IRRIGATION: PUMP SELECTION

The pump selected for an irrigation system must be able to provide the necessary flow rate at the operating pressure required for each irrigation zone. This involves selecting a pump that can:


Move the design flow rate of water from the draw down level in the well (or the low water level of a surface water supply) to the level of the irrigation system.
Produce enough pressure to overcome friction losses in the system (from water flow through pipes, fittings, risers, valves, etc.).
Provide the specified pressure for the sprinklers or emitters.

Each pump design is unique with respect to the flow rate (discharge) provided and the pressure (head) which may be developed. As the flow rate is restricted, the discharge pressure (head) increases.

The three most common types of pumps used for irrigating lawns and gardens are:

Surface Centrifugal Pumps: In centrifugal pumps energy is imparted to the fluid by the centrifugal action of an impeller. The velocity head imparted to the fluid by the impeller is converted into pressure head by means of a volute case. Placed on the surface have limitations in the height to which they can lift water by suction. The practical lift for a centrifugal pump is approximately 15’ to 20’. Therefore, if the water level with respect to the pump is deeper than this, due to draw down or other causes, the water column may separate, resulting in a loss of pump prime. In these cases a different type of pump such as jet or submersible pump may be required.

Jet Pumps: A jet pump consists of a pump (usually centrifugal) and a jet or ejector assembly. The jet assembly can be located in the pump for shallow-well units or in the well for deep-well units. The pump forces a portion of the discharge water through the nozzle and venturi of the jet-assembly. The rest of the water pumped is supplied to the distribution system. The practical lift of this pump is approximately 22’ for a shallow-well jet and up to 85’ for a deep-well jet. The amount of water that must be provided to the jet increases as the level of the water surface in the well decreases. At a lift of 50’ approximately 50% of the water pumped is provided to the jet. Jet pumps can produce a high capacity flow at low head. Their disadvantage is a low operating efficiency because of the need to recycle water to the jet.

Submersible Centrifugal Pumps: Submersible centrifugal pumps have one or more impellers mounted close together on a single vertical shaft, and the impellers and power unit (electric motor) are encased in a housing which is located below the water surface. Each impeller, its diffuser (guide vanes to the next impeller) and housing is called a stage. A 4” or larger well casing is required for submersible pumps. Any repair to the pump or motor requires removal from the well, making service difficult. Service requirements of these pumps and motors are generally minimal; however, in Florida, it may increase due to frequent lightning problems. Because submersible centrifugal pumps can lift water up to 1000’, they are adaptable for use in deep wells.

For more information and charts go to: http://edis.ifas.ufl.edu/WI003
Irrigation of Lawns and Gardens. Dorota Z. Haman, Gary A. Clark, Allen G. Smajstrla. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: May 1989.