Pre-swirl control allows a change in pump head. This requires the installation of a diffuser with variable pitch diffuser vanes upstream of an impeller. Usually diffusers with variable pitch vanes are only used on centrifugal pumps for pre-swirl control purposes.
The control range is broadened with increasing specific speed, thus limiting the use of these devices to mixed flow and axial flow impellers.
Compared with other types of control, both the effort required and the associated costs are relatively low.