Situations where reinforcement rings should not be used

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Note: The contents described in GB/T 150-2011, JB/T 4736-2002, and HG/T 20583-2021 will not be reproduced here. 1⃣️ At the large-size nozzles on convex end caps, since the reinforcement rings for openings at any position on such end caps must be curved plates in two directions, it is not possible to achieve the required specifications for these reinforcement rings by using a rolling machine alone. For small-sized reinforcement plates, they can be shaped by hammering, stamping, or other methods. However, it is difficult to achieve a satisfactory shape for reinforcement rings on large-sized pipes using these shaping methods, unless they are shaped in a head mold – but this is often not feasible in actual manufacturing. Therefore, for large-sized nozzles on convex end caps, if reinforcement rings are used, it is difficult to ensure a tight fit between the reinforcement rings and the shell; and even if they can be welded to the shell, it requires forced assembly, which results in significant residual stresses after welding. These stresses can easily cause cracks in the welds, thereby creating safety hazards. Therefore, for strengthening large-diameter pipes on convex heads, it is not advisable to use reinforcement rings; instead, it is better to employ methods such as overall thickening of the head or using thick-walled pipes for reinforcement. 2⃣️ Near the main weld: When the fitting is located close to the main weld, the superposition of welding residual stresses and local stress concentrations can result in stress peaks. This can lead to crack formation at the weld roots between the reinforcement ring and the outer peripheral welds of the shell, potentially causing safety accidents. 3⃣️ Beveled transition fittings (for larger sizes): In the case of beveled transition fittings, the shape of the opening often features sharp corners, resulting in a high stress concentration factor at those areas. In this case, peak stress may occur, leading to cracking and the delayed propagation of cracks. At the same time, since the equal-area method is used for the reinforcement calculations, this approach tends to result in insufficient reinforcement in such cases. Furthermore, this structure results in significant waste of reinforcement ring material, and it is also difficult to ensure the visual quality of the welds after welding. 4⃣️ Large opening condition: As can be seen from the stress concentration factor, the larger the diameter of the opening, the higher the peak stress near the connection point of the opening and the pipe. For large-opening structures with d/D>0.5, the stress concentration factor can reach as high as 7–8 depending on the opening ratio, and the stress concentration remains localized. In other words, for the case of large openings, the stress concentration remains confined to a small area, which is consistent with the theoretical analysis of small openings in shells. At the same time, due to the curvature of the shell, the additional bending moment generated at the edges of the openings worsens the stress conditions at those edges. For this reason, the equal-area method is not applicable to large openings. Reinforcement rings, on the other hand, cause stress to be dispersed and prevent them from forming a unified structure with the shell, which can lead to new stress concentrations. Therefore, in the case of large openings, using reinforcement ring structures is not conducive to safety.