0 Introduction With the rapid development of China's power industry, various types of large-capacity thermal power generating units have emerged, and boiler structures and operations have become more complex, inevitably leading to differences in flow rate and heat absorption in parallel tubes. When the working conditions of pressure-bearing heat-receiving components operating under harsh conditions deviate from the design conditions, boiler tube rupture is likely to occur. In fact, when tube rupture occurs, so-called quick repair methods, such as spraying or gasket welding, are often used to repair it, but the tube ruptures again after a period of time. The repeated occurrence of tube ruptures on the same tube, the same material, or the same cross-section in the same area of ​​the boiler indicates that the fundamental problem of boiler tube rupture has not been solved. Therefore, understanding the direct and fundamental causes of superheater tube rupture accidents, clarifying the mechanism of tube failure, and proposing preventive measures to reduce the occurrence of superheater tube ruptures are the primary issues at present. [b]1 Direct Causes of Superheater Tube Rupture[/b] There are many direct causes of superheater and reheater tube ruptures, which can be analyzed from the following aspects. 1.1 Design Factors 1. Discrepancy between Thermal Calculation Results and Actual Results The focus of inaccurate thermal calculations lies in the heat transfer calculation of the furnace, that is, how to reasonably determine the furnace outlet flue gas temperature and the heat transfer coefficient of the screen superheater from a theoretical calculation. Lack of experience leads to inappropriate arrangement of the superheater heating surface area, resulting in primary and secondary steam temperatures deviating from the design value or the heating surface overheating. [b]2. Unreasonable Selection of Coefficients During Design[/b] For example, the "W" type boiler designed and manufactured by B&W Company for Huaneng Shang'an Power Plant used an unreasonable heating surface coefficient, resulting in the measured value of the furnace outlet flue gas temperature being 80-100℃ higher than the design value; another example is the No. 2 boiler (HG-670/140-6 type) of Fularji Power Plant, which used an unreasonable boiler height-to-width ratio, resulting in the measured flue gas temperature at the furnace outlet being 160℃ higher than the design value. [b]3. Improper Furnace Selection[/b] In addition to problems with the calculation methods, early large-capacity boilers in my country lacked reliable basis for selecting furnace size based on fuel characteristics, making the designed furnace unable to adapt to the operating conditions of varying coal types. An unreasonable furnace structure led to overheating and tube rupture of the superheater. An excessively high furnace height caused the steam temperature to be too low. Conversely, an excessively low furnace height caused overheating. [b]4. Unreasonable Superheater System Structural Design and Heating Surface Arrangement[/b] The survey results show that for large-capacity power plant boilers, unreasonable superheater structural design and heating surface arrangement are one of the main reasons for the deviation of primary and secondary steam temperatures from the design value or overheating and tube rupture of the heating surface. The unreasonableness of the superheater system structural design and heating surface arrangement is reflected in the following aspects: (1) The improper arrangement of the inlet and outlet headers of the superheater tube group causes excessive static pressure changes when steam flows in the headers, resulting in a large flow deviation. (2) For parallel pipe groups where steam is radially introduced into the inlet header, a local vortex is formed at the tee between the inlet header and the inlet pipe, resulting in a smaller flow rate in the pipe group near the vortex area, thus causing a large flow deviation. The control circulating boilers designed with technology from the American CE company for 300MW and 600MW units do not have an intermediate mixing header between the screen reheat and the final reheat. Various deviations in the screen reheat are carried to the final stage, resulting in excessive thermal deviation in the final stage reheater. For example, the overheating of the reheaters in the 350MW units of Baosteel's self-owned power plant, Huaneng Fuzhou and Dalian power plants, the 300MW units of Shiheng power plant, and the 600MW units of Pingping power plant are all related to this. (3) Due to the differences in the structure (such as pipe length, inner diameter, number of bends) of the parallel pipes in the same screen (plate), the resistance coefficients of each pipe are significantly different, resulting in large flow deviation, structural deviation and thermal deviation in the same screen (plate). For example, the overheating of the high-temperature superheater of the Hitachi 850t/h boiler in Douhe Power Plant is due to this. (4) There is no intermediate mixing header between the front and rear stages of the superheater or reheater and they are directly connected, or there is no left and right cross, which causes the thermal deviations of the front and rear stages to be superimposed. In actual operation, the above-mentioned unreasonable structural design and layout often exist in several ways at the same time, which aggravates the occurrence of overheating and tube bursting of the heating surface. [b]5. Imperfect wall temperature calculation method leads to improper material selection[/b] In principle, when checking the wall temperature of the heating surface of the superheater and reheater, it should be ensured that the wall temperature of the deviation tube at the most dangerous point does not exceed the allowable temperature of the material used. In actual design, the combined effects of various deviations are often not fully considered, resulting in the calculated wall temperature at the check point being lower than the actual operating temperature, or the selection of the check point being unreasonable. In this case, the selected material may not meet the requirements of actual operation, or high-grade steel may not be fully utilized. [b]6. Thermal deviations were not fully considered in the calculation[/b] For example, in the design of the superheater of Boiler No. 5 in Huaibei Power Plant, the deviation caused by the front screen was not taken into account in the rear screen design, which affected the correct use of the pipe material and caused the superheater tube to burst. 1.2 Manufacturing process, installation and maintenance quality From the actual operation, superheater and reheater heating surface overheating tube bursting and leakage accidents caused by manufacturing process problems, on-site installation and power plant maintenance quality are also quite common. The main problems include the following aspects. 1. Poor welding quality For example, in Boiler No. 6 of Datong Power Plant, when the tube was replaced and repaired after the boiler superheater tube burst, the tube joint was misaligned, resulting in large residual stress after the tube was welded, the tube wall strength was reduced, and leakage occurred after long-term operation. 2. Welding issues with intermediate partition plates in headers: When installing partition plates in headers, they were not fully welded as required by design specifications, causing steam short circuits in the headers, leading to poor cooling of some pipes and ultimately pipe bursts. 3. Problems with fillet welds on header pipe seats: Investigations show that leaks or pipe bursts caused by quality issues such as incomplete penetration of fillet welds are quite common. For example, at Shentou Power Plant's Boiler No. 5 (a Czech 650t/h subcritical once-through boiler), the steam pipe between the outlet header and the mixing header of the superheater wall suddenly broke and detached during a hydrostatic test. The main reason was cold cracking in the fillet weld connecting the steam pipe and the header. 4. Welding issues with dissimilar steel pipes: In the heating surfaces of superheaters and reheaters, austenitic steel parts are often used as pipe clamps and plates. Austenitic tubes are also sometimes used as heating surfaces to improve safety margins. When welding austenitic steel and pearlitic steel, due to the significant difference in their coefficients of thermal expansion, several pipe tearing accidents have occurred on the heating surfaces. Furthermore, the welding of steel pipes often results in uneven wall thicknesses on both sides of the joint, leading to frequent accidents involving welded joints of main steam pipes with varying wall thicknesses. Some manufacturers believe that short sections should be used in such cases to ensure consistent wall thickness on both sides of the welded joint and within its heat-affected zone. 5. Common Weld Quality Issues: The majority of boiler heating surfaces are pressure-bearing components, especially superheaters and reheaters, where the temperature and pressure of the working fluid inside the tubes are very high, resulting in poor operating conditions. Therefore, the requirements for weld quality are particularly stringent. However, in actual operation, tube bursts and leaks caused by substandard weld quality (such as weld burrs and pinholes) from manufacturers, installers, and power plant maintenance are quite common, with serious consequences. 6. Issues with Tube Elbow Ovality and Tube Wall Thinning: GB9222-88, the standard for strength calculation of pressure-bearing components in water-tube boilers, specifies the ovality of elbows and considers the additional thickness required for bend thinning. This standard stipulates that for elbows with a radius R > 4D, the ovality of the bend should not exceed 8%. However, actual measured data often exceed this value, reaching a maximum of 21%, with a considerable number of elbows having an ellipticity between 9% and 12%. Furthermore, measured data indicates that the thinning of many pipe elbows reaches 23% to 28%, less than the minimum required wall thickness for straight pipes. Therefore, it is hoped that the bending process can be appropriately improved to reduce ellipticity and thinning, or to increase the wall thickness of the elbows. [b]7. Foreign Object Blockage in Pipelines[/b] During long-term operation, boilers accumulate significant rust, but due to the small pipe diameter, it is impossible to completely remove it. Rust deposits at the bottom horizontal section or elbows of the pipes, causing overheating and pipe bursts. A significant proportion of superheater pipe bursts are caused by manufacturing, installation, or maintenance residues in the main pipes. For example, the No. 1 boiler at Changchun Thermal Power Plant No. 2 experienced a short-term overheating pipe burst due to pipe blockage. [b]8. Pipe Material Quality Issues[/b] Poor steel quality. The pipes themselves have defects such as delamination and slag inclusions. During operation, these defects expand due to temperature and stress, leading to pipe bursts. While bursts caused by substandard pipe materials are not as common as the aforementioned problems, they do occur during operation. [b]9. Incorrect Steel Use[/b] For example, during the manufacturing and maintenance of Boiler No. 4 at Jingyuan Power Plant, carbon steel was mistakenly used for the outlet header pipe seat of the high-temperature superheater, which should have been made of alloy steel, causing the carbon steel pipe seat to overheat and burst over time. Therefore, strict quality checks should be conducted on pipe materials during manufacturing and power plant maintenance to avoid this. [b]10. Installation Quality Issues[/b] For example, the wall-mounted superheater of the DG-670/140-8 solid slag discharge pulverized coal boiler at Yangzhou Power Plant was not constructed according to the drawings, causing problems with pipe arrangement, fixing, and expansion gaps, resulting in pipe bursts. This type of problem is more common during unit trial operation. 1.3 Unreasonable Design or Malfunction of Temperature Control Devices To ensure the safe and economical operation of the boiler, in addition to striving for accurate design calculations, steam temperature regulation is also a crucial aspect. Large-capacity power plant boilers employ various steam temperature regulation methods. However, in actual operation, numerous problems arise due to issues with the temperature control devices. According to relevant departments, over 80% of the reheat steam temperature control devices in boilers equipped with 200MW units are not functioning properly. 1. Inadequate Desuperheating Water System Design: Some boilers use a single spray regulating valve to adjust the total amount of primary spray water in their spray desuperheating system design, then distribute the spray water into two separate loops (left and right). In this case, when there is a significant deviation in combustion conditions or steam temperature between the left and right sides, it is impossible to balance the steam temperature by adjusting the spray water volume on both sides. 2. Inappropriate Spray Desuperheater Capacity: Spray desuperheaters are generally designed to spray approximately 3% to 5% of the boiler's rated evaporation capacity. However, boilers equipped with 200MW units have a particularly prominent issue of steam temperature deviating from design values, and many power plants have found that the spray desuperheater capacity is insufficient. For example, Xingtai Power Plant, Shajiao A Power Plant, and Tongliao Power Plant have all enlarged the original desuperheating water inlets to meet temperature regulation requirements. For reheat steam, because large amounts of water spray have a significant impact on the economics of unit operation, the design typically involves very small amounts of water spray, or no spray at all. However, in actual operation, some power plants have had to increase the water spray volume to address reheater overheating. 3. Performance issues of the regulating valve in the water spray desuperheater: The regulating performance of the water spray regulating valve in the water spray desuperheater is also a factor affecting the temperature regulation effect of the desuperheating system. Research results show that many domestically produced valves have poor regulating performance and serious leakage, which to some extent affects the reliability and economy of the unit. 4. Desuperheater malfunctions: For example, at Baling Petrochemical Company's Power Plant No. 5 boiler, the desuperheater's first-stage regulating valve is fixed, and the second-stage regulating valve is used for adjustment. Because the first-stage desuperheater, which plays a major regulating role, receives less desuperheating water, the cooling effect of the screen-type superheater and high-temperature superheater is poor, increasing the possibility of superheater overheating. 5. Reheater Adjustable Heating Surface: The reheater adjustable heating surface refers to an additional heating surface that alters the heat absorption of the reheated steam by changing the amount of steam passing through, thereby regulating the reheated steam temperature. The Soviet-made Efl670/140 boiler utilizes this device to regulate the reheated steam temperature. However, due to the steam weight velocity being lower than the design value during operation, while the boiler load was higher than the design value, overheating accidents occurred in the reheater adjustable heating surface tubes of boilers No. 5 and No. 6 at the Matou Power Plant. The overheating problem was resolved by reducing the area and flow cross-sectional area of ​​the adjustable heating surface. 6. Baffle Temperature Control Device: Boilers using flue gas baffle temperature control devices have better reheated steam temperature control than those using steam-to-steam heat exchangers. Baffle temperature control can change the distribution of flue gas volume and is more suitable for temperature control of reheat steam in pure convection heat transfer. However, there are some problems in the actual application of flue gas baffles: (1) The baffles are not very flexible in opening, and some power plants have experienced rusting and seizure; (2) It is difficult to match the opening of the baffles on the reheater side and the superheater side, and the optimal operating point of the baffles is not easy to control. It is inconvenient for operators to operate, and they often do not adjust them as long as the main steam temperature is met. Some power plants have also reported that when using the regulating baffles, the steam temperature change is significantly delayed. 7. Flue gas recirculation Flue gas recirculation is to send a portion of the flue gas with a temperature of 250-350℃ after the economizer into the furnace through a recirculation fan to change the heat absorption ratio of the radiant heating surface and the convective heating surface in order to regulate the steam temperature. This temperature control method can reduce and equalize the flue gas temperature at the furnace outlet, prevent slagging of the convective superheater and reduce thermal deviation, and protect the safety of the screen superheater and the high-temperature convective superheater. Generally, flue gas is fed into the furnace from the bottom when the boiler is under low load, serving a temperature control function; under high load, it is fed into the furnace from the top, protecting the high-temperature convective heating surfaces. Furthermore, flue gas recirculation can reduce the furnace's heat load, prevent deterioration of boiling heat transfer within the tubes, and suppress NOx formation in the flue gas, reducing atmospheric pollution. However, this method requires additional recirculation fans operating at high flue gas temperatures, consuming considerable energy. Moreover, the corrosion and wear prevention issues of recirculation fans are far from resolved, thus limiting the application of flue gas recirculation. Furthermore, the impact of flue gas recirculation on the dynamic field of the flue gas and combustion within the furnace requires further research. Therefore, in principle, flue gas recirculation is a relatively ideal temperature control method, highly beneficial for the operation of large power plant boilers. However, due to various reasons, it is rarely used in actual power plant operations. 8. Flame Center Adjustment: Changing the position of the flame center in the furnace can increase or decrease the heat absorption of the furnace heating surface and change the flue gas temperature at the furnace outlet, thus regulating the superheater and reheater steam temperatures. However, to control the flue gas temperature at the furnace outlet during operation, the aerodynamic field inside the furnace must be well-organized, and the burner operation mode must be rationally selected according to changes in boiler load and fuel. Depending on the burner type, methods for changing the flame center position are generally divided into two categories: oscillating burners and multi-stage burners. Oscillating burners are mostly used in boilers with a four-corner arrangement. They are particularly common in boilers equipped with 300MW and 600MW units. Tests show that changes in the burner nozzle inclination angle have a significant impact on both reheater and superheater temperatures. When using multi-stage burners, the flame position can be changed by shutting down one stage of burners or adjusting the ratio of primary and secondary air; for example, shutting down the lower row of burners can raise the flame position. Unfortunately, the effect is not ideal in actual operation. 1.4 The Impact of Operating Conditions on Superheater Overheating and Tube Rupture While the design and layout of the superheater temperature control device plays a decisive role in the reliable operation of the superheater system, the operating conditions of the boiler and its related equipment also have a significant impact, which is often influenced by a combination of factors. Therefore, ensuring that the boiler operates under ideal conditions is a problem that requires in-depth research. 1. Poor steam quality leads to severe scaling inside the tubes, resulting in overheating and tube rupture. For example, the No. 6 boiler (DG-670/140-8) of Zhenhai Power Plant experienced seven tube ruptures due to this problem. 2. In-furnace combustion conditions: As the boiler capacity increases, the impact of in-furnace combustion and airflow conditions on the superheater and reheater systems increases accordingly. If the dynamic and temperature fields of the flue gas inside the furnace become skewed during operation, the flue gas temperature deviation along the width and depth of the furnace will increase. This will correspondingly increase the flue gas temperature and velocity deviations along the height and width of the horizontal flue heating surface and along the width and depth of the tail shaft heating surface. Furthermore, an increase in the primary air rate during operation may cause combustion delay and a rise in the flue gas temperature at the furnace outlet. For example, the four-corner tangential combustion technology, commonly used by CE in the United States and widely applied in large-capacity boilers in my country, often results in significant flue gas temperature or velocity deviations at the furnace outlet. When the flue gas rotates to the right, the temperature on the right side is higher; when it rotates to the left, the temperature on the left side is higher. Sometimes, the temperature deviation between the two sides is quite large (the maximum deviation reached 250℃ in the No. 6 1025t/h boiler at Shiheng Power Plant), thus causing a significant steam temperature deviation. 3. Low High-Pressure Heater Activation Rate: The activation rate of high-pressure heaters in large-capacity units in my country is generally low, with some units experiencing long-term shutdowns. For a 200MW unit, the activation or deactivation of the high-pressure heater affects the feedwater temperature by approximately 80℃. Calculations and operational experience show that for every 1°C decrease in feedwater temperature, the superheated steam temperature rises by 0.4–0.5°C. Therefore, when the high-pressure heater is shut down, the steam temperature will rise by 32–40°C. This demonstrates the significant impact of feedwater temperature changes on steam temperature. 4. Differences in Coal Types Most large-capacity boilers in my country operate with non-designed coal types, mainly manifested in discrepancies between actual and designed coal types, frequent coal type variations, and declining coal quality. When the coal type deviates from the design type, the ignition point is delayed, the flame center shifts upward, and when the furnace height is insufficient, the superheater will overheat and rupture. The influence of fuel composition on steam temperature is complex. Generally speaking, the main factors directly affecting combustion stability and economy are the fuel's lower heating value, volatile matter, and moisture content. Furthermore, the ash melting point and ash composition are closely related to furnace coking and heating surface fouling. When the fuel calorific value increases, the theoretical combustion temperature and furnace outlet flue gas temperature rise, potentially leading to furnace coking and overheating of the superheater and reheater. Increased ash content worsens combustion, delays the combustion process, and lowers the flame temperature. Generally, the higher the ash content in the fuel, the greater the temperature drop in actual operation. Furthermore, increased ash content exacerbates wear and fouling of heating surfaces. Increased volatile matter accelerates the combustion process, increasing the heat absorption of the evaporating heating surfaces, thus leading to a decrease in steam temperature. Increased moisture content, assuming a constant fuel quantity, lowers flue gas temperature, increases flue gas volume, and ultimately raises steam temperature. According to calculations by relevant departments, a 1% increase in moisture content raises the superheater outlet steam temperature by approximately 1°C. 5. Heating Surface Fouling: Some large-capacity domestic boilers lack soot blowers (early models), or their soot blowers are not functioning properly, often resulting in ash accumulation on the furnace and superheater heating surfaces. This is particularly problematic when burning high-ash fuels, easily causing coking in the furnace and leading to superheater overheating. For boilers with already low steam temperatures, ash accumulation on the superheater will further lower the steam temperature. Therefore, the proper functioning of soot blowers has a significant impact on the safe and economical operation of the boiler. 6. Wear and Corrosion: The flue gas produced during boiler fuel combustion contains a large amount of ash particles. These ash particles, carried by the flue gas, impact and cut against the heating surface tubes, causing wear. This is more severe when burning fuels with low calorific value and high ash content. When burning fuels containing certain amounts of sulfur, sodium, and potassium compounds, high-temperature corrosion occurs on the metal tube walls at 550–700℃, and also when the flame impacts the water-cooled walls. Furthermore, low-temperature corrosion occurs when SO2 and SO3 are present in the flue gas and the heating surface wall temperature is below the flue gas dew point. High-temperature corrosion is the most common type of corrosion occurring on the heating surfaces of superheaters and reheaters. The degree of wear on the heating surface tubes varies within the same flue cross-section and the same tube circumference. Wear is most frequently observed on the low-temperature heating surfaces located in the tail shaft of the superheater and reheater systems. Generally, the serpentine tubes near the rear wall of the shaft suffer severe wear. When the design flue gas velocity is too high or a flue gas corridor exists due to unreasonable structural design, wear of the heating surface tubes in localized areas is easily caused. High-temperature corrosion of boiler heating surfaces occurs in areas where the flue gas temperature exceeds 700℃. When burning coal with high K, Na, and S content, K2SO4 and Na2SO4 in the ash react with iron oxide on the tube surface in flue gas containing SO2 to form alkali metal complex sulfates K2Fe(SO4)5 and Na5Fe(SO4)5. These complex sulfates melt into a liquid state in the range of 550-710℃ and are highly corrosive, with the most severe corrosion occurring at wall temperatures of 600-700℃. According to investigations, the main causes of high-temperature corrosion of heating surfaces are poor combustion in the furnace and an unreasonable flue gas dynamic field. Controlling the tube wall temperature is the main method to reduce and prevent external corrosion of the superheater and reheater. Therefore, in China, the superheated steam temperature of boilers for high-pressure, ultra-high-pressure, and subcritical pressure units tends to be set at 540℃. When designing and arranging superheaters, the steam outlet section should be avoided from being located in areas with excessively high flue gas temperatures. Inter-tube vibration and wear are also contributing factors. For example, in the No. 1 boiler of Leiyang Power Plant, the weld connecting the fixing parts and the superheater tube screen cracked, causing vibration in the tube screen. Friction between the fixing parts and the inner ring of the tube screen caused wear and thinning of the tube wall, leading to tube rupture under internal pressure. Scale buildup on the inner wall and oxidation on the outer wall are also issues. For example, in the No. 2 boiler of Luohe Power Plant, 0.7mm of scale buildup on the inner wall of the tubes increased the superheater wall temperature by 20-30℃; 1.0mm of oxide scale on the outer wall further thinned the tube wall, resulting in frequent tube ruptures. Finally, excessive service life, such as in the No. 2 boiler of Huangtai, where the superheater tubes have been in operation for over 230,000 hours, has led to severe spheroidization and oxidation of the tube material, and creep cracks have appeared. If not replaced in time, tube rupture will inevitably occur sooner or later. 8. Operation Management In actual operation, accidents such as superheater or reheater tube ruptures due to operator error and failure to follow regulations or meet requirements during maintenance are not uncommon. Improper operation adjustments are also a factor. For example, in the Hunjiang Power Plant's No. 3 boiler, the superheater materials used were generally operating within their permissible temperature limits. Improper adjustments when operating conditions changed led to instantaneous overheating and tube rupture. 2. Root Causes and Countermeasures of Superheater Tube Rupture In the early 1980s, the Electric Power Research Institute (EPRI) in the United States, after extensive research, classified boiler tube rupture mechanisms into six categories, totaling 22 types. Of these 22 mechanisms, 7 were influenced by circulating chemicals, and 12 were influenced by power plant maintenance practices. Chinese scholars, based on extensive research into superheater tube rupture accidents in Chinese power plant boilers, have summarized the following nine different mechanisms of superheater tube rupture in power plant boilers. 2.1 Long-term overheating 1. Failure Mechanism Long-term overheating refers to the tube wall temperature remaining above the design temperature but below the material's lower critical temperature for an extended period. The overheating amplitude is not large, but the duration is long. This leads to carbide spheroidization in the boiler tubes, thinning of the tube wall due to oxidation, decreased creep strength, and accelerated creep rate, causing uniform expansion of the tube diameter. Ultimately, this results in brittle fracture and tube rupture at the weakest point of the tube. Thus, the tube's service life is shorter than its design life. The higher the degree of overheating, the shorter the service life. Under normal conditions, long-term overheating tube rupture mainly occurs in the outer ring of the high-temperature superheater and the fire-facing surface of the high-temperature reheater. Under abnormal operating conditions, long-term overheating tube rupture can occur in both the low-temperature superheater and the fire-facing surface of the low-temperature reheater. Long-term overheating tube rupture can be classified into three types based on the working stress level: high-temperature creep type, stress oxidation cracking type, and oxidation thinning type. [b]2. Causes of Failure[/b] (1) Uneven distribution of steam and water flow in the tube; (2) Local heat load in the furnace is too high; (3) Scale buildup inside the tube; (4) Foreign objects blocking the tube; (5) Incorrect material used; (6) Unreasonable initial design. 3. Failure Location (1) High-temperature creep and stress oxidation cracking mainly occur on the fire-facing surface of the outer ring of the high-temperature superheater; under abnormal conditions, it may also occur in the low-temperature superheater; (2) Oxidation thinning mainly occurs in the reheater. 4. Bursting Characteristics The morphology of the bursting tube after long-term overheating has the general characteristics of creep fracture. The tube bursting has brittle fracture characteristics. The bursting is rough, with uneven blunt edges, and the tube wall thickness at the bursting point is not much thinning. The tube wall undergoes creep expansion, and the tube diameter expansion is related to the tube material, with carbon steel tubes expanding more. The rupture of the low-temperature superheater tube of the No. 20 steel high-pressure boiler resulted in a maximum expansion of 15% of the tube diameter, while the rupture of the high-temperature superheater tube of 12CrMoV steel resulted in an expansion of only about 5% of the tube diameter. (1) High-temperature creep type a. The creep of the tube significantly exceeded the specified value of the metal supervision, and the edge of the rupture was relatively blunt; b. There were dense longitudinal cracks in the oxide scale around the rupture, and the oxide scale on the inner and outer walls was thicker than that of the tube ruptured by short-term overheating. The lower the degree of overheating and the longer the time, the thicker the oxide scale and the wider the distribution range of the longitudinal cracks of the oxide scale; c. Creep cavities and microcracks existed in a large area around the rupture; d. The surface of the tube on the fire side was completely glossed; e. The structure at the elbow may recrystallize; f. The degree of spheroidization of carbides on the fire side and the back fire side was quite different. Generally, the carbides on the fire side were completely glossed. (2) Stress oxidation crack type a. The creep of the pipe is close to or lower than the specified value of metal supervision, the edge of the burst is blunt, and it is typical of thick lip shape; b. There are multiple longitudinal cracks on the oxide layer of the outer wall on the fire-facing side near the burst, and the distribution range can reach the entire fire-facing side. The oxide scale of the inner and outer walls is thicker than the oxide scale of the pipe during short-term overheating burst; c. The longitudinal stress oxidation cracks extend from the outer wall to the inner wall, and there may be a small number of voids at the crack tip; d. Severe spheroidization occurs on both the fire-facing and unfire-facing sides, and the strength and hardness of the pipe decrease; e. The oxide scale of the inner and outer walls of the pipe is delaminated; f. Elements such as S, Cl, Mn, and Ca in the combustion products are deposited and enriched in the oxide layer of the outer wall. (3) Oxidation thinning type a. The inner and outer walls of the pipe on both the fire-facing and unfire-facing sides produce an oxide scale with a thickness of 1.0 to 1.5 mm; b. The pipe wall is severely thinned, only 1/3 to 1/8 of the original wall thickness; c. The oxide scale of the inner and outer walls is delaminated and uniformly oxidized. The inner layer of the inner oxide scale is in the form of ring-shaped stripes; d. The structure on the fire-facing side is completely spheroidized, while the structure on the unfire-facing side is severely spheroidized, and the strength and hardness are reduced; e. Elements such as S, Cl, Mn, and Ca in the combustion products are deposited and enriched in the outer oxide layer, promoting outer wall oxidation. 5. Prevention measures For high-temperature creep type, prevention can be achieved by improving the heating surface, making the medium flow distribution reasonable; improving combustion in the furnace and preventing the combustion center from being too high; and performing chemical cleaning to remove foreign matter and deposits. For stress oxidation crack type, since the tube life is close to the design life, the damaged tubes can be replaced. For oxidation thinning type, the protection measures of the superheater should be improved. 2.2 Short-term overheating 1. Failure mechanism Short-term overheating refers to when the tube wall temperature exceeds the lower critical temperature of the material, the material strength decreases significantly, and under the action of internal pressure, expansion and tube bursting occur. 2. Causes of failure (1) Uneven distribution of working fluid flow in the superheater tubes. In tubes with smaller flow rates, the working fluid has poor cooling capacity for the tube wall, causing the tube wall temperature to rise and resulting in tube wall overheating; (2) Excessive local heat load in the furnace (or deviation of the combustion center), causing the temperature of the nearby tube wall to exceed the design allowable value; (3) Severe scaling inside the superheater tubes, causing the tube wall temperature to exceed the design allowable value; (4) Foreign objects block the tubes, preventing the superheater tubes from being effectively cooled; (5) Incorrect use of steel. Incorrect use of low-grade steel can also cause short-term overheating. As the temperature rises, the allowable stress of low-grade steel decreases rapidly, resulting in insufficient strength and causing the tube to burst; (6) Oxide scale peels off the inner wall of the tube, causing blockage at the lower bend; (7) Improper addition of desuperheating water during low-load operation, resulting in excessive injection and water blockage in the tube, thus causing local overheating; (8) Abnormal flue gas temperature in the furnace. 3. Fault location often occurs on the heat-receiving surface tubes of the superheater that are in direct contact with the flame and directly exposed to radiant heat. 4. Burst shape (1) The burst has large plastic deformation, the tube diameter is obviously expanded, and the tube wall is thinned into a knife-edge shape; (2) Under normal circumstances, the burst is large and trumpet-shaped; (3) The burst is a typical thin-lip burst; (4) The microscopic structure of the burst is dimples (the fracture surface is composed of many pits); (5) The hardness of the tube material around the burst increases significantly; (6) The thickness of the oxide scale on the inner and outer walls around the burst depends on the degree of long-term overheating before the short-term overheating bursts the tube. The more severe the long-term overheating, the thicker the oxide scale. 5. Prevention measures Methods to prevent short-term overheating include improving the heat-receiving surface to make the medium flow distribution reasonable; stabilizing the operating conditions, improving combustion in the furnace, and preventing the combustion center from deviating; performing chemical cleaning; removing foreign matter and deposits; preventing the misuse of steel: take measures in time when misuse is found. 2.3 Wear 1. Failure mechanism includes fly ash wear, slag wear, soot blowing wear and coal particle wear. Take fly ash wear as an example for analysis. Fly ash wear refers to the high-speed scouring of the tube surface by hard particles such as SiO2, FeO3, and Al2O3 entrained in fly ash, which causes the tube wall to thin and burst. 2. Causes of failure (1) Hard particles are entrained in the fly ash of coal-fired boilers; (2) The flue gas velocity is too high or the local flue gas velocity of the tube is too high (such as when ash accumulates, the flue gas channel becomes smaller, which increases the flue gas flow velocity); (3) The ash concentration of flue gas is unevenly distributed, and the local ash concentration is too high. 3. Failure location often occurs at the elbow, the outgoing tube and the tube with uneven transverse pitch at the flue gas inlet of the superheater. 4. Characteristics of bursting (1) The tube wall is thinned at the fracture point, which is knife-edged; (2) The wear surface is smooth and gray; (3) The metallographic structure does not change and the tube diameter generally does not expand. 5. Prevention measures usually involve reducing the number and speed of fly ash impacting the pipe or increasing the wear resistance of the pipe to prevent fly ash wear, such as: changing the flow direction and velocity field by adding screens; adding dust removal devices in the furnace; eliminating excessive local smoke velocity; and adding wear-resistant covers to the surface of easily worn pipes. It is also necessary to select a furnace type suitable for the coal type, improve the fineness of coal powder, adjust the combustion, and ensure complete combustion. 2.4 Corrosion fatigue (or oxygen corrosion on the steam side) 1. Failure mechanism Corrosion fatigue is mainly caused by the chemical properties of water. The oxygen content and pH value in the water are the main factors affecting corrosion fatigue. Due to the depolarization effect of oxygen, the medium in the pipe undergoes an electrochemical reaction, and pitting occurs at the rupture of the passivation film in the pipe to form a corrosive medium. Under the combined action of the corrosive medium and cyclic stress (including internal stress caused by start-up, shutdown and vibration), corrosion fatigue and pipe bursting occur. 2. Causes of failure (1) Stress concentration at the elbow promotes pitting; (2) Thermal shock at the elbow causes fatigue cracks in the neutral zone of the inner wall of the elbow; (3) Water accumulates in the lower elbow when the furnace is shut down; (4) The medium inside the tube contains a small amount of alkali or free carbon dioxide; (5) The number of times the unit is started up and chemically cleaned is too many. 3. The fault location often occurs on the water side and then extends to the outer surface. The inner wall of the tube elbow of the superheater produces pitting or pitting corrosion, mainly during the shutdown of the furnace. 4. Characteristics of the burst: (1) Pitting or pitting corrosion occurs on the inner wall of the tube of the superheater, and the typical corrosion shape is shell-shaped; (2) During operation, the product of corrosion fatigue is black magnetic iron oxide, which is firmly bonded to the metal; during shutdown, the product of corrosion fatigue is brick red iron oxide; (3) The metal structure of the pitting and pitting corrosion areas does not change; (4) The corrosion pits develop along the tube axis, the cracks are transverse cracks, relatively wide and blunt, and there is oxide scale at the cracks. 5. Prevention measures To prevent oxygen corrosion, attention should be paid to boiler shutdown protection; when a new boiler is put into use, chemical cleaning should be carried out to remove rust and dirt and form a uniform protective film on the inner wall; during operation, the water quality should meet the standards, and the pH value should be appropriately reduced or the chloride and sulfate content in the boiler should be increased. 2.5 Stress corrosion cracks [b]1. Failure mechanism[/b] This refers to the pipe rupture phenomenon caused by static tensile stress or residual stress under the conditions of chloride ion content and high temperature. [b]2. Causes of failure[/b] (1) Chloride ion content in the medium, high temperature environment and high tensile stress are the three basic conditions for stress corrosion cracks; (2) Stress corrosion cracks can also be caused by the action of humid air; (3) Chlorine and oxygen-containing water masses may enter the steel pipe during startup and shutdown; (4) Thermal stress caused by residual stress caused by processing and welding. [b]3. Failure location[/b] It often occurs in the high temperature zone pipe and sampling pipe of the superheater. [b]4. Burst characteristics[/b] (1) The burst mouth is brittle and is generally a transgranular stress corrosion fracture. (2) There may be corrosive media and corrosion products on the burst mouth. (3) The crack has a dendritic branching feature. The crack originates from the corrosion site and there are many crack sources. [b]5. Prevention measures[/b] To prevent stress corrosion cracks, attention should be paid to removing residual stress in the pipe; strengthen protection during the installation period and pay attention to corrosion prevention during shutdown; prevent condenser leakage and reduce the content of chloride ions and oxygen in the steam. 2.6 Thermal fatigue [b]1. Failure mechanism[/b] Thermal fatigue refers to the fatigue damage of the furnace tube caused by thermal stress caused by boiler start-up and shutdown, repeated appearance and disappearance of steam film, and alternating stress caused by vibration. [b]2. Causes of failure[/b] (1) S, Na, V, Cl and other substances in the flue gas promote corrosion fatigue damage. (2) Water blowing in the furnace causes a rapid change in pipe wall temperature, resulting in thermal shock. (3) Overheating leads to a serious decrease in the fatigue strength of the pipe material. （4）按基本负荷设计的机组改变为调峰运行。 [b] 3.故障位置[/b] 常发生在过热器高热流区域的管子外表面。 [b] 4.防止措施[/b] 防止热疲劳产生的措施有改变交变应力集中区域的部件结构；改变运行参数以减少压力和温度梯度的变化幅度；设计时应考虑间歇运行造成的热胀冷缩；避免运行时机械振动；调整管屏间的流量分配，减少热偏差和相邻管壁的温度；适当提高吹灰介质的温度，降低热冲击。 2.7高温腐蚀[b]1.失效机理[/b] V2O5和Na2S04等低熔点化合物破坏管子外表面的氧化保护层，与金属部件相互作用，在界面上生成新的松散结构的氧化物，使管壁减薄，导致爆管。 [b]2.产生失效的原因[/b] （1）燃料中含有V、Na和S等低熔点化合物； （2）局部烟温过高，腐蚀性的低熔点化合物粘附在金属表面，导致高温腐蚀； （3）腐蚀区内的覆盖物、烟气中的还原性气体和烟气的直接冲刷，将促进高温腐蚀的产生。 3.故障位置高温腐蚀常发生在过热器及吊挂和定位零件的向火侧外表面。 4爆口特征（l）裂纹萌生于管子外壁，断口为脆性厚唇式； （2）沿纵向开裂，在相当于时钟面10点和2点处有浅沟槽腐蚀坑，呈鼠啃状； （3）外壁有明显减薄，但不均匀，无明显胀粗； （4）外壁有氧化垢，呈鳄鱼皮花样，垢中含黄色、白色、褐色产物，垢较疏松，为熔融状沉积物，最内层氧化物为硬而脆的黑灰色。 5.防止措施 　　防止高温腐蚀的方法有控制局部烟温，防止低熔点腐蚀性化合物贴附在金属表面上；使烟气流程合理，尽量减少热偏差；在燃煤锅炉中加入CaSO4和MgSO4等附加剂；易发生高温腐蚀的区域采用表面防护层或设置挡板;除去管子表面的附着物。 2.8异种金属焊接[b]1.失效机理及原因[/b] 焊接接头处因两种金属的蠕变强度不匹配，以及焊缝界面附近的碳近移，使异种金属焊接界面断裂失效。其中，两种金属的蠕变强度相差极大是异种金属焊接早期失效的主要原因。 [b] 2.故障位置[/b] 常发生在过热器出口两种金属的焊接接头处，当焊缝的蠕变强度相当于其中一种金属的蠕变强度时，断裂发生在另一种金属的焊缝界面上。 3.防止措施 　　稳定运行是减少异种金属焊接失效最关键的因素；当两种金属焊接时，在其中加入具有中间蠕变强度的过渡段，使焊缝界面两侧蠕变强度差值明显减少；在过渡段的两侧选用性质不同的焊条，使其分别与两种金属的性质相匹配。 2.9质量控制失误质量控制失误是指在制造、安装、运行中由于外界失误的因素所造成的损坏。质量控制失误的原因有：维修损伤；化学清理损伤；管材缺陷（管材金属不合格或错用管材）；焊接缺陷等。加强电厂运行、检修及各种制度的管理是防止质量控制失误出现的有效手段。 3结论 　　造成大型电站锅炉过热器爆管的原因很多，只有对过热器爆管的直接原因和根本原因进行综合分析，才能从根本上解决锅炉爆管问题，有效地防止锅炉过热器爆管事故的发生。 4参考文献1.张建国等. 电站锅炉屏式过热器爆管分析[J]. 能源研究与利用，1999,（ 5）:34～36 2.吴磊等. 1025t/h锅炉高温过热器爆管原因分析[J]. 湖北电力, 2004, 28（1）: 23～26 3.郭鲁阳等. 再热器结构布置对超温爆管的影响[J]. 中国电力, 1999, 32（5）: 8～11 4.程绍兵等. 大容量锅炉高温受热面超温失效原因及对策[J]. 广东电力, 2004, 17（2）: 38～41 5.王仁志等. 1025t/h锅炉末级过热器爆管原因分析[J]. 华东电力，2000,（11）:35～36 6.王莹等. 大型电站锅炉过热器爆管原因综述及对策[J]. 中国电力，1998,31（10）:26～29 7.肖向东等. 电厂锅炉高温过热器爆管事故分析[J]. 热力发电，1999,（2）:56～58,62 8.赖敏等. 电站锅炉对流过热器爆管机理研究[J]. 湖南电力，1998,（6）:1～5 9.周昊等. 根据爆口特征判断爆管原因[J]. 热力发电，1995,（5）:43～48 10.严立等. 大型电站锅炉再热器爆管原因分析及对策[J]. 华北电力技术，1999,（10）:13～15