Hello Auwis

There are two different issues here.

1 We have to ensure the nozzle wall is thick enough to withstand the pressure (internal or external). In this case, the undertolerance is important, and should be taken into account to ensure the thinned wall thickness still has enough metal to cope with the pressure.

2 When doing a nozzle reinforcement calculation in accordance with UG-37, the nomenclature for ‘tn’ states that the nominal wall thickness be used without regard for the mill undertolerance. This, as I explained, is because metal lost on one side of the pipe is gained on the other side.