Brochure

2222
1. Introduction to Centrifugal Pumps
The volute casing consists of three main components:
Ring diusor, volute and outlet diusor, see figure 1.21.
An energy conversion between velocity and pressure oc-
curs in each of the three components.
The primary ring diusor function is to guide the fluid
from the impeller to the volute. The cross-section area in
the ring diussor is increased because of the increase in
diameter from the impeller to the volute. Blades can be
placed in the ring diusor to increase the diusion.
The primary task of the volute is to collect the fluid from
the ring diusor and lead it to the diusor. To have the
same pressure along the volute, the cross-section area in
the volute must be increased along the periphery from
the tongue towards the throat. The throat is the place
on the outside of the tongue where the smallest cross-
section area in the outlet diusor is found. The flow con-
ditions in the volute can only be optimal at the design
point. At other flows, radial forces occur on the impeller
because of circumferential pressure variation in the vo-
lute. Radial forces must, like the axial forces, be absorbed
in the bearing, see figure 1.21.
The outlet diusor connects the throat with the out-
let flange. The diusor increases the static pressure by
a gradual increase of the cross-section area from the
throat to the outlet flange.
The volute casing is designed to convert dynamic pres-
sure to static pressure is achieved while the pressure
losses are minimised. The highest eciency is obtained
by finding the right balance between changes in velocity
and wall friction. Focus is on the following parameters
when designing the volute casing: The volute diameter,
the cross-section geometry of the volute, design of the
tongue, the throat area and the radial positioning as well
as length, width and curvature of the diusor.
Figure 1.21:
The components of the
volute casing.
Tongue
Volute
Ring diusor
Outlet diusor
Throat
Outlet flange
Radial force vector
Radial force vector