Experiment No. 2
EFFECT OF PRESSURE ON WATER DISTRIBUTION
Object:-To investigate the effect of pressure on the radial distribution of water from a Rotary Sprinkler.
Apparatus:-Sprinkler Test Rig (Catch Assembly, Display Unit, Pump Set and Storage Tank) and Stop Watch.
Theory:-The sprinkler supply water in the form of spray, somewhat as in ordinary rain. A sprinkler operated at the correct pressure should give the distribution of water which is highest close to the sprinkler and diminishes towards the edge of the wetting circle.
If the pressure at the sprinkler is too low, the jet produced by the nozzle will not break up sufficiently. It will therefore contain a greater number of large drops and these will be thrown to the outer edge of wetting circle because of their greater momentum. This gives a ‘doughnut ring’ pattern of distribution. Conversely if the pressure at the sprinkler is too high the jet will break up into very small drops which will not carry the required distance.
Both pressure conditions will result in poor uniformity when the sprinklers are operated as part of the system. When compared to good sprinkler system uniformity, a poor system will use more water to irrigate a given area. Improving the application uniformity of a sprinkler system can reduce the water supply necessary to irrigate a given area. This savings in water will lower pumping and operating costs.
The increase in CU from 70 to 90% would reduce the water requirement by 38.3% with such a savings 62% more land can be irrigated.
CU (%) | Water applied over entire area to get at-least 1” on 90% of the area | Water saved with CU > 70% | Greater area irrigated with CU > 70% |
90 | 1.19” | 38.3% | 62.0% |
80 | 1.47” | 23.8% | 31.3% |
70 | 1.93” | 00.0% | 00.0% |
Where,CU = Coefficient of Uniformity
= Mean value of readings (Sum of all the readings i.e. )
n = Number of readings
Experimental
Setup:-The rig is set up for normal use with the display tubes primed. Catch cans positioned 1.0 m apart from each other. The riser height is adjusted and Control Valve is opened to adjust pressure. The sprinkler will be adjusted according to the desired pressure. Now during adjustment if any water is collected in catch cans, empty the display tubes.
Now start the sprinkler and stop watch simultaneously. The sprinkler should be operated for 10-20 minutes or until there is reasonable height of water in display tubes. Note the height of water in display tubes, which shows the depth of water in each catch can. Repeat the experiment with changing pressure, take reading and plot the graph (Depth vs. Distance).
Observations:-
Nozzle size = _______ mm;Riser height = ______ cm
Catch Can No. | Pressure (kg/cm2) | |||||
d1 = | d2 = | d3 = | ||||
(mm) | (mm) | (mm) | (mm) | (mm) | (mm) | |
1 | ||||||
2 | ||||||
3 | ||||||
4 | ||||||
5 | ||||||
6 | ||||||
7 | ||||||
8 | ||||||
9 | ||||||
10 | ||||||
∑ | ||||||
mm | mm | mm. | ||||
CU | % | % | % | |||
Graph:-Depth of water is plotted against distance from the sprinkler for various pressures, as shown in the attached Figure.
[Attach the graph]
Result:-Based on the value of Coefficient of Uniformity i.e. _______%, is evident that the pressure of _________ kg/cm2, with the nozzle size of ____ mm at Riser height of ____ cm, gives the best radial distribution of water within the wetting circle of the rotary sprinkler. Hence it is suitable for the field conditions.
Conclusion:-