Hi Doren,
Thanks for the reply.
The question is why it is not considered in sustained calculation? Para. 320.2 ASME B31.3 2014, Z is defined as "Sustained section modulus. Z in eqs. (23b) and (23c) is described in para. 319.4.4 but is computed in this paragraph using nominal pipe dimensions less allowances; see para 320.1."
In para 320.1 says "Section moduli used to compute stresses in this paragraph shall be based on nominal pipe dimension less allowances, i.e., the sum of mechanical (thread or groove depth), internal and external corrosion, and erosion allowances."
The question is why mill tolerance is not included in the term allowances?
There is a debate regarding the 12.5% mill tolerance; some say it is due to the misalignment of the mandrel during the pipe forming and if the thickness at one point of the section is less, it would be larger on the other point of the section and the modulus effectively remains the same. Other say mill tolerance allows the whole cross section of the pipe to be of 12.5% smaller thickness. In this case mill tolerance should be considered in the allowance to be taken out of the pipe dimensions for sustained stress calculations.
Just my thoughts
Regards
MW