Methods for dealing with leaks in shell and tube heat exchangers

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Preface: Shell-and-tube heat exchangers are currently the most widely used type of heat exchange equipment. Compared to 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. Due to its compact and robust design, as well as the ability to use a variety of materials in its construction, it has strong adaptability, and is widely used especially in large-scale installations as well as in high-temperature and high-pressure environments. I. Introduction to Tubular Heat Exchangers: Over the years, among the various faults that occur in the plant’s water supply heat exchangers, 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 expansion differences, 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 piping leakage in shell-and-tube heat exchangers is mainly divided into tube-specific leakage and port leakage. 1 Reasons for 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 cause differences in temperature between the shell and the tube walls. This difference causes the shell and tubes to expand thermally at different rates; when the temperature difference between them is significant, it may bend or loosen the tubes from the tube sheet, or even damage the entire heat exchanger. In this regard, it is necessary to structurally consider the effects of thermal expansion and employ various compensation methods. During the start-up and shutdown of heat exchangers, 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 shaving operations, 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 halted, 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 permissible values for the temperature drop rate are only 1.7°C/min–2.0°C/min; this is stricter than the permissible values for the temperature rise rate, which are 2°C/min–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. On the water side of the high-pressure tube sheet, the pressure is high and the temperature is low, while on the steam side, the pressure is low and the temperature is high; 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 low and the temperature is high. 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 variations when fixed-speed feedwater pumps are used; these variations can even cause the pressure to exceed the rated value for the feedwater to the high-temperature heaters. Such changes can lead to deformation of the tube sheet, resulting in leaks at the tube 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 pipe holes; this affects the joints between adjacent pipes and the tube sheet, leading to damage and new leaks. During welding, factors such as inadequate preheating, as well as inappropriate weld position and size, can cause damage at the joints between adjacent tubes and 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 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 perforations 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 the water level in the heat exchanger’s drain line remaining too low or being absent, the drain fluid temperature being much higher than the designed value, high flow resistance in the drain line, 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. Click to view -- Summary of Chemical Engineering Skill Training Courses for 2023. Stress corrosion cracking refers to the cracking of metals or alloys resulting from the combined effect of tensile stress and specific corrosive agents. 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 typical 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 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 elastic tube bundle may vibrate under the influence of the perturbative forces from the fluid on the shell side. When the frequency of these excitation forces matches the natural vibration frequency of the tube 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 tube bundle. The mechanisms underlying tube bundle vibration 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 adjacent pipes at the middle of the span rub against each other, 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 copper is used as the material for the tubes in low-pressure heat exchangers, the copper tubes in low-pressure heaters often have to be replaced due to severe leaks. 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 led to corrosion of the copper pipes. The main factors affecting the corrosion of carbon steel tube bundles include oxygen content and feedwater pH: high dissolved oxygen levels or low pH values in the feedwater can cause corrosion of the inner walls of the high-temperature water tubes; therefore, the dissolved oxygen concentration 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 carbon steel surfaces: 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 pipe material, uneven wall thickness, defects in the pipes prior to assembly, excessive expansion at the flared ends, and pull marks on the outside of the pipes – all these factors can lead to significant damage to the pipes when the heat exchanger is subjected to abnormal operating conditions. III. Countermeasures 1. Measures to take after a leak occurs A leak leads to a decrease in feedwater pressure, resulting in less feedwater being delivered 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 tube plugging process used, to ensure the quality of plugging, the end portion of the tube 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. 2 Preventive measures
2.1 Preventive measures against port leakage
In the manufacture of heat exchangers, the tube sheet must be of sufficient thickness. In addition to proper processing of tube holes, surfacing welding, tube expansion, and welding techniques, during operation the rate of temperature rise and fall when the heat exchanger is started up or shut down must not exceed the specified limits. On the water side, a safety valve should be installed to prevent overpressure. During maintenance, a correct tube plugging procedure must be followed. 2.2 Preventive measures against leaks in the tubes themselves (1) Preventive measures against erosion due to flow: Limit the flow velocity of steam or drain water on the shell side and prevent flashing within 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 a normal water level on the shell side; operation at low water levels or without water is prohibited. (2) Preventive measures against pipe vibration: Install a steam-side safety valve on the steam side of the heater ； Limit the flow rate of steam or water on the shell side ； The spacing between tubes should be sufficiently large; this reduces the shell-side flow velocity on one hand, and minimizes the likelihood of mutual collision and frictional damage between tubes on the other hand: it also limits the length of the free section of the tube bundle. (3) Measures to prevent erosion 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 a flow rate 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 select an appropriate flow rate. Limiting the feedwater flow rate, shutting down one 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 ； Control the oxygen content in the feedwater to be less than 7 μg/L, and maintain the feedwater pH value between 9.2 and 9.6. (4) Corrosion prevention measures: Eliminate stress. Stress can originate 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 ； At the time of manufacture, effective anti-corrosion measures must be taken to prevent corrosion during storage and transportation. For carbon steel tube heat exchangers, nitrogen purging is commonly used as an anti-corrosion measure on both the steam side and the water side ； 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 tube plugging: Carry out 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. (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.