Causes and preventive measures for boiler four-tube leakage
Boiler seamless pipe is a kind of boiler tube, belonging to the category of seamless steel pipe. Seamless boiler tube refers to steel with openings at both ends, hollow cross-section, and a large ratio of length to circumference.
The main pressure-bearing pipes within the boiler body, economizer tubes, water-cooled wall tubes, superheater tubes, and reheater tubes (hereinafter referred to as "four tubes") are one of the important equipment of thermal power plant boilers, and they respectively undertake the important tasks of heating feed water, evaporating feed water, heating steam, and heating work steam.
Causes of boiler four-tube leakage
1. Wear
Mechanism of wear: The flue gas of the pulverized coal furnace carries a large amount of fly ash particles. These fly ash particles have a certain kinetic energy. When the flue gas flushes the heating surface, the fly ash particles continuously flush the tube wall, and each time they shave extremely small metal chips from the tube. Over time, the tube wall will become thinner, which is wear.
There are many factors that affect the wear of the heating surface, mainly the following aspects:
(1) Fly ash velocity: The amount of wear is proportional to the cube of the fly ash velocity. For every doubling of the flue gas flow rate, the amount of wear increases by 7 times.
(2) Fly ash concentration: As the fly ash concentration increases, the number of fly ash impacts increases, which intensifies the wear.
(3) Ash particle characteristics: The coarser and harder the ash particles are, the more severe the wear is. The increase in carbon content in fly ash will also intensify the wear, because the hardness of coke in the ash is higher than that of ash particles.
(4) Fly ash impact rate: If the fly ash particles are large, the specific gravity is high, the flue gas velocity is fast, and the flue gas viscosity is low, the fly ash will have more chances to impact and the wear will be serious.
2. High temperature corrosion
High temperature corrosion mainly occurs in the water-cooled wall area of the furnace. The corrosion site is usually above and below the burner. The power plant with concentrated primary air is more serious, and the water-cooled wall pipes facing the fire side are more serious. The high temperature corrosion of the water-cooled wall is mainly due to the formation of a molten ash layer on the surface of the pipe. The local high concentration of H2S has a great influence on the corrosion. The boiler burns in the absence of oxygen, and a strong reducing atmosphere is formed in the main combustion area. HCL aggravates the damage to the oxide film. The furnace power field is not good, and the diameter of the airflow cut circle in the furnace is too large, resulting in serious airflow sticking to the wall. These are all the reasons for the high temperature corrosion and burst of the water-cooled wall.
3. Unsmooth water circulation causes operation problems
In power plant boilers, one of the common problems that cause four-tube leakage is unsmooth water circulation. The root cause is that the power plant stays at low load for a long time, resulting in unsmooth water circulation. There are many reasons that lead to low load in power plants, which is mainly determined by the economic development situation. At the same time, in the northern region of my country, due to the long period of water freezing, the water-cooled wall will often be unsmooth. Because the unsmooth water circulation is a very subtle phenomenon, it is also very difficult to find in the maintenance process of actual equipment. Therefore, this problem will cause problems in the operation of the boiler to a large extent.
4. Mechanical damage caused by improper maintenance by maintenance personnel
During the maintenance process, due to the negligence of maintenance personnel or improper use of maintenance tools, the metal surface is injured, the strength is reduced, and the equipment cannot withstand the pressure of the medium and bursts; when sawing the pipeline during maintenance, other pipes are accidentally injured and not discovered in time, or not handled in time; welding pipes are not strictly carried out in accordance with the welding process operating procedures or heat treatment is not handled properly, which are all mechanical damage. As long as the maintenance personnel are responsible and supervise well during maintenance, and the damaged pipes are handled in time, this type of leakage can be completely avoided.
5. Unscientific boiler design
Based on the author's actual experience, it is known that unscientific boiler design in power plants is the key reason for boiler water wall leakage. This is because under huge pressure, the welding points of the water wall are very likely to leak because they cannot withstand huge pressure. Under normal circumstances, the root cause of this phenomenon is the unscientific boiler design. The specific weight of the cold water wall pipeline has not been given enough attention, resulting in a relatively large load at the hanging point, causing one hanging point to be unable to withstand its pressure, and then leakage.
Control measures to reduce four-pipe leakage accidents
1. Improve anti-wear and anti-corrosion measures, and adopt new technologies and methods to increase the service life of equipment
For example, the wear of water-cooled wall tubes is generally more serious. The grid anti-wear warp and weft structure technology can be used to install wear-resistant alloy plates on the surface of the water wall in the horizontal and vertical directions, forming a stepped grid-like protective barrier on the heating surface to block the formation of high-speed wall flow. The anti-wear effect in the protection area is outstanding, which can significantly solve the wear problem of CFB boiler water wall.
2. Strengthen operation management
Improve over-temperature monitoring during operation of heating surfaces, strengthen technical analysis of over-temperature phenomena, and strengthen technical guidance for operators. Operators are required to correctly handle the relationship between safety and efficiency, master information such as coal, equipment, and operation methods in advance, and strengthen combustion adjustments. Strictly control the phenomena of excessively fast temperature rise and fall, over-temperature and over-pressure operation, and unstable combustion during the start-up and shutdown, water pressure test, and operation of the unit, and strengthen supervision of water quality and coal quality. It is recommended that each plant conduct over-temperature statistics and incorporate over-temperature and steam temperature airborne monitoring into the metal technical supervision system in order to strengthen technical supervision and management.
(1) For economizers, focus on checking the flue gas corridors that are prone to wear; check whether the guide plates, anti-wear covers, anti-wear tiles are intact and the pipe discharge arrangement; check the pipe wear behind the flue gas damper temperature control device.
(2) For water-cooled walls, check the high-temperature corrosion and thinning of the water-cooled walls near the burner; check the fin welds, pipe diameter expansion and scaling of the water-cooled walls, and leave a certain expansion margin on both sides of the fins between the two medium temperatures.
(3) For superheaters and reheaters, focus on checking the corner welds of the short pipe joints with the headers. Improvements should be made to those with obviously unreasonable structures.
(4) Check for overheating phenomena such as scaling, high-temperature oxidation corrosion, and pipe diameter expansion in high-load areas.
(5) Check whether the tube row positioning clamping tubes are loose.
(6) In the convection zone, the heating surface should be checked for possible formation of flue gas corridors and tube wall wear in areas with concentrated ash concentrations. Untidy tube rows or out-of-line phenomena must be eliminated and straightened.
(7) Check for damage to pipes near the soot blower.
(8) Check whether the positioning screws of the desuperheater or the root of the flute-shaped water spray pipe are cracked.
4. Actively train maintenance operators
Excellent boiler maintenance operators 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, so as to improve their maintenance and overhaul technical level of power plant boilers and enhance the maintenance technical operation ability of boiler maintenance personnel. Under normal circumstances, technical personnel with rich experience, superb skills and deep qualifications should be hired to carry out technical training and explanations for boiler maintenance personnel.
If conditions permit, power plants can also actively cooperate with relevant colleges and universities, so as to use the rich resources of colleges and universities to improve the practical experience and theoretical level of maintenance operators.
5. Perfecting the design and appropriately increasing the safety margin during design is the fundamental measure to prevent and control the explosion and leakage of "four pipes"
If the layout of the heating surface, the thermal strength of the furnace, steel, cross mixing and expansion system are considered reasonably during the design, the occurrence of explosion and leakage will be greatly reduced. For example, the domestically imported 300MW and 600MW units that have been put into production in recent years have made modifications to the boiler design in the design, such as increasing the furnace height, selecting a lower furnace volume heat load, improving the steel grade, increasing the number of cooling stages and points, and reducing the smoke velocity of the tail heating surface, etc. Therefore, their explosions and leaks are relatively rare.
Summary
In the process of thermal power generation safety production, the safe operation of boiler equipment plays a decisive role. Since the boiler four-tube leakage and tube burst accidents must be stopped to eliminate the fault, the safe operation of the boiler four-tube is the top priority of the entire power plant safety production.