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A well-designed and properly adjusted device will save water and improve production
How to select and use a centre pivot machine ?
• Necessary design criteria. The daily amount to be applied (mm/day) will determine the design flow rate of the system. The amount applied depends on the soil water holding capacity and its permeability of the soil as well as the crop water requirements and any possible rainfall expected during the season. On an existing installation, knowledge of the pump’s pressure and flow rate will enable the designer to establish the dimensions of the device and the type of sprinklers to be used. The radius of the area irrigated by the sprinklers will determine the inten-sity of the precipitation rate.
• Selection criteria. The sprinklers are chosen according to the available pressure: the pivot’s sprinklers should have a low angle and operate within the pressure range for which they were designed.
• Sensitivity to wind drift. The smaller the droplets produced, the more vulnerable becomes the sprinkler to wind drift (Particularly in the case of higher pressures and when the boom is higher above the ground).
• Irrigation intensity. Intensity increases as the distance of the sprinklers’ throw decreases. In combination with the size of the droplets, this is directly responsible for a deterioration of the soil surface if irrigation is not suitable for the soil type: risk of induced crusting or capping and run-off.
Pressure regulators fitted to the sprinklers are often required on rough or uneven land. The manufacturers should comply with the European standards currently in force:
- EN 12325: description of the installations, performances and technical characteristics (sprinkler/nozzle pattern)
- EN 909: compliance with the safety requirements of the pivot and lateral move type machines.
All of this data will be applied to the calculated spray nozzle design chart (sprinkler package - type of nozzles used), which will include the information listed above. This document is contractual and must be sent to the person or company using the irrigation system.
Using the equipment
At the beginning of the season, the installation will have to be drained and checked to ensure that it is in good working order (rotation, obstructions) and the end gun properly adjusted (range and angle of the throw specified in the spray nozzle design chart/Sprinkler Package).
Secondly, using a portable pressure gauge, it is important to check the pressures at the inlet and at the end point. These pressures must comply with those specified in the nozzle design chart/ Sprinkler Package (do not leave the pressure gauge permanently connected to the installation).
The alignment of the spans must also be checked (against each other).
During the season, the pressures at the inlet and at the end point must be checked and it must be confirmed that the sprinklers and end gun are working correctly (rotation, obstruction, end gun shut-off device). If the pressures are different from those specified in the chart, the flow rate will vary and thus the amount applied will change and the water distribution will be poor. These measurements must be carried out regularly and they are essential when changing a nozzle.
Finally, the amount applied can be calculated as follows:
D= Q/S x 2nxR/Vx10
Q: Flow rate at the intake of the system in m³/h
S: Surface area irrigated (ha)
V: Average forward speed of the last tower (m/h)
R: Distance between the pivot’s centre axis and the last wheel (m) ?: 3.1416
In the case of a device operating by sector, correct S and R, without forgetting the time required for the machine to complete the return run. For an end gun that only operates intermittently, only correct S.
At the end of the season, the transmissions have to be greased, the tyres inflated to avoid buckling of the spans and the electrical installations protected.
A nozzle and the sprinklers have an average life span of 5 to 10 years.
An irrigation system can be considered to be efficient if an appropriate amount of water is applied. This should take into account the soil properties (water storage capacity, permeability) and the precipitation rate applied during irrigation and the crop water requirements must be met during the peak periods.
Rapid test for checking the settings
• Check the amount of irrigation water applied. This test can be carried out by placing containers under each span at an identical distance from each tower (e.g. 15 m.). This test can be carried out a number of times. If the results of the measurements show a difference in the amount applied of more than 10%, then a complete diagnosis will become necessary.
If the variation in the amount applied should be 10%, then this could lead to a 20% loss of yield in light sandy soils.
If the insufficient amounts applied by one span leads to extra runs being required, then this could lead to excessive amounts being applied to the rest of the field and, therefore, there will be water wastage.
• Checking the pressure of the last span. A connection piece ofitted to the last sprinkler will easily enable this test to be carried out. If there should be a difference in pressure compared with that given in the sprinkler package, check the pressure at the intake and then at the pumping station.
• Checking the flow-rate of the installation. This check can be carried out by calibrating the holding tank, or by using a flow meter. An ultrasonic flow meter is used to measure the flow rate when a full diagnosis is carried out on the device.
• Carrying out a flow dimension diagnostics test. This allows for the flow rate coming into the device to be checked as well as the pressure at the intake and at the end point. If there should be any discrepancies vis-à-vis the manufacturer’s specifications, then the cause will have to be ascertained and consideration given to studying the possibility of a new sprinkler package adapted to the hydraulic characteristics recorded on site.