Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 442

Submersible pumps can be mounted into a tank with the liquid media. The
pump's motor is normally sealed in an oil-filled cavity that is protected from
contact with the liquid.
12.3 Factors Affecting the Practical Suction Lift
of Suction-Mode Pump
The following factors affect or limit the practical suction lift of pump:
(i) Elevation above the mean sea level, or actual atmospheric pressure at specified
location
(ii) Density and viscosity of the fluid
(iii) Temperature of the fluid
(iv) Friction loss in suction pipe and well loss (entrance and formation loss, if
applicable)
(v) Air-bubbling point of the liquid
Maximum theoretical suction lift in a location is equal to the atmospheric pressure
at that location. Temperature affects the density and viscosity of the fluid, which
consequently affect the friction loss. Air-bubbling point depends on the temperature
of water and air pressure above it.
12.4 Centrifugal Pumps
12.4.1 Features and Principles of Centrifugal Pumps
Almost all irrigation pumps fall into this category. These types of pumps use cen-
trifugal force (hence the name, centrifugal pump) to push water out. Water entering
the pump hits an impeller (sort of like a propeller, but a little different) that imparts
circular motion to the water, causing it to spin. This spinning action moves the water
through the pump by means of centrifugal force, and forces it outward to the pump
wall. As this happens, the water picks up speed, which becomes pressure as the
water exits the pump. Centrifugal pumps may be “multistage,” which means they
have more than one impeller and casing, and the water is passed from one impeller to
another with an increase in pressure occurring each time. Each impeller/casing com-
bination is referred to as a “stage.” Almost all turf-irrigation pumps are centrifugal
pumps.
All centrifugal pumps must have a “wet inlet,” that is, there must be water in
both the intake (inlet) pipe and the casing when the pump is started. They can't suck
water up into the intake pipe like you can suck soda up into an empty straw. They
must be “primed” before the first use. To prime them, you simply fill the intake pipe
with water and then quickly turn “on” the pump. To put it simply, this type of pump
can not suck air, only water, so if there is no water already in the pump it would not
pull any water up into it. Once it gets water in it the first time, it will hold the water