imply endorsement by the authors or Kansas State University. The authors are Gary A. Clark, ...... Kincaid, D. C., K. H. Solomon, and J. C. Oliphant. 1996. Droplet.
MEASURED AND SIMULATED UNIFORMITY OF LOW DRIFT NOZZLE SPRINKLERS G. A. Clark, K. Srinivas, D. H. Rogers, R. Stratton, V. L. Martin ABSTRACT. Field measurements conducted on large–scale irrigation systems with fixed–plate, low drift nozzle (LDN) sprinklers showed that coefficient of uniformity (CU) values ranged from 70 to over 90. Measured CU values were typically lower in value for the lower operating pressure systems and for sprinkler packages with wider spacings. Measured single–sprinkler distribution patterns were then used in an overlapping sequence with specific sprinkler spacing scenarios to simulate multiple–sprinkler distribution patterns for moving systems. Scenarios included sprinkler operating pressures of 41, 69, 104, and 138 kPa; sprinkler spacings of 1.83, 2.44, 3.05, and 3.66 m; and nozzle orifice sizes of 4.76 to 7.94 mm with a flow range of 0.16 to 0.77 L/s. Simulated patterns and CU values compared well with field–measured patterns and CU values for the respective sprinkler size, spacing, and operating pressure combinations. CU values from simulated patterns were highest for closer sprinkler spacing scenarios (99 96 96
90 93 97 92 92
85 90 91 88 90
82 89 96 89 84
14S06 (1) 14S10 (1)[a] 14S15 (1) 14S20 (1)
93 >99 96 97
83 90 91 97
93 86 88 91
72 98 85 96
16S06 (2)[a] 16S10 (2) 16S15 (2) 16S20 (2)
92 93 95 98
88 82 >99 97
83 87 96 94
88 72 80 94
18S06 (2) 18S10 (2) 18S15 (2) 18S20 (2)
95 >99 97 99
88 94 91 >99
75 99 88 99
94 82 97 90
20S06 (3) 20S10 (3) 20S15 (3) 20S20 (3)
93 >99 >99 >99
86 98 95 >99
87 83 98 96
84 93 85 95
These four sprinklers had the same discharge rate (0.28 L/s).
(fig. 9) have similar shapes in comparison to the 16S10 pattern shown in both figures. As would be expected, the resultant CU values (table 3) are very similar for the four
328
Table 5. Average coefficient of uniformity values for the various simulated operating pressure and sprinkler spacing combinations and the analysis of variance results. Sprinkler Spacing (m) Operating 2.45 3.05 3.66 1.83 Significance[a] Pressure (kPa) 41 69 104 138
93 97 97 98
87 91 94 97
84 89 92 94
84 87 88 93
Significance[a]
**
***
*
NS
[a]
** NS ** *
Significance is expressed as p < 0.10 (*), p < 0.05 (**), p < 0.001 (***), or not significant (NS) using analysis of variance procedures.
spacing combinations with those nozzle combinations (16S15 and 18S10). Yet many of these single–sprinkler patterns are unique and difficult to predict, requiring individual measurement and characterization. Analysis of variance (ANOVA) results of the data in table 4 are summarized in table 5, showing the data grouped according to sprinkler spacing and operating pressure. Coefficient of uniformity (CU) values from simulated patterns significantly increased with operating pressure for the 1.83, 2.45, and 3.05 m spacing scenarios. The CU values for the widest spacing (3.66 m) were more varied, but still showed an increasing trend with operating pressure. While only the highest operating pressure (138 kPa) resulted in consistently acceptable uniformity values (CU > 90) for all spacing scenarios, closer sprinkler spacing is required with lower operating pressures to maintain CU > 90. Four of the scenarios identified in table 4 had the same discharge rate (0.28 L/s). Simulated CU values tended to decline for most
TRANSACTIONS OF THE ASAE
CU value
100 95 90 85 80 75 70 65 60 0.00
0.10
0.20
0.30
0.40
0.50
Spacing/Diameter Ratio 41 kPa
69 kPa
104 kPa
138 kPa
Figure 17. Relationship between simulated coefficient of uniformity (CU) and the ratio of sprinkler spacing to wetted diameter. Simulated scenarios are plotted with respect to operating pressure.
combinations with a wider spacing (3.05 and 3.66 m). This is consistent with fixed–plate spray sprinkler spacing recommendations of 1.5 to 2.4 m for operating pressures of 10 to 20 psi, respectively (Keller and Bliesner, 1990; Scherer et al., 1999). Figure 17 shows the relationship between CU value and the ratio of sprinkler spacing to wetted diameter for all simulated scenarios. In general, for design CU values of 90 or more, sprinkler spacing to wetted diameter ratios should be maintained at 0.20 or less. Lower operating pressures (41 and 69 kPa) had more varied CU values than the higher pressures (104 and 138 kPa). If wind is a problem and larger droplets (and coarser patterns) are needed, then the lower pressures should be used (Howell and Phene, 1983; Edling, 1985; Vories and von Bernuth, 1986; Kincaid et al., 1996). However, associated sprinkler wetted diameters should be assessed to determine an acceptable spacing. While this study did not evaluate spacing scenarios closer than 1.83 m, closer spacing criteria may be desired for smaller nozzle sizes with lower pressures that have wetted diameters less than 8 m. However, application rates should be closely monitored.
SUMMARY AND CONCLUSIONS
Field and laboratory studies were conducted to evaluate the uniformity of applied water from fixed–plate, low drift nozzle (LDN) sprinklers as used on center–pivot and linear–move irrigation machines. Field measurements of water application patterns were conducted on three center– pivot systems and one linear–move irrigation system in south central Kansas. Catch pan results showed that coefficient of uniformity (CU) values ranged from 70 to over 90, but were typically lower in value for lower operating pressure systems and wider sprinkler spacings. Distribution patterns from single fixed–plate sprinklers were measured and summed to approximate the cumulative pattern from a single moving sprinkler. Those patterns were then used in an overlapping sequence with specific sprinkler spacing scenarios to simulate the resultant multiple–sprinkler distribution patterns. Coefficient of uniformity (CU) values were calculated for the resultant patterns. Simulated application patterns and CU values compared well with field–measured patterns and CU values for the respective sprinkler sizes, spacing, and operating pressures.
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Simulated spacing scenarios ranged from 10% to 46% of the wetted diameters of the sprinklers. Resultant simulated spacing CU values ranged from 72 to 99 and tended to decrease with increased sprinkler spacing and/or decreased operating pressure. However, some lower pressure and wide spacing scenarios resulted in CU values in excess of 90. Manufacturers of these types of sprinklers recommend spacing sprinklers between 1.2 to 3.1 m and operating at the lower pressure ranges of 40 to 70 kPa. However, this research has shown that fixed–plate sprinkler package designs that use the higher end of that spacing range and/or the lower end of that pressure range may result in lower than acceptable uniformities (
