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Analysis of the Causes of Rusting on the Floating Roof of Floating Roof Oil Tanks and Methods for Preventing Rusting Abstract: Rusting on the floating roof of floating roof oil tanks poses a threat to the normal operation of these tanks. This paper analyzes the causes of rusting on the floating roof, and based on experience in using such tanks, proposes methods to prevent rusting on their floating roofs. Keywords: floating roof tank, corrosion, deformation, welding. I. Introduction to the problem: The four 5×105 m3 oil tanks in the Xishan tank farm of the oil depot have a double-disc floating roof design, and the upper and lower roof plates have developed rust-induced perforations in numerous areas over time. Cylinders 2# and 3# in the northern tank area of the oil depot have a capacity of 2×105 m3; they feature a single-disc floating roof design, and there are also multiple instances of rust-induced perforations. When a hole appears in the top plate of the oil tank’s floating roof, rainwater enters the floating compartment; in mild cases this accelerates the corrosion of the floating roof, while in severe cases it can cause the roof to tilt and lead to its sinking. After perforation of the lower roof plate (single disc), oil leaks out; in mild cases this leads to increased oil loss, while in severe cases it can cause a roof collapse accident. II. Causes of floating roof corrosion: The corrosion of floating roofs is mainly caused by factors such as disc deformation, the quality of the steel material, and the quality of welds; it results from the combined effect of these various factors. 1. The single disc adopts a \"strip\" layout, with the entire disc consisting of several interconnected strip-shaped plates. Due to the large number of long seams, this layout tends to cause significant wave-like deformation during assembly and welding, and the degree of deformation as well as its distribution are uneven. Furthermore, during welding, an uneven temperature field is generated in the vicinity of the weld area, causing the workpiece to expand and contract due to heat. However, due to the constraints imposed by the joint, in most cases the workpiece cannot expand and contract freely as its temperature changes. Therefore, during the cooling process after welding, the areas that have undergone plastic deformation due to welding heating are subjected to tension as they cannot contract freely. This results in an uneven internal stress field within the workpiece, causing welding residual stresses and welding deformation. This type of welding deformation also occurs near attachments such as floating roof supports. After the single disk deforms, since the rainwater on the floating roof cannot be completely drained out of the tank through the central drain pipe via the sump, corrosion and perforations occur in those areas that remain submerged in water for extended periods of time. Furthermore, after deformation of a single disk, certain areas (such as those around the welds of the floating roof supports) become separated from the oil surface, creating small pockets of air and oil. As the temperature changes, the pressure in these pockets also changes, which can lead to cavitation in those areas and ultimately result in perforation of the disk. 2. The elastic modulus of steel is directional; the greater the difference in elasticity between the longitudinal and transverse directions, the more difficult it is to control the degree of deformation. Moreover, the greater the degree of deformation, the more likely corrosion will occur. If the steel itself has defects such as sand holes, the likelihood of corrosion-induced perforation increases. 3. Defects in the welds are also a significant cause of corrosion-induced perforations; many leakage points occur at the welds. III. Methods to prevent rusting of floating roofs 1. From a design perspective, it is possible to adopt a layout pattern with minimal deformation, such as a “human”-shaped layout, which involves an interlaced arrangement of elements; this type of layout helps to avoid the problem of uneven stress concentration. Secondly, minimize the number of welds, arrange their positions carefully, and try to keep the welds symmetric with respect to the neutral axis of the cross-section, in order to reduce welding deformation. 2. When selecting steel materials, choose those with a low directional variation in elastic modulus; strictly ensure the quality of the steel by conducting inspections in accordance with relevant standards, and ensure that there are corresponding quality certification documents available. It is also necessary to prevent deformation and corrosion during the storage and handling of steel. 3. Measures should be taken from the aspects of installation and welding processes to reduce deformation. When assembling the floating roof on the temporary jig, the slope of the temporary jig during assembly should match the slope specified in the design. Additionally, diagonal supports should be installed at the columns of the temporary jig for the floating roof, and the outer crossbeam jigs should be connected and positioned in relation to the tank walls. When arranging the boards, cut the top layer at the overlapping area of the three boards at an angle; this prevents the edges from warping. During welding, welding methods with a more concentrated heat source should be used, such as gas shielded welding and submerged arc automatic welding. A proper welding sequence not only reduces welding deformation but also lowers welding internal stresses; during welding, start from the center of the tank, distribute the welders evenly, and carry out welding symmetrically. The welding sequence is: weld short seams first, then long seams ； Weld the middle weld first, then the welds on the sides. All welds must undergo visual inspection as well as vacuum and kerosene leak testing. The counter-deformation method is a common technique used to prevent welding deformation; it involves reserving a deformation amount in the opposite direction during welding, so as to counteract the welding deformation. Rigid fixation involves securing the components to limit welding deformation; the disadvantage of this method is that it increases welding internal stresses. 4. During the use of the oil tank, it is necessary to avoid water accumulation on the floating roof as much as possible. In rainy or snowy weather, the rain and snow accumulated on the floating roof should be cleared promptly to reduce corrosion damage. Try to avoid operating with empty tanks in order to reduce the damage caused by cavitation to the floating roof. References: 1. Handbook of Oil and Gas Storage and Transportation Projects 2. Oil and Gas Storage and Transportation