Is the straight section of an elliptical head considered part of the cylinder or of the head in terms of calculations? Is the formula for calculating the thickness of an elliptical head applicable only within the ellipsoidal surface?
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Dear teachers, may I ask: when calculating, should the straight sections of the head be considered part of the cylinder or of the head itself? As shown in Figure 1, the thickness of the elliptical head is 12 (10.5) mm, and the thickness of the cylinder shell is 16 mm. Should the thickness of the straight edge section of the upper head be taken as 12 mm or 16 mm for calculations? Is the formula for the head only applicable to the calculation of ellipsoidal shells above the elliptical equator and tangent line, and does it not include the calculation of the straight-sided sections? ——That is, when crossing the tangent line and entering the straight edge section of the head, does that straight edge section belong to the cylinder portion and should it be calculated based on the cylinder thickness? (As shown in Figure 2), for nodes like those in Figure 2, the hemispherical head has no straight edges, and the “tangent line” serves as the clear boundary between the upper shell and the head in calculations. What about an elliptical head? Should the “tangent line” still be used as the boundary between the head and the cylinder in calculations (as shown in Figures 3 and 4)? The straight-edge sections should be calculated based on the thickness of the cylinder, rather than considering them as part of the head, right? Beyond the tangent line, outside the range of the ellipsoidal shell, the relationship between σθ and σΦ would no longer conform to the calculation formulas for elliptical heads, right (as shown in Figures 3 and 4)?
With straight-edge end caps, the maximum value of the thickness of the elliptical end cap and that of the straight edges of the cylinder body is taken as the minimum end cap thickness; especially at high pressures, there is a significant difference between the thicknesses of the elliptical shape and those of the straight edges
See the attachment; the PV software calculates the values for both the elliptical shape and the straight edges, and takes the larger of these two values. China’s SW6 does not take such detailed considerations into account.
In pressure vessel design, the straight sections of the elliptical head are usually calculated as part of the cylinder. This is because the geometric shape of the straight-sided section is similar to that of the cylinder, and it is usually welded to the cylinder using the same material and thickness. During the actual calculation, design and verification should be carried out based on the parameters of the cylinder section, rather than those of the head. The formula for calculating the head thickness indeed only applies to the area within the ellipsoidal surface, that is, from the equator of the ellipse up to the tangent line. Once the tangent is passed and entry into the straight-edge section is reached, the calculation formulas for elliptical heads are no longer applicable. The calculation of the straight-edge sections shall be carried out in accordance with the specifications for cylinders, using the corresponding cylinder thickness for strength calculations. Therefore, when designing and calculating the structure for the connection between an elliptical head and a cylinder, the tangent line represents the clear boundary between the head and the cylinder. The head calculation formula is used within the elliptical shell area above the tangent line, while the cylindrical calculation method is applied to the straight-sided section below the tangent line. This ensures the structural safety and rationality of the entire pressure vessel. .
Hello, after reviewing the screenshot you sent and checking the ASME standards, the formula used for the straight edge section of the straight flange head in the screenshot is UG-27(C)-(1), which is the formula for cylindrical shells. Does this also mean that ASME uses the tangent line as a boundary for heads – the area above the tangent line is treated using the formulas for the head itself (Formulas in Appendix 1-4 of Volume VIII-1), while the straight edge section below the tangent line is handled with formula UG-27(C)-(1)? In other words, is the straight edge section below the tangent line considered part of the shell and calculated as such?
Yes, only when the pressure is extremely high does the difference in thickness between the straight edges and the elliptical shell become significant; at medium and low pressures, this difference is generally not noticeable
This post was last edited by Claymore2000 on 2025-1-16 at 15:58. Hello, is it to facilitate manufacturing with the same thickness that the straight edges are taken from the end caps and the thicker part of the ellipsoidal surface is used? If there is significant difference in thickness between the cylinder and the head under high pressure, by using the design options a~c (a–the head is manufactured with a gradual change in thickness from the elliptical surface to the straight edges; b–the welds on the head are located on the elliptical shell; c–the welds on the head are located at the tangent points), it is possible to calculate the thickness of the elliptical surface and those of the straight edges separately, right? There’s no need to follow design option d anymore, is there? And when the head is thin and the cylinder wall is thick, nodes a to d can all be used; only node e cannot be used, right—since the straight edge section does not meet the cylinder thickness requirement of δ2?
Your summary is quite comprehensive, but there are few actual situations in which it is used. Do not confuse ball heads with elliptical heads. It’s better to analyze each specific issue individually, by discussing a single device separately, so as to achieve a better understanding
This post was last edited by Claymore2000 on 2025-1-17 07:38. Okay, so specifically, if the diameter of the elliptical head of a device is 14 mm and that of the cylinder is 18 mm, then regardless of the method used to manufacture the straight sections of the elliptical head, it’s necessary to ensure that their diameter remains 18 mm, right?
Since the elliptical head thins out during forming, the steel plate is thickened according to the amount of thinning, to ensure that the thickness of the elliptical portion after formation is not less than the designed wall thickness. For the same material, in medium and low pressure applications, the straight edges of the head are generally thicker than those of the cylinder.
The equipment diameter, design pressure, design temperature, corrosion allowance, and material must all be specified