Page 88 - Know and Understand Centrifugal Pumps

Pump Classification

on the liquid. The blade in the bottom of the kitchen blender is a
macerating axial flow totally open impeller. The totally open axial flow
impeller moves a lot of volume flow (gpm), but not a lot of head or
pressure. With its open tolerances for moving and grinding solids, they
are generally not high efficiency devices.

Semi open impeller
~~ ~ ~~~~
A semi-open impeller has exposed blades, but with a support plate or
shroud on one side. Some people prefer the name semi-enclosed. These
types of impeller are generally used for liquids with a small percentage
of solid particles like sediment from the bottom of a tank or river, or
crystals mixed with the liquid (Figure 6-17).

Figure 6-17 .. __

The efficiency of these impellers is governed by the limited free space or
tolerance between the front leading edge of the blades and the internal
pump housing wall. Some pumps have a micrometer gauged jack bolt
arrangement on the axial bearing for performing an impeller setting.
The impeller setting corrects for erosion wear and thermal expansion in
this tight tolerance, returning the pump to its original efficiency.

Totally enclosed impeller
~
~~ ~~ __
Totally enclosed impellers are designed with the blades between two
support shrouds or plates. These impellers are for totally clean liquids
because tolerances are tight at the eye and the housing, and there is no
room for suspended solids, crystals or sediment, see Figure 6-18.