Bend Wall Thickness

Bend wall thickness

Preliminary. This result needs engineering review before anyone uses it. The bend formula is not a completed pipe specification.

Formula

I intrados = (4 × (R / D) − 1) / (4 × (R / D) − 2)

I extrados = (4 × (R / D) + 1) / (4 × (R / D) + 2)

t = P × D / (2 × ((S × E × W) / I + P × Y))

tm = t + c

T = tm / (1 − mill undertolerance)

ASME B31.3 paragraph 304.2.1. I is the bend factor. It is greater than 1 at the intrados and less than 1 at the extrados. The sidewall uses I = 1, which is the straight-pipe equation for internal pressure. P is internal design gauge pressure. D is outside diameter. R is the centreline bend radius. S is the allowable stress you enter. E here is the quality factor, not the modulus. W is the weld joint strength reduction factor. Y is the coefficient from the code table. c is the corrosion and mechanical allowance you add. T is the nominal thickness that still leaves tm after the mill undertolerance.

A long-radius elbow is often 1.5 times the nominal size, and a short-radius elbow is often 1 times the nominal size. Type the radius you are using. The nominal size is not the outside diameter.

These are minimum thicknesses after bending. Forming can thin the extrados. This page does not predict that thinning.

The W field starts at 1.0. That means no weld reduction. In the creep range the factor can be lower. Check ASME B31.3 paragraph 302.3.5 before you keep 1.0.

The Y field starts at 0.4. That is the Table 304.1.1-1 value for ferritic steel when t is less than D/6 and the temperature is at or below 482°C (900°F). It is not the value for austenitic stainless or for every other metal, and it is not the value when t is at least D/6. Check the table.

The mill field starts at 12.5%. That is the usual pipe-wall undertolerance: the nominal wall may be 12.5% under the ordered thickness. Confirm it on the pipe specification.

The straight-pipe limit is outside its stated range at one or more positions: t is at least D/6, or P/(S×E) is greater than 0.385. This page does not switch to another equation. The Y value 0.4 is not the table value for t at least D/6.