Boiler spray anti-corrosion coating
Circulating fluidized bed boilers with water-cooled wall tubes arranged in the furnace generally experience wear of the water-cooled wall tubes. The speed of wear and tear is related to factors such as different furnace designs, coal combustion types, and combustion adjustments, and some wear and tear can be quite severe. Ensure quality control in every aspect related to spraying. Checking the wear condition of the pipe wall, developing repair welding processes, selecting metal wear-resistant layers, rough treatment of the pipe wall before spraying, and quality inspection and acceptance after spraying are key steps in quality control during the spraying of water-cooled walls in the furnace.
1. On site inspection of wear and tear condition
When planning to shut down for maintenance, or during temporary repairs due to leakage or bursting of water-cooled wall tubes, the wear and tear of the water-cooled wall tubes should be checked; Checking the wear condition of the pipe wall is the first step in the anti-wear spraying process inside the furnace. A spraying plan should be developed based on the different wear conditions of each boiler, which can be applied over a large area or for individual repairs.
2. Pre spray treatment
After inspection, if there is wear on the water-cooled wall tube, it should be treated before spraying according to the wear condition. The processing principle of Huaneng power auxiliary equipment is: when the wear surface is thinned evenly without local pits, and the wall thickness measurement is greater than the theoretical strength calculation value, thermal spraying can be carried out after surface roughness treatment; If the wear surface is severely worn, the thinning is relatively uniform, the area is large, and the wall thickness is less than the theoretical strength calculation value, then pipe replacement treatment should be carried out (as long as the inspection is timely, this situation can be avoided); For locally worn pits, repair welding should be done first, followed by spraying; When the sprayed metal wear-resistant layer wears out new pits after long-term operation, it is also repaired by welding before spraying. In short, the water-cooled wall surface (including the fin wall surface) of the circulating fluidized bed boiler should be flat, and any protruding or concave parts will accelerate wear.
3. Repair welding before spraying
Due to the fact that the water-cooled wall tubes of circulating fluidized bed boilers are made of high-quality carbon steel pipes such as 20G, they have good welding performance. At the same time, the pits or surfaces that require repair welding are relatively small, so repair welding can achieve good results.
4. Sandblasting and polishing:
The surface of the substrate before spraying must be clean, free of oil stains, and meet the requirements of cleanliness and roughening. The purpose of sandblasting is to give the surface of the water-cooled wall tube a grayish white metallic appearance and uniform roughening. After sandblasting, the surface roughness of the substrate should reach Rz40~80um. And it is dry, dust-free, oil-free, oxygen free, and rust free. To carefully clean and effectively roughen the surface, in order to improve the bonding strength of the spray coating.
5. Anti corrosion spraying:
Spray the water-cooled wall using supersonic arc spraying technology, using a specialized 45CT spray material with a designed spray thickness of 0.8-1.0mm. The sprayed surface achieves uniformity and density. After sandblasting, the water-cooled wall tubes should be sprayed as soon as possible, with a shorter interval time. In sunny or non humid weather, the interval time should not exceed 12 hours, and in rainy, humid or salt spray environments, the interval time should not exceed 2 hours.
According to the operating conditions of circulating fluidized bed boilers, the anti abrasion thermal spray coating for water-cooled wall tubes should meet the following requirements:
a. The coating must have a good bond with the substrate to prevent detachment due to physical and chemical effects under operating conditions.
b. The coating should have a coefficient of thermal expansion that is generally consistent with the substrate.
c. The coating should not be severely insulated, and the sprayed coating should not affect the normal heat exchange function of the water-cooled wall tube.
d. The coating should not damage the substrate, especially when the workpiece is heated to high temperatures.
e. The coating should have good wear resistance, heat resistance, and oxidation resistance under operating conditions.
3、 Main physical and chemical indicators of boiler anti-corrosion spraying:
Thickness: 0.6-0.8mm
Particle velocity: 386m/s
Combined strength: ≥ 55Mpa
Hardness: HRC65
High temperature oxidation resistance:+12.16mg/cm2 (oxidation at 750 ℃ for 250h)
Porosity: ≤ 0.9%
Thermal conductivity: 45-55W/m2 · k
Linear expansion coefficient: 9.5-11.5 × 10-6/k
4、 Precautions for boiler anti-corrosion spraying:
a. A dedicated person is responsible for monitoring the wire feeding tray and wire feeding situation to ensure smooth wire feeding, prevent wire material from getting tangled, and ensure absolute insulation between the two wires to avoid short circuits.
b. If unstable wire feeding, fuse breakage, or intermittent arc is found during the spraying process, the use of the conductive nozzle should be checked first, and the installation of the wire guide tube should be checked again for firmness. If it is loose, it should be tightened in a timely manner.
c. Conductive nozzles burned during the spraying process should be replaced in a timely manner, and conductive nozzles with processing accuracy exceeding the standard are prohibited from use.
d. When spraying, it is necessary to avoid artificially dividing the coating that can be formed in one go into upper and lower zones for the convenience of work, and to increase the bonding transition zone.
Standard:
Visual inspection: After spraying, the coating should cover evenly and the surface should be free of pitting, peeling, cracking, bulging, and peeling. The surface should be silver white.
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