Correct choice of sprinkler for irrigation

Choosing the sprinkler for the irrigation project requires technical and theoretical knowledge, as it will determine all the parameters and characteristics of the irrigated area, such as length, diameter, spacing of the lateral lines and the power of the motor pump set. Furthermore, the availability of sprinklers and spare parts on the market must also be taken into consideration.

The latest agricultural report from the Brazilian Institute of Geography and Statistics, carried out in 2006, indicates approximately 4,536 million hectares irrigated. In 2013, the National Water Agency (ANA) indicated 6,04 million hectares were irrigated in Brazil.

Sprinkler irrigation in Brazil has expanded in recent years, mainly due to the application, via fertigation, of vinasse in areas cultivated with sugar cane. Currently, there are 2,486 million hectares in Brazil irrigated by sprinkler and central pivot, representing 41% of the total irrigated area.

Choosing the sprinkler to be adopted when preparing an irrigation project requires the designer to have operational knowledge of the sprinkler, that is, technical and theoretical knowledge. For a designer, it is not simple to define the sprinkler to be installed and used, as the choice of sprinkler will determine all other parameters and characteristics of the irrigated area, such as length, diameter, spacing of the lateral lines and, consequently, the power of the set. motor pump. Furthermore, the designer must evaluate the availability of sprinklers and replacement parts on the market, as well as their cost.

It is essential that the irrigation designer makes clear to the irrigator the intrinsic relationship that exists between the factors: distribution of sprinklers in the area (spacing and arrangement), uniformity of water application, water application efficiency, water savings and productivity obtained. .

In general terms and for the selection of sprinklers, it is suggested that the following criteria be initially adopted: flow rate, operating pressure, jet radius, uniformity of water application, jet angle, degree of spraying, rotation mechanism, durability, cost and spare parts. Therefore, the choice of the sprinkler to be used is of fundamental importance for the beginning of the preparation of a sprinkler irrigation project, characteristics that will allow the designer to begin preparing the system layout and hydraulic calculations.

Another important point when choosing a sprinkler is its application intensity (Ia), obtained by the relationship between its flow rate and its spacing. For example, a sprinkler with a flow rate of 1,56 m/h installed in a square spacing of 18x18 m has an Ia equal to 4,81 mm/hour, this same emitter installed in 6x6 m has an Ia equal to 43 mm/hour. This information is important, as the value of Ia can never exceed the speed of water infiltration into the soil (VIB), otherwise the applied depth will not be fully infiltrated, generating problems with surface runoff in the irrigated area.

Reinforcing the statement of the importance of choosing the sprinkler, we remember that the sprinkler nozzle is the last point of water control by the irrigator, that is, from the moment the water is sprayed into the atmosphere we no longer have control over it and at that moment The distribution of water in the irrigated area will be subject to environmental interference.

Several construction and dimensional factors affect the uniformity of water distribution through the sprinkler. Among constructive factors such as spring tension, rotation speed, jet angle, the internal architecture of the nozzle are very important characteristics. Nozzles can have different section shapes, variations in their length, as well as having grooves to promote ideal spraying of the water jet, and any change in these characteristics will result in changes in the flow rate, jet speed and distribution of water through the sprinkler.

Sprinklers can be classified under different aspects, depending on the mechanism of action (impact, reaction and turbine), the speed of rotation (fast and slow) and also the working pressure (low, medium and high). Sprinklers with impact action mechanism and two nozzles are the most common. The radius of reach, as well as the flow rate and the diameter of the sprinkler nozzle could also be classification parameters, but these characteristics are interdependent.

Sprinklers can also be classified as sectoral (partial rotation) or full rotation (360º). Sectorals are widely used in landscape irrigation, lawns and pasture irrigation, where you do not want to wet the animal circulation corridor, as well as the feeders.

The radius of the sprinkler's water jet, for example, is influenced by the rotation speed and water pressure. Therefore, the lower the rotation speed and the higher the operating pressure to which the water is subjected inside the tube, the greater the reach of the water jet will be.

A sprinkler nozzle is the subject of study and improvement by manufacturing companies and research institutes, as it is one of the main components of sprinklers and directly influences the uniformity of water distribution in the soil. In addition to the variation in diameter, the nozzles have different shapes in the flow section and in their length.

Stationary sprinklers are also common. These are widely used in landscape irrigation and central pivot. The advantage of this sprinkler is that it does not rotate, meaning there is no wear on parts subject to friction. However, spray sprinklers generally have a low flow rate and a short range of the water jet.

There is a wide range of sprinkler models on the market, with different water application capacities. There are mini-sprinklers, which have a flow rate of around 300 L/h, working with a pressure of 1,0 kgf/cm. And at the upper end, we can find sprinklers with a flow rate of 160.000 L/h, working at 7,0 kgf/cm.

To improve the uniformity of water distribution, there are self-compensating sprinklers. It is also common for designers to install pressure regulating valves at the base of the sprinkler, the purpose of which is to equalize overpressure fluctuations in the system, which lead to changes in the sprinkler's working characteristics. However, we must inform irrigation users that whenever a valve like this operates, it is causing an increase in pressure loss, consequently, an increase in energy expenditure.

When installing sprinklers in the field, the characteristics defined in the project must be respected, especially the layout of the pipes in the field. Normally the sprinklers are spaced 6x6 m apart, as the pipes have this factory standard, and the insertion of sprinklers at the junction of two pipes avoids cuts in the pipe. In general, larger spacings result in less uniform irrigation, in return for lower project costs. For example, changing spacing from 12 to 18 m means savings of 30%. It is recommended that, in order to balance this situation, simulations be carried out with software and uniformity tests in the field, under the conditions of the place where the sprinklers are used.

As important as spacing is the installation height of the sprinklers, whether fixed or mobile. It is interesting to note that when preparing the project, the designer focuses on the crop that will be irrigated at that time. However, subsequently the producer may migrate to other crops, which, being different in size from that previously used, can generate problems with uniformity of water application, due to the interception of the sprinkler jet. To reduce this problem, it is important to increase the sprinkler riser pipe, using extenders, adapting it to the height you wish to irrigate, remembering that the higher it is positioned, the more it will suffer the effect of winds. Raising the sprinkler generates an increase in the riser pipe and consequently an increase in the manometric height, requiring greater pressure for the system to function.

The current cost of a sprinkler can range from R$15,00 for low-flow sprinklers to R$4.000,00 for high-flow sprinklers. In general, and considering all the accessories for assembling a sprinkler system, the cost of sprinklers can represent 5% to 15% of the total. This variation may be a function of the price of the sprinkler and the arrangement spacing, which will imply a variation in the desired uniformity of water application.

Therefore, for the ideal selection of a sprinkler, that is, the one that will present the lowest cost to the irrigator and best uniformity of water application, in-depth studies are suggested, as well as consultation with specialists.

Alexandre Barcellos Dalri, Luiz Fabiano Palaretti, UNESP/FCAV

Article published in issue 158 of Cultivar Máquinas.