Points to note for opening reinforcement

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Reinforcement area: When performing reinforcement through drilling, it is necessary to carry out reinforcement within the designated reinforced area, and the amount of metal required for reinforcement must be no less than the amount of metal removed as a result of the drilling. When using pen and paper calculations, the problem of an inappropriate selection of the reinforcement area can often be avoided, but this error frequently occurs when using computer calculations. The main issues arising from an inappropriate selection of the reinforcement area are as follows: 1. The thickness reduction of the head is not taken into account; during the pressing process, the thickness of the head decreases. At this point, when calculating the amount of metal available for reinforcement within the effective width of the head, the amount of thinning resulting from processing must be deducted, with calculations being based on the minimum allowable thickness of the head. However, in practical engineering design, since the Sw6 software does not provide any specific guidance on this matter, many designers tend to carry out calculations based on its nominal thickness, resulting in insufficient reinforcement area. 2. The insufficient length of the cylinder is not taken into account; the “excess metal” within the effective width of the shell can serve as a reinforcement. Pressure vessels often have cylinders of relatively short length, whose length is insufficient to meet the requirements for an effective reinforcement width; in such cases, the actual effective reinforcement range should be used for calculations (as shown in the figure below). This problem is particularly common on the tube bank of heat exchangers. This issue should be given special attention in engineering design. Image 3: Reinforcement of thick-walled pipes. As shown in the figure below, when using thick-walled pipes for reinforcement, there is a certain difference between the values of h1 and h2; therefore, in calculations, the minimum protrusion height, namely h1, should be used. This phenomenon is more pronounced when the diameter of the cylinder is small and the diameter of the reinforcement tube is large; in such cases, the differences between the h1 and h2 values are significant, which often results in an insufficient reinforcement area. Image 4: An inappropriate selection of the weld metal area. The “excess metal” at the weld site can be used for reinforcement, which corresponds to the A3 value in reinforcement. However, different values need to be selected depending on the weld. When using SW6 for calculations, the value of the \"excess metal\" at the weld seam is generated automatically by the software; however, this value can differ significantly from the actual cross-sectional area of the weld seam, especially when thick-walled reinforcement pipes are used for opening reinforcements. This may pose a risk to the safety of the equipment due to an insufficient actual reinforcement area. 5. Combined reinforcement: This point is often overlooked. When using SW6 for calculations, the area of the overlapping portion between openings that are reinforced together should be allocated proportionally to the diameter of those openings; the value of the reinforcement width B must be entered manually by the designer. In terms of its calculation principle, the equal area method takes into account only the average stress in the cross-section of the opening; it considers only the overall strength of the cross-section, without addressing the stress concentration at the edges of the opening, nor does it verify the stability of the areas within the opening where high stresses occur. When there are longitudinal oblong (elliptical) openings in a cylindrical shell, and the ratio of the major to minor axes is large, very high local stresses occur at the vertices of the major axis, making instability likely – a phenomenon that is not taken into account in the equal-area reinforcement calculation method. Therefore, national standards all limit the ratio of the major to the minor axis when calculating elliptical holes using the equal area method. Generally, equal-area reinforcement is limited to openings with an axis ratio of ≤2. For cases where the ratio of major to minor axis is >2, stability checks must be performed on the locally high-stress areas around the hole. Many designers, for convenience, equate elliptical holes with circular holes for calculations, which can easily lead to potential problems with the equipment. Take over the larger end; the thicker it is, the greater its stiffness ； Correspondingly, the greater the thickness difference at the junction between the fitting and the shell, the larger the difference in stiffness between them, which in turn results in poorer coordination of deformation at that junction when the equipment is under pressure. Poor coordination deformability results in higher local stresses. Therefore, the thickness of the overlay should not be much greater than the thickness of the shell, especially for materials prone to cold cracking. Although GB150 does not specify any requirements regarding the thickness of the nozzle, in practical design, the thickness of the larger end of the nozzle should generally not exceed 1.5 times the thickness of the shell, with a maximum of 2 times the shell thickness. If the reinforcement requirements cannot be met, measures such as increasing the wall thickness of the cylinder and extending the nozzles should be considered to address the reinforcement issue. For the small-end connection, it is usually not necessary to calculate the thickness at that end; indeed, GB150 also does not specify any requirements regarding the minimum thickness of the connection neck. However, it is essential to ensure that the effective wall thickness of the connection meets the demands imposed by pressure loads. Therefore, the wall thickness of the small end of the pipe should be calculated in the same way as that of the cylinder wall thickness. Special attention should be paid to calculating the wall thickness of those pipes for which GB150 permits no additional reinforcement; the minimum required thickness at the small end of the pipe should be at least the minimum neck thickness required under internal and external pressures, plus a corrosion margin. Furthermore, except for manholes, handholes, and instrument interfaces used for access and inspection, all other connections are subject to additional stress from connections with external pipelines. ASME VIII-1 provides detailed considerations on this, but it is not mentioned in 150. It is recommended to design such pipe ends, especially those under high external stress, by performing calculations for external stress loads. No additional reinforcement is required. When all parameters meet all the requirements specified in clause 6.1.3 of GB150.2 regarding the \"maximum opening diameter for which no additional reinforcement is needed,\" the vast majority of designers choose not to apply any reinforcement, and SW6 will automatically eliminate the need for reinforcement calculations. When formulating this provision that eliminates the need for additional reinforcement, it was primarily taken into account that the wall thickness of the container shell often exceeds what is actually necessary; this increased thickness leads to a reduction in film stress, and as a result, the maximum stress values also decrease. At this point, the container has been fully reinforced, so no further reinforcement is needed. At the same time, the wall thickness after takeover is often greater than what is actually required, and the excess metal already serves as a reinforcement. Furthermore, when meeting the requirement for the maximum opening diameter without additional reinforcement, its stress concentration factor is low (generally below 3.0). However, when the design pressure approaches 2.5 MPa, the margin for the shell material is limited, and the outer diameter of the nozzles is large – that is, when all conditions are close to their boundary values – it is risky not to conduct calculations. Moreover, when calculations are performed using the equal area method under such conditions, the results are often unsatisfactory. Therefore, when all conditions approach the boundary conditions, Sw6 will automatically skip the calculations; it is thus recommended to use the equal-area method for manual verification in such cases.