Head loss

In static head, there is no flow in the system. But what happens when there is flow?

In short, every item in the irrigation system will cause friction, which will reduce the energy of the water. This is called head loss.

Back to our tanks example…

If the system is turned off, the pressure gauge on both would read 20 m (28 psi). This is the static pressure.

However, when there is flow, the pipes create friction (head loss) as the water passes. Therefore, neither pressure gauge will show 20 m (28 psi). The tank with the smaller diameter pipe will have a reduced pressure (greater head loss) than the tank with the larger pipe.


What increases head loss?

  • Smaller diameter pipes

  • Longer distance pipes

  • Higher flow rate/velocity of water through the system

  • Higher friction coefficient of the pipe


What decreases head loss?

  • Larger diameter pipes

  • Shorter distance pipes

  • Lower flow rate/velocity


In the section on Head loss, we look at how head loss “adds up” throughout the system. You have a goal of what you need for the emitter to perform, and you work back from there.

The example in Head loss had a table with each component, starting at 31 m (46 psi) at the pump, and 9 m (13 psi) at the emitter. Below is a graphical representation of the reduction of pressure from pump to emitter.

Although not all items are purely sequential in this order, the purpose of the above is to show the cumulative effect of head loss. From a design perspective, we need to ensure that there is enough pressure at the end.

Of course, the above is an example only. Every system has different components and topography, and as such, the m (ft of head or psi) reduction will vary.

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