The lowest design temperature of API standard 650 low-alloy steel tanks

The working pressure of the low-pressure storage tank is greater than that of the atmospheric storage tank, but its pressure is less than 0.1MPa. API 650 petroleum storage tanks are classified according to their environment: Above-ground storage tank: refers to the bottom of the storage tank at the design elevation of API standard 650 low-alloy steel tanks and above; the bottom of the tank is below the design elevation of +0.00 but not exceeding.

Some defects may be found after the regular safety inspection for the API standard 650 low-alloy steel tanks. The common API standard 650 low-alloy steel tanks defects are: Tank cracks, tank welding defects. The cracks on the base metal, surface and near surface of the tank wall shall be eliminated by grinding with circular arc, and then magnetic particle inspection or re inspection shall be carried out. Cracks in heat affected zone: generally, grinding method is used to eliminate cracks, and magnetic particle or dye flaw detection is used to confirm that there is no crack, and then repair welding or grinding smooth transition.

In this project, an elemental area of tank wall used is constructed of ASTM A442 Grade 55 steel plate subjected to a meridional unit force, T1, of 4000 lbf/in.2 tension and a latitudinal unit force, T2, of 5060 lbf/in.2 compression. The meridional radius of curvature, R1, is 75 in., and the length of the normal from the surface to the axis of revolution, R2, is 300 in. The joints in the wall are of double-welded butt-joint construction with a tensile efficiency of 85%. A corrosion allowance of 1/16 in. is required.
The problem in this example is to graphically find the minimum thickness of tank wall required for the given conditions (see 5.10.3.3).
Solution: As determined from Table 5-1, the maximum allowable tensile stress value, Sts, for ASTM A442 Grade 55 steel plate in simple tension is 16,500 lbf/in.2. Since the compressive stress is latitudinal, the governing value of R in this situation is R1, or 75 in.

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