The information you are looking for is located in table 341.3.2 in B31.3. The acceptance citeria varies depending upon the type of weld and fluid service category.
The acceptance criteria for UT in addition to that contained within the table is indicated in para 344.6 and is related to the length of the discontinuity.
An example for a girth and mitre groove weld would allow 1/32" or "T"/4 whichever is less.
Reinforcement is related to wall thickness an 1/4" wall would alow 1/16" Reinforcement >1/4" to <1/2" would allow 1/8".
Hope this helps. If this is being used for actual weld inspection, I suggest the actual code be referred to to assure I have not made any mistakes.
