Seamless boiler tubes refer to steel materials that are open at both ends and have hollow sections, with a large ratio between their length and the periphery.
The main pressure-bearing components within the boiler body, including economizer tubes, water wall tubes, superheater tubes, and reheater tubes (hereinafter referred to as "the four tubes"), are one of the important equipment of the boiler in thermal power plants. They respectively undertake the important tasks of heating feedwater, evaporating feedwater, heating steam, and heating power-generating steam.
In normal operation, if there is a leak in the "four tubes" of the boiler, it is necessary to take forced shutdown for emergency repair. The leakage severely affects the normal production of the thermal power plant and poses a great threat to the power plant and the power grid, resulting in direct economic losses of hundreds of thousands to millions of yuan. The direct and indirect losses to users caused by tripping and power outages are even greater. The leakage of the "four tubes" severely threatens the safe and full power generation of the thermal power plant units, and the hidden dangers of the "four tubes" are a major hidden danger for the thermal power plant units.
In recent years, as coal prices have risen and the cost of power generation has continued to increase, the survival of thermal power plants has been continuously impacted, and higher requirements have been placed on the safe operation of thermal power plants. Preventing leaks in the boiler's "four tubes" is of great significance for improving the economic benefits of the enterprise, establishing the image of the enterprise, and increasing the competitiveness of the enterprise.
There are many factors that affect the wear of the heated surface, mainly the following aspects:
① Fly ash velocity: The amount of wear is directly proportional to the cube of the fly ash velocity. If the flue gas velocity is doubled, the amount of wear will increase by seven times.
② Fly ash concentration: The higher the fly ash concentration, the more frequent the impact of fly ash, which exacerbates the wear.
③ Characteristics of ash particles: The coarser and harder the ash particles, the more severe the wear. An increase in the carbon content in the fly ash will also exacerbate the wear because the hardness of coke in the ash is higher than that of the ash particles.
④ Fly ash impact rate: The larger the fly ash particles, the greater the mass, the faster the flue gas velocity, and the smaller the flue gas viscosity, the more opportunities for fly ash to impact, and the more severe the wear.
High-temperature corrosion mainly occurs in the furnace water wall area, and the corrosion parts are usually above and below the burner. The situation is more serious in power plants with concentrated primary air distribution, and the water wall tubes on the fire side are more severe. The main reason for high-temperature corrosion of the water wall is the formation of a molten ash layer on the surface of the tubes. Local high concentrations of hydrogen sulfide have a great impact on corrosion. Oxygen-deficient combustion in the boiler, a strong reductive atmosphere in the main combustion area, HCl accelerates the destruction of the oxide film, poor furnace dynamic field, large furnace air flow cut circle diameter, leading to serious air flow attachment to the wall. These are all reasons for the high-temperature corrosion and tube rupture of the water wall.
In power plant boilers, one of the common problems causing leakage of the "four tubes" is poor water circulation. The root cause is that the power plant operates at low load for a long time, resulting in poor water circulation. There are many reasons for the low load of the power plant, which is mainly determined by the state of economic development. Because poor water circulation has a very subtle phenomenon, it is also very difficult to detect during the actual equipment maintenance process, so this issue will greatly lead to problems in the operation of the boiler.
During maintenance, due to the negligence of maintenance personnel or the improper use of maintenance tools, the metal surface is injured, and the strength is reduced, causing the equipment to not withstand the pressure of the medium and resulting in tube rupture; during the cutting of pipes during maintenance, if other pipes are accidentally injured and not found in time, or not timely treated; or if the welding of pipes is not carried out in strict accordance with the welding process operation regulations or the heat treatment is not properly handled, it all belongs to mechanical damage. As long as the maintenance personnel are responsible and supervise during maintenance, and promptly deal with the damaged pipes, this kind of leakage can be completely avoided.
Unscientific boiler design is a key reason for the leakage of boiler water walls. This is because under the action of great pressure, the welding parts of the water wall cannot withstand the great pressure and are prone to leakage. Under normal circumstances, the fundamental reason for this phenomenon is the unscientific design of the boiler, which does not give sufficient attention to the specific weight of the cold water wall pipes, resulting in a larger load at the suspension point, causing a suspension point to not withstand the pressure, and thus causing a leakage phenomenon.
Enhance the monitoring of overheating during the operation of the heated surface, strengthen technical analysis of overheating phenomena, and strengthen technical guidance for operators. Operators are required to correctly handle the relationship between safety and benefits, to grasp information such as coal, equipment, and operating methods in advance, and to strengthen combustion adjustment. Strictly control phenomena such as too fast temperature rise and fall, overheating and overpressure operation, and unstable combustion during the start-up, shutdown, water pressure testing, and operation of the unit, while also strengthening the supervision of water quality and coal quality. It is recommended that each plant conduct overheating statistics and include overheating and steam temperature sudden drop monitoring in the metal technical supervision system to strengthen technical supervision and management.
① For economizers, focus on checking the flue gas corridor positions that are prone to wear and tear;
Excellent boiler maintenance personnel can effectively reduce the probability of boiler leakage in power plants. Therefore, power plants should actively provide scientific and reasonable training for boiler maintenance personnel to improve their maintenance and repair technology level for power plant boilers and enhance the technical operation ability of boiler maintenance personnel. Under normal circumstances, it is necessary to invite experienced and highly skilled technical personnel with deep qualifications to carry out technical training and explanation for boiler maintenance personnel. If conditions permit, power plants can also actively cooperate with relevant colleges and universities to use the rich resources of colleges and universities to improve the practical experience and theoretical level of maintenance personnel.
(5)Improving design and increasing safety margins during the design phase are fundamental measures to prevent "four-pipe" ruptures and leaks.
If the layout of the heat-absorbing surface, furnace thermal intensity, steel material, cross-mixing, and expansion systems are reasonably considered in the design, the occurrence of ruptures and leaks can be greatly reduced. In recent years, many domestically introduced 300MW and 600MW units have been put into operation, and in these designs, manufacturers have made modifications to the boiler design. For example, increasing the furnace height, selecting a lower furnace volume heat load, improving the grade of steel material, increasing the number of temperature reduction stages and points, and reducing the flue gas speed of the rear heat-absorbing surface have been implemented. As a result, these units have relatively fewer incidents of ruptures and leaks.
In the process of safe production of thermal power generation, the safe operation of boiler equipment plays a decisive role. Since boiler "four-pipe" leaks and ruptures require shutdown for repairs, the safe operation of the boiler "four-pipes" is crucial to the overall safety production of the power plant. There are no shortcuts for boiler "four-pipe" inspection; only by genuinely investing a lot of manpower, material resources, and energy, and adopting an attitude that all necessary repairs must be made and done well, can we ensure the safety of the boiler "four-pipe" equipment. Although it is difficult to completely avoid "four-pipe" leaks for power plant boilers, preventive measures can minimize them. By understanding the basic patterns of "four-pipe" leaks and taking targeted measures, the reliability of boiler operation can be greatly improved.
The main pressure-bearing components within the boiler body, including economizer tubes, water wall tubes, superheater tubes, and reheater tubes (hereinafter referred to as "the four tubes"), are one of the important equipment of the boiler in thermal power plants. They respectively undertake the important tasks of heating feedwater, evaporating feedwater, heating steam, and heating power-generating steam.
In normal operation, if there is a leak in the "four tubes" of the boiler, it is necessary to take forced shutdown for emergency repair. The leakage severely affects the normal production of the thermal power plant and poses a great threat to the power plant and the power grid, resulting in direct economic losses of hundreds of thousands to millions of yuan. The direct and indirect losses to users caused by tripping and power outages are even greater. The leakage of the "four tubes" severely threatens the safe and full power generation of the thermal power plant units, and the hidden dangers of the "four tubes" are a major hidden danger for the thermal power plant units.
In recent years, as coal prices have risen and the cost of power generation has continued to increase, the survival of thermal power plants has been continuously impacted, and higher requirements have been placed on the safe operation of thermal power plants. Preventing leaks in the boiler's "four tubes" is of great significance for improving the economic benefits of the enterprise, establishing the image of the enterprise, and increasing the competitiveness of the enterprise.
1. Reasons for leakage of the boiler's "four tubes"
(1) Erosion
The mechanism of erosion: The flue gas in coal-fired boilers contains a large number of fly ash particles, which have a certain amount of kinetic energy. When the flue gas scours the heated surface, the fly ash particles continuously scour the tube wall, removing an extremely small piece of metal from the tube each time. Over time, this will cause the tube wall to thin, which is erosion.There are many factors that affect the wear of the heated surface, mainly the following aspects:
① Fly ash velocity: The amount of wear is directly proportional to the cube of the fly ash velocity. If the flue gas velocity is doubled, the amount of wear will increase by seven times.
② Fly ash concentration: The higher the fly ash concentration, the more frequent the impact of fly ash, which exacerbates the wear.
③ Characteristics of ash particles: The coarser and harder the ash particles, the more severe the wear. An increase in the carbon content in the fly ash will also exacerbate the wear because the hardness of coke in the ash is higher than that of the ash particles.
④ Fly ash impact rate: The larger the fly ash particles, the greater the mass, the faster the flue gas velocity, and the smaller the flue gas viscosity, the more opportunities for fly ash to impact, and the more severe the wear.
(2) High-temperature corrosion
High-temperature corrosion mainly occurs in the furnace water wall area, and the corrosion parts are usually above and below the burner. The situation is more serious in power plants with concentrated primary air distribution, and the water wall tubes on the fire side are more severe. The main reason for high-temperature corrosion of the water wall is the formation of a molten ash layer on the surface of the tubes. Local high concentrations of hydrogen sulfide have a great impact on corrosion. Oxygen-deficient combustion in the boiler, a strong reductive atmosphere in the main combustion area, HCl accelerates the destruction of the oxide film, poor furnace dynamic field, large furnace air flow cut circle diameter, leading to serious air flow attachment to the wall. These are all reasons for the high-temperature corrosion and tube rupture of the water wall.
(3) Poor water circulation
In power plant boilers, one of the common problems causing leakage of the "four tubes" is poor water circulation. The root cause is that the power plant operates at low load for a long time, resulting in poor water circulation. There are many reasons for the low load of the power plant, which is mainly determined by the state of economic development. Because poor water circulation has a very subtle phenomenon, it is also very difficult to detect during the actual equipment maintenance process, so this issue will greatly lead to problems in the operation of the boiler.
(4) Mechanical damage
During maintenance, due to the negligence of maintenance personnel or the improper use of maintenance tools, the metal surface is injured, and the strength is reduced, causing the equipment to not withstand the pressure of the medium and resulting in tube rupture; during the cutting of pipes during maintenance, if other pipes are accidentally injured and not found in time, or not timely treated; or if the welding of pipes is not carried out in strict accordance with the welding process operation regulations or the heat treatment is not properly handled, it all belongs to mechanical damage. As long as the maintenance personnel are responsible and supervise during maintenance, and promptly deal with the damaged pipes, this kind of leakage can be completely avoided.
(5) Unscientific boiler design
Unscientific boiler design is a key reason for the leakage of boiler water walls. This is because under the action of great pressure, the welding parts of the water wall cannot withstand the great pressure and are prone to leakage. Under normal circumstances, the fundamental reason for this phenomenon is the unscientific design of the boiler, which does not give sufficient attention to the specific weight of the cold water wall pipes, resulting in a larger load at the suspension point, causing a suspension point to not withstand the pressure, and thus causing a leakage phenomenon.
2. Control measures to reduce boiler tube leakage accidents
(1) Improve anti-wear and anti-corrosion measures, and use new technologies and methods to increase the service life of equipment
For example, add wear-resistant guard plates on both sides of the economizer where it penetrates the wall, and supplement and improve the wear-resistant cover plates on the tubes to enhance the anti-wear effect; in the combustion area of the water wall tubes, especially at the weld positions, the high-temperature corrosion is serious. To prevent high-temperature corrosion, use new anti-corrosion materials, eliminate the previous infiltration aluminum and other anti-corrosion technologies, and also solve the problem of high-temperature corrosion that is prone to occur at weld positions.
(2) Strengthen operation management
Enhance the monitoring of overheating during the operation of the heated surface, strengthen technical analysis of overheating phenomena, and strengthen technical guidance for operators. Operators are required to correctly handle the relationship between safety and benefits, to grasp information such as coal, equipment, and operating methods in advance, and to strengthen combustion adjustment. Strictly control phenomena such as too fast temperature rise and fall, overheating and overpressure operation, and unstable combustion during the start-up, shutdown, water pressure testing, and operation of the unit, while also strengthening the supervision of water quality and coal quality. It is recommended that each plant conduct overheating statistics and include overheating and steam temperature sudden drop monitoring in the metal technical supervision system to strengthen technical supervision and management.
(3) Strengthen boiler inspection and anti-wear and explosion-proof inspection
① For economizers, focus on checking the flue gas corridor positions that are prone to wear and tear;
② Check whether the guide vanes, wear-resistant covers, and wear-resistant tiles are intact and the tube arrangement;
③ Check the wear of tubes behind the flue gas damper temperature regulation device.
④ For water walls, check the high-temperature corrosion thinning of water walls near the burners;
⑤ Check the fin welds, tube diameter swelling, and scaling of the water walls, leaving a certain expansion margin on both sides of the fins for tubes with two different medium temperatures.
⑥For superheaters and reheaters, focus on checking the corner welds at the junctions with the headers, and improve those with obvious unreasonable structures;
⑦ Check for scaling, high-temperature oxidation corrosion, and tube diameter swelling in high-load areas; whether the tube row positioning clamps are loose;
⑧ Check the heat-absorbing surface in the convection area for possible formation of flue gas corridors and concentrated ash areas, and eliminate and align any uneven or out-of-line tube rows;
⑨ Check the damage to tubes near the soot blowers;
⑩ Check whether the positioning screws of the attemperators or the roots of the trumpet-shaped spray pipes are cracked.
(4) Actively train maintenance personnel
Excellent boiler maintenance personnel can effectively reduce the probability of boiler leakage in power plants. Therefore, power plants should actively provide scientific and reasonable training for boiler maintenance personnel to improve their maintenance and repair technology level for power plant boilers and enhance the technical operation ability of boiler maintenance personnel. Under normal circumstances, it is necessary to invite experienced and highly skilled technical personnel with deep qualifications to carry out technical training and explanation for boiler maintenance personnel. If conditions permit, power plants can also actively cooperate with relevant colleges and universities to use the rich resources of colleges and universities to improve the practical experience and theoretical level of maintenance personnel.(5)Improving design and increasing safety margins during the design phase are fundamental measures to prevent "four-pipe" ruptures and leaks.
If the layout of the heat-absorbing surface, furnace thermal intensity, steel material, cross-mixing, and expansion systems are reasonably considered in the design, the occurrence of ruptures and leaks can be greatly reduced. In recent years, many domestically introduced 300MW and 600MW units have been put into operation, and in these designs, manufacturers have made modifications to the boiler design. For example, increasing the furnace height, selecting a lower furnace volume heat load, improving the grade of steel material, increasing the number of temperature reduction stages and points, and reducing the flue gas speed of the rear heat-absorbing surface have been implemented. As a result, these units have relatively fewer incidents of ruptures and leaks.
