Seeking advice on pipeline welding issues

Coil winding and high-temperature demagnetization – these are the methods I usually use. But once again, I was unable to demagnetize the pipes on the steam drum in Maoming. In the end, I hired a demagnetization expert; I saw that he brought along a set of instruments, though I didn’t know what they were

Causes of residual magnetism and its impact on welding quality: During welding operations to construct or repair gas pipelines, magnetic deflection can sometimes occur, affecting the welding process. The formation of magnetic deflection is the result of residual magnetism present in the tube metal. Generally, residual magnetism is divided into induced magnetism and process magnetism. Induced magnetism often arises during the pipe manufacturing process in factories, such as during metal melting, when using electromagnetic cranes for loading and unloading, when steel pipes are placed in a strong magnetic field, during non-destructive testing using magnetization methods, and when steel pipes are positioned near high-voltage power lines. Process magnetism often arises during assembly and welding operations, as well as when magnetic clamps, fixtures are used, or when pipes are welded with direct current; for example, due to prolonged contact with electrical wires connected to a direct current power source, exposed sections of wires, or short circuits between welding electrodes and pipes. When welding magnetic steel pipes, difficulties in igniting the arc, disruptions in the stability of the arc’s combustion, deviation of the arc in a magnetic field, and splashing of liquid metal and slag from the welding pool are common phenomena. To stabilize the welding process and improve the quality of the welded joint, the magnetized steel pipe must be demagnetized before welding. It should be noted that it is difficult to achieve complete demagnetization of the welded steel pipes. Therefore, welding is permitted when the residual magnetism is not sufficient to affect the welding quality. 2 Demagnetization methods for Russian pipeline welding When welding and repairing pipelines in field conditions or at semi-finished product facilities, demagnetization is particularly necessary. The relevant Russian authorities have developed corresponding pipeline demagnetization procedure documents. The document contains advanced domestic and international experiences in carrying out similar tasks today. 2.1 Demagnetization Process For demagnetization prior to welding, demagnetization processes for individual steel pipes as well as for the joints between steel pipes have been developed, which include the following steps: ① Determining the magnitude and direction of the residual magnetic field in the steel pipes ； ② Select the demagnetization method, system diagram, and technical approaches ； ⑧ Demagnetize the steel pipe or the welded joint using the selected demagnetization method ； ④ Check the residual magnetism after demagnetization to see if it meets the requirements. 2.2 Demagnetization Methods The established process documents specify the following demagnetization methods: using direct current or alternating current, as well as magnetic field methods generated by electromagnets or permanent magnets. Analyze the residual magnetism parameters (see Table 1), and select the demagnetization method and system based on the specific conditions at the construction site (such as the available equipment, etc.). Table 1 Remanent Magnetism Level and Welding Conditions Remanent Magnetism Level Remanent Magnetization Intensity (×10⁻⁵ T) Welding Conditions Weak 100 Demagnetization Demagnetization is carried out using an electromagnetic coil made of welding wires with a cross-sectional area of 35–50 mm, for both direct current and alternating current. The wire is wound around steel pipes or two connected steel pipes, forming coils with different numbers of turns depending on the residual magnetism of the steel pipes. When demagnetizing with direct current, a welding rectifier or converter with a current of 500–1,000 A must be used, including those with multiple stations. When demagnetizing with alternating current, a welding transformer with a current of 500 to 1,000 A is used. All selected power supplies shall have remote control and current regulation devices, and ballast resistors may be used. When using a welding transformer for demagnetization, it is recommended to use portable ammeter meters such as LI-4505 and LI-4501 to measure the demagnetization current. To demagnetize with a dedicated electromagnet, a welding rectifier or transformer must be used as a power source, as shown in Figure 1(a). When demagnetizing with a permanent magnet, no power source is required, as shown in Figures 1(b) and (c). The demagnetization of steel pipes is divided into three grades, as shown in Table 1. VIP Information http://www.cqvip.com Welding of Pipes, September 2002 Figure 1 Schematic diagram of the degaussing system for butt-welded pipe ends using an electromagnet (a), a C-shaped permanent magnet (b), and a cylindrical permanent magnet (c). 1 – Pipe to be degaussed; 2 – Electromagnet; 3 – Welding wire; 4 – DC welding power supply; 5 – C-shaped permanent magnet; 6 – Cylindrical permanent magnet. During degaussing, the magnetic field strength should be greater than the residual magnetic field strength: H = (1.2–1.5)H, where H represents the magnetic field strength used for degaussing ； H. Remanent magnetic field strength. The demagnetizing magnetic field strength is determined by the formula: H = I · N, where I is the current flowing through the coil. A ； N number of turns of the degaussing coil ； L – length of the winding, in meters. To measure magnetism. It is recommended to use the lIMI1 97 X magnetic meter. A magnetometer is a portable instrument used to measure the magnetic induction intensity of pulsating magnetic fields as well as leakage magnetic fields in the air gap of magnetic systems. The instrument consists of a measurement converter, electronic devices, and a charging unit. The power supply for the instrument is a 9V built-in battery pack; the technical specifications of the magnetometer are shown in Table 2. (1) Demagnetization using direct current: The process of demagnetizing with direct current is as follows: ① Determine the magnitude and direction of the residual magnetic field in the steel pipe using a magnetometer. Table 2: Technical specifications of the magnetometer. Range of magnetic flux density to be measured (×10⁻⁶ T): 1–1,999. Lower limit of sensitivity (×10⁻⁶ T): 1. Time required for adjustment (s): 30. Continuous operating time after battery charging (h): 8. Dimensions: Electronic unit: 170×60×35 (mm×mm×mm); Charging unit: 70×70×30. Weight: Electronic unit: 0.35 kg; Measurement converter: 0.35 kg ； ② A coil composed of flexible welding wires with a cross-section of 35–50 mm is installed on the steel pipe; this coil is connected to one or two welding inverters connected in series, so that the direction of the magnetic field generated by it is opposite to the direction of the residual magnetic field of the steel pipe, as shown in Figure 2 ； ③ In the demagnetization process, Figure 2 shows the diagram of a demagnetization system that uses direct current for a single steel pipe (the middle section). Figure 1 depicts the steel pipe being demagnetized. 2 represents the welding wires, and 3 refers to the direct current welding power supply. Initially, the current strength is between 80 and 100 A. ④ During the demagnetization process, it is necessary to periodically use a magnetometer to check the effect of the demagnetizing magnetic field on the steel pipe (measurements are taken with the power supply connected). If necessary, control the current or change its direction (using the method of swapping wires on a welding inverter). ⑤ After demagnetization is complete, in order to reduce the magnetic flux smoothly, the current should be gradually decreased over a period of lmin until it reaches zero, after which the power supply should be turned off. Demagnetization using direct current can be accomplished in several ways. To demagnetize a single steel tube, first wind 8 to 12 turns of wire around the outer circumference at one end of the tube, using the highest possible magnetic field strength for demagnetization ； Then, using the same method as that described in Volume 25, Issue 5 of VIP Information http://www.cqvip.com, compiled by Xu Guizhi: Modern demagnetization methods before pipeline welding in Russia, demagnetization is carried out on the other end of the steel pipe. When a single steel pipe is demagnetized to the joint of the pipes, the two pipes are separated by a distance of at least 300 mm; coils consisting of 18–20 turns are wound 80–100 mm from the end face of each pipe, and demagnetization is carried out in accordance with the method shown in Figure 3(a). Figure 3: Schematic diagram of the system for demagnetizing steel pipes using direct current before butt welding. 1 – Demagnetized steel pipe; 2 – Welding wire; 3 – Direct current welding power supply; 4 – Welding torch with electrode; 5 – Metal plate. In some cases, it is recommended to use a method in which the welding torch and the metal plate are connected to the electrical system for demagnetization, as shown in Figure 3(b). The electrode placed in the welding torch was short-circuited with a metal plate at a current of 300 A for 10 seconds. Then disconnect. After each short-circuit and disconnection cycle, check the magnetism using a magnetometer, and repeat the demagnetization process if necessary. When demagnetizing the assembled joint, welding wires with a cross-section of 35–50 mm are wound around the ends of the connected steel pipes, forming a common coil for the two pipes, as shown in Figure 4(a). The coils can be wound overlappingly (clockwise or counterclockwise), with a total number of turns ranging from 16 to 22. At this time, the steel pipes with more turns should have a greater residual magnetism. This demagnetization process is often the best. Once the residual magnetism level measured is less than 2O×10 T, welding at the root of the weld is completed. At this time, it is recommended to perform supplementary demagnetization at a low current of 10–20 A. (2) Demagnetization using alternating current: Demagnetization with alternating current can be applied to the ends of individual steel pipes before they are assembled, as well as to the joints of assembled steel pipes with a wall thickness of 25 mm. At this time, in addition to demagnetizing the butt joint pipe ends using the common welding wire as shown in Figure 4 above – namely, demagnetization with direct current in (a) and with alternating current in (b): 1–the steel pipe to be demagnetized; 2–common welding wire; 3–direct current welding power supply; 4–device for gradually reducing current (steel wire); 5–insulating material pad; 6–welding transformer – there is also the following additional method: As per the demagnetization system shown in Figure 4(b), a coil composed of one welding wire is used, with a steel wire that is 0.5–1.0 m long and 1.5–3.0 mm in diameter connected in the return circuit. This steel wire is placed on a backing plate made of insulating and non-flammable material, such as asbestos bricks. The wire can smoothly adjust the magnitude of the current flowing through it, thereby changing the strength of the demagnetizing field. When the power is turned on, the wire is heated and burns out after a certain period of time. The burnout time depends on the wire diameter, length, and current value. After the steel wire is broken, use a magnetometer to check the residual magnetism level. When the demagnetization effect is insufficient, demagnetization must be repeated (sometimes 4–5 times). The removal of the demagnetization system can be carried out after welding the root weld; it is recommended to remove it immediately after demagnetization. For demagnetizing alternating current, an electrical regulator can also be used to smoothly change the magnitude of the current. (3) Demagnetization using electromagnets and permanent magnets is mainly applied to individual sections 100–200 mm long on the already joined steel pipes, especially in the vicinity of areas where the direction of the magnetic field changes. At this point, after demagnetizing individual sections, the root weld should be completed, followed by demagnetizing the next section. To demagnetize, an electromagnetic iron with a specialized structure was selected. The electromagnet is installed at the joint of the steel pipes, as shown in Figure 1(a), with the N pole of the electromagnet positioned at the edge of the steel pipe that has a magnetic S pole, while the S pole of the magnet comes into contact with the magnetic N pole of the pipe. During the demagnetization process, it is necessary to use a magnetometer to regularly measure the direction and magnitude of the residual magnetism in the steel pipe (when the power is turned on). The magnitude of the demagnetizing field is adjusted by changing the current value, while the direction of the field is adjusted by changing the direction of the current, that is, by reversing the positive and negative poles of the power supply. Demagnetization is carried out using permanent magnets; C-shaped or cylindrical permanent magnets made of the IoH UuKT5 alloy are used, as shown in Figures 1(a) and (b). When the magnet is installed correctly, its poles should be opposite to those of the steel pipe to be magnetized. Whether the magnet is installed correctly can be checked using a magnetometer. To enhance the demagnetization effect, magnets can be connected to each other (two, three or more, with the same function). After demagnetizing the butt joint section, it is necessary to complete the welding of the root weld at this location. Thereafter, the magnet should be moved to the next demagnetization section. To increase the demagnetizing field, the magnet needs to be brought closer to the demagnetization area; conversely, the magnet can be removed. By moving a magnet along the surface of the steel pipe, the residual magnetism at the welded joint can be reduced to a minimum. To change the direction of the demagnetizing flux, the C-shaped magnet must be rotated 180 degrees in the horizontal plane. The cylindrical magnets mounted on the edges of the joint need to be swapped positions or rotated 180 degrees in the vertical plane. . After each demagnetization step, it is necessary to check the level of residual magnetism using a magnetometer. 3 Conclusion Russian demagnetization experience shows that the demagnetization methods outlined in existing process documents are highly effective. This method can be used in the installation, welding, and repair of various pipelines in the industrial sector.