Meshing flow domain

I need to simulate the flow of water into a valve.
At the moment I'm able to do that in this way:

1. Import IGES or STEP geometry
2. Repair the geometry (SEWING) - optional but almost always requested
3. Explode faces
4. Build the shell using faces
5. Build the solid using the shell (this something goes wrong with STEP geometry, solid is builded but actually it is still a shell)
6. Create a box sorrounding the solid but the inlet/outlet part of the valve outside it.
7. Cut the box using the solid
8. Define group of faces for inlet/outlet/wall
9. Mesh everything
10. Group the meshed faces for inlet/outlet/wall
11. Export the mesh as UNV (to openfoam)

As I said I was able to compute everything. Now some questions:
1. Is this the right way ? In the mesh I obtain there is still the portion of the box outside the the valve. This mesh is not affected by any flow. How can i remove it ?
2. I tried to partition the box using the solid instead of cutting it, but when i use ideasUnvToFoam conversion utility (to go into OpenFoam) it fails because there are internal boundaries. It is right but how can i define (this will be the next step) an internal component that has to interact with the flow then ?

Thanks.