A bellmouth, also called a suction bellmouth in connection with centrifugal pumps, is a nozzle-shaped inlet casing component (see Fitting), often employed with vertical tubular casing pumps.
The flow acceleration resultant of the bellmouth's shape minimises irregularities in the velocity distribution. Even velocity distribution ensures a uniform approach flow (see Inlet conditions); this is especially important for high specific speed pumps (see Specific speed).
In the case of vortex flow at the inlet, a flow straightener should be fitted to provide a degree of flow straightening. See Fig. 1 Bellmouth
If pre-swirl control has been provided downstream, there is no need to fit a flow straightener in the bellmouth. See Fig. 2 Bellmouth