Number 4. Flushing regime

Flushing is one of the most important preventative maintenance activities for your irrigation system.

Although you have filtration, very small debris (e.g. silt and clay) will pass through the filtration system. They will usually not create problems unless they remain in the system and coagulate, especially with organic material, over time to become much larger problematic material. Flushing expels this foreign matter before it becomes a problem.

First – some key concepts.

What is flushing?

Contaminants like minerals or organic materials can be found in any water source. When the water is traveling quickly, this isn’t an issue as it stays suspended in the water, but when it slows down, it can begin to settle out and build up over time in the pipes.

Flushing, as the name implies, sends a higher velocity of water through the system to push out any contaminants that have built up to keep the pipes clean.

A key point is that a higher velocity, and therefore a higher volume of water, is required to flush.

Poor, infrequent, or no flushing will cause clogging, and result in the loss of crop yield and ultimately the cost replacement of the system over time.

Due to the increased water requirement to flush a system, this additional water needs to be considered in the hydraulics when first designing the system.

Water velocity and flushing

Basically, velocity is the speed the water travels through the pipe. Getting the speed right is critical to the effectiveness of the flush.

  • Submains should have a flushing velocity of at least 0.5 m/s (1.5 ft/sec).

  • Drip laterals should have a flushing velocity of at least 0.3 m/s (1.0 ft/sec) at the end of the laterals.

In a perfect world, flushing water velocity should be 0.5 m/s (1.5 ft/sec) for drip laterals, but in reality, this can be difficult due to the different hydraulic requirements between irrigation and flushing. Therefore, 0.3 m/s (1.0 ft/sec) is acceptable. Below 0.3 m/s (1.0 ft/sec) is not sufficient to flush a system.

The higher the flushing velocity, the better, however, there is always the constraint of available flow rate and pressure.

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