Technical Instructions for the Installation of Spherical Storage Tanks

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I. Project Overview: The 1000m3 ethylene sphere tank in the storage and transportation area was supplied by a foreign party, which provided the sphere shell plates as well as all the necessary accessories. The material of the ethylene sphere tank is Japanese LT50-A-50G. The shell plates are of a mixed type (orange-petal shaped in the equatorial zone, and football-petal shaped in the upper and lower covers), and it consists of 26 shell plates in total across three zones and two poles, along with 7 struts. The diameter of the sphere is 12.41 m, and the thickness of the spherical shell plates is 36 mm. II. Construction Preparation 1. Materials for the project and those for construction measures (1) Main materials for the project: The main materials for spherical tanks include 14 equatorial plates, 2 top polar plates, 4 upper temperature zone plates, 2 bottom polar plates, 4 lower temperature zone plates, 7 steel pillars and tie rods, as well as steel components such as manholes, connections, and steel ladders, all provided by the manufacturer. (2) Materials for the construction measures: These materials mainly include the steel platforms and fixtures required for the sectional welding of spheres, as well as materials used for lifting and deformation prevention reinforcement; in addition, there are materials such as welding protection sheds and central umbrella frames utilized for welding. Steel plates, pipes, shaped steel, wooden planks, sleepers, and insulation materials are primarily used. 2. Main construction equipment The main construction equipment includes: truck cranes, welding machines, control cabinets, plate-type infrared heaters, electrode drying ovens, gamma-ray equipment, angle grinders, manual chain guides and gas cutting tools, as well as theodolites, levels, hydraulic pumps, high-pressure fans, etc. 3. Preparation and conditions for on-site work: (1) Conduct re-checks of the geometric dimensions of components such as shell plates, struts, steel ladders, and manhole connections, as well as random non-destructive testing inspections. At the same time, it is also necessary to verify the quality certificates for the aforementioned materials, and to keep records of the testing data as well as archive them. (2) Re-measure the axis, elevation of the spherical tank foundation, and the dimensions of the anchor bolts, and simultaneously carry out the acceptance procedures for the intermediate handover. (3) All materials required for the assembly of spherical tanks, such as steel platforms, jigs, tooling, central umbrellas, and wind protection shelters, must be fabricated and made fully available. (4) The storage order for electrodes, drying equipment, as well as electrode insulation sleeves and preheating equipment must all be available, and it is necessary to ensure that these devices are in good condition. (5) Ensure smooth access for cranes at the construction site and a reliable supply of electricity for construction, and provide adequate fire-fighting equipment and safety devices. III. Labor Organization: To form a construction team specialized in the installation of pressure vessels, 8 riveters, 14 welders, 7 grinders, 2 scaffolders, 1 gas welder, 3 signalmen, 2 maintenance electricians, and 1 surveyor are required. IV. Main Construction Techniques 1. Construction Procedure In accordance with the installation guidelines provided from abroad, the assembly and welding of the sphere are carried out using a zonal assembly method. The main construction procedures are as follows: (1) The upper and lower pole plates as well as the temperature zone plates are assembled and welded separately on the ground-based steel platform and molds. (2) After the welding of the current plate and the temperate zone plate is completed, lift them onto the jig inside the tank foundation. Then, the steel pillars and equatorial belt plates are welded together. (3) Weld the lower hemispherical part and the lower ring opening of the equatorial band together to form a single unit, and finally weld the upper hemispherical part and the upper ring opening of the equatorial band together. (4) After the entire sphere has been assembled and welded, the installation of accessories such as spiral ladders is carried out, along with non-destructive testing of the welds. (5) Once all welding work is completed, the sphere can undergo hydrostatic testing and airtightness testing. 2. Construction techniques (1) Assembly and welding of the upper and lower dome covers: On the steel platform on the ground, steel pipes are used to create molds for the plate and the tempering plate, which are employed for the assembly of the lower cover and the welding of the outer welds. A supporting jig was fabricated using spherical shell plates to facilitate the assembly of the upper cover and the welding of the inner welds. The assembly of the lower cover involves welding the positioning blocks of the assembly fixture to the inner side of the spherical shell plate, while the upper cover is welded to the outer side of the spherical shell plate, in order to facilitate welding after the assembly is completed. The assembly procedure for the lower cover is as follows: first, the four temperate plates are lifted onto the jigs on the steel platform for alignment and adjustment; then, the two polar plates are lifted into place; finally, the overall geometric dimensions are adjusted. The assembly procedure for the upper cover is as follows: first, lift the two electrode plates and align them on the proper position within the spherical tank foundation, then lift the four temperature control plates into place, and finally adjust the overall geometric dimensions. Before welding, it is necessary to implement support and reinforcement measures to prevent deformation due to welding as well as during lifting and turning. (2) Lifting of the upper and lower dome covers by turning them over. The diameter of the ring opening of the upper and lower dome covers is ф=10.19m, the arch height is H=3.25m, and their weight is 33.7t. Once the welding of the inner weld on the upper cover and the outer weld on the lower cover is completed, the lifting and turning of the upper and lower dome covers can be carried out. Two 70t truck cranes were used to lift the upper dome cover off the jigs in the foundation and move it horizontally to the ground. Then, tie the lifting points of the 2 cranes to the two lugs at the symmetric points of the upper cover, and lift it off the ground simultaneously. At this point, another 15t crane is used to pull down the bottom of the lower cover, turning the upper cover over onto the fixtures on the steel platform, after which the root areas of the outer welds are cleaned and welded. The flipping procedure for the lower cover is the same as that for the upper cover. After flipping the lower cover, lift the lower cover body onto the mold inside the spherical tank foundation. Subsequently, the root cleaning of the inner weld can be carried out, followed by step (3): the lifting of the equatorial band. Before lifting the equatorial band plates, it is necessary to first install the central column umbrella frame inside the tank, then install the spherical struts and tie rods, and finally lift the equatorial band plates. First, lift the 2 equatorial strip plates with upper support pillars, then lift the middle plate and place it between the two plates for assembly; repeat this process until all the equatorial strip plates have been lifted. After adjustment and successful geometric dimension inspection, welding of the inner and outer welds can proceed. (4) Welding of the upper and lower dome covers with the equatorial band: A 160t hydraulic crane is used to lift the lower cover first, followed by welding it to the equatorial band. Then, lift the upper cover to the ring opening on the equatorial band to carry out the welding of the ring opening. During the assembly of the upper and lower dome covers with the equatorial band ring, any misalignment or gaps at the ring area must be adjusted properly before the crane lifts the structure, and everything must be secured firmly. The alignment quality of the upper and lower ring covers with the equatorial band ring opening is extremely important, and it is also very difficult. Therefore, careful adjustments and thorough retesting are necessary; only after confirmation of compliance can the spot welding and welding work on the ring joints be carried out. (5) Welding of spherical tanks: The welding material used for ethylene spherical tanks is the L-60S electrode produced by Nippon Steel in Japan. After being baked for 1.5 hours, the welding rod is stored and used in an incubator at a temperature of 100–150°C; the maximum time it can be taken out at one time is 4 hours. Full-position manual arc welding is employed, using a DC arc welder from the AX-320 series with the electrode connected in reverse polarity. The welding preheating temperature is 100–150°C, the interpass temperature during welding is 100–200°C, the post-heating treatment is at 150–200°C for 1 hour, and the welding heat input is controlled within the range of 13–26.9 kJ/cm. The principle of welding procedures is: weld the longitudinal seams first, then the transverse seams, and finally the circumferential seams ； First, weld the bead on the side with the larger groove, and then weld the bead on the side with the smaller groove ； Each weld pass should be laid in segments for multi-layer, multi-pass welding. The welding distribution requirements for the welds are as follows. 1) The upper and lower temperate longitudinal seams are welded simultaneously by 4 welders each; the end of the longitudinal seam where the arc stops falls into the groove of the circumferential seam. When welding the circumferential seam, this portion is removed before continuing with the welding process. 2) For the butt weld between the two plates, two welders shall simultaneously weld back in segments from the middle of the weld toward both ends; the requirements for the end of the weld where the arc is stopped are the same as those in (1). 3) The circumferential joint between the polar plate and the temperate plate was welded simultaneously from the right by 4 welders distributed evenly. The starting point should be about 500 mm away from the Y-shaped joint; it is strictly prohibited to start or end the arc at the intersection of the Y shape. 4) For each longitudinal seam in the equatorial zone, two welders divide it into two sections from the middle; one welder welds upward from the middle, while the other welder welds downward from the middle. 5) The circumferential seams of the equatorial zone and the upper and lower dome covers were welded simultaneously in segments by 7 welders distributed evenly. The distance between the segmentation points and the T-joint at the intersection of the equatorial zone longitudinal seam and the circumferential seam is 500 mm. 6) When welding the first and second layers of the root pass on large grooves, it is required that the welding be completed continuously on the same day, with a welding interval between the two layers not exceeding 15 minutes. 7) Arc starting plates and arc ending plates must be provided at the start of welding for all longitudinal seams. (6) Weld inspection: 48 hours after welding, 100% fluorescent magnetic particle testing (MT) and penetrant testing (PT) shall be conducted on the weld seam; ultrasonic testing (UT) shall be carried out on the weld seam and the heat-affected zone; furthermore, gamma-ray panoramic radiography shall be used to assess the quality of the weld seam. (7) Hydrostatic and airtight tests of the spherical tank: Once all welding work on the spherical tank is completed and all inspections confirm it to be satisfactory, hydrostatic and airtight tests can be carried out. Specifically, a pressure testing plan is prepared separately, along with dedicated technical briefings. V. Quality Standards 1. Mainly comply with construction specifications (1) Technical documents provided by the Italian company TPL ； (2) Regulations on Safety Technical Inspection of Pressure Vessels ； (3) \"Steel Spherical Storage Tanks\" ； (4) Volume 8, Book 2 of the American ASME standards. 2. Key points for quality inspection (1) Quality requirements after assembly (before welding): After the spherical shell plates are aligned, a comprehensive re-inspection should be carried out. Its misalignment should not be greater than 2 mm ； The angular deformation at the weld shall not exceed 7 mm ； The unevenness at the end of the plate shall not exceed 2 mm ； The out-of-roundness of the upper and lower ring openings shall not exceed 10 mm ； The diameter deviation shall not exceed 20 mm. The verticality deviation of the pillar shall not exceed 10 mm. (2) Post-welding quality requirements: The misalignment of the spherical shell plates after welding shall not exceed 3 mm ； The edge angle at the weld shall not be greater than 10 mm ； The out-of-roundness of the sphere shall not exceed 124 mm ； Sphere diameter deviation ±80mm ； The verticality of the pillar shall not exceed 24.2 mm. The pass rate for all non-destructive testing methods regarding weld quality is 100%. The hydrostatic and airtightness tests passed on the first attempt. VI. Precautions (1) When installing the spherical tank using a zoned welding method, it is necessary to carefully inspect the welding of the lifting lugs and reinforcing supports on the sphere to ensure that they meet the specified requirements, thereby guaranteeing the safe turning and lifting of the spherical cover ； (2) The actual weight of each lifting piece must be calculated accurately; the type of lifting equipment chosen should be appropriate, and there must be a sufficient safety factor. (3) When performing various tasks inside a closed sphere, rubber-soled shoes should be worn, and the tank should be equipped with ventilation and dust removal facilities. (4) After the internal and external scaffolding for the spherical tank are erected, a dedicated person shall conduct inspections and acceptance; it may be put into use only after it is confirmed to be safe and up to standard. (5) When lifting the first few equatorial strip plates, it is essential to use steel cables and hand-cranked winches to tighten and secure them firmly. (6) When conducting gamma-ray testing and pressure testing on spherical tanks, sufficient safety exclusion zones must be established; all non-operational personnel are strictly prohibited from entering these zones.