Hi tech - booleans between objects that have overlapping surface areas or spots that barely graze each other are much more difficult to process cleanly. It's harder for the surface/surface intersector to get a good result in those cases. That's because when surfaces overlap each other they have something more like a "zone of intersection" rather than a crisply defined intersection.

It can help if you have a very tight accuracy on the pieces involved, like in this case instead of using a sweep for the pipe part, use Construct > Revolve. Revolve will make a 100% precise surface of revolution, while sweep will make a fitted surface.

Please try this attached version where I've used Revolve to build the bent piece, it should boolean union ok now.

Another possibility with your original objects is if you cut your objects in half and use ShrinkTrimmedSrf so there are no closed surfaces, that will also avoid some of the other difficult areas of surface/surface intersection. It's then possible to use Construct > Curve > Isect to generate intersection curves and then trim surfaces to those curves.

- Michael