ASME B31.3 intra-extrados

[schlebb]

In calculating the required wall thickness of a straight pipe I use the formula in section 304.1.2. This is quite straight forward.

My problem is in understanding section 304.2.1.
Can anyone explain to me what I am calculating when I use the formula 3c? It is supposed to be the minimum wall thickness in a bend. However, I need to use the value for "I" in the formula. If I calculate the intrados (inside bend radius) and insert it in the formula, I get a larger value for the wall thickness. If I calculate the extrados (outside bend radius), I get a smaller value.

What am I actually calculating here, and which value determines the required wall thickness?

 

[Achilleas]

ASME B31.3, par.304.2.1

Equation (3c) gives you the pressure design thickness for internal pressure of a pipe after bending. During bending plastic deformation of the straight pipe occurs. It is quite natural to obtain greater values at the intrados and smaller ones at the extrados since the inside fiber of the pipe is under compression while at the same time the outside fiber is under tension, during bending.

These points - at the mid-span of the bend at the intrados (max. compression) and extrados (max. tension)- are those that get more influenced from the pipe bending. What you are calculating is the minimum wallthickness that is allowed to appear to the pipe after bending.

Remember that bending is usually a site activity and you will find this formula very usefull if you have to decide whether the bent pipe is accepted or not - as far as its wallthickness is concerned.

 

[schlebb]

Thanks:

If I understood you correctly, then I calculate the minimum wall thickness as shown in 304.1.2. Then I use 304.2.1 to determine the acceptable wall thickness in the bend. This means that the wall thickness at the outside of the bend could be less than the thickness calculated for a straight pipe in 304.1.2.

This has just resolved a couple of issues for me.

Schlebb.