Pressure vessel inspection error

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Errors in the testing tool itself: The testing tool is a prerequisite for conducting tests. Currently, the collaborative processes of cold working, metal machining, and welding remain the main methods for manufacturing pressure vessels. However, cold welding and metal machining yield parts with different levels of precision, and accordingly, different testing tools and methods are used. Gold processing involves precise measurements using tools such as calipers and micrometers, ensuring a high level of accuracy. Cold welding processing is largely manual, with limited testing tools, and human factors determine the precision of the products. Traditional measuring tapes, square rulers, and even non-standard tools such as template rulers and welding inspection rulers exhibit instability in the testing process. Especially when it comes to weld inspection tools, those still in use are the homemade tools from the past; they rely on visual inspection and solely on the inspector’s own experience, which reduces the accuracy in detecting dangerous defects and sometimes leads to misjudgments. Although welding inspection tools have seen significant innovation and improvement as production has developed and technology has advanced, this method inherently involves errors. For example, when checking the depth of undercut and the height of burrs in circumferential welds, an arc is treated as a straight line; although this error is very small, it becomes quite noticeable when the radius is small enough. The flaws of such testing tools are quite evident; they affect the assessment of product quality to a certain extent, and moreover impact the authenticity of the tests. However, this type of non-standard tool is still widely used for measuring various angles of welded parts, as well as the height, width, gap, and root penetration of welds. Although there are currently no established standards and regulations regarding manufacturing precision and calibration, users have strict requirements for the products, which inevitably leads to conflicts between manufacturing and usage. By optimizing the selection of materials, it is possible to control warping during the production of homemade testing tools, prevent deformation, and make timely improvements and adjustments. However, the errors inherent in the tools themselves already pose potential problems for the inspection of products. The testing standards lack consistency and rigor. The degree to which the weld is discontinuous with respect to the extension of the straight lines or curves on either side is what constitutes the corner angle of the pressure vessel; in other words, it is the degree to which the longitudinal or circumferential weld forms a corner with the actual arc or straight line. GB150.4-2011 stipulates that the angular deviation of longitudinal welds shall be inspected using an internal or external template whose chord length is equal to 1/6 of Di and not less than 300 mm ； The edge angle of the circumferential weld is checked using a measuring ruler with a length of not less than 300 mm. In actual inspections, the designed inner and outer diameter dimensions are usually used as references; that is, the theoretical dimensions from the drawings are compared with the corners at the fillet welds, and the difference obtained serves to determine the corner angle. It can be seen that error-free detection results can only be ensured when the arcs in the remaining parts of the container match the theoretical arcs; otherwise, errors will occur. Actual inspections have shown that the actual arc value of the container cannot be guaranteed to be exactly equal to the theoretical value. If the shell around the weld deforms, abnormalities will occur during the rolling process of the cylinder section, which to some extent will cause the measuring gauge not to fit properly against the cylinder, making it difficult to ensure the accuracy of the measurements. It is difficult to determine the detection standard. The main tools for measuring the misalignment of weld joints are welding inspection rulers and custom-made templates. Although efforts are made to measure at a position as close as possible to the root of the weld, errors are evident due to the uncertainty of the reference points. When using a template to measure the misalignment, only the distances from the heat-affected zones on both sides of the weld to the template can be determined; however, it is not possible to measure the sharp angles present in the weld or the local deformation of the shell. When using a welding inspection gauge to measure the misalignment, the final value is often determined by comparing measurements from the left and right sides, relying entirely on the inspector’s experience, which results in significantly poor accuracy. If a template is used, it is necessary to measure the angle of the weld edge; in this case, the scale line at the center of the template must be aligned with the center of the weld and coincide with the measurement center line. Taking the inspection of longitudinal seams with a welding gauge as an example, it is necessary not only to ensure that the gauge is perpendicular to the axial centerline, but also that it remains parallel to the circumferential tangent of the container. When choosing an arc-shaped template for measurement, if the diameter of the template’s arc does not match the actual diameter of the container, adjustments have to be made. However, these measurement methods are all carried out manually by humans through observation, and the data obtained cannot be absolutely accurate. The inspection locations and methods are not standardized. There are numerous inspection items for pressure vessels, and a variety of inspection methods exist; when these are used together, it can lead to neglect of some aspects or to mistakes and omissions. For example, when checking the edge angle, factors such as ovality, misalignment, or weld excess often have an impact; without taking these into account comprehensively, it is difficult to ensure accurate measurements. Therefore, identifying the specific testing sites and standardizing the testing methods and procedures is essential to effectively avoid interference from other factors. GB150.4-2011 specifies the specific method for measuring the straightness of the shell: four symmetric measurement reference points are designated on the cylinder (0°, 9°, 180°, 270°), the generatrix is identified, a steel wire with a length of 0.5 mm is used, and the distance between the cylinder and this steel wire is measured ； It specifies the measurement location, that is, the distance of the measurement point from the center of the longitudinal and circumferential welds, as well as the replacement criteria when the length exceeds the specified range. However, in the actual process of measuring the ellipticity of a pressure vessel shell, inspectors randomly select several points on the cylinder to measure its diameter, and the difference between the maximum and minimum values of these measured diameters constitutes the ellipticity. The measurement process is neither scientific nor rigorous; randomness and arbitrariness increase the inaccuracy of the measurements