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# Waviness and Flatness in Shell

## Waviness and Flatness in Shell

(OP)
Hello I have a doubt about how to measure and analyze the flatness and waviness tolerance in the shell, API 650 12th states this in the paragraph 7.5.2 a), pointing that we should use ASTM A6, A20 or A480 for checking the tolerance for out plumbness in one shell course.

My main doubt it's how we measure, we just measure points in the shell course up and down and take the difference of the inner diameter of the tank, and then compare with the tables in those norms?

### RE: Waviness and Flatness in Shell

No, measuring the diameter will give you the roundness, not plumbness. To measure plumbness you need a plumb line, basicly a rope with a weight attached at he end of it from the roof to the ground level You will use this as a vertical reference. Next, you need to measure horizontal distance from this line to the top and bottom of each shell course and take the difference.

### RE: Waviness and Flatness in Shell

Or a long spirit level ( length of one course)

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### RE: Waviness and Flatness in Shell

(OP)
Thanks a lot both of you for your answer, I got this issue since we start construction, and always insisted for use a plumb line instead we use topography for measuring.

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