What to do if a heat exchanger is leaking? This article explains the causes of the leakage and the corrective measures

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Heat exchanger leaks often cause the entire installation to come to a stop, which not only affects the economic efficiency of production but also frequently poses a direct threat to the safe operation of the main machine or other equipment, and can even lead to serious equipment damage. Introduction to tube heat exchangers: Tube heat exchangers are currently the most widely used type of heat exchange equipment. Compared with several other type of partitioned heat exchangers, it offers a much larger heat transfer area per unit volume of equipment, as well as better heat transfer performance. A tubular heat exchanger is composed of tubes, tube sheets, baffle plates, a shell, and end caps (tube boxes). During heat exchange, one fluid enters at the connection pipe of the end cap, flows through the pipes, and exits through the outlet pipe at the other end of the end cap; this is referred to as the tube side ； Another fluid enters through a nozzle on the shell and exits through another nozzle on the shell; this is referred to as the shell side. Causes of leakage in shell and tube heat exchangers: Leaks within the tube assembly of a shell and tube heat exchanger can be primarily categorized as leaks in the tubes themselves or leaks at the tube ends. Reasons for leakage at the ends of heat exchanger tubes: 1. Excessive thermal stress. During operation of a shell-and-tube heat exchanger, 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 temperature drop exceeds the specified limits, resulting in significant thermal stress on the tubes and tube sheets of the high-pressure heater. This leads to damage at the welds or expansion joints that connect the tubes to the tube sheets, causing leaks at those connections. Deformation of the tube sheet is mainly due to processing-induced deformation as well as deformations that occur 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. 3 Improper pipe plugging technique: Conical plugs are commonly used for welding to plug the pipes. 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, issues such as inadequate preheating, or improper weld position and size, can cause damage at the joints where adjacent tubes are connected to the tube sheet. Other tube plugging methods, such as expansion plugging and explosive plugging, can also cause leakage at adjacent pipe openings if the process is not proper. Therefore, a strict pipe plugging process should be followed. Reasons for leakage in heat exchanger tubes: 1. Erosion by scouring – One reason is that when the steam flow velocity is high and the steam stream contains large water droplets, the outer wall of the tube 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. 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 steam flow velocity at the inlet. Vibration of the tubes: In situations such as excessively low feed water temperature or excessive load on 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, can vibrate due to 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 leads to repeated stress on the connections between the tubes and the tube sheet, ultimately causing damage to the tubular bundle. 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 metal 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. When the tubes of the low-pressure heat exchanger are made of copper, the copper tubes in the low-temperature heater 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. 5. Poor material quality and manufacturing processes: The pipe material is of poor quality, with uneven wall thickness; the pipes have defects prior to assembly, the flaring areas are over-flared, and there are pulling damage marks on the outside of the pipes. Under abnormal operating conditions in the heat exchanger, this can lead to significant damage to the pipes. Measures for dealing with leaks in shell-and-tube heat exchangers: Actions to take after a leak occurs. When a leak takes place, the feedwater pressure drops, 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 minimize 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 pipe plugging process used, to ensure the quality of plugging, the ends of the pipes to be plugged must be properly treated, so that the tube sheet and tube holes are smooth and clean, providing a good contact surface with the plug. In the event 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. Preventive measures against heat exchanger leaks: 1. Preventive measures against leaks at tube ends. In addition to having tube sheets of sufficient thickness during manufacturing, as well as proper tube hole processing, cladding, tube expansion joining, and welding techniques, it is also necessary to ensure that the temperature rise and drop rates of the heat exchanger during startup and shutdown do not exceed specified limits. A safety valve should be installed on the water side to prevent overpressure, and correct methods for blocking tubes must be employed during maintenance. 2 Measures to prevent leakage in the tubes themselves; measures to prevent erosion caused by scouring; controlling the flow rate of steam or drain water on the shell side and preventing flashing in the subcooling 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 a normal water level on the shell side; operation at low water levels or without water is prohibited. Vibration prevention measures for pipes: Install steam-side safety valves on the steam side of the high-pressure heater ； Limit the flow rate of steam or water on the shell side ； The spacing between the tubes must be sufficient; this reduces the flow velocity on the shell side and, at the same time, 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. Erosion prevention measures at the water inlet end of pipes: 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. 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. Corrosion prevention measures involve 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 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 to achieve anti-corrosion effects ； When the heat exchanger is out of use, anti-corrosion measures such as filling it with water, steam, or nitrogen are typically applied depending on the duration of inactivity; additionally, the pH value of the deoxygenated water on the water side is adjusted appropriately to provide protection. Preventive measures against pipe leakage caused by poor materials and manufacturing processes: The pipe wall thickness should be at least 2.0 mm to enhance its resistance to erosion. Before assembly, each tube 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. Preventive tube plugging: Perform preventive tube 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 the heat exchangers and reduce the frequency of leaks. Selection of the process: 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 for making the choice: 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 under high pressure flow inside the pipes, so the casing does not need 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 as it flows in the shell side equipped with baffles, turbulence can be achieved at low Re numbers (Re greater than 100), which helps to increase the convective heat transfer coefficient of the fluid outside the tubes.