There is no specific guidance on when an insert plate becomes a shell plate, at least none that I can find. I would think that a shell plate includes vertical sides that extend from horizontal weld to horizontal weld but there is language in the code that suggests that an insert plate can also intersect two girth welds so maybe that is not the only consideration. Other items in the code: shell plates may not be greater than 1 3/4" and they must be at least 72" tall.
". . shell plate includes vertical sides that extend from horizontal weld to horizontal weld but there is language in the code that suggests that an insert plate can also intersect two girth welds"
Exactly; my read is that 'shell plate' goes from existing seam to existing {parallel]seam. Insert plates are smaller, and do not reach two parallel seams.
What about the other direction? Does it have to be a full width plate? Is there some minimum width? The committee has been asked these questions, hopefully they will reply.
My interpretation is seams in either direction, i.e. from one vert to the next vert; from one round seam to the next one. When you stop short of a seam that is perpendicular to your added plate, that plate is an insert, regardless of direction. Seam-to-seam makes it a 'shell plate'. CAVEAT: vert seams, especially in the lowest 'tub' ring require great respect and care.
