Thanks guys for your help.  Just a couple of quick notes before I dive into another serries of tests on this.

The dispacement map starts at pure black; the only displacement on the main steam cylinder should be the bands and bolts.

All parts are using the same smoothing of 22.5 and it is rendered with smoothing off.

Other cylinder shapes on the model use the same type of map yet show no defects.

I'll render some more side-by-side tests and post here in a bit.

As an aside, the only time I have trouble with displacement at edges of maps is when I haven't carefully uvmapped and then drawn the displacement map to line up exactly.  I make the displacement black/zero at corners of cubes to avoid it pushing the surface outward and splitting it.