This process uses sound wave reflections to locate and size sub surface discontinuities. No , each Code only qualifies for itself. There is no hierarchy. However, sometimes the owner can be persuaded that one Code is as good or better than the other. No , there is no such thing as the perfect weld.
All welds contain discontinuities.
Each Code determines the extent of the discontinuity before it is classified as defective and must be repaired. For example, B31.1 does not allow any lack of penetration. No , these conditions require VT only. Inform him that for the service conditions the Code specifies that only VT be performed. This is due to the fact that the Code has a large enough safety factor built into the design that even if there were discontinuities contained inside the weld, that VT could not detect, that it would still give acceptable service life.