Design of under-bottom connections for API standard 650 unanchored liquid-storage tanks

The application of API 650 is as stated by the American Petroleum Institute: The production and installation of our ground welded steel API standard 650 unanchored liquid-storage tanks storage tanks are generally used in industry; under normal circumstances, the storage tanks installed on the industrial site in the United States are in accordance with API 650, API 620 and API 12d Standards are established.

The most popular standards for API standard 650 unanchored liquid-storage tanks are API Std 650, JIS B8501, BS2654, GB50341, etc. The formulas used in each standard are derived from the hoop stress calculation formula of membrane theory. Combined with the stress characteristics of API standard 650 unanchored liquid-storage tanks, plus the allowable stress, weld coefficient and additional wall thickness, the wall thickness calculation formula is complete. The allowable stress and weld coefficient are determined according to the material performance characteristics and construction technology level of each country. According to different API standard 650 unanchored liquid-storage tanks standards, the calculation results of tank wall thickness are sometimes quite different.

In this project, a cylindrical tank 62 ft 6 in. in diameter is designed for an internal pressure of 4 lbf/in.2 gauge in the vapor space. The plate material is ASTM A131 steel with appropriate grades for different plate thicknesses in accordance with Table 4-1 for a design metal temperature less than 65°F but not less than 25°F. No corrosion allowance is required for any portion of the tank. The top course of the butt-welded cylindrical sidewall is 1/4 in. thick. The roof is a single lap-welded spherical dome with an internal radius of 50 ft and a thickness of 1/4 in. The maximum design liquid level is 6 in. below the plane of the juncture of the tank API standard 650 unanchored liquid-storage tanks roof and sidewall.

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