I really appreciate your interest. We are building three identical carbon steel (A516 gr.70) sulphuric acid storage tanks. The design metal temp is -10 degrees Celsius. However, I'm not sure how this applies to the question so I will provide the code material from API 650: Number and location of radiographs- 8.1.2.2 The following requirements apply to vertical joints:
a) For butt joints in which the thinner plate is less than or equal to 3/8" thick, one spot radiograph shall be taken in the first 10 ' of completed vertical joint of each type and thickness welded by each welder.
b) For butt joints in which the thinner plate is greater than 3/8" thick, but less than or equal 1"in thickness, spot radiographs shall be taken according to item a.
Each welder must be "qualified" to each thickness of metal he welds, not? Seems pretty simple, but I am notorious for missing the forest for the trees. Not to mention the previous inspector did not cover his bases(10 shots missing by my count) and the first tank is complete and supposedly ready for testing(luckily it's - 30 degrees celsius here), in the spring now apparently.