# 4.5 Example of a Real Sprinkler System

An example of a real sprinkler system illustrates the discussion of flow, pressure, and friction loss.

Real Sprinkler System is an illustration of a small overhead sprinkler system having eight sprinklers on three laterals. A pump supplies water. The illustration does not illustrate the effect of elevation changes, but it does show:

1. Water pressure at the pump and at each sprinkler,

2. Water flow rate in the pipes within the system, and

3. Friction loss in psi in sections of the pipeline (shown in parentheses).

Initial conditions:

• Pump discharge pressure = 50 psi

• Pump discharge flow = 16 gpm

• 8 sprinklers discharge an average of 2 gpm each

• Given: All pipe diameters are the same

Follow the water flow in the Real Sprinkler System diagram:

A pump discharges a flow rate of 16 gpm into the system; thus, 16 gpm must come out somewhere. There are 8 sprinklers that average 2 gpm discharge at 45 psi. Since flow = velocity x pipe cross-sectional area, think about what is happening to flow and water velocity in each pipe segment (between sprinklers and tees). Remember the pipe diameter stays constant throughout the system.

Start with 16 gpm flowing to the first tee where flow splits to supply sprinklers in each direction. There are 3 sprinklers to one side and 5 sprinklers to the other side. At the first tee there is 48 psi pressure and the water flow is split 6 gpm to one side and 10 gpm to the other side. Continue from there to follow the water.

The pump provides an operating pressure of 50 psi. There is no elevation change so the pressure does not change due to elevation. There is flow in the pipes, so there is friction as the water flows through the pipe. Some energy is lost and the operating pressure gradually decreases.

Friction loss in the mainline from the pump to the field was 2 psi, giving the 48 psi at the first tee. Follow the pressures through the system and understand why the friction losses are what they are and why the sprinkler operating pressures are what they are.

Note - remember the connection between flow and pressure is water velocity. Remember the relationship between velocity and friction loss. Keep this in mind when studying the irrigation system in the example.

The discharge from the sprinklers will vary with pressure. The following chart illustrates this.

Note the discharge rate and throw distance vary with operating pressure.

2019 Sprinkler, # 7 Nozzle

 Pressure, psi 30 35 40 45 50 Flow, gpm 1.59 1.74 1.87 2 2.12 Diameter, ft 76 77 78 79 80