube hole machining, welding, heat exchanger tube expansion, and welding processes, the heat exchanger’s operation must ensure that the temperature rise and fall rates during start-up and shutdown do not exceed the specified limits. A safety valve must be installed on the water side to prevent overpressure. Proper tube plugging procedures must be followed during maintenance.
2.2. Leakage Prevention Measures for Heat Exchanger Steel Pipes:
(1) Erosion Prevention Measures: Limit the flow rate of steam or condensate on the shell side and prevent flash evaporation in the cooling section; ensure sufficient residual superheat of the steam at the outlet of the steam cooling section; ensure the anti-erosion plates are firmly fixed, have sufficient area, and are made of good material; maintain a normal shell side water level and prohibit operation with low or no water level.
(2) Vibration Prevention Measures for Heat Exchanger Steel Pipes: Install a steam-side safety valve on the blast furnace steam side; limit the flow rate of steam or condensate on the shell side; ensure sufficient spacing between heat exchanger steel pipes, which reduces the shell side flow rate and the possibility of collision and friction damage between heat exchanger steel pipes; limit the length of the free section of the tube bundle.
(3) Erosion Prevention Measures at the Feedwater Inlet End of Heat Exchanger Steel Pipes: The flow rate of the fluid in or within the tube side affects not only the value of the convective heat transfer coefficient but also the fouling thermal resistance, thus affecting the overall heat transfer coefficient. Especially for fluids containing easily deposited particles such as silt, excessively low flow rates may even lead to pipe blockage, seriously affecting the use of the equipment. However, increasing the flow rate will significantly increase the pressure loss. Therefore, choosing an appropriate flow rate is very important. Limiting the feedwater flow rate, shutting down a heat exchanger, or having a large number of blocked heat exchanger tubes will significantly increase the flow rate inside the tubes. In this case, some feedwater should be bypassed to enter the boiler or the unit load should be reduced; control the oxygen content of the feedwater to be less than 7μg/L, and control the pH value of the feedwater to 9.2-9.6.
(4) Corrosion Prevention Measures: Eliminate stress. Stress can have various sources, such as applied stress, residual stress, welding stress, and stress generated by corrosion products. When selecting materials, the unit should be made into a copper-free system, which is beneficial for the corrosion prevention and vapor crystal quality control of the entire unit; a complete air venting system is required, and it is generally recommended not to use a series connection method for pipeline connection to prevent non-condensable gases from accumulating in the heat exchanger at lower pressure; ensure the normal operation of the air venting system, and during startup, air should be vented from both the water and steam sides, and the feedwater quality should be qualified; good anti-corrosion measures should be in place at the factory to prevent corrosion during storage and transportation. For carbon heat exchanger steel tubes, nitrogen purging is usually used for both the steam and water sides for corrosion prevention; when the heat exchanger is not in use, water purging, steam purging, or nitrogen purging are usually used for corrosion prevention, depending on the length of the downtime, and the pH value of the deoxygenated water on the water side should be appropriately adjusted for protection.
(5) Preventive measures for leakage of heat exchanger steel tubes caused by poor materials and processes: The tube wall thickness should be at least 2.0 mm to improve erosion resistance. Before assembly, each heat exchanger steel tube should be inspected for flaws and subjected to hydrostatic testing; the tube bundle should be heat-treated and free of visual defects; the tube sheet bores should maintain a certain roughness, tolerance, and concentricity, and the chamfers or roundings of the bores should be smooth and burr-free.
(6) Preventive Tube Blocking: Preventive tube blocking should be implemented. It is recommended to block a portion of the tubes while simultaneously opening bypass holes of a certain size on the tube sheet to reduce the feedwater flow rate and mitigate corrosion. This method has been used in many power plants both domestically and internationally, proving that it can appropriately extend the life of the heat exchanger and reduce the number of leaks.
(7) Flow Selection: In the heat exchanger, the following points can be considered as general principles for selecting which fluid flows through the tube side and which flows through the shell side:
a) Impure or easily decomposed materials should flow through the side that is easier to clean. For straight tube bundles, the above materials should generally flow inside the tubes, but when the tube bundle can be removed for cleaning, they can also flow outside the tubes.
b) Fluids requiring increased flow velocity to enhance their convective heat transfer coefficient should flow inside the tubes, as the cross-sectional area inside the tubes is typically smaller than the cross-sectional area between tubes, and it is easier to use multiple tube passes to increase flow velocity.
c) Corrosive materials should flow inside the tubes, allowing the shell to be constructed from ordinary materials, with only the heat exchange steel tubes, tube sheet, and end caps requiring corrosion-resistant materials.
d) High-pressure materials should flow inside the tubes, so the shell does not have to withstand high pressure.
e) Very high or very low temperature materials should flow inside the tubes to reduce heat loss. Of course, for better heat dissipation, high-temperature materials can also flow through the shell side.
f) Steam generally flows through the shell side, as this facilitates the removal of condensate, and steam is relatively clean, with its convective heat transfer coefficient less dependent on flow velocity.
g) High-viscosity fluids generally flow through the shell side, because when flowing in the shell side with baffles, the flow channel cross-section and flow direction are constantly changing, resulting in a surge at low Reynolds numbers (Re > 100), which is beneficial for improving the convective heat transfer coefficient of the fluid outside the tubes.
The above points cannot be satisfied at the same time, and sometimes they may even contradict each other. Therefore, one should focus on the main aspects and make an appropriate decision based on the specific circumstances.
Post time: May-21-2026