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In pressure vessels, the head is one of the key pressure-bearing components. There are various types such as elliptical heads, disc heads, hemispherical heads, and conical heads, among which the elliptical head is the most commonly used. So why is a straight edge section provided on the elliptical head? What is the function of the straight edge section? I. Force analysis: After the curved surface of the elliptical head is connected to the cylinder, under internal pressure the cylinder expands radially while the bottom edge of the elliptical shell contracts radially; this results in shear stresses and bending moments at the connection point. These stresses, combined with the membrane stresses, give rise to the maximum stresses in the head. The presence of straight edges prevents the bending stresses at the head from combining with the stresses in the welds between the head and the cylinder. Image: Stress diagram at the junction of the cover and the cylinder body. II. Manufacturing analysis: 1. The inclusion of straight edges facilitates cutting off some or all of these edges in cases where severe wrinkling at the edges occurs after molding, making repair difficult ； 2. The setting of straight edge sections facilitates assembly during the alignment of the head and the cylinder body ； 3. The setting of straight edge sections facilitates the operation of RT non-destructive testing. III. Height of the straight edges: Requirement for the height of the straight edges of elliptical heads are specified in 6.3.10 of GB/T25198-2010. When the nominal diameter DN of the head is ≤ 2000 mm, the height h of the straight edges should be 25 mm ； When the nominal diameter DN of the head is >2000 mm, the straight edge height h should be 40 mm, with a tolerance for the straight edge height of (–5%~10%) h. 2. The author believes that the height of the straight edge section of the elliptical head should also be related to its thickness; when δ≤10mm, h should be 25mm ； When 10＜δ≤30mm, h should be 40mm ； When δ > 30 mm, h should be 50 mm