Mobile Raingun Systems

Mobile raingun systems (also called traveling sprinklers or travelers) use a large rotary sprinkler operating at high pressure. The term raingun is used because of the large size of sprinkler used and its ability to throw large quantities of water over wide areas. They have become popular because of their relatively low capital cost and low labor requirements. They are well adapted to supplemental irrigation. Because of the high pressure requirements, they have high energy costs.

Rainguns normally operate at high pressure from 400 to 800 kPa with discharges ranging from 30 to 200 m3 h-1. They can irrigate areas up to 100 m wide and 400 m long (4 ha) at one setting. Application rates vary from 7.5 to 25 mmh-1. Information below should be complemented with other background literature [11, 94-99].

Hose-Reel Systems

The hose-reel machine has a raingun mounted on a sledge or wheeled carriage. Water is supplied through a semirigid hose that is flexible enough to be wound onto a large reel. The 200- to 400-m-long hose is used to pull the raingun toward the hose reel.

In a typical layout for a hose-reel system (Fig. 5.26), the mainline is across the center of the field. The hose-reel is placed close to the mainline at the start of the first run and connected to the water supply. The raingun is slowly pulled out across the field by a tractor and the hose is allowed to uncoil from the reel. The pump is started and the valve coupler is opened slowly to start the irrigation. The raingun then is pulled back slowly across the field by winding the hose onto the hose reel. Power to drive the hose reel can be provided by a water motor or, more often, by an internal combustion engine. At the end of a run, the hose reel automatically stops winding and shuts down the water supply.

Figure 5.26. Hose-reel system: 1) hose reel, 2) valve coupler on the mainline, 3) semirigid hose, 4) raingun mounted on a sledge carriage, 5) towpath of the raingun, 6) initial position of the raingun to irrigate the dashed area, 7) field location of the hose reel, 8) mainline.

Figure 5.26. Hose-reel system: 1) hose reel, 2) valve coupler on the mainline, 3) semirigid hose, 4) raingun mounted on a sledge carriage, 5) towpath of the raingun, 6) initial position of the raingun to irrigate the dashed area, 7) field location of the hose reel, 8) mainline.

When the hose reel is located in the center of the field, it is rotated 180° and the raingun is pulled out in the opposite direction to start the next irrigation run. When irrigation is completed in this position, the hose reel and the raingun are towed by tractor to the next outlet along the mainline.

Hose-Pull or Cable-Drawn Systems

The hose-pull machine has a raingun mounted on a wheeled carriage. Water is supplied through a flexible hose up to 200 m long and 50-100 mm in diameter, which is pulled along behind the machine. The mainline is laid across the center of the field. A strip up to 400 m long can be irrigated at one setting of a 200 m long flexible hose.

The raingun carriage is positioned at the start of its first run at a distance equal to 1 Dw from the field edge. The flexible hose is laid along the travel lane and connected to the raingun and the valve coupler on the mainline.

A steel guide cable on the sprinkler carriage is pulled out to the other end of the field and firmly anchored. The valve coupler is opened slowly to start the irrigation. The raingun carriage is moved either by a "water motor" powered from the water supply using a piston or turbine drive, or, more often, an internal combustion engine.

At the end of a run the carriage stops automatically and shuts down the main water supply to the raingun. Labor is required only to reposition the hose, cable, and machine to start the next run.

The pressure at the raingun determines the application rate. The forward speed of the machine controls the depth of water applied. Typical machine speeds vary from 10 to 50 m h-1. The faster the machine travels, the smaller the depth of water applied.

The required machine speed, Vtg (mh-1) can be calculated from

where qs is the sprinkler flow rate (m3 h-1), D is the desired irrigation depth (mm), and W is the width of the irrigated strip (mm).

The duration of operation for each set, ti (h) is ti = Lf / Vtg, (5.144)

where L f is the length of the irrigated field (m). Side-Move Systems

These are traveling systems that combine hose-reel machines with moving laterals. In place of a raingun, booms with small rotary sprinklers or sprayers extend out to each side of the portable carriage. When irrigating, the carriage is positioned at one end of the field and slowly pulled across using a steel guide cable and winch in the same way as the hose-pull raingun. A strip of land up to 70 m wide and 400 m long can be irrigated at one setting (2.8 ha). For light systems, a rigid hose can be used as with the hose-reel raingun. These systems require less operating pressure than raingun systems, but the instantaneous application rate is higher.

Rainguns

There are two types of these large rotary sprinklers: swing-arm rainguns, which are large-impact sprinklers and water-turbine rainguns, which are gear-driven sprinklers. Rainguns have sector stops to adjust for the desired circular arc to be irrigated.

Rainguns are fitted with either taper or ring nozzles. Taper nozzles normally produce a good water jet that is less affected by wind, and they have a slightly greater throw than ring nozzles. Ring nozzles, however, provide better stream breakup at low operating pressures. They are less expensive and provide greater flexibility in size selection. Typical nozzle diameters vary from 15 to 50 mm. Typical discharges and wetted diameters corresponding to common nozzle sizes are in Table 5.8. The trajectory angle for rainguns varies between 15° and 28°. Generally, the higher the angle, the larger the throw for a given operating pressure. Low angles (<20°) are preferred under windy conditions (>15 km h-1). Because rainguns operate at high pressures, it is important that the jet of water leave the nozzle relatively undisturbed. Turbulence reduces the throw of

Table 5.8. Typical discharges qs (m3 h~1) and wetted diameters Dw (m) for raingun sprinklers with 24° trajectory angles and tapered nozzles

Nozzle Diameter (mm)

Table 5.8. Typical discharges qs (m3 h~1) and wetted diameters Dw (m) for raingun sprinklers with 24° trajectory angles and tapered nozzles

Nozzle Diameter (mm)

Sprinkler

20.3

25.4

30.5

35.6

40.6

(kPa)

qs

Dw

qs

Dw

qs

Dw

qs

Dw

qs Dw

415

32

87

51

99

75

111

——

480

35

91

55

104

81

116

109

133

——

550

37

94

59

108

86

120

117

139

153 146

620

40

97

63

111

92

125

123

143

162 151

690

42

100

66

114

96

128

130

146

171 155

760

44

104

69

117

101

131

137

149

179 158

825

46

107

73

120

105

134

143

152

187 163

Source: Adapted from [11].

Source: Adapted from [11].

the sprinkler. Modern rainguns have vanes, which "straighten" the flow and suppress turbulence.

Rainguns irrigate only part of a circle behind the machine. This ensures that the machine always moves on a dry towpath. The application depth profile is not uniform and varies across the strip irrigated by a traveling sprinkler as influenced by the effect of changing the wetted sector angle. The most uniform profile is with a sector angle m = 240°. The most commonly used is m = 270°, which is still fairly uniform. As m is increased further, the uniformity of the profile decreases.

Towpath Spacing and Application Rate

The application uniformity of raingun sprinklers is affected by wind velocity and direction, jet trajectory, nozzle type, wetted sector angle, sprinkler profile characteristics, and overlap. Variations in operating pressure also affect uniformity. Under calm wind conditions (0-3.5 km h-1), a towpath spacing of 80% to 90% of the wetted diameter produces good uniformity. Towpath spacing should be reduced about 5% for each 2 km h-1 of wind-speed increase, resulting in a towpath spacing of 55% when wind speed is expected to average 16 km h-1.

For a part-circle gun sprinkler spaced to give sufficient overlap between towpaths, the application rate ia (mm h-1) is approximately where qs is the sprinkler discharge (m3 h-1), Dw is the wetted diameter (m), and m is the wetted sector angle (degrees).

Constant travel speed is required for uniform water distribution over the irrigated area. Traveler speed should vary no more than 10%.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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