What are the methods for internal anti-corrosion treatment of large-diameter spiral welded steel pipes

Cement mortar coating should generally be applied after the spiral welded steel pipe has been laid, pressure tested, and backfilled and compacted according to the design requirements. This is because spiral welded steel pipes have low rigidity and thin walls, and must be kept stable during coating application. If the anti-corrosion coating is applied before laying the pipe, significant deformation during lifting, transportation, and backfilling can damage the anti-corrosion layer.

Before coating application, the inner wall of the spiral welded steel pipe must be cleaned to remove loose rust, dirt, scale, grease, welding slag, and other adhering substances. The protrusion of the inner wall of the spiral welded steel pipe should not exceed 1/3 of the designed thickness of the anti-corrosion layer. The vertical deformation of the pipe should not exceed the design specifications and should not exceed 2% of the inner diameter of the spiral welded steel pipe.

The cement mortar internal anti-corrosion layer can be applied using mechanical spraying, manual troweling, drag-tube application, or centrifugal prefabrication. If prefabrication is necessary for the internal anti-corrosion layer, protective measures should be taken during transportation, installation, and backfilling.

During construction, cement mortar should be mixed first, with a cement-to-sand ratio of 1:(1-2). The slump of the cement mortar should be 60-80 mm, and its compressive strength should not be less than 30 MPa.

The cement mortar internal anti-corrosion layer applied by mechanical spraying has a smooth, dense surface, uniform thickness, and good effect. Mechanical spraying began to be used abroad in the 1930s, and in China, it has been used in cities such as Shanghai, Qingdao, and Dalian since the 1960s. When using mechanical spraying, elbows, tees, special fittings, and pipe sections near gate valves can be manually coated, with a smooth transition section connecting to the mechanically sprayed material.

In addition, drag-tube or centrifugal prefabrication methods are also widely used. Regardless of the construction method, the maintenance process is a crucial step in ensuring that the anti-corrosion layer of the spiral steel pipe within the cement mortar does not crack or become hollow. Therefore, after the cement mortar anti-corrosion layer is formed, it is necessary to immediately seal the pipe to prevent air convection; after final setting, moist curing should be carried out. Generally, the curing time for silicate cement should not be less than 7 days, and for slag silicate cement, it should not be less than 14 days; before water is supplied, the pipe should be continuously sealed and kept moist.