Sprinklers

Types of Heads and Nozzles

Sprinkler heads are the most important component in the system because their performance determines the effectiveness and efficiency of the whole system. A sprinkler operates by forcing water under pressure through a small hole or nozzle and into the air. Nozzle size and water pressure determine the flow rate. Most sprinklers are designed to give a circular wetting pattern. The distance from the sprinkler to the outer edge of that circle is called the throw or wetted radius. Nozzle design, size, and pressure determine the pattern wetted diameter. Sprinklers may have special features that allow them to irrigate only a part of the circle.

In rotary sprinkler heads, the water jet commonly discharges at an angle above horizontal between 22° and 28°. The jet breaks up into small drops as it travels through the air, and falls to the ground like natural rainfall. Good water distribution is dependent on maintaining water pressure (jet velocity) within the range that produces the proper droplet sizes. The sprinkler rotates in a horizontal plane to produce a circular wetting pattern.

Sprinklers may have two nozzles discharging in opposite directions. The larger provides for larger throw and creates the sprinkler rotation. The small one provides for wetting the inner circle.

Impact sprinkler rotation is caused by the water jet impinging on a spring-loaded swing arm (Fig. 5.22). The water jet impulse forces the spring arm sideways. The spring returns the arm, which impacts the body of the sprinkler, rotating it a few degrees. Then the cycle repeats. The rotational speed (1 to 3 rotations per minute) is controlled by the swing-arm weight and spring tension. It is important that the sprinkler rotates correctly so that no area is left underirrigated.

In gear-driven rotary sprinklers the pressurized water entering the sprinkler rotates a small water turbine which, through reducing gears, provides for slow, continuous sprinkler rotation. Gear-drive mechanisms require clean water to prevent clogging and wear.

Figure 5.22. Impact sprinkler.

Spray heads discharge a jet of water vertically from a nozzle onto an impingement plate that redirects it into a circular pattern. This plate may be smooth or serrated and have a flat, convex, or concave surface, depending on the desired pattern shape (throw and droplet sizes). Sprayers generally operate at low pressure and have smaller pattern diameters than impact sprinklers. They are commonly used on continuous-move lateral systems. Recent adaptations of spray heads use rotating or wobbling plates with curved grooves that turn the plate by jet reaction. These variations are designed to increase throw at low pressures and improve water distribution patterns.

Characteristics

A sprinkler is characterized by

• operating pressure P (kPa) required to provide good water distribution,

• effective diameter of the wetted circle Dw (m).

The same sprinkler head may be used for different flow rates and diameters by changing the operating pressure and/or the nozzle diameter dn (mm). Sprinkler charts given by the manufacturer should provide the information on the best combination P-qs- Dw for each dn (Table 5.5). These characteristics are interrelated by

Table 5.5. Discharge qs (m3 h 1) and wetted diameter Dw (m) for typical rotary sprinklers with trajectory angles between 22° and 28° and standard nozzles without vanes

Nozzle Diameter dn (mm)

Table 5.5. Discharge qs (m3 h 1) and wetted diameter Dw (m) for typical rotary sprinklers with trajectory angles between 22° and 28° and standard nozzles without vanes

Nozzle Diameter dn (mm)

Sprinkler Pressure (kPa)

2.4

3.2

4.0

4.8

)

qs

Dw

qs

Dw

qs

Dw

qs

Dw

140

0.26

19.2

170

0.29

19.5

0.51

23.2

0.80

25.0

205

0.32

19.8

0.56

23.5

0.88

25.9

1.20

27.8

240

0.34

20.1

0.61

23.8

0.94

26.5

1.36

28.7

275

0.37

20.4

0.65

24.1

1.01

26.8

1.45

29.3

310

0.39

20.7

0.69

24.4

1.07

27.1

1.54

29.9

345

0.41

21.0

0.73

24.7

1.13

27.4

1.63

30.5

380

0.43

21.3

0.77

25.0

1.19

27.7

1.71

30.8

415

0.45

21.6

0.80

25.3

1.24

28.0

1.78

31.1

450

0.83

25.6

1.29

28.3

1.86

31.4

485

1.34

28.6

1.93

31.7

Source: Adapted from [11].

and, more empirically,

Parameters Kd and Kr primarily depend on the nozzle diameter but they also vary with nozzle design and shape. The exponent in Eq. (5.141b) may be slightly different from 0.25. The value 0.272 is given by Rochester and Hackwell [85] for small rainguns. The wetted diameter can be increased by increasing the water pressure and the nozzle angle (within limits) or using nozzles with vanes [11].

Application Rate

The rate at which sprinklers apply water when operating is called the application rate ia (mmh-1). The application rate depends on the sprinkler discharge qs (m3 h-1) and, for set systems, the spacing between the sprinklers, which determines the irrigated area aw (m2):

Increasing the nozzle size or the pressure, or spacing the sprinklers closer together will increase the application rate. Manufacturers supply the information required to compute ia for their sprinklers. The application rate should always be less than the rate at which the soil can infiltrate water to avoid surface water ponding, redistribution, and runoff. Redistribution lowers application uniformity. Runoff not only wastes water but also may cause soil erosion. Tillage management practices including surface residue management and reservoir tillage are used to increase infiltration and/or surface storage

Table 5.6. Suggested maximum continuous sprinkler application rates (mmh 1) for average soil, slope, and tilth

Table 5.6. Suggested maximum continuous sprinkler application rates (mmh 1) for average soil, slope, and tilth

Soil Texture and Profile

0-5%

5-8%

8-12%

12-16%

Deep coarse sandy soils

50

38

25

13

Coarse sandy soils over more compact soils

38

25

19

10

Deep light sandy loams

25

20

15

10

Light sandy loams over more compact soils

19

13

10

8

Deep silt loams

13

10

8

5

Silt loams over more compact soils

8

6

4

2.5

Heavy textured clays or clay loams

4

2.5

2

1.5

Source: Adapted from [11].

Source: Adapted from [11].

and reduce runoff [86]. Table 5.6 provides suggested maximum continuous application rates according to soil infiltration conditions and slope.

Drop Sizes

A sprinkler normally produces a wide range of drop sizes from 0.5 mm up to 4.0 mm in diameter. The small drops usually fall close to the sprinkler whereas the large ones travel much farther. Information about drop-size distributions along the wetted radius as influenced by pressure is given by Kincaid et al. [87]. The range of drop sizes can be controlled by the size and shape of the nozzle and its operating pressure. At low pressures, drops tend to be large. At high pressures they are much smaller and misting may occur. Noncircular nozzle shapes have been developed to produce smaller drops at low pressures. A detailed analysis is provided by Li et al. [88, 89].

Large drops have high kinetic energy and can damage delicate crops. They also break down the surface structure of some soils, resulting in reduced infiltration rate and crusting. Sprinklers producing large drops should not be used on soils that tend to crust.

Water Distribution

Sprinklers generally cannot produce an even water distribution over the whole of the wetted radius. Often the application is highest close to the sprinkler and decreases toward the edge, resulting in a radial pattern of distribution shaped like a triangle. To make the distribution more uniform over the field, several sprinklers must operate close enough together that their distribution patterns overlap. The sprinkler pattern determines the desired spacing between sprinklers. Uniformity usually is improved by putting sprinklers close together, but this increases water application rates and cost of the system.

Fixed set sprinklers usually are placed in a square or rectangular grid, although triangular grids improve pattern overlap and distribution uniformity (Fig. 5.23). In continuous move systems, only spacing along the lateral affect distribution (assuming the movement is adequately continuous). Continuous move systems usually produce better uniformity than set systems.

Sprinkler Classification

Sprinklers can be classified according to several factors including • type—rotary or spray;

i

RECTANGLE

| C j _

!

TRIANGLE

e

Figure 5.23. Rectangular and triangular sprinkler grids, with spacing e along the lateral and l between laterals.

Figure 5.23. Rectangular and triangular sprinkler grids, with spacing e along the lateral and l between laterals.

• area shape wetted—full or part of the circle;

• throw or wetted diameter—small (< 5 m) to large (> 50 m);

• operating pressure—low (<150 kPa) to high (>350 kPa);

• discharge—small (<0.5 m3 h-1) to large (>50 m3 h-1);

• application rate—low (<5 mmh-1) to high (>15 mmh-1);

• angle of the jet with the horizontal—very low (<10°) for undertree irrigation, low (18°-21°) to reduce wind drift, or normal (25°-28°); and

• drop-size distribution.

In general, these classifications should be combined and considered under the perspective of adaptability of the sprinkler to the field conditions. According to Keller and Bliesner [11],

• Low-pressure (35-140 kPa) impact or spray sprinklers have a small wetted diameter (6-15 m), produce large water drops with a fair water distribution and relatively high application rate (>10 mmh-1). They are mostly suitable for small areas or continuous-move systems.

• Rotary sprinklers with low to moderate pressure (105-210 kPa) produce a medium wetted diameter (18-24 m), water drops are fairly well broken up, and application rates can be selected over a wide range (>3 mmh-1). Water distribution is good when the pressure is near 200 kPa. They are suitable for most crops, including vegetables and undertree irrigation and are also suitable for continuous-move systems.

• Low- to medium-pressure (70-245 kPa) spinners or sprayers for undertree orchard irrigation produce wetted circles with moderate diameters (12-27 m), moderate-size drops, and fairly good water distribution. A large range of application rates is obtainable (>5 mm h-1). Ideal for orchards in windy areas.

• Medium pressure (210-410 kPa) rotary sprinklers are available with one or two nozzles, irrigate a medium to large circle (23-37 m), and produce excellent water distribution with well-broken water drops, with application rates also in a very wide range (>2.5 mmh-1). They are suitable for all type of soils, including those with low intake, and all crops.

• High-pressure rotary sprinklers (340-690 kPa), either single or dual nozzle, wet large diameters (34-90 m), drops are well broken, water distribution is good when wind speed does not exceed 6 km h-1, but application rates are relatively high (>10 mmh-1). They are suitable for field crops, soils with nonlimiting infiltration rate and regions without excessive wind. They can be used as center-pivot end guns and as traveller guns.

• Very high pressure (550-830 kPa) gun sprinklers, generally single nozzled, irrigate circles of large diameters (60-120 m), have high application rates (>15 mmh-1) and produce very well broken water drops. Water distribution is good under calm conditions but is distorted easily by wind. They are suitable for field crops in soils with good infiltration characteristics and are mostly used as traveling rainguns.

Water Distribution Profiles and Recommended Spacing

In choosing a sprinkler, the aim is to find the combination of sprinkler spacing, operating pressure, and nozzle size that provides the desired application rate with the best distribution uniformity. The uniformity obtainable with a set sprinkler system depends largely on the water distribution pattern and spacing of the sprinklers. The uniformity is strongly affected by wind and operating pressure.

Small droplets from sprinklers are blown easily by wind, distorting wetting patterns and reducing irrigation uniformity. Losses due to wind effects are analyzed by Yazar [90]. The distortion of the precipitation distribution patterns caused by the wind is analyzed by Han et al. [91]. Although 15 km h-1 is only a gentle breeze, it seriously disrupts the operation of a sprinkler system [92]. Sprinklers need to operate close together under windy conditions to distribute water evenly. In prevailing wind conditions, the orientation of the laterals should be at right angles to the wind direction and the sprinkler spacing along the lateral should be reduced.

A sprinkler performs best within a pressure range that normally is specified by the manufacturer. If the pressure is too low, the water jet does not break up adequately, and most of the water falls in large drops near the outer diameter of the pattern. If the pressure is too high, the jet breaks up too much, causing misting, and most of the water falls close to the sprinkler. Both of these patterns have a reduced throw.

Manufacturers of sprinklers specify a wetted diameter for all nozzle-size and operating-pressure combinations for each type of sprinkler. These diameters, together with the water distribution profile are used when making sprinkler spacing recommendations.

Indicative spacing recommendations based on the wetted diameter Dw are given in Table 5.7 for the most common water distribution profiles. Triangular and elliptic profiles are characteristic of sprinklers operating at the recommended pressure. A donut profile generally is produced with sprinklers operating at pressures lower than those recommended and by sprinklers with straightening vanes just upstream from the nozzle [11].

Table 5.7. Water distribution profiles and suggested spacings (% Dw) for fixed set sprinklers

Sprinkler Spacing Grid (% Dw)

Triangular

Profiles

Square Equilateral Rectangular

Triangular

55 66 40 X 60

Elliptic

60 66 40 x 60 or 65

Donut

40 80 40 X 80

80 (fair)

Source: Adapted from [11].

Source: Adapted from [11].

The wetted diameters listed in manufacturers' brochures usually are based on tests under essentially no wind conditions. Under field conditions with up to 5 km h-1 wind, such diameters should be shortened by 10% from the listed figure to obtain the effective diameter. A reduction of 1.5% for each 1 km h-1 over 5 km h-1 is proposed for the usual range of wind conditions under which sprinklers are operated [11].

Detailed information on water distribution profiles produced by several agricultural sprinklers with different nozzles and operating at various pressures is provided by Tarjuelo et al. [93]. Information also includes expected uniformity (CU) at various spacings and as influenced by wind.

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