Learn how oversized pumps waste energy and how you can save energy


Irrigation pumps are typically over-specified at the design stage, resulting in significantly higher power consumption and operating costs. A pump is generally oversized when it is not operating at or within 20% of its best efficiency point (BEP), although it is normally considered acceptable if the duty point fall within 50 to 110% of the BEP flow rate. By replacing oversized pumps with smaller ones, energy and maintenance savings can be achieved due to lower power consumption requirements and less wear and tear.

Introduction.


Energy used in irrigation can account for upwards of 50% of a farm’s total energy bill. Pump duties are often overestimated at the design stage of an irrigation scheme, which can result in the installation of oversized pumps. As well as costing more to install, larger pumps consume more electricity and so have higher operating costs.
Operating too far from a pump’s best efficiency point (BEP) can risk overloading the pump’s motor. Cavitation occurs when pumps exert excess force, creating rapid changes of pressure within the pump liquid. This can cause bubbles and air pockets to form and then, subjected to higher pressure, these air pockets can implode, damaging equipment.
Selecting the right pump for the job will ensure that the pump runs efficiently, increasing the lifespan and reliability of the equipment. By replacing oversized pumps with smaller ones, or by reducing the size of the impeller, energy and maintenance savings can be achieved due to lower power consumption and less wear from excess flow energy.

Design for the best efficiency point (BEP)


Generally, a pump is oversized when it is not operated within 20% of its BEP, although normally acceptable if the duty point falls within 50 to 110% of the BEP flow rate. This allows for a greater margin for error in the event that the system designer overestimates the actual resistance curve.
Is your pump too big?
Pump operates to the right of the best efficiency flow rate. Using the pump curve provided by the manufacturer, check your pump is not running too far from its BEP, outside the recommended operating range.
Increased risk of high-flow cavitation. Check for signs of cavitation, such as abnormal sound and vibration. Check if pump internals have been damaged by cavitation during pump maintenance.
Increased risk of overloading pump motor. Check that the pump motor is not running beyond its maximum current (FLA) rating and that the load factor (in amps/FLA) is not greater than the manufacturer’s acceptable service factor.

Guidelines for correct pump sizing.


Consult with irrigation planners or engineers and follow the following guidelines:
Determine the total dynamic head (TDH) of the system using flow rate requirements (L/min), pipe length and diameter, and height between suction and discharge points. TDH = static head + dynamic head (line friction).
Using manufacturer pump curves for different pump and impeller sizes, select the combination (pump + impeller) that gives the best efficiency for the required operating conditions (flow rate and TDH).

Reference: Farm Energy Innovation Program - Energy & Innovation
NSW Farmers