Study on Production Technology and Process Parameter Control of Double Layer Coiled Welded Pipe

The structure and composition of double layer coiled welded pipe production line are introduced. The key processes of coiled pipe forming and brazing in production process are analyzed emphatically. The detection and control of key process parameters in production line are studied.


1. Structural Composition of Continuous Pipe Production Line

The double-layer coiled pipe is made by forming machine, which coils the double-sided brazed strip steel 720 degrees along the direction of bandwidth, and then solders the tube into a whole through heating device – steel pipe. According to the continuous forming and brazing process, the production process of double-layer coiled pipe can be divided into continuous and semi-continuous. The continuous production line has compact structure, fast production speed and easy to realize automatic control. As shown in Figure 1, the continuous pipe production line is mainly composed of uncoiler, forming unit, brazing furnace, cooling pipe, tractor, straightening roll system, eddy current flaw detector and coiler.

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1-uncoiler; 2-forming unit; 3-brazing furnace; 4-cooling pipe; 5-primary tractor; 6-vertical straightening device;
7-eddy current flaw detector; 8-level straightening device; 9-secondary tractor; 10-pneumatic calibration device; 11-winder
Fig. 1 Pipe production line structure sketch
When the production line is in operation, the double-sided copper-plated steel strip placed on the uncoiler enters the forming unit under the action of traction and rolls 720 degrees from the forming roll system to form a double-layer coiled pipe barrel. After sizing by the mandrel and roller, it enters the resistance direct-heating brazing furnace, and the copper between the tube barrels melts and penetrates into each other at 1200 . After cooling, the double-layer coiled pipe comes out firmly. After the furnace, the eddy current flaw detector is driven by a tractor to inspect the welding quality. The defect is marked automatically by the calibration device. Finally, the coiled pipe is automatically discharged on the coiler. The tractor adopts closed-loop DC speed regulation to ensure the stability of pipe speed and the continuous operation of the production line. During flaw detection, the welded pipe forms multiple fulcrums under the action of the front and rear vertical and horizontal straightening wheels. At the same time, the welded pipe is tensioned by the tension of the secondary traction device without shaking, which ensures the normal eddy current detection.

2. Manufacturing Process Analysis of Double-Layer Coiled Welded Pipe

The manufacturing process of double-layer coiled pipe can be divided into uncoiling, coiling, brazing, cooling, straightening, flaw detection, coiling and other processes. The whole process is completed continuously, in which the forming quality of coiled pipe is the key process affecting the size of double-layer coiled pipe, and brazing is the key process affecting the forming quality of double-layer coiled pipe, so this paper focuses on the coiled pipe and brazing two processes.

2.1 Rolling Tube Forming
In the production of double-layer coiled welded pipe, tube forming process is the most difficult and important process in all production processes. The quality of tube forming not only directly affects the size and shape accuracy of finished pipe, but also has a decisive influence on the success or failure of subsequent welding process. The forming of double-layer coiled welded pipe is accomplished by the forming unit arranged alternately by horizontal roll and vertical roll. The forming process is shown in Fig. 2.
The forming machine device is composed of groove roll, forming roll and sizing roll, mandrel and transmission device. The forming process of double-layer coiled welded pipe can be divided into three stages.

  • (1) Edge rolling stage. Firstly, under the roll action of groove roll, the edge of the strip is rolled into a groove with a certain angle. The purpose is to make the inner and outer layers of the double-layer pipe lap smoothly.
  • (2) Rolling stage. On the first horizontal roll which takes part in the main forming, the inner ring is bent by a single radius and the outer ring is bent into a right-angled edge with a transition corner. Then the right-angled edge of the forming is positioned. Under the combined action of the vertical roll and the horizontal roll, the other side is gradually curled along the radial direction to form the inner ring, and the rest of the inner ring is formed into the outer ring. At this time, it is not completely circular.
  • (3) Sizing stage. The sizing roll cooperates with the mandrel to carry out the second shaping of the unformed pipe, so that the inner and outer layers are closely joined and become a complete circular tube, which meets the basic size requirements of the inner and outer diameters of the welded pipe, and provides the basis for the next brazing process to proceed smoothly.

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Figure 2 Forming process of double-layer coiled pipe
2.2 Brazing
As shown in Figure 3, the brazing device is mainly composed of heating zone, welding zone, pressure roll, heat preservation and cooling zone, electrodes, etc. The tube supplied by the forming machine enters the well-insulated brazing chamber through sliding electrode at a certain speed and contacts with rolling electrode. The tube between the two electrodes forms a circuit through the electrode and power supply. During the operation of the tube from sliding electrode to rolling electrode, the tube is gradually heated and reaches the highest temperature when it reaches the positive electrode, i.e. brazing temperature. At this time, there is no need to impose any external load on the tube, because copper has good liquid fluidity and rapid diffusion in steel, it will be firmly bonded between layers after cooling. The key is the temperature control in the furnace and the anti-oxidation of the surface coating. Brazing is carried out in the reducing atmosphere of the conduit and the protective gas is hydrogen [3,4].

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1-Welded Pipe; 2-Sliding Electrode; 3-Welded Cavity Insulation Tube;
4-sliding electrode support rod; 5-rolling electrode
Fig. 3 Structure sketch of brazing device
When the steel pipe comes out of the heating section of the brazing furnace and enters the cooling zone, it is cooled in the furnace under the protective atmosphere. At this time, the copper layer between the layers and the surface begins to solidify. The surface temperature of the steel tube at the exit of the cooling zone should be reduced to about 100 C before it can be discharged from the furnace, so as to avoid the oxidation discoloration of the copper coating due to the contact with air (the oxidation temperature of copper is about 350 ). The annealing quality of finished pipes can be guaranteed by using furnace cooling, air cooling and water cooling.

3. Detection and Control of Technological Parameters

3.1 Process parameters
In order to meet the requirements of dimension accuracy and surface quality of double-layer coiled pipe, improve the welding quality and performance of the product, and reduce the reject rate, the process parameters are strictly controlled in the whole production process. The key points of control are diameter pressure, brazing temperature and running speed.

  • (1) sizing pressure. In order to ensure the tight fit between layers, a certain sizing pressure should be applied to the tube in the sizing stage. If the sizing pressure is too small, the bonding rate between layers will be reduced. But if the sizing force is too large, not only will the distortion and instability between layers be too large, but also the axial tension of the mandrel will increase sharply and fracture. Due to the size limitation of finished pipe, the cross section of mandrel tie rod can not be obtained very large, especially when forming small diameter double-layer coiled welded pipe, the cross-section area of tie rod is very small and becomes the weakest link.
  • (2) Brazing temperature. In the heating zone, the temperature of the coiled welded pipe increases from room temperature to over 1200 , and the resistance value of the coiled welded pipe is extremely uneven at all points in the direction of operation, which is a variable, but the resistance value is related to the maximum temperature. If the temperature is too low, the pipe can not be welded firmly; if the temperature is too high, the pipe will burn or the copper on the surface of the pipe will melt completely and flow downward to form copper nodules. Therefore, the key to the welding process is to ensure the temperature stability in the furnace.
  • (3) Running speed. When the temperature is constant and the diameter of the coiled pipe is small, the running speed of the pipe in the production line should be faster, otherwise the coiled pipe will be burnt out. Therefore, the requirements of pipe-making speed are different for different diameter reels. The production speed control range of double-layer coiled welded pipe production line is 10 m/min~27 m/min. The production speed of forming unit should match the traction speed of primary tractor and secondary tractor in order to ensure the operation stability of the whole production line, and the traction speed of primary tractor is slightly higher than that of pipe production, so that the steel pipe is in tension state.

3.2 Control of process parameters
As shown in Fig. 4, the control system consists of touch screen, central controller, field bus, sensor and actuator. The control objects are pipe-making speed, primary traction speed, secondary traction speed, mandrel tension and brazing temperature.

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Figure 4 Process parameter control block diagram
The control principle of each process parameter is as follows:

  • (1) On-line monitoring of mandrel pressure. In the pipe-making process, the tension of the mandrel is transmitted to the programmable control device through the pressure sensor. The adjusting rod is adjusted according to the pre-determined parameters, and the position of the mandrel is confirmed by the parameters, thus avoiding the uncertainty of manual adjustment of the mandrel. At the same time, the device collects and analyses real-time data. If the measured value is zero, that is, when the mandrel breaks, the stop command is issued immediately to reduce waste of raw materials and waste rate.
  • (2) Detection and control of brazing temperature. After the start-up of the production line, the temperature of the pipe starts to rise automatically. The infrared temperature probe converts the detected temperature signal into current signal and feeds it back to the programmable control device. The programmable control device converts the detected temperature signal into digital signal and outputs it to the display screen. At the same time, the controller compares the received temperature with the set temperature and controls the power regulator after the conversion process to adjust the voltage. The temperature in the furnace is gradually stabilized.
  • (3) Inspection and control of running speed. The process speed is set before production. After the start of the production line, the detected speed pulse signal is fed back to the PLC of the central control system. After processing by the PLC, it is output to the control end of the frequency converter. The speed of the frequency converter is controlled by adjusting the frequency of the frequency converter, so that the pipe speed of the production line can automatically reach the set speed and keep stable.

4. Summary

In this paper, the key technology and control parameters of double-layer coiled pipe production line are studied, which lays a foundation for the realization of automatic control of double-layer coiled pipe production line.

Source: China Stainless Steel Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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[1] Liu Chengjie, Yu Enlin, Wu Jian. Production Research of Double Layer Coiled Welded Pipe [J]. Anshan Steel Technology, 1994 (5): 42-44.
[2] Yu Enlin, Lai Mingdao, Wu Jian. Theoretical analysis and experimental study on forming quality of double-layer coiled welded pipe [J]. Steel, 1996 (1): 40-44.
[3] Mi Junru. Continuous welding process and equipment for double-layer coiled welded pipe [J]. Electric welding machine, 1994 (1): 28-30.
[4] Zhou Zhantong. Study on Neural Network Adaptive Control Method for Welding Process of Double Layer Coiled Welded Pipe [D]. Shijiazhuang: Hebei University of Science and Technology, 2009: 5.

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