The term moving system applies to side-roll, moving rain-guns and centre-pivot systems (amongst others). This type of system moves across the field while applying water. Irrigation ef­ficiency depends largely on the type of system used. A common characteristic of moving systems is that they tend to be expensive and that some of them re­quire specialised training and support.

Moving systems operate at high, medium, or low pressures. Due to the high initial capital costs and the need for highly specialised technical support, some of these systems are not normal­ly used by small-scale farmers.

Centre pivots are the most com­monly used moving system. Irrigation efficiency should theoretically exceed 85%, but soil limitations, poor main­tenance, or even the use of incorrect sprinkler packages, will reduce meas­ured efficiency. A significant considera­tion among large commercial farmers who use centre pivot systems is labour savings.

Pumps

Where pressure is required to operate an irrigation system, the farmer will normally need to pump water. A pump is simply a tool used to raise water to a higher level. A pump that raises water pressure by 100 kPa can be used to fill a tank raised 10 m above the water level at the source.

 The simplest form of pumping would be to use buckets to carry the wa­ter needed to fill the tank. When we need to move large volumes of water, or raise that water to a significant height, carrying water quickly becomes impractical. It then becomes neces­sary to use some form of mechanical energy, and to transform that energy into useful water pressure. In irrigation systems, the most common form of pump used is a centrifugal pump.

How a centrifugal pump works:

Take a bucket filled with water and swing it in a wide circle on a piece of string or rope. For as long as the bucket revolves around one at the end of its tether, is can be seen that the water it contains does not spill out.

Water is held inside the bucket by centrifugal forces. Even if the same bucket is swung about a vertical axis water stays in the bucket, although the bucket is upside down half of the time. If, while swinging the bucket around a vertical axis, the rope breaks and the bucket flies up vertically into the air, the bucket will rise to a height above the ground before water spills from the inverted bucket. This height represents the water pressure created in the bucket by centrifugal forces.

A centrifugal pump uses this princi­ple to add energy (pressure) to water. The pump can be seen as a number of buckets spinning together. The buckets are replaced by an impeller with a se­ries of blades that spin at high speed.

The rotating impeller throws water un­der pressure against the volute casing of the pump. This pressurised water is diverted into the delivery pipeline, as seen in the image below.

Performance characteristics of a centrifugal pump are influenced by:

• Impeller diameter
• Type (design) of impeller used
• Rotation speed
• Volute design

The correct alignment of couplings between the pump and motor is very important.

Incorrect alignment will cause vibra­tion, which uses energy that could have been used to raise water pressure. The same vibrations will also cause rapid wear on the bearings, drive-shaft and other elements of the pump. The pump will break down and need expensive repairs.

Suction

Most centrifugal pumps are placed above the surface level of water at the source. This means that, in the same way that a person sucks water up a straw, the pump must draw water through its suction line. Each pump has a maximum suction lift and will no longer work properly if this negative pressure head is exceeded.

Friction losses that occur through the intake filter, foot valve and suction pipe wil all reduce the allowable suction head. It is important for the farmer, or pump operator, to know the maximum suction lift that can be allowed and to be able to identify the problems that may arise if the filter screen on the foot valve is partially blocked, or the water level at the source drops too low.

Priming the pump

A centrifugal pump will not work if air is trapped in the suction line. The impeller will spin uncontrollably and the pump will not draw up water. Small air bubbles in the system will implode and damage the impeller. Before the pump is started, the volute must be filled with water (primed). The pump must be started with the delivery valve closed, and the valve must be opened slowly.

The reflux valve (foot valve) should be positioned deep enough in the water to prevent suction vortexes from being formed, allowing air to be sucked into the system. The foot valve must be positioned high enough off the stream bed to prevent mud and silt from being sucked into the line.

This series is published with acknowledgement to the ARC Agri­cultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.