Maintenance method for eliminating under-deposit corrosion of water-cooled wall of boiler

Abstract

The present application discloses a maintenance method for eliminating under-deposit corrosion of a water-cooled wall of a boiler. The method comprises two stages: in the first stage, quickly determining that under-deposit corrosion occurs in a water-cooled wall; and in the second stage, checking an under-deposit corrosion range and thoroughly eliminating under-deposit corrosion. By quickly determining a failure property, thoroughly checking a corrosion range, and rationally planning a pipe replacement scheme, the present application efficiently eliminates systematic damage caused by under-deposit corrosion of the water-cooled wall. The present application not only is conducive to overall planning of various maintenance operations such as pipe cutting, checking, welding and detection, but also greatly reduces the leakage risk of the under-deposit corrosion repeatedly occurring within a short time period.

Description

A maintenance method for eliminating corrosion under scale on boiler water-cooled walls

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202410485828.4, filed on April 22, 2024, entitled "A Maintenance Method for Eliminating Under-Scale Corrosion on Boiler Water-Cooled Walls", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of thermal power generating unit maintenance, and in particular to a maintenance method for eliminating under-scale corrosion on boiler water-cooled walls. Background Technology

[0004] Under-scale corrosion of boiler water-cooled walls stems from a combination of factors, including abnormal feed water quality, excessive scale buildup on tube walls in high-load areas, and poor localized water circulation. Because the corrosion occurs on the inner wall of the water-cooled wall tubes, conventional anti-wear and anti-explosion inspection methods are insufficient for detection. Once corrosion leads to tube rupture, it signifies extensive damage to the water-cooled wall. Limited by knowledge, detection methods, and maintenance cycles, it is very difficult to efficiently and completely eliminate under-scale corrosion of water-cooled walls in a single instance. Often, repeated tube ruptures or even simultaneous leaks at multiple points occur within a short period, posing a significant safety hazard.

[0005] Summary of the Invention

[0006] The purpose of this application is to provide a maintenance method for eliminating under-scale corrosion on boiler water-cooled walls. By quickly determining the nature of the failure, thoroughly investigating the extent of corrosion, and rationally planning a tube replacement scheme, the under-scale corrosion defect on the boiler water-cooled walls can be completely eliminated in one go.

[0007] To achieve the above objectives, this application adopts the following technical solution:

[0008] A maintenance method for eliminating under-scale corrosion in boiler water-cooled walls includes two stages: the first stage is to quickly determine that under-scale corrosion has occurred in the water-cooled walls; the second stage is to investigate the extent of under-scale corrosion and completely eliminate it.

[0009] A further improvement of this application is that the method specifically includes the following steps:

[0010] S1: The water-cooled wall tube burst occurred in the high-load area of ​​the front and rear walls. The burst opening was brittle and cracked, with some of the base material missing. There were hard ulcer-like corrosion products on the inner wall. The tubes around the burst opening were cut open for inspection immediately. A strong flashlight was used for initial inspection, and an endoscope was used to check whether there was similar corrosion on the fire side.

[0011] S2: Metallographic analysis of the bursting opening: Severe decarburization and a large number of intergranular cracks were found on the inner wall side of the water-cooled wall tube, which was further confirmed as under-deposit corrosion damage.

[0012] S3: Determine the pipe cutting area: Burner burnout, pipe wall coking and other issues can easily cause local high temperatures in the water-cooled wall. Determine the pipe cutting area based on the conventional high-load area and the high-temperature sulfur corrosion area.

[0013] S4: Pipe cutting inspection, welding restoration, and weld joint inspection are carried out on a rolling basis. A strong flashlight is used to vertically illuminate the upper and lower cuts for initial inspection, and an endoscope is used for inspection. All pipe sections with corrosion pits are cut off.

[0014] S5: Control the number of tubes cut at one time to avoid overloading and deformation of the furnace due to excessive number of tubes cut at one time. At the same time, it is conducive to the overall coordination of tube cutting, inspection, welding and testing operations.

[0015] S6: Inspection of special locations: For water-cooled wall tubes with complex structures around the burner and water-cooled jacket, cut the tube at the top to open an inspection window and use an endoscope to probe. For areas that cannot be reached by the length of the endoscope wire, use X-ray inspection or high-frequency ultrasonic inspection.

[0016] A further improvement in this application is that the high-powered flashlight uses vertical illumination, and the endoscope cable length is not less than 8m.

[0017] A further improvement of this application is that, in step S2, the metallographic microscope is used at a magnification of not less than 200× to clearly identify decarburization and intergranular cracks in the microstructure.

[0018] A further improvement of this application is that, in step S3, the pipe cutting length is not less than 3m and the width is not less than 4 times the pitch, so as to facilitate pipe panel assembly and reduce fin welding.

[0019] A further improvement of this application is that, in step S4, the pipe cutting should preferably be done by mechanical methods or by heat processing methods such as flame cutting, with a machining allowance of not less than 5mm left at the cut.

[0020] A further improvement of this application is that, in step S5, the number of tubes cut in a single operation is controlled to be between 40 and 60.

[0021] A further improvement of this application is that, in step S6, the frequency of the high-frequency ultrasonic detection probe is not less than 5MHz; when using X-ray detection, attention should be paid to the impact of the high-temperature corrosion zone of the water-cooled wall on image evaluation, and the outer wall should be polished at the same time.

[0022] This application has at least the following beneficial technical effects:

[0023] This application establishes two key stages: defect identification and investigation / elimination. Each stage is further subdivided into two and four work steps, respectively. The first key stage rapidly determines the nature of under-scale corrosion defects through macroscopic morphology and metallographic characteristics, enabling immediate expanded inspection. The second key stage provides specific methods for investigating and eliminating under-scale corrosion defects, ensuring work efficiency and maintenance effectiveness. This application effectively links the two key aspects of defect identification and hazard investigation for under-scale corrosion in water-cooled walls, providing relatively specific operational suggestions on inspection tools, key technical parameters, and process optimization, greatly reducing the risk of recurring under-scale corrosion leaks in water-cooled walls within a short period. Detailed Implementation

[0024] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] This application provides a maintenance method for eliminating under-scale corrosion on boiler water-cooled walls, comprising the following steps: Steps S1-S2 are the first stage, quickly determining that under-scale corrosion has indeed occurred on the water-cooled wall; Steps S3-S6 are the second stage, investigating the scope of under-scale corrosion and completely eliminating it.

[0026] S1: The water-cooled wall tube rupture occurred in the high-load area of ​​the front and rear walls. The rupture site showed brittle cracking, partial loss of the base material, and relatively hard ulcer-like corrosion products on the inner wall. The tubes around the rupture site were immediately cut open for inspection. A strong flashlight was used for initial inspection, and an endoscope was used to check for similar corrosion on the fire-facing side.

[0027] S2: Metallographic analysis of the burst opening: Severe decarburization and numerous intergranular cracks were found on the inner wall of the water-cooled wall tube, which was further confirmed as under-deposit corrosion damage.

[0028] S3: Determine the pipe cutting area: Burner burnout, pipe wall coking, etc. can easily cause local high temperature of water-cooled wall. Determine the pipe cutting area according to the conventional high load area combined with the high temperature sulfur corrosion area.

[0029] S4: Pipe cutting inspection, welding restoration, and weld joint inspection are carried out on a rolling basis. A strong flashlight is used to vertically illuminate the upper and lower cuts for initial inspection, combined with endoscopic inspection. Pipe sections with corrosion pits are cut off.

[0030] S5: Control the number of tubes cut at one time. This avoids overloading and deformation of the furnace due to excessive tube cutting at one time, and also facilitates the overall coordination of tube cutting, inspection, welding, and testing operations.

[0031] S6: Inspection of special locations: For water-cooled wall tubes with complex structures around the burner and water-cooling jacket, shorten the tube at the top and open an inspection window for endoscopic access. For areas inaccessible by wire length, use X-ray inspection or high-frequency ultrasonic inspection.

[0032] Example 1:

[0033] A tube ruptured in the front water-cooled wall of the boiler in Unit #5 of a thermal power plant, causing an unplanned shutdown. Due to the winter heating season and insufficient assessment of the defect's hazard, a second tube rupture occurred in the rear water-cooled wall less than 72 hours after startup. To efficiently and thoroughly eliminate under-scale corrosion defects, the proposed solution includes two key stages: defect identification and investigation / elimination. Each stage is further broken down into two and four work steps, respectively. The first stage rapidly determines the nature of the under-scale corrosion defect through macroscopic morphology and metallographic characteristics. The second stage outlines the specific methods for investigating and eliminating under-scale corrosion defects.

[0034] The vent appeared at the height of the burner in the middle layer of the rear wall of the water-cooled wall. The vent was brittle and cracked, with part of the base material missing, and relatively hard ulcerative corrosion products were present on the inner wall. Ten pipes adjacent to the vent were immediately cut. A strong flashlight was used to illuminate the vent vertically, providing uniform light and a large depth of penetration. Combined with endoscopic examination, it was found that the inner walls of three pipes had severe corrosion on the fire side.

[0035] Metallographic analysis provides a field of view at magnifications ranging from 50× to 1000×, with the range above 200× being more conducive to evaluation. Significant decarburization is observed near the inner wall, and cracks initiate on the inner wall and propagate along the grain boundaries towards the outer wall, exhibiting typical characteristics of under-deposit corrosion.

[0036] Based on the spare parts dimensions, the cut tube length is determined to be 4m, with 2m above and below the rupture site. The endoscope lead wire is 8m long, stacked to reach a total inspection length of 20m, covering and exceeding the range of high load and high temperature corrosion.

[0037] Sixty workers from various trades were assembled on-site and divided into two shifts to carry out emergency repairs. Taking into account workload, furnace wall load-bearing capacity, overlapping operations, and net working time, the work was coordinated to include insulation removal, scaffolding erection, pipe cutting, endoscopic inspection, beveling, welding, and non-destructive testing. Initially, 50 pipes were to be cut at a time, which was adjusted to 60 pipes after two rounds based on the progress on site.

[0038] The burner and water-cooled jacket surrounding water-cooled wall tubes were inspected by cutting open inspection windows at the top. Endoscopic inspection of a length of 6m reached the bend in the tube structure, where further access was impossible. Two water-cooled wall tubes exhibiting under-scale corrosion were selected; X-ray examination of the corrosion area revealed clear images. Using the same method, a 2m section of film was randomly inspected in the endoscopic blind zone. Film number 5 showed a suspected defect, which was confirmed on-site to be high-temperature corrosion of the outer wall.

[0039] Analysis 1: There is a high probability of severe under-scale corrosion in the pipe screens 1-2m above and below the rupture point. The effective inspection length of the strong flashlight vertically is about 2-3m, which can ensure that no one is missed.

[0040] Analysis 2: A total of 352 pipes were replaced on-site. From pipe cutting to the completion of inspection and restoration of all welds, the total time taken was 97 hours, including the time for rectifying any non-conformities during construction. In terms of manpower and overall workload, this is significantly better than the progress of conventional emergency repairs.

[0041] Analysis 3: Common methods for detecting under-deposit corrosion include high-frequency ultrasound, eddy current testing, and thickness measurement, but these methods are insufficient for emergency repairs in terms of accuracy and efficiency. Radiographic testing, on the other hand, has excellent identification capabilities for corrosion defects that have thinned the wall.

[0042] in conclusion:

[0043] Based on Example 1, it can be concluded that:

[0044] 1. The nature of under-deposit corrosion defects can be quickly determined by combining macroscopic morphology with metallographic features;

[0045] 2. The number of pipes cut in a single operation should be matched with the manpower and the number of workers involved. Each pipe panel should ideally have 4 to 6 pipes to prevent difficulties in alignment due to changes in the pipe pitch.

[0046] 3. Radiographic testing has good identification accuracy for corrosion defects that have thinned the wall to a certain extent. However, it should be noted that when there is high-temperature corrosion on the outer wall, its imaging has a certain similarity to that of under-deposit corrosion.

[0047] 4. Ultrasonic testing can be used to inspect the weld joints of water-cooled wall pipes, avoiding the need to stop all other work areas during radiographic testing.

[0048] Although this application has been described in detail above with general descriptions and specific embodiments, some modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, such modifications or improvements made without departing from the spirit of this application are all within the scope of protection claimed in this application.

Claims

1. A maintenance method for eliminating corrosion under scale on boiler water-cooled walls, characterized in that, The method consists of two stages: the first stage is to quickly determine that under-deposit corrosion has occurred on the water-cooled wall; the second stage is to investigate the extent of under-deposit corrosion and completely eliminate it.

2. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 1, characterized in that, The method specifically includes the following steps: S1: The water-cooled wall tube burst occurred in the high-load area of ​​the front and rear walls. The burst opening was brittle and cracked, with some of the base material missing. There were hard ulcer-like corrosion products on the inner wall. The tubes around the burst opening were cut open for inspection immediately. A strong flashlight was used for initial inspection, and an endoscope was used to check whether there was similar corrosion on the fire side. S2: Metallographic analysis of the bursting opening: Severe decarburization and a large number of intergranular cracks were found on the inner wall side of the water-cooled wall tube, which was further confirmed as under-deposit corrosion damage. S3: Determine the pipe cutting area: Burner burnout, pipe wall coking and other issues can easily cause local high temperatures in the water-cooled wall. Determine the pipe cutting area based on the conventional high-load area and the high-temperature sulfur corrosion area. S4: Pipe cutting inspection, welding restoration, and weld joint inspection are carried out on a rolling basis. A strong flashlight is used to vertically illuminate the upper and lower cuts for initial inspection, and an endoscope is used for inspection. All pipe sections with corrosion pits are cut off. S5: Control the number of tubes cut at one time to avoid overloading and deformation of the furnace due to excessive number of tubes cut at one time. At the same time, it is conducive to the overall planning of tube cutting, inspection, welding and testing operations. S6: Inspection of special locations: For water-cooled wall tubes with complex structures around the burner and water-cooled jacket, cut the tube at the top to open an inspection window and use an endoscope to probe. For areas that cannot be reached by the length of the endoscope wire, use X-ray inspection or high-frequency ultrasonic inspection.

3. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, The high-powered flashlight should be used for vertical illumination, and the endoscope cable should be at least 8m long.

4. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, In step S2, the metallographic microscope is used to observe the microstructure at a magnification of no less than 200× in order to clearly identify decarburization and intergranular cracks.

5. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, In step S3, the pipe length is not less than 3m and the width is not less than 4 times the pitch, so as to facilitate the assembly of the pipe screen and reduce the welding of fins.

6. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, In step S4, the pipe cutting should preferably be done mechanically, or by heat processing methods such as flame cutting, with a machining allowance of not less than 5mm left at the cut.

7. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, In step S5, the number of tubes cut in a single operation is controlled to be between 40 and 60.

8. The maintenance method for eliminating under-scale corrosion on boiler water-cooled walls according to claim 2, characterized in that, In step S6, a high-frequency ultrasonic probe with a frequency of not less than 5MHz is used; when using X-ray inspection, attention should be paid to the impact of the high-temperature corrosion zone of the water-cooled wall on image evaluation, and the outer wall should be polished at the same time.