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 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.
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,