Key technologies for coating thick walled steel pipes with 3PE

1. Preface

With the development of the pipeline industry, the improvement of sheet metal smelting technology, and the change in design concept based on pipeline stiffness, large-diameter thick walled steel pipes have been widely used in the construction of long-distance transportation pipelines.

For thick walled pipes, the application and curing of FBE coating require a stable temperature field to fully cure. Due to the problem of penetration depth caused by intermediate frequency heating, usually only 60-70% of the steel pipe wall thickness is heated. When using a far-infrared thermometer to measure temperature, only the surface temperature of the steel pipe is measured, not the actual pipe body temperature. When the production speed is too fast, a large amount of heat is absorbed by the unheated part through thermal conduction. Therefore, the temperature of the steel pipe entering the water cooling section cannot guarantee the full curing of the coating, which will inevitably reduce the adhesion between the coating and the steel pipe surface, and ultimately lead to the failure of the anti-corrosion layer.

2. Some quality issues of thick walled steel pipes in 3PEE external anti-corrosion

In recent years, steel pipes with a diameter of D1016-D1219 mm and a wall thickness of 20-30mm have become the mainstream of pipeline construction. Due to their large diameter, thick wall, and high self weight, usually exceeding 400-1500 kg/m, the inertia force of the steel pipe is relatively large when it is in motion on the work line. The rotation of the steel pipe can only go in one direction, and the reverse direction causes a large change in pitch and unstable movement, which is one of the reasons for mass deviation.

The second and most important reason is that the heating of the tube body is not thorough enough, resulting in the bottom FBE not meeting the process requirements for sufficient curing and entering the water cooling section, which affects the adhesion of the bottom FBE coating and seriously causes the bottom FBE layer to detach and curl.

Thirdly, without measuring the temperature distribution curve of the heated steel pipe from the sensor to the inlet of the water cooling section based on the existing conditions of the production line, the production speed is determined to meet the curing temperature requirements of the coating, but only blindly pursuing the production speed, resulting in insufficient adhesion and an increase in waste. It is necessary to have a detailed understanding of the key technology of properly matching the intermediate frequency preheating temperature and production speed before coating.

3. Key technologies

The most important and crucial issue is to store enough heat in the steel pipe to fully cure and adhere the coating, and improve the adhesion of the powder layer to the steel pipe substrate; Another thing is to adjust the matching equipment parameters reasonably on the anti-corrosion operation line to meet the coating requirements. We cannot blindly pursue production efficiency, but only scientifically increase production under the premise of quality assurance.

As is well known, the intermediate frequency used in anti-corrosion production lines is a type of heat transfer equipment that can roughly equalize the temperature of the outer and inner surfaces of steel pipes. The main advantages of induction heating for heat transfer are improving workers' working conditions, high heating efficiency, fast speed, easy temperature control, ensuring heating quality, and easy assembly of production lines.

Permeation induction heating is a parallel circuit of inductance and capacitance. The depth of the heat transfer layer is mainly determined by the frequency of the heating power source. The higher the frequency, the smaller the penetration depth. Conversely, the lower the frequency, the greater the penetration depth.

During the induction heating process of steel pipes, the main heat energy is generated at a certain depth of the pipe wall itself. The heat energy of the heated steel pipe is transferred from two directions. Due to the skin effect, the temperature of the outer surface of the pipe wall is higher than that of the inner surface, so most of the heat energy is transferred towards the center of the pipe. The second is that a small amount of heat energy is radiated outward from the outer surface of the steel pipe. This is determined by the heat transfer rate, thermal conductivity, specific heat, density, and other factors during the heat transfer process.