B31.1 Section Modulus Question

[JGard1985]

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

 

[McEng29]

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

 

[BigInch]

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.

 

[KevinNZ]

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 t_n for all the other calculations but I would not recommend it. I would use t_n - A everytime.