First, Introduction to Corrosion Protection of Large-Diameter Steel Pipes.
1. Before processing, the steel plate on one side of the inner wall of all large-diameter steel pipes should be derusted until the metal shows a metallic luster (whitening). It should meet the sa2.5 standard required in GB8923-88, “Rust Grades and Rust Removal Grades of Steel Surfaces Before Painting”.
2. The derusted steel should be processed immediately (to prevent secondary rusting). After processing, use a clean cloth to wipe away any residual rust powder on the inner wall of the steel pipe (if secondary rusting is found, perform secondary rust removal to ensure proper painting). Immediately apply two coats of GZ-2 gray new polymer anti-corrosion coating (thick paste type) as a primer, and two coats of sky blue paint as the topcoat, achieving a total paint film thickness of 250-300μm. 3. The primer should be applied before the steel pipe fittings leave the factory. The topcoat should preferably be applied before the steel pipes are installed. Do not paint within 200mm of both ends (where the weld seams are installed) of all steel pipe fittings; apply the topcoat after the pipe fittings are installed.
4. Use only a brush when applying the primer; do not use spraying. The topcoat can be applied by spraying.
5. The painting area should have adequate ventilation to prevent the metal surface from becoming damp. Painting work should not be carried out in adverse weather conditions such as strong winds, dust, or direct sunlight (e.g., temperatures above 70℃, below +5℃, or rainy days). Painting is best performed at temperatures between 10℃ and 25℃ and relative humidity below 70%.
Second, Construction Preparations Before Welding Large-Diameter Steel Pipes.
1. Organize relevant operators, familiarize them with construction drawings and specifications, develop welding procedures, and prepare welding operation instructions.
2. Welders should receive relevant training and possess the corresponding operating qualification certificates.
3. Check whether the roundness and misalignment of the delivered steel pipes meet the relevant technical requirements.
4. Check the surface of the steel pipe sections for defects such as blemishes, cracks, and severe corrosion.
5. Before installation, each large-diameter steel pipe section should be measured and numbered. It is advisable to select steel pipe sections with small diameter differences for welding.
Third, the stages of the mechanical expansion process for large-diameter steel pipes.
1. Press the steel plate into a U-shape in a forming mold, then press it into an O-shape, and then perform internal and external submerged arc welding. After welding, the diameter is usually expanded at the ends or along the entire length, called UOE welded steel pipe; if not expanded, it is called UO welded steel pipe.
2. Roll-bending the steel plate, then performing internal and external submerged arc welding. After welding, the diameter is expanded to an RBE welded steel pipe, or not expanded to an RB welded steel pipe.
3. Forming the steel plate in the order of J-type-C-type-O-type, and then expanding the diameter after welding, called JCOE welded steel pipe or not expanded to JCO welded steel pipe. Among the LSAW welded steel pipes mentioned above, UOE is the most commonly used. The expansion process for large-diameter steel pipes begins with a preliminary rounding stage. The sector blocks open until all sector blocks contact the inner wall of the steel pipe. At this point, the radii of all points within the inner circle of the steel pipe within the step length are almost uniform, and the steel pipe achieves preliminary rounding. The sector blocks reduce their speed from the initial position until they reach the required position, which is the required inner circumference position of the finished pipe. The expansion process for large-diameter steel pipes then progresses to the springback compensation stage. The sector blocks further reduce their speed from the position in stage 2 until they reach the required position, which is the inner circumference position of the steel pipe before springback, as required by the process design. The sector blocks remain stationary at the inner circumference position of the steel pipe before springback for a period of time; this is the pressure holding and stabilization stage required by the equipment and expansion process. Finally, the expansion process for large-diameter steel pipes enters the unloading and return stage. The sector block rapidly retracts from the inner circumference of the steel pipe before the spring-loaded expansion until it reaches the initial expansion position. This is the minimum shrinkage diameter of the sector block required by the expansion process.
Post time: Dec-03-2025