>because these pressures are mostly
>balanced with only pressure differences
>of around 100 psi

Nope. If you have ten walls, each of which is sized for a 100 psi differential, that is the exact thickness needed for a 1000 psi differential in a single wall. If the outer shells are increasingly larger to hold more volume, they need to be correspondingly thicker to handle the additional stresses.

>BUT a bundle of pipes welded together
>with a hollow center which also holds
>pressure should hold more volume at
>higher pressure than a single walled
>large pipe (cylindrical tank

Also doesn't hold, because for it to contain any pressure in the center, the pipes on the outside will have additional stress trying to pull them apart, so they will need to be thickened.

With pressure vessels, TANSTAAFL. Really.

John Carmack