API-653 AST Minimum Shell Thickness Determination

Please refer to API-653, Section 4.

Following an inspector's initiative to scan about a corrosion area on the tank shell, a point of minimum wall thickness is located at t2.

And then, L is calculated by,

L = 3.7 x SQRT (D x t2).

The question is,

How should the range of L be located within the given corrosion area ?

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Please comment as follows :-

1. If t2 is located approximately central within the given corrosion area,

should the mid-point of L be aligned at t2 ?

2. If t2 is located near the upper border of the given corrosion area,

should the range of L extend down-wards

such that the top end-point of L remains at t2 ?

3. If t2 is located near the lower border of the given corrosion area,

should the range of L extend upwards

such that the bottom end-point of L remains at t2 ?

4. There's a note stating that, the given corrosion area may be vertically larger than

the linear dimension of L calculated.

However, if D = 200 feet, t2 = 0.5 inches ; then L = 37 inches.

That means that, if the given corrosion area is relatively small,

the range of L would extend beyond the border thereof.

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Please share your experience and enlighten the above.

Regards.