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This post was last edited by Freestyle-sky on 2022-5-15 18:42. A spherical storage tank, as the name suggests, has a spherical shape; it is an effective and economical pressure vessel used for storing and transporting various gases, liquids, or liquefied gases, and it is widely utilized in industries such as chemistry and petroleum. Under the same wall thickness conditions, compared to containers of other shapes, it has uniform stress distribution, high load-bearing capacity, and can significantly save material costs. Based on the characteristics of spherical tanks, in a certain natural gas transmission project, there was a storage vessel (10 m3) used for measuring natural gas flow, designed as a spherical tank. Due to the design requirements, strict limits were imposed on the weight of the equipment, with the goal of minimizing its weight. The volume of this spherical tank is 10 m3, and the GB12337-2014 standard does not apply to spherical tanks with a nominal volume of less than 50 m3; therefore, this spherical tank cannot be designed in accordance with GB 12337-2014. The reason why GB12337 is not applicable to spherical tanks with a volume of <50 m3 is that the GB12337 standard for \"Steel Spherical Storage Tanks\" does not set an upper limit on the engineering volume of such tanks; only a lower limit is specified, namely the nominal volume of a spherical tank must be ≥50 m3. The main reasons for this are as follows: 1) Spherical tanks with a volume of less than 50 m3 are mostly used under high pressure for special purposes, and the manufacturing and inspection requirements associated with them are also special, differing significantly from those applicable to regular spherical tanks. Their scope of application is limited. This requirement is consistent with the standard’s stipulation that the design pressure cannot exceed 6.4 Mpa (for example, the spherical tank in question has a volume of 10 m3 and a design pressure of 10 Mpa under high pressure, and it is primarily used for storing natural gas). 2) Compared with cylindrical storage tanks, spherical tanks with a volume of less than 50 m3 require less material, but due to the large number of panel pieces involved, on-site assembly and welding are more difficult; the weld lengths are long, the manufacturing cycle is prolonged, and the overall economic efficiency is poor. Therefore, when the volume is less than 50 m3, it is more economical to use a cylindrical tank rather than a spherical tank. Main design parameters of the spherical tank: This spherical tank needs to be analyzed and designed in accordance with the JB4732 standard. Its design conditions and main parameters are shown in Table 1 below: Table 1 Main design parameters of spherical tanks. Selection of spherical tank materials: For the selection of materials, steel plates used for spherical shells are high-strength quenched and tempered steel plates 07MnNiMoVDR suitable for low-temperature applications ； 10Ni3MoVD forgings are used for pressure-bearing spherical tank forgings. All the aforementioned materials have undergone safety registration and passed the technical evaluation by the \"Boiler and Pressure Vessel Standardization Technical Committee.\" The chemical composition and material properties of the materials used for spherical tanks are shown in Tables 2 to 5 below: Table 2: Technical requirements for the chemical composition of 07MnNiMoVDR steel plates (melting analysis, %); Table 3: Technical requirements for the chemical composition of 10Ni3MoVD steel forgings (melting analysis, %); Table 4: Design stress strength values for spherical tanks; Table 5: Material properties of spherical tanks. Description and schematic diagram of the spherical tank structure: This spherical tank structure consists mainly of a spherical shell plate along with seven nozzles, and its support type is a skirt foundation. Square manholes (450×450) are provided on both sides of the skirt base for equipment installation and maintenance. To ensure the safety of the equipment, the openings in the sphere shell are reinforced using integral biconical section forgings, and the connection between the pipe fittings and the sphere shell plates is made by butt welding, in order to meet the requirements for 100% radiographic inspection. After deducting the material’s corrosion allowance and forming thinning, the calculated thickness of the shell plate is 29.5 mm. The main structure and dimensions of the spherical tank are shown in Figure 1 below: Figure 1: Schematic diagram of the spherical tank’s structure; Figure 2: Schematic diagram of the nozzle structure. A finite element model of the spherical tank was developed. To ensure the integrity of the tank’s structural framework, this analysis adopted an overall approach, using the advanced finite element software ANSYS along with SOLID95 solid elements to create a three-dimensional finite element model of the tank. SOLID95 is a 20-node high-order element; each node has 3 translational degrees of freedom (in the X, Y, Z directions). It allows for certain irregular shapes while ensuring better computational accuracy, and it exhibits good compatibility with offset shapes, making it suitable for simulating models with curved boundaries. Number of elements in this model: 159,108; number of nodes: 732,617. The finite element model is shown in Figures 3 and 4 below: Figure 3 shows the overall finite element model of the spherical tank, while Figure 4 shows the finite element model of the local nozzles on the spherical tank. Design load conditions for the spherical tank: The analysis and design of this spherical tank take into account two main types of loads: internal pressure and its own weight. For the overall analysis of the spherical tank, the following combinations of loads need to be considered: 1) Condition with only internal pressure acting ； 2) Only the self-weight of the spherical tank is considered (this factor is not required according to standards under this condition; it has been included in the calculations here for a more straightforward explanation) ； 3) Consider the combined condition of internal pressure + the self-weight of the spherical tank as a whole. Note: Since the spherical tank is installed and operated indoors, with no insulation or ladder platforms, and given its small size and relatively low weight, the effects of wind load, snow load, and seismic load are ignored ； Furthermore, depending on the pipeline conditions, there is no mechanical external load on the spherical tank nozzle. Application of boundary conditions for the spherical tank: 1) Internal pressure: A pressure load of (10.0 MPa) is applied to the inner surfaces of the spherical shell and the nozzles. At the same time, to ensure balance in the pressure system within the equipment, an axial balancing pressure generated by internal pressure is applied at the end of the connection pipe ； 2) Self-weight of the spherical tank: The operational mass of the spherical tank, including the weight of the shell, nozzles, skirt, contents, and other accessories such as those used for pre-welding of the tank. In the finite element model, the self-weight load of the spherical tank is handled using the equivalent density method. The equivalent density of the material is calculated (ρ=m/V), and this value is converted into a force per unit volume within the acceleration field in the form of an inertial load (G=mg), which is then applied to all the shell elements. 3) Displacement boundary conditions: Vertical axial and circumferential displacement constraints are applied to the bottom surface of the spherical shell skirt. The application of the load and displacement boundary conditions is shown in Figure 5 below: Figure 5 shows the load and displacement boundary conditions for the finite element model. The stress calculation results are presented in Figures 6 to 7: Figure 6 shows the stress under internal pressure conditions, with the maximum stress value being 470.82 Mpa; Figure 7 shows the stress under self-weight conditions, with the maximum stress value being 7.54 Mpa; Figure 8 shows the stress under both internal pressure and self-weight conditions, with the maximum stress value being 470.67 Mpa. Analysis indicates that the combined condition of self-weight and internal pressure represents the most hazardous scenario for the equipment. The stress generated by self-weight is relatively low, far less than that caused by internal pressure, and constitutes a very small proportion of the total stress level, thus having little impact on this spherical tank. Therefore, for the strength verification of the spherical tank, it is sufficient to evaluate the condition of \"self-weight + internal pressure\". At the welds of various parts of the spherical tank, the minimum value of the corresponding design allowable stress strength among different materials is used to evaluate the strength of the equipment. Stress intensity assessment: In accordance with the provisions of JB4732-1995, the critical sections of the finite element model are selected to determine the dangerous cross-sectional dimensions for stress linearization classification, and then the various stresses are classified for assessment. Meanwhile, to simplify the process of evaluating the computational workload, in this analysis, the combined second stress intensity SⅣ is considered based on the design conditions. At the locations where the maximum stresses may occur, 12 custom paths are defined, as shown in Figure 9 below: Figure 9 Schematic diagram of path definitions. The stress intensity assessment results are shown in Table 6 below: Table 6 Material properties of the spherical tank. Note: 1. The film stress for path PATH_01 is considered to be the primary film stress SI, and it is evaluated using a value of 1.0·K·Sm ； 2. The film stress in other paths is considered as a local film stress SII, to be evaluated at 1.5·K·Sm ； 3. The membrane + bending stress on all paths is considered a secondary stress SIV, to be evaluated as 3.0·Sm ； The content of this article is taken from the proceedings of the ANSYS User Conference. The authors are Qiu Bo and Cheng Wei from China Global Engineering Company; it has been compiled and published by the author. The views expressed in this article represent solely those of the paper’s authors, and it is intended for academic purposes, as well as for discussion and exchange. Welcome to search for and follow the WeChat official account \"ANSYS Analysis and Design Professionals\" – a platform dedicated to the analysis and design of pressure vessels. 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