Introduction to anti-corrosion of the inner wall of large-diameter steel pipes and preparations before welding

Introduction of anti-corrosion on the inner wall of large-diameter steel pipes:

1. Before processing all steel pipes, the steel plate on the inner wall side of the large-diameter steel pipe shall be derusted until the metal is exposed to metallic luster (whitening). It should meet the sa2.5 standard required in GB8923-88 of "Steel Surface Corrosion Grade and Rust Removal Grade Before Painting".

2. The steel after de-rusting should be processed immediately (to prevent secondary rusting). After the processing is completed, use a clean rag to wipe off the residual rust powder on the inner wall of the pipe (if secondary rust is found, perform secondary rust removal to ensure painting), and use GZ-2 gray new polymer in time Anti-corrosion paint (high-build type) is used as a primer twice, and the top coat is painted with sky blue paint twice so that the total thickness of the paint film reaches 250-300μm.

3. The primer should be finished before the pipe fittings leave the factory, and the top coat should be painted before the pipe installation. All pipe fittings should not be painted within 200mm of both ends (installation welds) and should be painted after the pipe fittings are installed.

4. You only need to use a brush when applying the primer, and the spraying method is not allowed. The spraying method can be used for the top coat.

5. Ventilation conditions should be provided in the painting site to prevent the metal surface from getting wet, and the site should not have strong winds, dust, hot sun exposure, and other bad weather, (such as high temperature > 70 ℃, or too low < + 5 ℃ rainy days, etc.) Do not carry out painting work, it is best to paint at 10°C-25°C, and the relative humidity is below 70%.

Construction preparations before welding of large-diameter steel pipes:

1. Organize relevant operators, familiarize themselves with construction drawings and specifications, formulate welding procedures, and prepare welding operation instructions.

2. Welders should be trained for corresponding items and have operation qualification certificates.

3. Check whether the roundness and misalignment of the incoming steel pipes meet the relevant technical requirements.

4. Check whether there are scars, cracks, serious corrosion, and other defects on the surface of metal pipe joints.

5. Before the installation of large-diameter steel pipes, the pipe joints should be measured and numbered one by one, and it is advisable to use pipe joints with small diameter differences for butt welding.

Several stages of the mechanical expansion process of large-diameter steel pipes:

The steel plate is first pressed into a U shape in the forming die, and then pressed into an O shape, and then internal and external submerged arc welding is performed. After welding, the diameter is usually enlarged at the end of the entire length, which is called UOE welded pipe, and the one without diameter expansion is called UO welded pipe. . The steel plate is rolled into shape (RollBending), and then internal and external submerged arc welding is carried out. After welding, the diameter is expanded into RBE welded pipe or RBE welded pipe without expansion. The steel plate is formed in the order of J type-C type-O type, and after welding, the diameter is expanded into JCOE welded pipe or JCO welded pipe without diameter expansion. Among the above-mentioned LSAW welded pipes, UOE is commonly used. The diameter expansion process of large-diameter steel pipes is first a preliminary full circle stage. The fan-shaped blocks are opened until all fan-shaped blocks touch the inner wall of the steel pipe. At this time, the radii of all points in the inner circular pipe of the steel pipe within the step range are almost the same, and the steel pipe obtains a preliminary full circle. The fan-shaped block starts to reduce the moving speed from the front position until it reaches the required position, which is the required position of the inner circumference of the finished tube. The expansion process stage of large-diameter steel pipe is the spring back compensation stage. The fan-shaped block starts to lower the speed at the position of the second stage until it reaches the required position, which is the position of the inner circumference of the steel pipe before spring back is required by the process design. The fan-shaped block remains stationary for a period of time at the inner circumference of the steel pipe before springing back, which is the pressure-holding and stable stage required by the equipment and diameter-expanding process. Afterward is the unloading return stage of the diameter expansion process of large-diameter steel pipes. The fan-shaped block retracts rapidly from the inner circumference of the steel pipe before springing back until it reaches the initial diameter expansion position, which is the minimum shrinkage diameter of the fan-shaped block required by the diameter expansion process.