Sect I, PG-31(g)

[GenB]

Sect I, PG-31(g) reads: "If a value of m less than 1 is used in calculating t, the shell thickness ts shall be maintained along a distance inwardly from the inside face of the head equal to at least 2 (square root of d*ts)." The question is, what do they mean by "maintained along a distance inwardly"?

 

[gr2vessels]

The idea is to create a curved transition from the flat end to the cylindrical shell, by means of the weld thickness, to minimise the local stresses. Now, the wording is a bit strange indeed, because is trying to cover every situation for m = 0.6 - 1.0 (that's where the 0.7ts fillet weld thickness is coming from).
Inward is the direction from the flat end internal face towards the other end of the shell.
Also, the inside and the outside cumulated weld thickness (flat end and the cylinder) shall never be less than the shell thickness ts.
Cheers,
gr2vessels