Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 443

with a small valve so the pump does not need to be primed again every time you
turn it on.
The force created by centrifugal action of the pump depends on the density of
the fluid within the pump. As the density of air is too low ( 1.2 kg/m 3 , in contrast

1,000 kg/m 3 for water), the force created is not enough to suck water from the water
source.
With respect to type of impeller, all centrifugal pumps can be classified into the
three following groups:

∼

- Radial-flow pumps
- Axial-flow pumps
- Mixed-flow pumps

Specific speed range of different pumps is given in Table 12.1 :

Table 12.1 Specific speed of
different centrifugal pumps

Specific speed range Pump type

Below 5,000 Radial flow pumps

4,000-10,000 Mixed flow pumps

9,000-15,000 Axial flow pumps

12.4.2 Some Relevant Terminologies to Centrifugal Pump

Important concepts associated with the operation of centrifugal pumps include
pump efficiency, net positive suction head, specific speed, affinity laws, cavita-
tion, and priming. Good design, efficient operation, and proper maintenance require
understanding of these concepts.

12.4.2.1 Suction Lift

The absolute pressure on the water at the water source is the driving force for the
water moving into the eye of the impeller. Theoretically, if a pump could create a
perfect vacuum at the eye of the impeller, and if it were operating at sea level, the
atmospheric pressure of approximately 14.5 psi would be the driving force pushing
water into the eye of the impeller. This pressure could lift water a distance of about
8m(1psi

=

2.31 ft of water). In practice, this lift is much smaller due to lack of
perfect vacuum in the impeller and friction losses in the intake pipe. The practical
value of maximum lift differs between pumps, but it is usually no greater than 24 ft.

12.4.2.2 Net Positive Suction Head Available

Net positive suction head available (NPSHa) is the absolute pressure of the water at
the eye of the impeller. It is atmospheric pressure minus the sum of vapor pressure of
the water, friction losses in the intake pipe, and suction head or lift. Since any varia-
tion of these four factors will change the NPSHa, NPSHa should be calculated using