Regarding API 650, 11th ed. with Addendum 1, I am doubtful about some statements regarding wall thickness.
According to para. 5.6.4.2,"required shell thickness for each course shall be the greater of the design shell thickness plus corrosion allowance (t1d) or the hydrostatic test shell thickness (t1t), but the total shell thickness shall not be less than the shell thickness required by 5.6.1.1, 5.6.1.3 and 5.6.1.4".
Under appendix K, para. K.2.1,for wall thickness calculation in accordance with the variable point method, for the hydro test condition, t1=t1t, and t1<= tpt

But, following 5.6.4.2, what if t1d is greater than t1t? I have tried to calculate wall ticknesses for a 44 m diameter, 21 m tall atmospheric tank. Checked calculated values for first course are t1t=22.06 mm and tpt=21.46 mm. This is not logical according to App. K.
Taking into account these results, it appears that there is a conflict between the mentioned paragraphs.
I insist, the calculations have been checked. I would like to know either if my interpretation is right or if there is a conflict in the code
 

I think API-650 is saying that for the test condition, t1 need not be greater than tpt, not that it will always calculate smaller.

If you had a very light design specific gravity, the test condition (tt)could conceivably be of a greater thickness than that of the design condition (td).

rab1944,
Fluid is methanol. Specific gravity is 0.77


 

This is not enough information.  Pick you material, get your Sd and St stresses from Table 5-2b, make sure the material is appropriate for the design metal temperature and run it through the 1-foot design in Section 5.6.3.  I hope you the best.