×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

B31.1 Section Modulus Question

B31.1 Section Modulus Question

B31.1 Section Modulus Question

(OP)
Greetings

I have a question regarding the use of "Z" pipe section modulus for piping calculations within B31.1 for straight-run systems (not counting branches, tees etc).

For sections 104.8.1, 104.8.2 and 104.8.3, what is the pipe section modulus that should be used for these design equations?

Is it the section modulus calculated from nominal wall thickness (and published by manufacturers), or is it the section modulus calculated from the minimum permissible wall thickness (i.e. discount wall thickness by 12.5%)

I cannot locate any references within B31.1 to require a reduction from nominal wall thickness for the calculation of section modulus, and the calculation of longitudinal stress due to pressure, (P*D)/(4*t), in the same equation utilizes nominal wall thickness.

I've seen some postings regarding this imposition in B31.3

When using B31.1 I have only recomputed section modulus when corrosion or erosion has determined a loss of wall thickness greater than the 12.5% tolerance permitted by the manufacturing tolerance and I am evaluating as-found conditions.

Thank you

RE: B31.1 Section Modulus Question

When doing calculations with piping, never use nominal anything, errors will occur. Section modulus is calculated by 0.0982(do^4-di^4)/do

RE: B31.1 Section Modulus Question

B31.4 and B31.8 both use NOMINAL thicknesses in their calculations, as the DESIGN FACTORS account for any under-tolerance possible within the limitations of the pipe fabrication specifications therein referenced.

RE: B31.1 Section Modulus Question

No need to take off the 12.5%. This margin is mainly for seamless pipe where the inner is off centre from the outside surface. The Z value is much same as if the hole was in the middle. If the pipe was 12.5% thinner everywhere then it would not meet the weight/length requirement of the pipe material code.

The 12.5% (+ corrosion allowance) is only used for the hoop stress calculation.

B31.1 code also allows you to use tn for all the other calculations but I would not recommend it. I would use tn - A everytime.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources