API 650 tanks roof and top-angle joints

Corrosion allowance, if specified, shall be added to calculated API 650 tanks shell and self-supporting roof thicknesses; to minimum nominal thicknesses of bottom and supported API 650 tanks roof plates; to nozzles and manholes; and to all exposed surfaces of internal structural members and non-removable internals.

Corrosion of the API 650 tanks body caused by external conditions such as high temperature, rain, humidity, or inadequate anti-corrosion measures of the oil storage tank itself, the water-bearing deposits in the oil storage tank caused corrosion and perforation of the inner surface of the oil storage tank bottom plate. Defects fatigue cracks caused by stress, with the accumulation of time, the compressive strength of the API 650 tanks body decreases and corrosion cracks occur.

Allowable compressive stresses shall be in accordance with 5.5.4 except that for aluminum alloy plate the allowable compressive stresses shall be reduced by the ratio of the modulus of compressive elasticity to 29,000 for values of (t – c)/R less than 0.0175 and by the ratio of the minimum yield strength for the aluminum alloy in question to 30,000 for values of (t – c)/R equal to or greater than 0.0175 (see 5.5.2 for definitions). In all other equations in this standard API 650 tanks where yield strength or modulus of elasticity is used, such as Equations 27 and 28, similar corrections shall be made for aluminum alloys.

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