Sprinkler nozzles work on the basis of several factors; these are nozzle orifice diameter to allow water to pass through, discharge flow rate, and pressure. For a given nozzle size, it is the pressure that governs the amount of water that is forced through the nozzle and it is the pressure that provides the force to create the diameter of throw of the water stream.
Nozzles can be tested to determine if there is a pressure at which they provide a good distribution profile. When found, the nozzle diameter, its angle from the horizontal, its operating pressure and radius of throw are recorded. This information is used for design purposes. It is the information found in catalogs and nozzle charts.
A sprinkler catalog generally shows performance for a sprinkler and nozzle combination. Shown will be a chart giving for several pressures the pressure, nozzle orifice diameter, discharge rate in gallons per minute (gpm) or liters per minute (lpm), and the radius of throw, in feet (ft).
Sprinkler distribution patterns will be poor for too low a pressure and for too high a pressure. Too low a pressure causes the water to come out of the nozzle in a stream that falls heavier in one place along the radius of throw; a donut pattern is formed. Too high a pressure causes the water stream to break up too much when passing through the nozzle to create a mist that falls heavier close to the sprinkler head.
As a learning exercise, go to a page in Rain Bird’s catalog to look at the specs for the Rain Bird 20JH sprinkler or Rain Bird 20JH metric. General specs for the sprinkler are given on the spec sheet on the left under the photo as:
Trajectory Angle: 23° (degrees above the horizon of the water stream)
Operating Range: 35 – 80 psi (pounds per square inch)
Flow Range: 2.0 – 5.47 gpm (gallons per minute)
Radius: 35 – 44 ft (feet)
In the chart for Straight Bore Nozzle (SBN-1) there are four nozzle sizes that cover the range given in the sprinkler specs. Note in column one for the 7/64 inch nozzle that both the radius of throw (Rad.) and the flow rate (gpm) of the nozzle increases as the pressure (psi @ nozzle) increases. These four nozzles apparently give an acceptable distribution pattern with pressures from 35 to 60 psi. The radius of throw increases a small amount. The application rate increases for the higher pressures.
The information tells the system designer that sprinkler spacing can be about 30 to 50 feet maximum apart (40% to 60% of diameter), depending on wind and specific nozzle size and operating pressure. It is important that this information be used designing the system.
Another point to observe here is that if a 7/64 inch nozzle wears and it becomes a 1/8 inch nozzle, the water demand increases and the pump may not be able to supply more water at the same pressure. Most irrigation pumps have a pressure – flow rate relationship that changes, often inversely, so a greater demand for water (the larger nozzle) results in the pump supplying the greater flow at a lower pressure. This leads to a distorted water application pattern and non-uniform application.