Hi everyone,

I would like to ask some question regarding ASME's minimum thickness requirement for pipe bends.

As per ASME B31.3 para. 304.2.1 eq. (3c), minimum thickness can be calculated differently from the intrados(inside bend radius) to extrados(outside bend radius). For the extrados, minimum thickness can be below the minimum thickness of the straight run pipe at the bend centerline. I know that from a straight run pipe after bending, the inner bend radius is compressed resulting to slightly bigger thickness from before and vice versa on the outer bend radius. But if ASME already calculated the minimum required thickness for a straight run pipe base on all parameters then why is it that in the bend extrados, it goes beyond the minimum? I would to know the reason behind this.

Any answers will be very much appreciated.

Thank you.

Hi,

For Thickness calculation as per B 31.3 if you have checked the thickness for straight pipe then I don't think you have to check the thickness for bend/elbows. Because the minimum thickness at the end tangents should not be less than the required as per para 304.1, So basically the minimum thickess will remain unchange for straight pipe and bend as well.

You will get the higher thickness (most of the time) at the extrados. This is specified for the bend manufacturer to check the bend angle. I hope this help

Regards,
R.K.

Hi,

Looking at your case otherwise, assume that the thickness be uniform around the cross-section of the bend.

In this case, for a same (assume small) included angle,
1) inner curvature called CROTCH or INTRADOS, has less area at disposal (or exposed) to resist the pressure force.
2) Outer curvature called CROWN or EXTRADOS, has comparitively more exposed area at disposal to contain the pressure force.

As a result,
due to differences in area, more Hoop stress is developed at intrados than that at extrados.

Meaning to say, for the SAME ALLOWABLE STRESS, a thicker wall will be required at the intrados and a thinner wall can be used at the extrados.

Now, this thinner wall thickness mentioned above is what seems worrying you and i hope now you have your answer as you can now say :
as Hoop stress in a curved segment of pipe varies around the circumference of the cross-section, one need to have different thickness.

Refer to Peng's pipe stress engineering, 'B31.3 allow a thinner wall thickness than that of the straight pipe extrados'

But B31.1, not allowed. Bend extrados minimum thickness after bending shall not be smaller than straight pipe thickness. They even have the Table for recommended thickness of straight pipe for bending.

Hi everyone,

Thank you for the answers to my question.

Dinesh,

I think your answer very much explained my question above.
And yes, it does make sense that the difference in area on which the circumferential forces acting on the intrados and extrados makes the difference in the thickness.

Regards,
Mandz