Pumps and pump selections

Configuration & efficiency

Pumps for irrigation use come in many different configurations:

• end suction center centrifugal’s
• self priming centrifugal’s
• submersible’s
• line shaft turbines.

Motors may require either single phase or three-phase power depending on the horsepower. In many rural locations, three-phase power is simply not available so hp limitations require the use of smaller motors, 15 horsepower or less using single phase power. In certain situations the pump might be powered with either a gasoline or diesel engine. The particular pump selected needs to be matched to the irrigation system's hydraulic requirements.

We are often asked by customers what size and type of pump to use when the requirements of the irrigation system have not yet been defined. Most important thing to remember here is that other than the cost of water, the pump is the only component in your irrigation system that will consume something you have to pay for: either electricity or fuel. For this reason pump selection should be the last piece of the irrigation puzzle.

The pressure created by your pump is more a function of pump design than horsepower. Two pumps of the sime horsepower with different pump designs will produce entirely different results in terms of gallons/minute and PSI produced. Energy, the consumable resource you will pay for, is directly related to the gallons/minute pumped. For this reason it is very important to make a selection that best meets your system requirements.

All pumps have a volume versus pressure curve, available from the manufacturer, that defines the pump performance at various flow rates and pressure levels. Once your system duty is defined, the designers at Poly Drip will determine what pump best suits your needs.

A word now about pump efficiency: All pumps are not created equal. If there are several pump models that will all suit your application, the appropriate one to select is the one that has the best efficiency. This simply means that for each energy dollar you spend you will get the maximum value in converting that energy into pressure and volume. Since energy costs are on-going, a pump with a lower initial cost may seem like a good buy; but in the long run it will cost you much more to operate.

Mounting the pump

How the pump is mounted is determined by the type of pump and type of source. Well pumps obviously must be installed in the well or, in the case of a shallow well, on the surface of the well casing. When pumping from a pond there are two options:

1. The pump may be installed on the bank of the waterbody with the suction line extending an appropriate distance into the water. Care must be taken not to exceed the pumps maximum lift capability. Care must also be taken in assuring a means to keep the pump primed.
2. A preferred method of installation when using surface water is to float the pump on the surface of the water source. This eliminates any problems with loss of prime and suction lift. Additionally the discharge pipe is configured so that when the pump is cycled off a portion of the pipeline drains back through the pump to give a flushing action to the intake strainer. We don't have to worry about the pump freezing unless the pond freezes.

Pump Operation

There are also options available for controlling the operation of the pump motor.

1. The irrigation controller, a device that automatically controls the opening and closing of the valves in the irrigation system can also be used to turn on the pump. The disadvantage of this method is that the pump is not readily available for manual irrigation.
2. A preferred method of operating the pump is through the use of a specialized valve that, along with a pressure switch and a small pressure tank, allows the pump to run when water is called for in the irrigation system. The advantages of this method are: elimination of water hammer, elimination of frequent cycling, the ability to have a constant downstream pressure at various flow rates, and the option to irrigate manually when desired.

Remember that the amount of energy consumed is directly related to the volume of water pumped. If your system has irrigation zones of differing flow rates, the same pump (whatever the horsepower), can be made to function more economically using this equipment. Motor manufacturers tell us that the worst thing that can happen to an electric motor is to cycle too frequently. Energy consumption and heat produced is highest at the moment of startup for all motors. As a rule of thumb the pump should not cycle more than four times an hour. By controlling the pump in this manner we can eliminate many of the problems associated with pump motor failure and irrigation system component failure.

The bottom line: Don't choose your pump until everything else associated with your irrigation system has been designed.