I think I found a bug in the [bug] archpipe tool. Some parts of the pipe disapper depending on the diameter. [/bug]
In the attachet file you can see that by changing the diameter between 50-100-200.
The bug seems quite arbitrary. If you change the diameter one millimeter and than recompute you get a lot of appearing and disappearing of parts of the pipe.
edit: if you add a filet radius to the dwire, the pipes are also shown correctly