Dear sirs.
I'm trying to calculate by hand the required wall thickeness as per formula (34a) and (34b).
I understand that if we have internal corrosion, formula 34b shall be used.
In this formula we have inside diameter.
1) Is this the formula used by CAESAR II when dealing with B31.3 IX?
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

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.

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.

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.

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.

Thank you very much Michael.
I appreciate a lot your answer.

I'll try to make an example
Let's say we know:
- the external diameter D of the pipe
- the design pressure P of the line
- the allowable S
- mill tolerance m
- the internal corrosion allowance
Let's say we have no external corrosion allowance

I understand mill tolerance shall not be considered in pressure thickness calculation, but only in required thickness calculation.
So mill tolerance is not part of ci and co.
This is what I do with "low" pressure wall thickness calculation as per B31.3, and I get the same results of CAESAR II

If I understand correctly, equation 34b shall be used.
I see around some people using equation 34a in the above situation, but I think it is wrong. You can't use equation 34a when you deal with c0=0 and ci ≠ 0.
In this equation we have d = internal diameter
The question is, if we know D and we don't know t, d should be an unknown.
Michael, if I understand your suggestion, I should use equation 35a (or 35c if appropiate).
From this equation I can get the value of T , then calculating d = (D-2T-2ci) and use d in eq 34a.

Before posting, I want to say why I am asking.
I know CAESAR II calculates the tm value. I'm trying to get the same results to check if I am fully aware of how the Code works.

Best regards

I would read the paragraph above and below 34a/b more carefully.

I'll paraphrase:
"t shall be no less than 34a or b when you have D or d."
"You may use 35a/b to correlate design pressure with D, T, co, ci, etc."

Let's change the subject matter, but use similar wording:
"If you pick the wrong thickness, the amount of money you owe me shall be no less than $5000."
"You may use this formula to correlate %error and the amount of money owed to me..."