A semi-open impeller is a compromise between an open and an enclosed impeller. It incorporates a single shroud, usually located on the back of the impeller. A semi-open impeller has a solids passing capability similar to that found in an open impeller. With only a single shroud a semi-open impeller is easy to manufacture and completely accessible for applying surface hardening treatments. For moderately abrasive slurries, especially if plugging is a concern, a semi-open impeller is a good choice. A semi-open impeller operates more efficiently than an enclosed impeller because of lower disc friction and tighter axial clearances. It has an advantage over an open impeller in that it can be adjusted axially to compensate for casing wear.