Brochure
1818
InletOutlet Leak flow Gap
In pumps with a magnetic drive, the motor and the fluid are separated by
a non-magnetizable rotor can which eliminates the problem of sealing a
rotating shaft. On this type of pump, the impeller shaft has a line of fixed
magnets called the inner magnets. The motor shaft ends in a cup where the
outer magnets are mounted on the inside of the cup, see figure 1.11. The
rotor can is fixed in the pump housing between the impeller shaft and the
cup. The impeller shaft and the motor shaft rotate, and the two parts are
connected through the magnets. The main advantage of this design is that
the pump is hermitically sealed but the coupling is expensive to produce.
This type of sealing is therefore only used when it is required that the pump
is hermetically sealed.
In pumps with a rotor can, the rotor and impeller are separated from the
motor stator. As shown in figure 1.12, the rotor is surrounded by the fluid
which lubricates the bearings and cools the motor. The fluid around the ro-
tor results in friction between rotor and rotor can which reduces the pump
eciency.
1.2.4 Impeller seal
A leak flow will occur in the gap between the rotating impeller and stationary
pump housing when the pump is operating. The rate of leak flow depends
mainly on the design of the gap and the impeller pressure rise. The leak flow
returns to the impeller eye through the gap, see figure 1.13. Thus, the impel-
ler has to pump both the leak flow and the fluid through the pump from the
inlet flange to the outlet flange. To minimise leak flow, an impeller seal is
mounted.
The impeller seal comes in various designs and material combinations. The
seal is typically turned directly in the pump housing or made as retrofitted
rings. Impeller seals can also be made with floating seal rings. Furthermore,
there are a range of sealings with rubber rings in particular well-suited for
handling fluids with abrasive particles such as sand.
1. Introduction to Centrifugal Pumps
Figure 1.12: Canned rotor type pump.
Impeller seal
Figure 1.13: Leak flow through the gap.
Fluid
Rotor
Stator
Rotor can
Outlet
Impeller
Inlet
Bearings