Bend Wall Thickness — Companion Article

Open the tool: https://pipingdesigners.com/tools-training/bend-wall-thickness/

What this tool does

Bend Wall Thickness works out the minimum wall needed after bending at the inside of a pipe bend (the intrados), the outside (the extrados) and the sidewall. It uses the ASME B31.3 paragraph 304.2.1 approach for bends.

You type every material factor. No code table is stored on the page.

When to use it

Use it when you are specifying a pipe bend and want to check that the wall left after bending is enough for the pressure. It is the bend version of the straight-pipe wall thickness tool.

It does not predict how much the wall thins in forming. That depends on the bending method and the bender.

Inputs

• Design pressure P.
• Outside diameter D.
• Bend radius R, to the centreline. R must be more than half of D.
• Allowable stress S, quality factor E, weld strength reduction factor W, and coefficient Y.
• Corrosion and mechanical allowance c.
• Mill undertolerance, as a percentage.
• Units: metric or US customary. The page remembers your choice.

How it works

The page works out a factor I from the ratio R/D:

• At the intrados, I = (4(R/D) − 1) / (4(R/D) − 2). This is greater than 1, so more wall is needed.
• At the extrados, I = (4(R/D) + 1) / (4(R/D) + 2). This is less than 1.
• At the sidewall, I = 1, the same as straight pipe.

The pressure thickness is t = P D / (2 (S E W / I + P Y)). Then tm = t + c, and the nominal wall is tm / (1 − mill undertolerance).

Understanding the result

You get three minimum thicknesses. The intrados is always the largest. Compare it with the wall you expect at the intrados after bending, not with the wall of the straight pipe you start from.

Engineering considerations

• W starts at 1.0. Check paragraph 302.3.5 before you keep it.
• Y starts at 0.4. Check the code’s Y table for your material and temperature.
• The mill field starts at 12.5%. Confirm it against the pipe specification.
• Tight bends thin the extrados and thicken the intrados. Ask the bender what wall they can guarantee.

Example

At R/D = 3, the intrados factor is 1.1 and the extrados factor is about 0.929. At I = 1, the sidewall gives the same result as the straight-pipe equation.

Standards/references (licensed vs interim)

The method follows ASME B31.3 paragraph 304.2.1. The page does not reproduce any code table. You supply S, E, W and Y from your licensed copy of the code.

Limitations/disclaimer

• Minimum thickness after bending only. No prediction of forming thinning.
• Internal pressure only.
• The result is preliminary and needs engineering review.

Use the tool

Calculate now: Bend Wall Thickness

Related tools

• Wall thickness for straight pipe under internal pressure. Read the wall thickness companion article.
• Bend Cutting, for mitre and elbow cutting geometry (new tool, being added to the site).