API Std 650 carbon and low-alloy steel tanks with free vents
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.
The calculated thickness of the above-mentioned wall panels required by the hydrostatic pressure is also It API Std 650 carbon and low-alloy steel tanks should be increased by the same ratio, unless special analysis is performed to determine the seismic overturning moment and the corresponding stress at the bottom of the wall of each layer of API 650 tank.
In January, 2019, our South Africa customer purchased 669 tons S235J2+N carbon steel plates to use in their tank fabrication project . S235J2 steel, as a non-alloy structural steel grade, is mainly used in structural projects and tanks. J2 symbol -20 temperature impact test. The client required CEV 0.38%, S≤0.01%, 100% UT, Charpy V-notch impact test at-20℃, Z25 for S235J2+N plates. After they receiving the S235J2+N steel plates, the customer are very satisfied with the quality of this batch of API Std 650 carbon and low-alloy steel tanks products.
}Generally, external environment corrosion should be considered before design and construction of API Std 650 carbon and low-alloy steel tanks and so on storage tanks. The corrosion on the outer surface of storage tank is often worse than that on the inner surface. Especially in the chemical industry area, there are often acid fog, alkali or salt dust in the atmosphere. These impurities form an active corrosive medium with dew or steam and oxygen in the atmosphere. The corrosion of the external environment of the storage tank shortens the maintenance period of the storage tank, even causes the storage tank to be scrapped in advance, which affects the normal operation of the storage and transportation.
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