Peaking & Banding Inspection API 650 Tanks

[Raminshah Muhammed]

As per API 650, local deviations from the theoretical shape (for example, weld discontinuities and flat spots) shall be limited as follows:

a) Deviations (peaking) at vertical weld joints shall not exceed 13 mm (1/2 in.). Peaking at vertical weld joints shall be determined using a horizontal sweep board 900 mm (36 in.) long. The sweep board shall be made to the nominal radius of the tank.

b) Deviations (banding) at horizontal weld joints shall not exceed 13 mm (1/2 in.). Banding at horizontal weld joints shall be determined using a straight-edge vertical sweep board 900 mm (36 in.) long.

How is this value verified at site? For example, at a vertical joint, if one side shows 6 mm and the other side shows 7 mm, is it acceptable? Or should the average be considered? Also, how can we determine whether the deviation is negative (-) or positive (+)? Can anyone please explain?

[Bhavani Prasad Emani]

What API 650 is asking you to check

API 650 limits local shape distortion at welds:

Vertical seam → peaking (a local “kink”/step at the vertical joint)

Horizontal seam → banding (a local “belt”/flat/step around the circumference at a horizontal joint)

In both cases the maximum allowed deviation is 13 mm.

A) How to verify peaking at a vertical weld (sweep board – 900 mm)

Tools: 900 mm long curved sweep board (curved to the nominal tank radius) + feeler gauge / wedge gauge (or a dial gauge).

Field method:

Put the sweep board horizontally across the vertical weld seam so it spans ~450 mm each side of the weld (board centered on the weld).

Hold it snug to the shell (usually you rotate/shift it slightly to ensure it’s seated properly).

Measure the maximum gap between the sweep board and the shell (commonly at/near the weld) using a feeler gauge.

That maximum gap reading is the peaking for that location.

Repeat at multiple heights along the seam (as required by your inspection plan).

Your example: “6 mm on one side and 7 mm on the other”

For API 650 acceptance, do not average. What matters is the worst (maximum) deviation found with the board.

Two common situations:

If you measured two separate gaps (left side gap = 6, right side gap = 7) while the board is seated: the controlling value is 7 mm (still OK).

If the joint forms a ‘kink’ and you’re thinking 6 + 7 = 13 mm as a “step across the weld”: API 650 peaking is typically judged by the single maximum gap to the sweep board, not by averaging. If the maximum gap you can insert anywhere is ≤ 13 mm, it’s acceptable.

(Practically: inspectors record the maximum gap they can measure with the sweep board in that area.)

B) How to verify banding at a horizontal weld (straight-edge – 900 mm)

Tools: 900 mm long straight edge + feeler gauges.

Field method:

Place the straight edge vertically, spanning the horizontal weld (center the weld in the middle of the straight edge).

Measure the maximum gap between straight edge and shell (often just above/below the weld).

The largest gap is the banding value. Must be ≤ 13 mm.

Positive (+) vs Negative (–) deviation

In practice:

“+” (positive) = shell is proud/outward (convex) relative to the intended curve/line.

“–” (negative) = shell is inward/concave (dished in) relative to the intended curve/line.

How to tell on site:

If the shell touches at the weld and you see gaps away from the weld → weld area is likely outward (+).

If the sweep board/straight edge touches on both sides and you get a gap at the weld → weld area is likely inward (–).

Important: API 650’s limit is generally applied to the magnitude (absolute value). Unless your project spec says otherwise, both inward and outward deviations are unacceptable if they exceed 13 mm.

Thanking you,

With best regards,                                            

           

Bhavani Prasad Emani,

Associated Engineering Services, Hyderabad

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