2.2 Pressure and Nozzle Discharge Uniformity

Several good quality pressure gauges on an irrigation system are be valuable for management purposes. Figure 2.2a illustrates a pressure gauge. The range of the gauge in psi or in kilopascals (kPa) should exceed the water pressure that the gauge will experience; however, accuracy in reading will be better if the range matches the application. Sprinklers may require a range of 0 – 60 psi, 0 – 100 psi or greater while a drip system may require a range of 0 – 30 psi.

Figure 2.2a. A good quality pressure gauge with correct operating range is a critical management tool.

Pressure is going to vary within an irrigation system but variation within a lateral or within an irrigation zone is a limitation on achieving uniformity. Within a lateral one wants to keep the pressure fairly uniform. A lateral is allowed to have a pressure variation from one end to the other of about 20% (10 % + or -) of the average pressure. With a 20% variation in pressure there will be a corresponding 10% variation in discharge.

Another way to understand this is there was an average pressure for the nozzle when it was selected. This pressure should have been in the range of the manufacturer’s recommended operating pressure for the sprinkler used. The pressure should not vary more than +/- 10 % of the nozzle pressure (average pressure) selected as the basis of the lateral design.

Figure 2.2b. An economical pipe size allows a 20 percent variation in pressure in an irrigation lateral. Check to see that pressures do not vary more than this.

There is chance for error in measuring the pressure of a system but measurements ensure that there are reasonable values to compare in checking the design. A pressure gauge that will read the lateral pressures near its mid-range is most accurate. A portable pressure gauge with a pitot tube on it is used to measure the operating pressure of an operating sprinkler. Figure 2.2c illustrates a pressure gauge with a pitot tube. Figure 2.2d illustrates a pressure gauge with a pitot tube in front of a nozzle for measuring the water pressure. In the field the pitot tube is held about 1/8 inch in front of the nozzle in the middle of the flow stream from the nozzle.

Measure the pressure (Pm) in the first sprinkler on the lateral near the main line. Measure the pressure (Pe) of the last sprinkler on the lateral (end away from main line). The difference in pressures divided by the higher pressure at the main should be less than 20%.

Use this equation above to determine if the difference is pressure is less than 20 percent as allowed.

[ (Pm - Pe) / Pm ] x 100 < 20%

Figure 2.2c. Pitot tube on end of pressure gauge is used to measure pressure in the flow stream from a nozzle. The pitot tube is placed into the flow coming out of the nozzle.

Figure 2.2d. The pilot tube on the pressure gauge is positioned about 1/8 inch in front of the nozzle in the center of the flow stream to measure pressure in the field.

The nozzle discharge can be collected for a known period of time at each end of the first and last laterals (first and last nozzles) and compared in a similar manner. The volume difference should be less than 10 percent difference. Collect nozzle discharge for a timed minute or more.

[ (Vm - Ve) / Vm ] x 100 < 10%

Taking several readings at each sprinkler and averaging them will give a more precise value.

Pressure Variation Allowances:

Main Supply Lines:

Main supply lines deliver water to one outlet. They are sized such that the total cost of the irrigation system, including hardware and energy, is minimized. Pipes are sized by selecting pipes that have a certain level or amount of friction. To minimize friction, however, it is necessary to use very large pipe, which is very expensive. Therefore, there is a compromise needed between the cost of the pipe and the cost of the energy required to operate the pump.

Mains are sized to the smallest pipe size that will cause the friction in the pipeline to be less than about 1.0 psi per 100 feet of length. This will generally give the most economical pipe size for pipes carrying water less than half the hours in one year (<4,000 hr/yr). For a pipeline used more than half of the time annually, the friction in the pipeline should be less than 0.2 psi per 100 feet of length. Another consideration is that the velocity of flow in main transmission pipelines should never exceed 5.0 ft/sec, especially if quick closing valves follow the location of high velocity water flow.

Main line pressures should be measured at the pump, at the highest point on the line, and at the point farthest from the pump. Differences in these pressures may result from elevation change and friction loss. The total loss should not exceed an economically practical value.

Laterals:

Laterals are sized, in a good design, such that the volume of water discharged from each sprinkler on the lateral will be within 10 percent of the volume discharged from the lateral’s average sprinkler. This 10 percent difference in volume of discharge is equivalent to a 20 percent change in pressure.

Reference:

Page 336-337. Water Management. A.R. Jarrett 2nd Edition, 2000, Kendall/Hunt Publishing Company, Dubuque, Iowa

Pair, C. H., et al., Irrigation, 1983, 5th Edition, Chapter 17, Operation and Maintenance, The Irrigation Association,