Pipe bend in Section VIII, Div 1

[jamesl]

I would like to know how pipe bend (carbon steel material) is handled in Section VIII, Div.1:
1. Is there any requirement on minimum radius?
2. How to calculate minimum required thickness?
3. How to handle the thinning out after bending?

 

[aditya707]

Jamesl,
The pipes which are bent to make elbows are having limitations on the dimensions upto which they can be bent.
If i recall correctly we were not allowed to use those pipes which were cold bent after the variation in the dimensions was found to exceed 8%.
When we bend a pipe, it becomes elliptical at the bent area.The OD at both the axes is to be measured and the difference should not exceed 8% of the original OD.
The thickness should be measured at all the stressed locations and compared with the design for usage.Also, the flow required should be calculated after measuring the new dimensions(ID=OD-2t).

 

[deanc]

See UG-31,43,44,45,96 Check with your AI. The ANSI Stds. are a good reference.

 

[jamesl]

Thanks for replies. If the pipe is straight, we use nominal thickness minus 12.5% manufacture tolerance. When the pipe is bent, pipe wall thins out at extrados. If we measue the thickness, it is very possible the thickness at extrados is actually higher than nomial minus 12.5% tolerance since 12.5% is a very conservative number. So my question is: which thickness, measured one or calcualted one (nominal minus tolerance minus some theoritical thinning out due to bending), shall be used to calculate MAWP.

 

[deanc]

In my opinion the 12.5% is for base material. After forming the calculation values apply. Believe you should measure for t min after forming. ie.UT See UG-96a

 

[prex]

If you bend a pipe, then your pipe could be 12.5% minus in thk, and then you will thin it out by bending. So...
The standard minus tolerance of 12.5% is for sure conservative, however if you take it fully into account you'll avoid any discussions with the inspector...

prex

http://www.xcalcs.com

Online tools for structural design

 

[twjag]

deanc is correct. Minimum thichness after forming must equal or exceed the required calculated thickness. You must confirm this by final measurement, as would be required for any pressure part.

 

[davefitz]

One does not want to be in the position of having an installed piping system that does not ahve the required min wall. To avoid that, you would need to have available a chart of expected wall thinning vs bend radius for thsame material and bendig process. Then you would need to calculate what the minimum required as-recieved wall thickness is for the pipe section to be bent. The bending shop would need to confirm this min wall prior to bending using a UT device and mark the pipe section which is scheduled to be the extrados of the bend. After bending, the QC inspector would need to confirm via UT the after-bent min wall and certify it on a QC document.

Alternatively, you can add margins of 12.% to min wall plus the standard wall thinning based on the B31.1 table.

 

[aditya707]

min. thickness measured should be used for calculations.