Brochure

2121
Large cross-section:
Low velocity, high static
pressure, low dynamic
pressure
Small cross-section:
High velocity, low static
pressure, high dynamic pressure
A fourth method to balance the axial thrust is to mount fins on the pump
housing in the cavity below the impeller, see figure 1.19. In this case, the pri-
mary flow velocity in the cavity below the impeller is reduced whereby the
pressure increases on the shroud. This type of axial balancing increases disc
friction and leak loss because of the higher pressure.
1.2.6 Volute casing, diuser and outlet flange
The volute casing collects the fluid from the impeller and leads into the
outlet flange. The volute casing converts the dynamic pressure rise in the
impeller to static pressure. The velocity is gradually reduced when the cross-
sectional area of the fluid flow is increased. This transformation is called
velocity diusion. An example of diusion is when the fluid velocity in a pipe
is reduced because of the transition from a small cross-sectional area to a
large cross-sectional area, see figure 1.20. Static pressure, dynamic pressure
and diusion are elaborated in sections 2.2, 2.3 and 5.3.2.
Figure 1.18: Axial thrust reduction through
blades on the back of the hub plate.
Figure 1.19: Axial thrust reduction using fins
in the pump housing.
Diusion
Blades
Fins
Figure 1.20: Change of fluid velocity
in a pipe caused by change
in the cross-section area.