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Summary of 8 common causes and solutions for overheater tube failures: Tube failures in boiler overheaters represent a significant threat to the safe operation of thermal power plants today. Based on actual production operations and maintenance, this paper conducts a targeted analysis of tube failures in various boiler superheaters, and proposes the main measures to prevent such failures. Keywords: superheater tube rupture ； Superheater scaling ； Superheater corrosion ； Through a comprehensive analysis of numerous cases of tube failures in superheaters of power plant boilers, stress corrosion in superheaters can be attributed to the following 8 common root causes along with corresponding solutions. Reasons for tube failure in boiler superheaters: 1. Overheating of the superheater tubes refers to a situation where the temperature of the tube walls exceeds the designed operating temperature. This leads to the spheroidization of carbides within the tubes, oxidation and thinning of the tube walls, a decrease in the strength of those walls, an increase in the rate of creep, and an expansion of the tube diameter. Eventually, brittle fracture occurs at the weakest parts of the tubes. Based on the duration of overheating, tube failures due to overheating can be divided into two types: long-term overheating-induced tube failure and short-term overheating-induced tube failure. Based on the working stress level, tube overheating and rupture can be classified into three types: high-temperature creep type, stress-induced oxidation crack type, and oxidation-thinning type. Tube rupture caused by overheating occurs mainly in the outer ring of the high-temperature superheater and the fire-facing side of the high-temperature reheater. Under abnormal operating conditions, overheating, over-temperature, and tube rupture may also occur on the fire-facing sides of the low-temperature superheater and low-temperature reheater. Oxidative thinning-induced superheat tube failure mainly occurs in the reheater. Over-temperature and overheating are the causes of tube rupture in boiler superheaters. The main reasons for tube failure in boiler superheaters due to overheating are as follows: ① The distribution of steam and water flow inside the superheater tubes is uneven; in the tubes with lower flow rates, the steam cannot cool the tube walls effectively, resulting in overheating of those tube walls ； ②Severe scaling inside the superheater tubes caused the tube wall temperature to rise above the limit ； ③Foreign objects have blocked the superheater tubes, preventing them from being cooled effectively ； ④A high local heat load inside the furnace or a deviation of the combustion center causes the wall temperature of the nearby superheater tubes to exceed the design allowable values ； ⑤Improper use of superheater steel led to overheating, which caused the allowable stress of lower-grade steel to drop rapidly; insufficient strength resulted in the bursting of the superheater tubes ； ⑥The oxide scale or salt scale on the inner wall of the pipe peels off, causing blockage at the elbow ； ⑦During low-load operation, improper use of the cooling water results in an excessive amount of water being injected, which causes a water blockage inside the pipes and leads to localized overheating ； ⑧Abnormal flue gas temperature in the furnace ； ⑨The initial design was unreasonable, and the actual operating conditions did not match the designed conditions. ▲Figure 2 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the type and causes of tube failures in this boiler’s superheater? Characteristics of tube ruptures and cracks in boiler superheaters caused by overheating. Over-temperature overheating leading to tube failure in boiler superheaters can be classified into three types: superheater high-temperature creep-induced tube failure, superheater stress-oxidation crack-induced tube failure, and superheater oxidation-thinning-induced tube failure. The failure mode of tube rupture due to overheating creep in superheaters exhibits the following characteristics. ①The creep expansion of the superheater tubes significantly exceeds the specified values for metal materials, and the edges of the cracks are relatively blunt ； ②The scale around the burst opening features dense longitudinal cracks; when the scale on the inner and outer walls is thin, the scale on the tube due to overheating is thicker ； ③Creep cavities and microcracks exist around the burst area of the superheater tubes ； ④There is a significant difference in the degree of carbide spheroidization on the fire-side and backfire-side of the tubes at the superheater burst site; the surface on the fire-side has already achieved complete spheroidization ； ⑤Recrystallization may occur in the microstructure at the superheater elbow. Overheating stress-induced oxidative crack-type tube explosions in superheaters exhibit the following characteristics. ①The creep expansion of the superheater tubes is close to or below the specified value for metal materials; the edge of the crack is relatively blunt, showing a typical thick-lipped shape ； ②Multiple longitudinal cracks exist on the oxidation layer of the outer wall on the fire-facing side near the overheater burst opening, and these cracks can be found throughout the entire fire-facing side; the thickness of the oxidation scale on the inner and outer walls is greater than that in the case of tube rupture due to short-term overtemperature ； ③In the longitudinal stress oxidation cracks caused by overheater ruptures, these cracks extend from the outer wall toward the inner wall; there may be small voids at the tips of the cracks ； ④Severe spheroidization occurred on both the fire-side and backfire-side of the location where the superheater tube burst, resulting in a decrease in the strength and hardness of the tube material ； ⑤The oxide scale on the inner and outer walls of the superheater tubes has delaminated ； ⑥Elements such as S, Cl, Mn, and Ca in the combustion products deposit and accumulate in the oxide layer on the outer wall of the superheater tubes. ▲Figure 3 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? The failure modes of superheater tubes due to overheating, oxidative thinning, and bursting exhibit the following characteristics. ①On the fire-side and back-fire-side inner and outer walls of the tubes that burst in the superheater, an oxide layer with a thickness of 1.0–1.5 mm can form ； ②The wall thickness of the overheater tubes that have burst has been significantly reduced, to only 1/3 to 1/7 of the original thickness ； ③The oxide scale on the inner and outer walls of the superheater tubes exhibits uniform oxidation stratification, with the oxide scale on the inner wall showing ring-shaped patterns ； ④In the superheater tubes that have experienced tube rupture, the microstructure on both the fire-side and backfire-side of the tube walls has become severely spheroidized, with a decrease in strength and hardness ； ⑤Elements such as S, Cl, Mn, and Ca in the combustion products deposit and accumulate in the outer wall oxide layer, promoting oxidation of the outer wall of the superheater tubes. Solutions for tube rupture in boiler superheaters caused by overheating. For the situation of tube failures in boiler superheaters caused by high-temperature creep and excessive overheating, this can be addressed by improving the heating surfaces and ensuring a proper distribution of the fluid flow ； Improve combustion inside the furnace, prevent the combustion center from being too high, and stabilize operating conditions ； Scaling on the superheater tubes can be removed through chemical cleaning; prevention can be achieved by removing foreign substances and deposits. In the case of tube rupture in the boiler superheater caused by stress-induced oxidation cracks due to overheating, the damaged tubes can be replaced since their service life is approaching the design life. To address the issue of tube failures in boiler superheaters caused by oxidative thinning and excessive overheating, the protection measures for the superheaters need to be improved. Prevent the misuse of steel. ▲Figure 4 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failure in boiler superheaters: 2. Wear. The main types of tube failure resulting from wear in boiler superheaters include tube failure caused by fly ash wear, tube failure due to slag fall wear, tube failure resulting from soot blowing wear, and tube failure caused by coal particle wear – a total of 4 types. Overheater tube failures due to wear usually occur at the elbows at the flue gas inlet of the overheater, as well as on tubes with uneven lateral spacing. Taking the overheater tube rupture caused by fly ash wear as an example, fly ash wear refers to the phenomenon where hard particles such as SiO2, Fe2O3, and Al2O3 contained in the fly ash scour the surface of the overheater tubes at high speeds, resulting in thinning of the tube walls and eventual tube rupture. Reasons for tube rupture in boiler superheaters due to wear. The three main reasons for tube failures in boiler superheaters due to wear are the presence of hard particles in the fly ash from coal-fired boilers ； An excessively high smoke velocity, or a locally high smoke velocity within the tubes, or a reduction in the smoke flow path due to ash accumulation, all contribute to an increased smoke flow velocity ； The ash concentration in the flue gas is unevenly distributed, with excessively high local ash concentrations. Characteristics of tube ruptures in boiler superheaters caused by wear. The cracks caused by overheater wear and tube failure exhibit the following characteristics: ① The tube wall at the fracture site becomes thinner, appearing blade-like ； ②The worn surface is smooth and gray in color ； ③The microstructure of the tube walls in the superheater tubes remains unchanged, and the tube diameter generally does not increase. Solutions for tube bursts in boiler superheaters caused by wear. Fly ash wear is typically prevented by reducing the number and speed of fly ash impacts on the superheater tubes, or by increasing the wear resistance of these tubes; for example, the flow direction and velocity field are altered through the use of screens and other methods ； Install an in-furnace dust removal device ； Lower local smoke velocity levels ； Anti-wear covers are installed on the surface of the superheater tubes in areas prone to wear. It is also necessary to select a furnace type that matches the coal type, improve the fineness of the coal powder, adjust the combustion conditions properly, ensure complete combustion, and reduce shocks. ▲Figure 5 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failure in boiler superheaters: 3. Corrosion on the steam side. Tube failure in the boiler superheater due to corrosion on the steam side is mainly caused by the chemical properties of water; the oxygen content and pH level in water are the key factors affecting corrosion fatigue in the superheater. Steam-side corrosion in superheaters often occurs on the inner wall of the tube elbows in the superheater, where spot-like or pit-like corrosion appears on that inner wall. The main types of corrosion on the steam side of superheaters are oxygen corrosion, acidic corrosion caused by low pH levels, and under-scale corrosion. Due to the depolarizing effect of oxygen, electrochemical reactions occur within the tubes of the superheater; pitting forms at the areas where the passivation film is damaged, leading to the formation of corrosive agents as well as under-scale corrosion. Under the combined action of these corrosive agents and cyclic stresses, including internal stresses resulting from startup/shutdown and vibrations, corrosion fatigue can cause tube failure. Steam-side corrosion is the cause of tube failure in boiler superheaters. The six main reasons for tube failure in boiler superheaters due to steam-side corrosion are: ① Stress concentration at the superheater elbows, which leads to pitting corrosion ； ②Thermal shock at the superheater elbow caused fatigue cracks to form in the neutral zone of the inner wall of the elbow ； ③Water accumulation at the lower elbow of the superheater during plant shutdown ； ④The medium inside the superheater tubes contains a small amount of alkali or free carbon dioxide ； ⑤Excessive number of superheater unit startups or excessive number of superheater chemical cleanings ； ⑥Under-scale corrosion occurs due to iron oxide scale resulting from oxygen corrosion in the superheater, salt scale in the superheater, or mixed scale in the superheater. Characteristics of tube ruptures in boiler superheaters caused by steam-side corrosion. The rupture patterns caused by corrosion on the steam side of superheaters exhibit the following characteristics: ① Spot-like or pit-like corrosion occurs on the inner wall of the superheater tubes, resulting in pitting or corrosion of such tubes; the typical shape of this corrosion is shell-like ； ②During operation of the superheater, the product of corrosion fatigue is black magnetic iron oxide Fe3O4, which bonds firmly to the metal ； During shutdown, the product of corrosion fatigue is brick-red iron oxide Fe2O3 ； ③The metallic structure in the pitted and cratered corrosion areas, that is, pitting or cratering, does not change ； ④The corrosion pits develop along the tube axis, while the cracks are transverse fractures that are relatively wide and blunt, with scale present at the crack sites. Solutions for tube bursts and leaks in boiler superheaters caused by steam-side corrosion. To prevent oxygen corrosion in the superheater, attention should be paid to shutdown protection ； When a new furnace is put into use, the superheater should be chemically cleaned to remove rust and dirt from it, thereby forming a uniform protective layer on the inner wall of the superheater ； Keep the water quality in the boiler at standard levels during operation, and appropriately reduce the levels of chlorides, phosphates, and sulfates in the boiler water. ▲Figure 6 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failure in boiler superheaters: 4. Stress corrosion. Stress corrosion-induced tube failure in boiler superheaters refers to the cracking of superheater tubes that occurs under conditions of chloride ions in the medium and high temperatures, as a result of static tensile stress or residual stress; this phenomenon often occurs in the tubes located in the high-temperature areas of the superheater, as well as in the elbows and sampling tubes of these tubes. Stress corrosion as the cause of tube failure in boiler superheaters. The three main causes of tube failure in boiler superheaters due to stress corrosion are: ① the presence of chloride ions in the medium, high temperatures in the environment, and high tensile stresses – these are the three basic conditions for the formation of stress corrosion cracks ； ②During startup and shutdown, water masses containing chlorine and oxygen may enter the superheater tubes ； ③Thermal stress caused by residual stresses resulting from machining and welding. Stress corrosion induces the failure characteristics of tubes in boiler superheaters. The fracture characteristics of tube failures in boiler superheaters caused by stress corrosion are as follows: ① Stress corrosion fractures exhibit a brittle morphology, typically being transgranular stress corrosion fractures ； ②There may be corrosive agents and corrosion products on the crack opening ； ③The cracks exhibit a dendritic branching pattern; they originate from areas of erosion, with multiple crack sources. Solutions for tube bursts and cracks in boiler superheaters caused by stress corrosion. ①To prevent stress corrosion cracking in the superheater, it is necessary to remove the residual stress in the tubes ； ②Strengthen protection during the installation period, and pay attention to corrosion prevention when shutting down the furnace ； ③Prevent overheater leaks and reduce the levels of chloride ions and oxygen in the steam. Colleagues and students, has your facility experienced tube failures in the boiler superheater? What was the cause of the tube failure in the superheater of your unit’s boiler, and how was it resolved? Regarding issues such as superheater tube failures due to scaling, corrosion, etc.; problems related to corrosion and scaling in the condenser; online cleaning and descaling techniques that allow operations to continue without interruption; as well as online descaling methods for boiler scaling without shutting down the boiler – plus issues like red-colored boiler drainage water and excessive iron levels in steam condensate – Yan Hui from Beijing University of Chemical Technology at I86OO475З86 is always available to discuss these problems, exchange experiences and insights, and to talk with everyone about new technologies related to condenser cleaning. Next time, we will discuss the mechanisms behind scaling and tube failure in superheaters, as well as the solutions for these issues. There are 8 common causes of superheater tube failures along with corresponding solutions. ▲Figure 7: Based on the characteristics of the cracks caused by superheater tube failures in the boiler shown in this figure, what are the types and causes of such failures? Reasons for tube failure in boiler superheaters: 5. Thermal fatigue. Thermal fatigue-induced tube failure in boiler superheaters refers to the fatigue damage that occurs as a result of thermal stresses caused by the start-up and shutdown of the boiler, as well as the repeated formation and disappearance of steam films, coupled with alternating stresses resulting from vibrations. Overheating fatigue tube rupture often occurs on the outer surface of tubes in the high heat flux areas of the superheater. Reasons for thermal fatigue tube rupture in boiler superheaters. 4 main causes of thermal fatigue tube failure in boiler superheaters: ①) Substances such as S, V, Cl, and Na in the flue gas contribute to corrosion-induced fatigue damage ； ②Water soot blowing is used in the furnace, causing rapid changes in the temperature of the superheater tube walls and resulting in thermal shock ； ③) Over-temperature leads to a significant reduction in the fatigue strength of the pipes ； ④Units designed for base load are switched to peak-shaving operation. ▲Figure 8 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Solutions for thermal fatigue tube failure in boiler superheaters. Measures to prevent thermal fatigue in superheaters include: ① Changing the component structure of the areas where alternating stress concentrations occur ； ②Adjust the operating parameters to reduce the magnitude of pressure and temperature gradients ； ③Thermal expansion and contraction caused by intermittent operation should be taken into account during design ； ④Avoid mechanical vibrations during operation ； ⑤Adjust the flow distribution between the tube banks to reduce thermal gradients and temperatures of adjacent tube walls ； ⑥Appropriately increase the temperature of the soot-blowing medium to reduce thermal shock. ▲Figure 9 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failure in boiler superheaters: 6. High-temperature corrosion. High-temperature corrosion causes tube failure in boiler superheaters because low-melting-point compounds such as Na2SO4 in the flue gas destroy the oxide protective layer on the outer surface of the tubes. These compounds interact with the components that make up the superheater tube walls, resulting in the formation of new, loose-structured oxides at the interface between the tubes and the wall surfaces. This leads to thinning of the tube walls and ultimately to tube failure. High-temperature corrosion often occurs on the outer surface of the fire-facing side of the superheater tubes, hangers, and positioning components. Reasons for tube failure in boiler superheaters caused by high-temperature corrosion. The reasons for tube failure in boiler superheaters due to high-temperature corrosion are: ①) The flue gas contains low-melting-point compounds such as V, Na, Cl, and S ； ②The local smoke temperature in the superheater is too high; corrosive low-melting-point compounds adhere to the metal surface, leading to high-temperature corrosion ； ③The coverings in the corrosion area, the reducing gases in the flue gas, and the direct scouring by the flue gas all contribute to the occurrence of high-temperature corrosion. Characteristics of tube ruptures and cracks in boiler superheaters caused by high-temperature corrosion. Four main characteristics of tube ruptures and cracks in boiler superheaters caused by high-temperature corrosion: ① The cracks resulting from high-temperature corrosion originate on the outer wall of the superheating tubes, and the fracture pattern is a brittle thick-lipped type ； ②The rupture site of the tube-bursting superheater showed longitudinal cracks, as well as shallow grooved corrosion pits in a fine, wavy pattern ； ③There is a noticeable thinning of the outer tube wall at the site of the burst, but it is uneven; there is no significant bulging ； ④The outer wall at the location where the superheater tube burst was covered with oxidized scale; this scale sometimes contained yellow, white, and brown substances. The scale was loose and in the form of molten deposits, while the innermost oxide layer was hard, brittle, and black-gray in color. Solutions to tube burst in boiler superheaters caused by high-temperature corrosion. Measures to prevent tube rupture in boiler superheaters due to high-temperature corrosion include: ① Controlling local flue gas temperature to prevent low-melting-point corrosive compounds from adhering to the metal surface ； ②Optimize the flue gas flow to minimize thermal deviations as much as possible ； ③Additives such as CaSO4 and MgSO4 are added to coal-fired boilers ； ④Use a surface protective layer or install baffles in areas prone to high-temperature corrosion ； ⑤Remove the deposits from the surface of the superheater tubes. ▲Figure 10 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failure in boiler superheaters 7: Welding of dissimilar metals. Tube failure in boiler superheaters due to welding of dissimilar metals occurs because the mismatch in creep strength between the two metals at the weld joint, along with carbon migration near the weld interface, leads to the failure of the weld junction between these two metals. Among them, the significant difference in creep strength between the two metals is the main cause of early failure in dissimilar metal welding. Tube failure in boiler superheaters due to welding of dissimilar metals often occurs at the weld joint where the two metals meet at the superheater outlet; when the creep strength of the weld is equivalent to that of one of the metals, fracture takes place at the weld interface of the other metal. Main characteristics of tube ruptures and cracks in boiler superheaters caused by welding of dissimilar metals. ①Cracking on one side of the blowhole weld ； ①The crack is horizontal to the overheated tube and parallel to the weld seam ； ③Leakage usually occurs at the weld crack before the blowout. Solutions for tube bursts in boiler superheaters caused by welding of dissimilar metals. Stable operation is the most critical factor in reducing welding failures in dissimilar metals ； When two metals are welded, a transition zone with intermediate creep strength is introduced, which significantly reduces the difference in creep strength on both sides of the weld interface ； Weld rods with different properties are selected on both sides of the transition zone to match the properties of the two metals respectively. ▲Figure 11 shows the characteristics of the burst openings in the boiler superheater as depicted above; what are the types and causes of tube failures in this boiler’s superheater? Reasons for tube failures in boiler superheaters 8: Errors in equipment quality management control. Tube failures in boiler superheaters caused by errors in equipment quality management control refer to damages that occur during manufacturing, installation, or operation due to human errors. Reasons for tube rupture in the boiler superheater due to errors in equipment quality management and control include: ① Damage caused by superheater maintenance ； Corrosion damage from chemical cleaning of superheaters ； ②Superheater tube defects, namely substandard tube metal or incorrect use of tubes ； ③Welding defects in equipment manufacturing or maintenance, etc. Solutions for tube failures in boiler superheaters caused by errors in equipment quality management control. Strengthen the management of power plant equipment operation, maintenance, and various systems, as well as enhance equipment quality control. 8 Common Causes of Superheater Tube Failure and Solutions (Yan Hui)