PIPE FLEXIBILITY CALCULATIONS B31.3 CHAPT II PART 5

[rfitts]

Occasionally we are requested to fabricate a header system. Typically the header system will be mounted horizontally. It will have a main feed pipe with several branch pipes that are perpendicular to the main feed pipe. The branch pipes will be connected to the main pipe by using a branch connection (pipe saddle). The entire header system is fixed only at the connection flange and is mounted on pipe slides to allow the pipes to move because of thermal displacement. Is there anywhere I can go to view some sample calculations involving pipe flexibility calcs? Bob

 

[BigInch]

Flexibility is the reciprocal of stress. Have a look at the free pipe stress course and downloads available at this web site,

http://www.sstusa.com/freepipestress.htm

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JohnBreen]

Hi Bob,

With all due respect, I am guessing from your description of the task, you are unaware of, or underestimating, the level of effort needed to comply with the referenced Code section. You should be aware of the all the requirements within B31.3, including paragraph 301.1 (Qualifications of the Designer). The Designer is the person who will be held responsible for the design. The flexibility of the header will likely not be an issue. The flexibilities of the branches will determine the loadings (forces and moments) acting at the branch connections (also take a look at paragraph 304.3.3 on reinforcing branch connections).

It would be ill advised to allow the header to "float freely" without some "fixed" point. Left without such fixity, you would not be able to reliably predict how it would move on successive thermal cycles and therefore calculating stresses at the branch connections (for comparison to Code allowable stresses) would be inaccurate (i.e., invalid). Of course, supporting your header is another non-trivial topic and I would not want to trivialize it with a brief comment.

"Flexibility calculations" were done by hand in the past but are now addressed by modern software. There are reasons for that and the reasons include accuracy and the amount of work involved.

On the Internet you can download without cost some documents that may provide some background and you can do some “homework”.

http://www.anvilintl.com/db/datasheets/Pipe_Hanger_Design-05.pdf

http://engstandards.lanl.gov/engrman/6mech/pdfs/D20-AppA-ASME_B31.3-r1a.pdf

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-1-4008/toc.htm

Respectfully, John Breen