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Abstract: Considering the structural characteristics and layout of the economizer, this paper focuses on analyzing the underlying mechanisms by which factors such as wear, corrosion, and vibration lead to overheating and tube failure in the economizer. Furthermore, based on the mechanisms of wear, corrosion, and vibration, some practical and useful measures were proposed to prevent overheating and tube rupture in economizers. Keywords: power station boiler ； savings burner ； Over-temperature tube rupture ； Solution 1: Analysis of the mechanism behind tube rupture due to overheating in the economizer. The reasons for tube rupture due to overheating in the economizer are very complex, mainly caused by wear, corrosion, and vibration. The mechanism of superheat tube explosion in economizers is mainly explored from these three aspects below. 1.1 Wear: Among tube failures caused by wear, fly ash wear is the primary factor, with influencing factors including fly ash concentration, flue gas flow velocity, and the wear resistance of the fly ash ； Furthermore, the structure of the economizer also wears out. 1.1.1 Fly ash concentration: A high fly ash concentration indicates a large amount of ash in the flue gas, which increases the number of times that ash particles strike the heated surfaces, thereby exacerbating wear. The diversity of coal types in our country and the uncertainty regarding the coal used in power plants result in the coal burned in many current power plants having a high ash content, exceeding the design values. Some fuels have an ash content as high as 40. The quality of coal deteriorates, with an increase in ash content; meanwhile, the amount of coal used also increases, leading to a sharp rise in the concentration of fly ash in the flue gases and thereby increasing wear on the economizer. 1.1.2 Flue gas velocity The flue gas velocity is the most important factor affecting the wear of the heated surfaces. Some studies indicate that the wear rate is proportional to the 2.3rd power of the flue gas velocity. The higher the flue gas velocity, the more severe the wear on the economizer. The wear amount can even be in an n-th (n＞3) power relationship with the flue gas velocity. The reason can be explained as follows: erosion wear arises from the kinetic energy of the ash particles, and the kinetic energy of a particle is proportional to the square of its velocity. Wear is also related to the ash concentration (which is proportional to the square of the velocity), the frequency factor of ash particle impacts, and the relative velocity of the ash particles with respect to the worn object. If it is approximated that vp≈vg, then the wear amount will be proportional to the cube of the flue gas. An increase in flue gas velocity enhances the effect of the aforementioned factors, leading to rapid erosion and wear; therefore, the higher the flue gas velocity, the greater the value of n. Furthermore, numerical experiments show that when the particle diameter is small, the n value will be large. Finally, it should be noted that although the n value recommended in the boiler thermal calculation standards is 3.3. However, we believe that it is more accurate to use a diameter-based classification method: first determine the erosion wear amount for each particle diameter category, and then calculate a weighted average. 1.1.3 Influence of the economizer structure: Different types and structures of economizers result in varying degrees of wear. (1) Under the same conditions, the wear resistance of tube bundles made of optical tubes, finned tubes, and membrane tubes decreases in that order ； (2) The tube bundle of the economizer suffers less wear when arranged in series rather than in a staggered arrangement ； (3) In the staggered arrangement, the most severely worn tubes are those in the second row; in the aligned arrangement, the most severely worn tubes are those after the fifth row ； (4) The higher the fins of the fin-tube economizer, the more severe the wear. When the fin height is small (h=3 mm), the degree of wear is relatively similar to that of the light tube. Therefore, installing fins of small height is beneficial for wear prevention ； (5) When the membrane economizer is arranged in a staggered pattern, tubes with larger diameters suffer less wear than those with smaller diameters. When designing or modifying a economizer, the type and structure adopted for the economizer should be considered comprehensively. 1.2 Corrosion 1.2.1 Types of economizer corrosion Economizer corrosion includes internal tube corrosion and external tube corrosion. Internal pipe corrosion is a type of oxygen corrosion, also known as oxygen absorption corrosion. It occurs because although boiler feed water has been treated, it still contains a certain amount of oxygen. Oxygen is highly reactive chemically and can react with the iron elements in steel equipment, causing corrosion in such equipment. This results in the formation of iron oxides such as Fe2O3 and Fe3O4, which are what we commonly refer to as rust. Based on the aforementioned principles of oxygen corrosion, when water flows through the tubes of the economizer, the high temperature makes oxygen corrosion highly likely to occur within these tubes, resulting in ulcer-like corrosion pits on their inner walls and posing a threat to the safe operation of the economizer. Oxygen corrosion inside the tubes of the economizer is generally less severe in the high-temperature section than in the low-temperature section, which is the result of the gradual consumption of oxygen in the feed water. External tube corrosion is a type of sulfuric acid corrosion, also known as low-temperature corrosion. It occurs when boiler flue gas passes through the economizer section; due to the low temperature of the economizer tube walls, the sulfuric acid vapor in the flue gas condenses into acidic liquid, which then adheres to the outer surfaces of the economizer tubes, thereby causing acid corrosion to the economizer. External corrosion of the economizer tubes usually occurs only in the low-temperature section. 1.2.2 Cause analysis: The most severe problem facing the economizers in power plant boilers is low-temperature corrosion outside the tubes; therefore, the internal mechanism of this corrosion is discussed in detail. Sulfur in the fuel burns to produce sulfur dioxide; a small portion of it also forms sulfur trioxide, which then reacts with water vapor in the flue gases to create sulfuric acid vapor. When the sulfuric acid vapor in the flue gas cools down to the acid dew point, it condenses into liquid acid. This liquid acid adheres to the fly ash present in the flue gas and settles on the walls of the pipes at the cooling points, thereby causing acid corrosion to the steel pipes in those areas. The acid dew point increases as the concentration of acidic vapors in the flue gas rises; when the content of acidic vapors in the flue gas is 0.005%, the acid dew point can reach 130–150°C. Practice has shown that the higher the acid dew point, the greater the corrosion on the economizer, and in some cases it can even pose a threat to the economizers in the high-temperature section. 1.2.3 Factors affecting low-temperature corrosion in the economizer (1) The coal used in the power plant contains a high level of sulfur. A high sulfur content is an important factor causing corrosion in the economizer. High sulfur and moisture content in the fuel result in a large amount and high concentration of sulfuric acid vapor generated during combustion. This raises the dew point (i.e., condensation temperature) of the acidic vapor in the flue gases. Meanwhile, the wall temperature of the economizer in the substitution zone is relatively low; as a result, the acidic vapor easily condenses on the walls of this economizer, causing corrosion there. (2) Low feed water temperature is a major cause of economizer corrosion. Low feedwater temperature reduces the wall temperature of the economizer; when it falls below the dew point of the acidic gases in the flue gas, these acidic gases cause condensation on the economizer tube walls, where they combine with fly ash and thus lead to continuous corrosion of the economizer tubes. A low feed water temperature has the greatest impact on newly installed economizers. (3) An excessively high excess air coefficient indicates an increased oxygen content in the flue gases, which creates favorable conditions for the formation of sulfur dioxide and sulfur trioxide during combustion; it also has a certain impact on the low-temperature corrosion of the economizer. Based on the above theoretical analysis, on the one hand, a low feedwater temperature results in a lower wall temperature of the economizer tube banks ； On the other hand, high sulfur and moisture content in the fuel, combined with a high excess air coefficient for the fuel, increase the proportion of acidic vapors in the flue gases, leading to an elevation in the dew point of these acidic vapors. The combined adverse effects of these two factors exacerbate the condensation of acidic vapor on the walls of the economizer tubes, leading to corrosion. 1.3 Vibration 1.3.1 The form in which the tube bundle moves and ends up hitting its own feet. Based on the flow of air over the tube bundle, heat exchangers can be divided into three main categories: (1) airflow perpendicular to the centerline of the tubes (transverse flow) ； (2) Gas flow parallel to the tube’s centerline (parallel flow): (3) Gas flow that moves in an S-shaped path around the tube’s centerline (S-shaped flow). During lateral flow, excitation is caused by the separation of Karman vortices in a single tube. In the case of parallel flow, vortices in the airflow are the source that leads to excitation. When the tube starts to lift stones and hit its own foot, the path of the airflow attached to the tube (due to the formation of a critical layer) becomes curved. Thus, the airflow exerts a centrifugal force on the tube, causing it to bend even more. In this way, self-excited vibration is generated between the airflow and the pipe. In S-shaped flow, not only the vortices that form during lateral flow but also those that occur during parallel flow can induce vibrations in the tube. Vibration is induced by the separation of eddies and amplified by those eddies. This vibration mainly occurs in the heat exchanger. 1.3.2 Tube bank vibration in economizers The tube banks in economizers can be divided into two types: in series and out of series. Most vibration accidents occur in a tandem arrangement. For example, in a serious vibration incident at a thermal power plant abroad, occurring in the superheater and economizer flues, the amplitude on the flue walls reached a value of ±0.211 kg/cm2, causing permanent outward deformation of those wall surfaces. (1) In-line tube bundle economizer: In an in-line economizer, the external fluid flows continuously from top to bottom, which causes the temperature of the fluid as well as the sound speed of the medium to change accordingly. The natural frequency f/Hz of the flue gas under transverse restraint is: Equation (1), where b--characteristic width, m; n--harmonic order ； c--flue gas velocity, m/s. As a general rule, the natural frequency determination for flue gas is relatively low. On the other hand, as the flue gas passes across the tube bundle, vortices are generated on the backside, leading to paired separation phenomena (i.e., Karman vortices). As the vortices alternately detach, creating alternating forces on the object’s surface, this may lead to fatigue failure of the structure ； At the same time, vortex separation is also a significant cause of other vibrations that occur. When the vortex shedding frequency coincides with the natural frequency of the transverse flue gas, the vibrations will be further intensified, resulting in very strong vibrations throughout the fluid. The economizer is also forced to experience intense vibrations, resulting in loud noise being emitted into the surrounding environment. (2) Staggered tube bank economizer For a staggered tube bank economizer, the natural frequency of the flue gas vibration is primarily related to the free width of the tube bank, and can be calculated using the following formula: In equation (2), ƒi represents the i-th natural frequency, in Hz ； i--resonance order serial number, i=1, 2, 3… ； T--gas temperature, K ； L--free width of the tube bundle group, m. For misaligned tube bundle coal burners, vibration has its own characteristics. In other words, when the frequencies at which vortices detach are equal, it does not necessarily lead to intense vibrations, mainly because in the flue of the boiler unit, the proportion of sound energy scattered is quite large compared to the energy that drives the vortices. 2. Solutions to overheating and explosion in the economizer: (1) Reasonably control the flow rate of flue gas, reduce the ash content in coal, appropriately control the fineness of coal powder, avoid operating under overload conditions as much as possible, and using anti-wear coatings can effectively prevent wear of the economizer ； (2) Selecting an appropriate number of bends z in the economizer and the gap in the flue gas corridor can reduce the coefficient of velocity non-uniformity kv; moreover, installing comb-shaped tubes as well as protective tiles or curtains can effectively mitigate the impact of the flue gas corridor ； (3) Raising the flue gas temperature, using corrosion-resistant materials, installing equipment for heating cold air, and adopting other types of economizers such as rotary air preheaters can all help avoid or prevent corrosion of the heated surfaces of the economizer ； (4) Making the lateral natural frequencies of the entire economizer different from each other can improve the vibration of the tube bundle ； (5) Replacing the straight tubes with threaded finned tubes not only improves the flow field and enhances heat exchange efficiency, but also effectively reduces ash accumulation in the economizer ； (6) In operation, it is important to strengthen the monitoring of the excess air coefficient in order to prevent the generation of SO2 and SO3; timely maintenance can also avoid tube bursts due to overheating in the economizer.