4 Preliminary Analysis
The Van de Klundert’s have been irrigating for a number of years and found that the irrigation system, because of physical limitations, did not provide the flexibility to irrigate the right crops at the right time with the appropriate depth of water. In addition, the electricity costs seemed to be excessive when considering the amount of water used. A preliminary analysis was carried out in October 1997 to find out what was causing these difficulties.
4.1 OPERATING PRESSURE OF SYSTEM
The pump was oversized for the amount of water being pumped. This resulted in the system being operated with a partially closed gate valve to prevent the mainline from being subjected to excessive pressures. The mainline, being Class B PVC pipe, is rated at a maximum working pressure of 600 kPa, while the pump, at the flow rates being used, is capable of supplying a much higher pressure. The result is high electricity use for the amount of water pumped.
The Ritz submersible pump has been designed to operate at a flow of about 200 m3/hour. Operating at lower flows results in excessive pressure available, and as it has been found, requires throttling of the system at the gate valve at the pump. The lower the flows, the worse this problem becomes.
4.2 TRAVEL SPEED OF IRRIGATOR
The Rotorainer irrigator could not operate at a fast enough speed to allow two shifts per day. This resulted in excessive depths being applied, and a rotation time that was too long for the soil type on the property. At times, crops were under stress, with losses in yield resulting.
The nozzle sizes on the Rotorainer had been increased to try to increase travel speed, but with the limitation of 600 kPa at the start of the mainline and the higher pipe friction losses that resulted from the increased flow, the desired effect was not achieved.
Normally, this type of irrigator should have the capability to travel 500 metres in approximately 10 hours, making it possible to operate two shifts per day with a margin to spare. The fact that the machine could not operate at that speed indicated problems with low pressure or wear due to its age.
A recommendation was made to return the nozzles to their original size.
4.3 OTHER PROBLEMS
In terms of property layout, paddock shapes and run lengths, the Rotorainer irrigator was suited to irrigating about 100 ha of the property – that is paddocks A, H, J, L, N, P Q and R (see Figure 2 for location of paddocks). The remainder of the paddocks (i.e. paddocks B, C, E, F, G, K, M and Z) have short run lengths or in the case of Paddock D, a triangular shape, which makes them less suitable for irrigation by this particular machine. Even if the main part of the property could have been irrigated with twice daily shifts, it would have been difficult to irrigate the farm on a rotation short enough to prevent moisture stress in some crops.
Other difficulties included a power line running from the house across Paddock G and along the water race between Paddocks A and H. This prevented the Rotorainer irrigator from being moved directly from Paddock A to H. To move the irrigator from Paddock A to H required that it be taken around a very long route.
4.4 ORIGINAL RECOMMENDATIONS
A number of general recommendations were made at that time. These included:
- Increasing the travel speed of the Rotorainer irrigator by reducing the nozzle sizes, adjusting the angle of the reaction nozzles, and checking/repairing the drive mechanism.
- Adding another irrigator to the system to increase system capacity and make better use of the pump capacity.
- Re-evaluating the system objectives, decreasing irrigation return interval and decreasing depths applied.
Contact for Enquiries
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PO Box 2526
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