Introduction to non-destructive testing of pressure vessels such as boiler tubes

Inside the pressure vessel and pressure vessel components such as boiler tubes, there are often defects that are not easily found, such as unfused, incomplete penetration, slag inclusions, pores, cracks, etc. in the weld. In order to know the location, size and nature of these defects, it is impossible to perform a destructive inspection of each boiler or pressure vessel. For this purpose, non-destructive testing is required. That is, physical deformation of the workpiece or structure is inspected and measured by physical methods without damaging the structure to infer the internal tissue condition and defects of the workpiece or structure.

The purpose of nondestructive testing is to:

• (1) Improve the manufacturing process to ensure product quality.
• (2) In the manufacturing process of the product, defects can be found in advance to avoid product scrapping, thereby saving man-hours and costs, and reducing the cost of manufacturing the product.
• (3) Improve the reliability of the product, ensure the safety of the use of the product, and avoid accidents. Apply non-destructive testing to all aspects of product design, manufacturing, installation, use and maintenance; through a series of tests, determine the design, raw materials, manufacturing processes and operation, and identify the factors that may cause damage, and then Improve to improve product reliability.

Commonly used non-destructive testing methods are: radiographic flaw detection, ultrasonic flaw detection, magnetic particle inspection, penetrant inspection, and eddy current inspection. There are also leak detection, acoustic emission detection, stress testing, visual inspection and so on.
Radiographic inspection The method of examining the quality of a weld by the ability of the ray to penetrate metals and other materials is called radiographic inspection. The basic principle of radiographic inspection is the projection principle. When the ray passes through the weld metal, when there are defects in the weld metal (such as cracks, slag inclusions, pores, incomplete penetration, etc.), the radiation is attenuated differently in the metal and the defect, and the sensitivity is different on the film. The ray decays rapidly in the metal, and the ray decays slowly in the defect. Therefore, the use of radiographic inspection can determine the size, shape and position of defects in the weld. Since radiographic inspection is a projection principle, this method is sensitive to volumetric defects such as slag inclusions. Because this method can record and save, China's boiler pressure vessels have more confidence in this method. China's boiler regulations stipulate that the longitudinal steam seam of the boiler drum with the rated steam pressure of 0.1MPa or more and less than 3.8MPa, the longitudinal joint of the header and the joint of the head shall be 100% radiographic inspection; For a .8 MPa boiler, 100% ultrasonic flaw detection plus at least 25% radiographic inspection is required.

Ultrasonic flaw detection Ultrasonic flaw detection is a method of non-destructive testing when the sound waves are transmitted through the medium and encounter the characteristics of reflection at different medium interfaces. Since the elasticity of gas, liquid and solid medium is very different, the influence on the propagation of ultrasonic waves is different, so reflection, refraction and waveform conversion will occur at the hetero interface. When the ultrasonic wave propagates in the weld, if the weld has defects, the interface that encounters the defect will be reflected, received by the probe, and formed a waveform on the screen, so that the nature, position and size of the defect can be judged. The traditional ultrasonic flaw detection can not record the results of the flaw detection, and the evaluation of the defects depends on the human factor is too large, so at present, China mainly uses radiographic inspection in low-pressure boilers. Ultrasonic flaw detection is sensitive to area-shaped defects such as cracks, incomplete penetration, and the like. Therefore, the advantage of ultrasonic flaw detection in thicker boards is more than that of radiographic inspection. Once the ultrasonic flaw detector can record and save the results, the scope of ultrasonic flaw detection will be further expanded.
Magnetic particle inspection Magnetic particle inspection uses the leakage magnetic field formed at the defect to attract magnetic powder to show defects that are difficult to observe by the naked eye. The magnetic particle flaw detection first magnetizes the external magnetic field applied to the weld to be inspected, and after the weld is magnetized, the fine magnetic powder of the particles is uniformly sprayed on the surface of the weld (the average particle size of the magnetic powder is 5 to 10 μm). If the weld to be inspected has no defects in the near surface, it can be regarded as a uniform body with no change in magnetic permeability after magnetization, and the magnetic powder is evenly distributed on the surface of the weld. When there is a defect in the weld near the surface, the defect (crack, pore, non-metallic slag) contains air or non-metal, and its magnetic permeability is much smaller than the permeability of the weld metal. Due to the change of the magnetic resistance, a leakage magnetic field is generated at the defect on the surface of the weld or near the surface to form a small magnetic pole, the magnetic powder is attracted by the small magnetic pole, and the defect is displayed due to the accumulation of more magnetic powder, which can be seen by the naked eye. The resulting defect graph. Weld surface or near-surface defects generate a leakage magnetic field due to their low magnetic permeability. When the strength of the leakage magnetic field reaches the level at which the magnetic powder can be adsorbed, the surface of the weld or near-surface defects can be observed. The applied magnetic field has a large intensity, and the strength of the leaked magnetic field formed is also large, and the sensitivity of the magnetic particle flaw detection is also higher.
Magnetic particle inspection is easy to find defects on the surface or near surface, especially cracks, but the degree of defects is related to the relative position of the defects and the magnetic lines. When the defects are perpendicular to the magnetic lines, they appear most clearly. When the defects are parallel to the magnetic lines, they are not easy to appear. .
Magnetic particle inspection has been widely used in the manufacture, installation and inspection of boiler pressure vessels, especially in the inspection of spherical cans.
Penetrant testing Liquid penetrant inspection is a method of inspecting weld surface or near-surface defects. This method is not limited by the magnetic properties of materials and can be used in a variety of metallic and non-metallic materials, magnetic and non-magnetic materials. The liquid penetrant inspection method is based on the wetting ability and capillary phenomenon of liquids in solids in physics. In the case of liquid penetrant inspection, the surface of the weld to be inspected is first dip coated with a permeate having a high permeability. Due to the wetting ability and capillary phenomenon of the liquid, the permeate penetrates into the defects on the surface of the weld, and then the permeate on the outer surface of the weld is cleaned, and then a layer of white developer with strong affinity is applied. The permeate that penetrates into the surface crack of the weld is sucked out, and a sharp pattern reflecting the shape and position of the defect is displayed on the white coating. Liquid penetrant flaw detection can be divided into color display coloring method and fluorescence display fluorescence method according to different defect display methods.
The coloring flaw detection method uses the color of the dye to display defects, and the dyeing substance dissolved in the permeate should have a clear and easy-to-see color. The fluorescence display flaw detection method uses the luminescence of a fluorescent substance to display defects. In the flaw detection, the fluorescent substance adsorbed in the defect is irradiated by ultraviolet rays, and absorbs light energy to reach an excited state, enters an unstable state, and is bound to return from the unstable state to a stable state, lowering the potential energy, and releasing the light quantum. That is, it emits fluorescence.
Eddy current flaw detection is a workpiece flaw detection method in which an exciting coil is used to generate an eddy current in a conductive workpiece, and a change amount of the eddy current of the object to be detected is detected by a detecting coil.
The detection coil of the eddy current flaw detection can be divided into three types: a through coil, a probe coil and a plug coil. Through-coil is used to inspect wires, rods and tubes, and its inner diameter fits snugly over the round bars and tubes. The probe coil is placed on the surface of the workpiece for local inspection. The plug-in coil is also called the internal probe and is placed inside the tube and in the hole for internal wall detection.

Eddy current testing is applicable to workpieces made of conductive materials such as steel, non-ferrous metals, and graphite. It is not suitable for non-conductive materials such as glass and synthetic resins.
Its advantages are:

• (1) Since the flaw detection result can be directly output by the electric signal, automatic detection can be performed.
• (2) Since the non-contact method (the probe does not directly contact the workpiece to be inspected), the detection speed can be fast.
• (3) Suitable for surface or near surface defect detection.
• (4) A wide range of applications, in addition to the detection of damage, but also to detect changes in materials, changes in size and shape.

Acoustic emission detection The probe is used to detect the sound wave emitted by the deformation or crack initiation due to the applied stress, so as to infer the position and size of the defect.
The ultrasonic flaw detection method is that the ultrasonic signal emitted by the probe is reflected back and received after encountering the defect. The function of the defect in this process is to passively reflect the ultrasonic signal back. The acoustic emission detection enables the object (defect) to be actively involved in the detection process. The sound emission is generated due to the development of the defect, so the acoustic emission Detection is a dynamic non-destructive testing method. According to the characteristics of the emitted sound waves and the external conditions that cause the acoustic emission, it is possible to check the location of the sounding (the location of the defect) and the microscopic structural characteristics of the acoustic emission source. This detection method can not only understand the current state of the defect, but also understand the defect. The formation process and the tendency to develop and increase under actual use conditions.
Acoustic emission detection can be divided into single channel detection, dual channel detection and multi-channel detection according to the number of detection probes. Single-channel detection can only detect the presence or absence of defects in the object, and the position of the defect cannot be determined. The two-channel detection can only be used for linear positioning, and is generally used for the detection of known condition welds. Multi-channel detection is generally 4 channel, 8 channel, 16 channel, 32 channel acoustic emission detection, mainly used for acoustic emission detection of large components, not only can detect the existence of acoustic emission source, but also can locate the acoustic emission source.
Source: China Stainless Steel Boiler Tubes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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