Common defects and inspection methods of pressure vessels

Thread Content

Common defects 1. Surface defects: After a certain period of use, pressure vessels are prone to developing surface defects. The inspection of surface defects is relatively simple; it can generally be done by visual inspection. When surface cracks appear, air will come into direct contact with the medium stored inside, causing the surface cracks to expand further and posing a serious threat to the safety of using pressure vessels. There are many types of surface defects, the most common being surface cracks, which pose a great threat. When surface cracks are detected, their causes should be carefully analyzed, and targeted measures should be taken promptly to eliminate them completely. At the same time, weld undercutting is also a type of surface defect; it occurs at areas with geometric discontinuities and stress concentrations, and can easily lead to cracks. When the total length of undercut on both sides of the weld is less than or equal to 10% of the weld length, no action is required; otherwise, it must be removed by grinding or rewelded after grinding. 2. Buried defects: Buried defects are also one of the common types of defects that occur during the use of pressure vessels, and their inspection methods and procedures are relatively complex. Usually, certain testing equipment is required to determine the main types of defects and their causes. There are many common types of buried defects, including buried cracks, lack of penetration, lack of fusion, pores, and inclusions. (1) Buried cracks: Buried cracks are in contrast to surface cracks; they do not come into direct contact with the corrosive medium, and as a result, the stresses generated and the level of harm they cause are relatively low. However, under conditions of alternating loads and frequent intermittent operation, it tends to propagate to the surface or even penetrate through it, thereby damaging the pressure vessel. (2) Undercut and lack of fusion: On the radiographic film, if neat, thin straight black lines appear on both sides in the middle of the weld, the type of defect is undercut ； If the thin black line is neat on one side and has greater blackness, it indicates a lack of fusion defect. The above two types of defects occur during the manufacturing process due to lack of fusion; as a result, their severity is relatively low, but they still have the potential to induce cracks. If induced cracks are found during inspection, they must be repaired by patching. (3) Pores and inclusions: Pores and inclusions occur inside pressure vessels; they are generated during the manufacturing process. Porosities form inside, on the surface, or near the surface of castings; their shape is often irregular. They are residues of non-metallic solid substances such as inclusions, which appear during the welding process. Since it does not develop into cracks, the potential risk is low. During the inspection, if new cracks appear at the edges of the pores, they can be ignored and no action taken. If the slag inclusions are relatively large in height or have sharp edges, patching repairs are required. Inspection methods 1. Visual inspection. Visual inspection is a common method in non-destructive testing; it includes macroscopic examination and visual checking, and is primarily used to assess the container’s structure, geometric dimensions, corrosion and insulation properties, signs of leakage, degree of corrosion, as well as the condition of welds. Visual inspection can detect obvious surface defects such as cracks and corrosion pits, making it suitable for checking surface imperfections. 2. Magnetic particle testing: When ferromagnetic materials are magnetized, a strong magnetic induction intensity and a high magnetic flux density are generated within them. When there are discontinuities in the material, the magnetic field lines tend to become distorted, and some of them escape from the surface of the material, thereby creating a leakage magnetic field. When magnetic powder is scattered on a workpiece, it is attracted by the magnetic flux leakage; the accumulated magnetic powder forms a shape similar to that of the defect, known as a magnetic mark. This allows for the identification of the location, size, shape, and severity of the discontinuities, making it suitable for detecting surface defects. 3. Penetrant testing: Based on the principle of capillary action, penetrant testing involves applying a penetrant containing fluorescent or colored dyes to the surface of a part. Capillary action causes some of this penetrant to penetrate into any openings or defects on the surface after a certain period of time. Removing the excess penetrant from the surface of the part and then applying the developer to that surface will cause capillary action to occur once again. At this point, the penetrant retained in the defects is attracted by the developer and seeps into it; under the action of a certain external light source, the shape and distribution of the defects become visible. Penetrant testing is suitable for detecting surface openings in most non-absorbing materials, and it is not affected by the geometry of the workpiece or the orientation of the defects; it is thus appropriate for inspecting surface defects. 4. Ultrasonic testing: In the same homogeneous medium, the speed and direction of ultrasonic waves remain constant. When encountering another medium, phenomena such as reflection, refraction, and diffraction occur. Pressure vessels are mostly made of steel, and when defects exist inside them, ultrasonic testing can be used to accurately detect the presence of such defects. It is suitable for testing various test pieces and workpieces with large thicknesses, and it offers good detection performance for buried defects. 5. Radiographic inspection: When a workpiece is exposed to radiation, transmission occurs, and its intensity varies depending on the type, thickness, and density of the material. Under the influence of properties such as photography and fluorescence, the changes are recorded on the film, and after development, variations in darkness are produced on the resulting negative. The internal structural condition of the workpiece can be analyzed through changes in blackness, thereby achieving defect detection. Ray inspection enables the acquisition of visual images of defects, featuring accuracy in both qualitative and quantitative analysis; moreover, the inspection results can be stored for a long time, making it suitable for detecting buried defects.