1. Introduction
A storage tank with a partition plate is an atmospheric pressure vessel, which separates the cylindrical shell to form multiple regions to storage different kinds of liquid contents in a single tank [
1]. According to the relevant regulations of GB 50160-2018, it is the “Standard for fire prevention design of petrochemical enterprises” [
2]. The development of hydrocarbon production in industrial plants and the close distance between these facilities usually lead to the spread of major accidents in the space, which leads to the expansion of the affected area [
3,
4,
5,
6]. In order to prevent fire from spreading due to the outflow of combustible liquid materials in case of tank leakage, a certain fire separation distance shall be maintained between adjacent tanks, and fire prevention devices such as fire dike and partition dike shall be set. However, in some cases, especially when the original plant is increased or reconstructed, due to many factors such as plant layout, site constraints, and so on, the storage tank with partition plate is often used to store different materials to meet production requirements. Compared with other types of storage tanks of the same volume, it has the advantages of more centralized overall layout, smaller floor area, easier tank farm management and lower investment cost. Therefore, it is widely used in petroleum, chemical, pharmaceutical, and other, industries [
7]. During the normal operation of the storage tank, the liquid level difference on both sides of the partition plate is very small. However, under extreme working conditions, that is, when one side of the partition plate is full of materials and the other side is empty while negative pressure exists, the partition plate will bear a large external pressure, which is likely to cause large displacement of the structure of the partition, resulting in overall strength and stiffness problems, strength and buckling failure, affecting the safe operation of the storage tank and causing economic losses [
8,
9].
During normal operation of the storage tank with partition plate, due to the small difference in liquid levels on both sides of the partition plate, the force on the partition is small, and its structure is safe. However, under extreme working conditions, that is, when one side of the partition is full of materials and the other side is completely empty and negative pressure exists, the partition needs to withstand greater external pressure, which is likely to cause greater displacement of the partition structure. There will be overall strength and stiffness problems and strength and buckling failure, and other problems will occur, which will affect the safe operation of the storage tank and cause economic losses. As NB/T 47003.1-2009 “Steel welded atmospheric pressure vessels” [
10] and AQ 3053-2015 “Safety technical code for vertical cylindrical steel welded tank” [
11] and other standards do not put forward the design requirements or contents for the structure of atmospheric vessel with partition plate, it is impossible to use theoretical formulas to design the structure of such vessels and guarantee the reliability. At present, the structural design of the storage tank with a partition plate mainly adopts the finite element analysis method in China. Commonly used methods of increasing wall thickness to ensure safety will also lead to failure and increase in manufacturing costs. Among them, Hong Ying [
12] investigated the loaded structure of partition plates with folded plates and S-shaped partition plates for storage tanks and studied the reinforced effect of the tie rod structure. In combination with the design of a water tank, Lu Xi [
13] analyzed the structure of the vessel with partition plate, and its characteristics proposed a new type of reinforcing structure combining plates, stiffening ring and strut. Zhu Weibo [
14] conducted FEA and experimental study on a large-diameter atmospheric-pressure vessel with a partition plate.
This paper analyzes the problems of the storage tank with S-shaped partition plates that have been in operation in the factory for many years.Problems such as cracks in the welded parts between the partition plate and center vertical columns are shown in
Figure 1, and buckling and depression of the partition plate is shown in
Figure 2. The strength and stability of the overall model are analyzed with finite element software. According to the analysis based on the numerical model, by adding stiffeners and tie rods, the overall rigidity of the optimized storage tank is significantly improved. At the same time, the improvement measures for the storage tank compartment to operate according to a certain safe operation curve are proposed.
2. Original Model Finite Element Analysis
The model dimensions are based on the original design parameters provided by the factory, and the influences of openings and nozzles on the tank, are ignored. According to process requirements, the inner diameter, height and wall thickness of the tank are 5000 mm and 6000 mm. The wall thickness of the tank is determined in accordance with the formula provided in article 5.3.1 of SH3046-1992 “Petro-chemical design specification for vertical cylindrical steel welded storage tanks”:
where t is the design thickness when storing the contents, mm; ρ is the density of the storage liquid medium, kg/m
3; H is the calculated height of the liquid, m; D is the diameter of the tank cylinder, m;
is the allowable stress of the tank wall steel plate at the design temperature, MPa; Φ is the weld coefficient, 0.9; C
1 is the negative deviation of the steel plate, mm and C
2 is the corrosion allowance, mm. Therefore, the design thickness of the storage tank in this paper is 8 mm, the thickness of bottom plate is 8 mm, the incurvature radius and thickness of S-shaped partition plate are 3300 mm and 8 mm and the outer diameter and wall thickness of the central column are 250 mm and 8 mm. The schematic drawing is shown in
Figure 3. The tank is made of stainless steel 304L with excellent corrosion resistance and heat resistance. When the design temperature is 150 °C, the elastic modulus is
, the Poisson ratio is 0.3, the allowable stress is 118 MPa, the corrosion allowance is 0 mm and the minus deviation of thickness is 0.3 mm.
The analysis of the straight partition plate of a vertical storage tank can usually be calculated by the theory of the original flat plate of the surrounding fixed branch. However, the storage tank with S-shaped partition plate has a complex structure and many influencing factors, so it cannot be calculated theoretically. Therefore, it is calculated by numerical analysis.
The structural dimensions of the model adopt the original design parameters provided on the drawings of the factory, and the influence of the opening nozzle on the tank body is not considered during the modeling. The upper part of the tank is welded to the head. Since the influence of loaded hydro-static pressure of liquid column on the head is slight, the model is built only for the parts below the head. The mesh of the finite element model is finely divided by the SOLID186 elements. The accuracy of the finite element analysis results is not only related to the correctness of the load and boundary strips, but also to the mesh size and density of the finite element model. It is necessary to verify the mesh independence of the finite element model of the storage tank and the mesh convergence. The results of the analysis are shown in
Table 1. When the number of meshes is 15 × 15 mm, and then the meshes are further refined, and the finite element calculation result is only about 1.7% different from that before the refinement. It can be considered that the division accuracy of the model unit meets the calculation requirements. In order to save calculation memory and calculation time, this paper adopts the mesh cell size of 15 × 15 mm to perform finite element analysis on the tank model.
During operation of the storage tank, the liquid height of the contents on both sides of partition plate is fluctuating, and there is a density difference between the contents on both sides of the partition plate. Liquid sloshing refers to the widely existing phenomenon that liquid moves in a container when the tank is subjected to the external excitation, and it will have serious impacts on the security and stability of the system [
15,
16].The design condition shall be considered for the design of the storage tank, that is, one side of partition plate is fully filled with relatively high-density contents, and the other side is completely empty with negative pressure. Wherein, the distributed load of pressure gradient on the inner surface of one side of the tank caused by the height of the filled liquids can be calculated by
where P is the pressure caused by the liquid height of the content, MPa; ρ is the content density,
and h is the liquid height of the content, mm. The calculation results are shown in
Figure 4. The pressure gradient distribution due to the liquid height of
and the internal pressure of
are applied to the inner surface of one side of the tank fully filled with content. The negative pressure of
is applied to the inner surface of the empty side of the tank. Because the tank is fixed on the foundation, to avoid rigid displacement of the model, the bottom surface of the storage tank is clamped to restrain. When the top of the tank is welded with head, if the reinforcement effect of the head on the tank is considered, the cross-section of the top of the tank is clamped to restrain.