Minimum thickness for tower design

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Minimum thickness: When the design pressure is low, and seismic and wind loads are also low, the cylinder wall thickness calculated using formulas turns out to be small; such thicknesses often fail to meet the requirements related to manufacturing, transportation, and lifting. Therefore, a minimum wall thickness for the cylinders of towers is specified. The main considerations are meeting the thickness requirements of the welding process during manufacturing and ensuring the tolerance requirements for geometric dimensions; in addition, the stiffness requirements necessary during manufacturing and transportation are also taken into account. It cannot be assumed that a smaller wall thickness always results in less steel usage and thus lower costs. The thinner the cylinder, the more auxiliary steel is required during manufacturing to maintain the necessary roundness and stiffness, as well as to prevent excessive deformation during transportation; this auxiliary steel is used to keep the cylinder sections round and ensure sufficient stiffness. In particular, the edges of the two joined cylinder sections must be made round, and after manufacturing, this auxiliary steel still has to be removed. Therefore, the more auxiliary steel is used, the higher the manufacturing costs become. Most design firms in our country, in the 1950s and 1960s, followed the formula δmin≥0.001D+2.54 recommended by the American Petroleum Institute and the American Society of Mechanical Engineers in 1944. There is also a formula based on the book \"Oil Production Machinery and Equipment\" written by A. BMXMAH of the former Soviet Union: when D≤1200 mm, δmin≥0.001D+2.54; when D>1200 mm, δmin≥0.001D+4. An additional corrosion margin is applied. With this selection, the thickness is relatively large when the diameter is small; therefore, many design agencies have made revisions to it. Based on actual production practices, this standard specifies that for cylinders made of carbon steel or low-alloy steel, the minimum wall thickness shall be δmin≥2Di/1000, with a minimum value of 3 mm; an additional corrosion margin shall also be applied. With this selection, the manufacturing requirements are generally met, and the stiffness requirements during transportation can also be adequately satisfied. Since there are many factors that affect stiffness during transportation, lifting, and other processes (such as the external dimensions, structure, weight of the tower, the length of its sections, and whether it is in a horizontal or vertical position), detailed calculations of the conditions under which it is located should be carried out when necessary, and appropriate measures should be taken. For stainless steel cylinders, the minimum wall thickness is specified to be no less than 2 mm. The reason why the requirement δmin≥2Di/1000 is not imposed is mainly to save stainless steel material. Therefore, appropriate measures should be taken according to the specific circumstances during manufacturing, transportation, lifting, and operation. For other metal towers, they should meet the requirements for the minimum thickness of corresponding pressure vessels. When there is no minimum thickness requirement, the stability requirements of the container need to be considered, provided that the strength requirements are met.