Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 456

12.5.5 Volute Pump
The centrifugal volute pump is the most common type of radial-flow centrifugal
pump. It has an impeller housed in a progressively widening spiral casing. Water
enters the eye of the impeller and is thrown radially outward. This type of pump
does not have diffuser vanes to reduce the velocity of the water. Instead, velocity
is reduced by the shape of the volute itself. This design creates an unequal pressure
distribution along the volute which may result in a heavy thrust load on the impeller,
creating deflection of the shaft, and increasing the probability of its failure.
Volute pumps can be single-suction or double-suction pumps. A single-suction
pump impeller is exposed to a large axial hydraulic thrust resulting from the
unbalanced hydraulic pressures on the impeller. In a double-suction pump, water
is fed from both sides of the impeller, significantly improving its hydraulic bal-
ance. As a result, double-suction volute pumps can produce higher pressures than
single-suction pumps.
Volute pumps are commercially available as single-stage (single impeller) or
multistage (multiple impellers) pumps. The main reason for the multistage con-
figuration is to increase the head produced by the pump. If a multistage pump has
single suction impellers, the impellers are usually arranged with equal numbers dis-
charging in opposite directions to counteract the hydraulic imbalance on each of the
impellers.
Volute pumps are used where irrigation water is obtained from depths generally
less than 20 ft.
12.5.6 Axial-Flow Pump
Axial-flow pumps, also called propeller pumps, produce flow by the lifting action
of the propellers. Axial-flow pumps are designed for conditions where the capacity
is relatively high and the head developed by the pump is low.
An axial pump does not produce high pressure or lift, but can have signifi-
cant flow capacity if the pump is large enough. Most axial-flow pumps operate on
installations where suction lift is not required. Generally, these pumps are mounted
vertically or on an incline from vertical, since it is necessary to submerge the
impeller of an axial-flow pump. In some applications where high volumes of water
are required and ample submergence above the pump is available, it is possible to
mount an axial-flow pump in a horizontal position.
12.5.7 Mixed-Flow Pump
Mixed-flow centrifugal pumps use both centrifugal force and some lifting action to
move water. Water is discharged both radially and axially into a volute-type casing.
The process is a combination of processes occurring in volute and axial-flow types