A story triggered by the fact that the ellipticity of a cylinder did not meet the standard requirements

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This post was last edited by Freestyle-sky on 2018-9-30 at 15:41. When manufacturing pressure vessel cylinders, it is necessary to control their ellipticity; a high level of ellipticity can result in significant misalignment when the cylinders are joined together. If forced assembly is used, it will generate significant residual stresses, which can affect the service life of the equipment; therefore, strict control over the ellipticity of the cylinder body is necessary during equipment manufacturing. The ellipticity of the cylinder is caused by: 1. The ellipticity that arises during the manufacturing process of the cylinder, which can generally be adjusted by rounding it out ； 2. After the cylinder is manufactured, during heat treatment, if no proper supports are provided inside the cylinder, it deforms; and if the wall thickness of the cylinder is large, it is difficult to straighten it out for adjustment ； 3. Welding connections with large openings in areas close to the edge of the cylinder can also affect the cylinder’s ellipticity ； Wait. Controls for ellipticity in the standards: 1. GB/T150.4-2011 specifies requirements for the ellipticity of cylinders, distinguishing between cases with no openings and those with openings, and provides corresponding range limits for each case, as follows: 2. According to ASME VIII-1 UG-80, the ellipticity of cylinders is controlled at a level of 1%, applicable either to the cylinder itself or to the junction between the cylinder and the head ； For cylinders with an open-section cross-section, the ellipticity allowance can be increased to 2%, as detailed below: If the standard ellipticity of a cylinder exceeds this limit and it is no longer possible to make it circular, does that mean the cylinder must still be used? Of course not; there must be a solution. The United States has published the API 579—1 Fitness-For-Service standard, which provides methods for safety assessment of defects identified during the use of pressure vessels, atmospheric storage tanks, and pressure pipelines – such as brittleness, overall thinning, local thinning, pitting, hydrogen embrittlement, hydrogen-induced cracking and stress-guided hydrogen-induced cracking, crack-like defects, creep, fire damage, weld misalignment and shell deformation, dents, scratches, and delamination. The ellipticity of the cylinder can be evaluated in accordance with Chapter 8; the evaluation is divided into 3 levels: (1) Level 1 Assessment – This evaluation procedure controls ellipticity based on the requirements set by relevant standards, and compliance with these standards constitutes acceptance. (2) Level 2 Assessment: This assessment procedure is used when the ellipticity of the cylinder does not meet the requirements of Assessment 1; in such cases, calculations can be carried out step by step according to the procedures specified in the standards, to determine whether the results satisfy the requirements of Assessment 2. (3) Level 3 Assessment: This assessment procedure uses stress analysis techniques to evaluate ellipticity, and may employ linear stress analysis and stress classification methods, or nonlinear stress analysis. Case study: A piece of equipment exported abroad was a modified unit, not a complete one, and required welding to the reactor body. Since this is a heat treatment equipment with a refractory lining, controlling the deformation of the cylinder during the manufacturing process becomes particularly important. This equipment requires a U stamp from the manufacturer, while on-site installation and welding require an R stamp. However, after completion of manufacturing, the ellipticity of the cylinder did not meet the requirements of ASME standards; although the domestic AI system approved it, the problem remained at the site. During on-site construction, an R stamp is required, so the construction party hired a foreigner from AI. foreigners are more diligent in their work than those in China; they even went ahead to inspect things personally and measured the dimensions of the cylinder’s ends. The inspection report indicated that the cylinder’s degree of ellipticity did not meet the ASME standard requirements, preventing further construction work from proceeding. Foreign AI offers two options: one is to round out the cylinder so that it meets the standards before installation ； Another approach is to persuade the owner not to use an R stamp on the equipment. But both of these methods are difficult to implement. The first method involves using external forces to round things out; not only does this delay the project timeline, but the deformation may also damage the internal refractory material, resulting in the failure of the equipment’s insulation system ； Another attempt to persuade the owner not to use the R stamp also failed; the owner insisted repeatedly that the R stamp was necessary. To ensure the smooth progress of the construction, AI agreed to release the R stamp; ultimately, the evaluation procedures 2 and 3 specified in the API 579—1 Fitness-For-Service standard were used for the calculations, a detailed calculation report was prepared, and AI’s approval was obtained, which prevented prolonged delays in the construction schedule. Based on the measured data, calculations related to the cylinder’s ellipticity were carried out using the Evaluation Procedure 2, and the cylinder’s ellipticity meets the requirements of the API 579—1 Fitness-For-Service standard. Generally, foreigners tend to be stubborn, especially when they haven’t met the standards. To enhance persuasiveness, finite element modeling was employed to compare the actual conditions of the cylinder with the results obtained from idealized calculations; both sets of results met the standard requirements, with only a small difference between them. Thoughts for us: 1. Manufacturers should strictly adhere to the specified tolerances; during the manufacturing process, appropriate internal supports should be used to ensure that no significant deformation occurs during heat treatment and pipe welding, so that the ellipticity of the cylinder remains within the limits allowed by the standards ； 2. Domestic AI systems must also fulfill their responsibilities as AI systems, adhering strictly to ASME standards; they cannot turn a blind eye and approve things in a careless manner when those standards are not met ； 3. Equipment problems that remain on site are very difficult to handle. If the external force alignment method suggested by the on-site AI is used, the resulting delay in project completion is difficult to estimate – losses of several million dollars per day can occur – and it also creates a lot of additional work for the site, leading to claims from the construction party ； 4. In foreign countries, the attitude toward AI work is very serious; people often conduct on-site inspections in person, examine each report carefully, make numerous annotations, and require many changes to be made. Although I think this foreigner is up to something, his work attitude is still worth learning from*. If you want to learn more about the analysis and design of pressure vessels, please follow the official account “ANSYS Analysis and Design Experts”!