Why are double saddles the best choice for horizontal containers?

Thread Content

Horizontal vessels are classified into internal-pressure horizontal vessels and external-pressure horizontal vessels based on the pressure they are subjected to. Horizontal containers are supported by brackets. When the container size is small, the load imposed on the container at the support points due to support is relatively small and can be ignored ； However, when the container diameter is large, the external loads caused by the equipment’s own weight, as well as wind and seismic loads, cannot be ignored compared to the operating pressure of the container, and these loads must be taken into account. Therefore, the design of horizontal vessels is based on the operating pressure (internal or external), and the wall thickness is determined in accordance with the requirements for the design of general pressure vessels (strength is generally controlled by circumferential stress, while stiffness is controlled by circumferential stability). Subsequently, various additional loads are taken into account, and the circumferential and axial strength and stability of the vessel walls under these additional loads are checked, so that the vessel can maintain both circumferential strength and stiffness as well as axial strength and stiffness under all operating conditions. The current design standard applicable to horizontal vessels is NP/T 47042-2014 \"Horizontal Vessels\". This standard applies to horizontal vessels with a design pressure not exceeding 35 MPa, which are supported by two symmetrically placed saddle supports under uniformly distributed loads. The design of horizontal containers also includes the design of horizontal supports. The supports for horizontal containers mainly include saddle supports, ring supports, and leg supports (or supporting supports). The currently applicable design code for supports is NB/T 47065-2018 \"Supports for Containers\". Various types of horizontal vessels, including storage tanks and heat exchangers, typically use saddle supports. Unless special circumstances require a custom design, the structure and dimensions of the saddle can generally be determined by selecting standard types of supports based on the nominal diameter of the equipment. For horizontal vessels, in addition to considering the film stress caused by operating pressure, it is also necessary to take into account the bending stress resulting from the vessel’s weight on the shell; therefore, even after using standard saddles, it is still required to perform checks on the vessel’s strength and stability. A horizontal container mounted on a double saddle experiences stress conditions similar to those of a beam with two supports; beams with multiple supports have lower bending stresses, but it is required that all supports remain at exactly the same level, which is difficult to achieve for various large-scale horizontal containers. Due to uneven settlement of the foundation, the support reactions at the supports of multi-support containers cannot be distributed evenly; therefore, it is generally better for horizontal containers to use dual supports. When using double supports, the selection of their location must take into account, on the one hand, the reinforcing effect of the end caps, and on the other hand, preventing excessive bending stresses in the shell due to loads. Therefore, the position of the supports is determined according to the following principles. ① According to mechanics of materials, for a simply supported beam under uniform loading at both supports, if the total length of the beam is L, then when the length of the overhanging end A equals 0.207L, the maximum bending moment at the midpoint of the span between the supports is equal in magnitude to the bending moment at the support sections. Therefore, it is necessary to ensure that the strength of the sections at the midpoint of the span and at the supports is as high as possible. Considering that there are other loads in addition to bending moments at the support sections, for horizontal containers it is generally adopted that A ≤ 0.2L (where A is the distance from the tangent line of the head to the centerline of the support, and L is the distance between the tangent lines of the two heads); otherwise, the stress at the support sections will become excessive due to the effect of the overhanging end of the container. ② Since the bending stiffness of the head is greater than that of the cylinder, the head provides local reinforcement to the cylinder’s resistance to bending. If the support is adjacent to the head, the reinforcing effect of the head on the cylinder section at the support can be fully utilized. Therefore, while satisfying A≤0.2L, it is advisable to keep A≤0.5Ri (where Ri is the inner radius of the cylinder). The size of the saddle wrap angle has a direct impact on the stress in the cylinder at the saddle area; generally, 120°, 135°, and 150° are used. To prevent additional stresses in horizontal vessels caused by thermal expansion, as well as bending of the vessel due to the weight of the cylinder and the material, only one of the two supports is allowed to be fixed, while the other must be movable. The foundation bolt holes of the movable support should be made as oblong holes, with the long axis aligned along the axis of the cylinder ； To enable the movable support to move flexibly, rolling bearings can sometimes be used. A precise theoretical analysis of the stresses in horizontal vessels supported on double saddles is very complex; currently, most domestic and international codes for vessel design adopt the approximate analysis and calculation methods proposed by Zick in 1951 based on experimental research. Ring supports are generally suitable for large-diameter thin-walled vessels and thin-walled vessels used in vacuum operations. Except for containers operating at normal temperature and pressure, at least one ring seat shall have a sliding support structure. The main advantage of leg supports is their simple structure, but the reaction forces they generate cause significant local stress on the housing; therefore, they are used only in relatively light, small-scale equipment.