Originally Posted By: Chakot
Dear sirs.
I'm trying to calculate by hand the required wall thickeness as per formula (34a) and (34b).

34a and b don't calculate "required" wall thickness. They calculate "pressure" thickness. If you control all the unknown factors and had perfectly measured pipe, that's the thickness necessary to contain the pressure.
Further, 34c and d are sometimes applicable.
tm is "required" wall thickness.
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I understand that if we have internal corrosion, formula 34b shall be used.

34a, 34b, 35a, and 35b are all the same equation, just solved from different perspectives. 34a assumes ci=0 and 34b assumes co=0. 35a and b don't. 34a and 35a assume you know exactly what your outer diameter needs to be. 34b and 35b assume you know exactly what your inner diameter needs to be.
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In this formula we have inside diameter.
1) Is this the formula used by CAESAR II when dealing with B31.3 IX?

I won't speak on behalf of Hexagon, but CAESAR II likely uses 35a, 35c, and relevant calculations as called out by K304.2.1. "Standard" bends have their own formula. Chase that backwards to 304.2.
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2) the Code specifies: "in accordance with this Chapter, the inside diameter of the pipe is the maximum value allowable under the specificiations"
What does it mean? How can I calculate the value of d?
I apologize if the question is trivial.
Best regards

Again, if you don't know d, then you should know D. Solve 35a (or 35c, if appropriate) for T, and you have your "pressure thickness."

"Minimum required" thickness requires you to add mill and mechanical tolerances to the "pressure" thickness.