An irrigation pump does
two things; it provides the water flow rate and the pressure head to make the
irrigation system function properly. The "flow rate" and "pressure head" are
the two factors used to size the pumping plant.
Flow rate is the
total discharge of all sprinklers or emitters operating together.
Add the total
water needed by all the sprinklers in the system. The total discharge cannot
exceed the supply available. The irrigation system can be divided into several
zones, if necessary, to avoid exceeding the supply. The power requirement will
be proportional to flow.
Head or Pressure Head
of an operating pumping system is dynamic. The dynamic head is made up of several
heads that are illustrated in the Total Pressure Head diagram, and are defined as follows:
- Static suction head.
The vertical distance from the water surface to the pump center-line to which
the pump must lift the water. For wells, estimate to the draw down point at
which the water level will be when the pump is operating. If the pump intake
pipe is lower than the water surface, subtract the submerged depth.
- Static elevation head.
The vertical distance (elevation) from the pump to the highest discharge pipe
or sprinkler. Consider this as a column of water the pump lifts when the pipes
are full of water.
- Pressure head.
Operating pressure, in psi, of the sprinkler or emitter. Convert psi to "feet
of head" before adding to the other pressures to get total dynamic head. Remember
2.31 feet equals 1 psi.
- Friction head.
Includes all pressure losses in the system due to friction. Since the pump
must overcome all the friction losses and still deliver water at the desired
pressure at the end of the pipe, add the friction head into the estimate of
the total dynamic head. Convert psi to feet of head.
The total pressure head
of an operating system can be summarized as follows:
- Total static head
= static suction lift + static elevation head. This is the total pressure
head to fill the pipes to the highest point.
- Total dynamic head
= total static head + pressure head + friction head. This is the total head
to make the system operate properly.