Coated steel pipe

Coated steel pipes can be roughly divided into two categories, namely outer wall coated steel pipes and inner wall coated steel pipes. According to the classification of coatings, there are polyethylene, epoxy, polyvinyl chloride, polyester, phenolic epoxy, and other coated pipes.

It can also be divided into liquid coating tubes and powder-coating tubes as well as coating tubes coated in the form of film and granular plastics.

According to the processing technology, there are cross-type double-layer extrusion method, T-type double-layer extrusion method, Coated tubes such as flow dip coating method, dispersion melting method, an electrostatic spraying method, spraying method, laminating method, coiling method, and heat shrinking method.

Selection of coating materials for large diameter steel pipes

The coating material of large diameter steel pipes is recommended to be lined with solvent-based synthetic resin (mainly epoxy resin) and mortar for the inner wall. The former is used for line pipes and waterway pipes, and the latter is mainly used for waterway pipes.

The outer wall coating is mainly asphalt coating, polyethylene coating, epoxy coating, etc. (for line pipes), and zinc-rich paint or tar epoxy is used for pipe piles. The purpose of the inner wall coating of the pipeline steel pipe is twofold: one is to resist oil and gas corrosion, and the other is to improve the flow efficiency. Due to the large roughness of the inner surface of the steel pipe, the coating can reduce the flow resistance of the medium to reduce power consumption, reduce the transportation cost and increase the distance between the pumping stations to save engineering investment.

In addition to improving the flow efficiency, the inner wall coating of the water pipe does not reduce the quality of drinking water when transporting tap water and does not cause impurities such as dissolved zinc to pollute the water supply. Generally, the large-diameter inner wall is coated with mortar, and the small diameter pipe is coated with plastic after galvanizing.

Epoxy powder coating

The epoxy powder anticorrosive layer is sprayed on the surface of the pre-treated and preheated pipe by electrostatic spraying, and the curing reaction occurs on the surface of the pipe to form a dense anticorrosive layer. The anti-corrosion layer has good corrosion resistance and low environmental pollution, and has the following advantages:

1) The operating temperature range is -60~100 ℃, which is wider than PE;

2) Strong adhesion to the tube body, good resistance to cathodic spalling;

3) Good impact resistance, toughness and temperature resistance, and small welding damage;

4) Good waterproof and electrical insulation;

5) The damage is easy to find and repair.

The main disadvantages are that the membrane is thin, susceptible to trauma, blisters, and high production technology requirements; the equipment is also complex, and the field construction management is difficult.

Polyethylene anticorrosive coating

Polyethylene has good physical and mechanical properties such as impact resistance, high hardness, and chemical stability. And its temperature application range is also relatively wide (adhesive type: -40~60 ℃; close type: -45~80 ℃). Especially in low-temperature conditions, it has better bendability and impact resistance than other materials, so it is widely used in pipelines in cold regions.

The coated polyethylene coating is an extruder extruding hot melt polyethylene onto the surface of the steel pipe to form a plastic shell to prevent corrosion. It is also called polyethylene thermoplastic coating or jacket.

Features of coated steel pipe

(1) The coating of chemically stable coated steel pipe has excellent stability in chemical media, and the range of resistance to alkali and acid is between pHZ~13.5. This basic feature greatly improves the service life of the steel pipe.

(2) Electrical insulation This performance is very obvious for improving the stray current corrosion of pipelines.

(3) Wear resistance The wear resistance of the coating can reach 13~200 cubic sand/m. The coating is as smooth as a mirror, does not breed bacteria or mold, effectively prevents wax and scales, and can increase the conveying capacity. The equivalent sand roughness of the ordinary steel pipe is KO. 07 mm, while the K of the coated steel pipe is 0.012 mm. The surface finish of the steel pipe is high, which can increase the transportation volume by 6-12% and reduce the power consumption of the pipeline.

(4) The impact resistance and the adhesion of the flexible coating have reached the yield limit elongation of the base metal material. The inner wall of the oil drill pipe can withstand the alternating stress of tension, compression, torsion, etc. and the impact of instantaneous tension in the case of stuck drilling. The shear adhesion of some organic coatings can reach more than 400 kg/cm, and the impact resistance can reach 1600 kg·m.

(5) High-temperature resistance. For example, the internal coating of the oil drill pipe can withstand the high temperature of 120~204℃ under the condition of mud circulation and adapt to the working conditions of thousands of meters of formation.

(6) Resistance to temperature changes At present, Japan has developed coated pipes for use in polar alpine regions, which can withstand temperature changes in the range of 60 to 120 ℃.

(7) The cathodic peel resistance of the fused epoxy coatings is almost nonexistent.

Coated steel pipe vs Cement pipe vs Iron pipe vs Galvanized steel pipe

Compared with cement pipes, cast iron pipes, and galvanized steel pipes, plastic-coated steel pipes have the following advantages: high strength, good toughness, easy to form, not easy to crack, less pipeline maintenance; corrosion resistance, not only resistant to long-term corrosion of general water media, Moreover, it is resistant to acid, alkali, salt solution, organic media and soil corrosion, which fundamentally eliminates the harm of pipeline corrosion, and the service life can reach 40-50 years; it is hygienic, and the plastic coating does not absorb water or dissolve harmful impurities, and keep the water transported. The purity of the product; the conveying resistance is small, which reduces the conveying energy consumption.

From the perspective of environmental protection, the steel pipe powder coating production process has no waste slag, waste liquid, exhaust gas discharge, will not pollute the environment, and the material utilization rate is more than 99%; it is corrosion-resistant during use, has a long life, and does not produce harmful impurities. The quality of the conveying fluid, especially drinking water, can be maintained. Therefore, the use of plastic-coated steel pipes is beneficial to ecological and environmental protection.

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