When you run the minimum wall thickness calculation, you end up with a value, that is increased for corrosion, then increased again for mill tolerance, and finally rounded up to the next commercial pipe size that you can purchase. This is the thickness you enter as an input value in CAESAR II for the thickness of your pipe.
Most Piping Codes suggest that a flexibility analysis (i.e. the analysis that CAESAR II performs) be done using the nominal (input) wall thickness. Certain Codes specifically state that certain load cases should be corroded or have mill tolerance deducted from the section modulus when making the stress calculation.
You referenced B31.3: this Code does not address mill tolerance in the flexibility analysis, and it corrodes the Sustained and Occasional load cases only.
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Regards,
Richard Ay - Consultant