Pipe Bend Thicknesses 1

[reichertc]

In the 1999 and earlier editions of B31.3 the minimum wall thickness for bends was the same as for straight pipe. The 2000 edition has some formulas for calculating the minimum required thicknesses at the intrados, sidewall, and extrados.

This is causing me some major headaches. Previously, bends could be specified by the corresponding straight pipe wall thickness taking into account manufacturers tolerances and wall thinning. Now, additional calculations need to be performed to determine whether the bends will acceptable at the design conditions.

Hence, bends appear to be moving from an item that can be "bulk ordered" to being an "engineered item". Of course, when the job started that I am currently on, the 1999 edition of the code was being used. Somewhere along the line, someone decided we should be using the 2000 edition of the code. This is causing some problems within our purchasing system.

Does anyone have any background on what brought about these changes to the code? Any good technical papers on pipe bending in general? I am finding that on larger-diameter, tight radius bends, the wall thickness requirements are substantially increasing. I would like to get a good technical argument together before I really push through a change on the project that I am on. (And no, the argument "because it's in the code" will not work. The project was initiated when the '99 edition was in effect, and the local regulations allow for the use of either edition.)

A common comment is that changes in the code are predicated by failures in industry. Anyone have knowledge of failures in bends due to older design guidelines? Any comments or suggested resources on pipe bending in general?

Thanks in advance for any input.

 

[mgp]

Checkout John Breens reply on may 22 and may 24 in thread378-22947

This will give you some good historic background information.

Regards
Mogens

 

Hello,

The B31 Pressure Piping Codes have always tried to provide design rules that are as simple as they could be, consistent with safety (including an approriate design margin).

Prior to the 2000 edition of B31.3 bends were required to have (after bending) a wall thickness at least equal to the minimum required wall thickness for straight pipe in paragraph 304.2.1. That rule had been in B31 from "the beginning of time". This was changed in the 2000 Addenda.

A bend or elbow is NOT a straight piece of pipe, that is to say it does not have a geometry as simple as straight pipe. The bend is a portion of a torus.

Allow me to quote from Dr. Charles Becht's new book, "Process Piping - The Complete Guide to ASME B31.3" (

http://www.asme.org/pubs/):

"......Note that the prior requirement that simply stated that the thickness should be the same as required for straight pipe was deleted. The new requirement is more conservative for the intrados (inside curve) and less conservative for the extrados (outside curve).......The normal process of making a bend by bending straight pipe results in a thickness variation with the extrados being thinner and the intrados being thicker. Part of the reason for providing these new rules is due to the practice of fabricating elbows by forming two "clamshells" out of plate and welding them together. This produces a bend of uniform thickness, and the thickness on the intrados would be too thin if it simply satisfies the required thickness for straight pipe"

Because the thickness required for pressure loading on the inside and the outside walls of the torus are (unlike a straight piece of pipe) different (for the "sides" the thickness is the same as for straight pipe), the simple rule was not (ever really) adequate. The "clamshell method of making bends just served to motivate the committee to act. So, the committee finally got aroung to "fixing it".

By the way the first printing of Chuck Becht's book will only be 1000 copies so it would be good to order soon if you want a copy.

Best regards, John.

 

[reichertc]

Thanks John, I was hoping you'd comment on my question. And I just ordered myself a copy of that book, it looks pretty good.