Pressure and Spacing of Distribution Hardware

Two inter-related elements that are vital to the success of any spray irrigation system are pressure and spacing of distribution hardware.  Pressure is the force behind the water.  There is static pressure, the pressure that registers when nothing is moving, and operating pressure.  Operating pressure is the pressure that is in the lateral systems, and most importantly at the base of the sprinkler (or emitter, etc.)  during operation.

Operating Pressure is critical

Ensuring operation of sprinklers at the stated optimum operating pressure is critical to the even distribution of water.  At optimum pressure nozzles produce a ratio of large droplets that carry toward the end of the radius, and a finer sized droplet that fill in around the sprinkler.  (With appropriate sprinkler spacing, the small and larger droplet from opposing heads infill and create a uniform application.  Even application is an Irrigation Essential!)   With the right operating pressure, a nozzle creates proportionally (more) large droplets and throws them to the end of the radius because there is a greater area to cover at the end of the radius, than the area near the head.  So it is easy to see that uniformity of coverage is dependent on spacing, pressure and one more thing – wind, or the absence of it.

With too much pressure, a percentage of the water droplets atomize and simply float away as mist without landing on the landscape.  With too little pressure, the water is not deployed evenly, and soaks some areas while leaving others dry.  A good rule of thumb is all hardware in a lateral system should operate within a 10 psi range of each other.  

Elevation impacts pressure as well.  Pressure is lost going uphill and gained going down hill.  Large lateral systems that include substantial elevation changes have several challenges “built in”.  Email us back with more specifics about this situation – you’ll want to get this right the first time!

Square Spacing

“Square” spacing is typically used for irrigating areas that are square or have borders that are 90-degree angles to each other.  The sprinklers are the same distance between sprinklers as between laterals.  It is best used when the border is a sidewalk, or building where it is vital to confine the water to not over spraying onto a non-landscape surface.   The shortcoming of “square” spacing is the distance down the “legs” of the square is approximately equal to 70% of the diagonal distance.   The diagonal dimension makes it difficult to achieve a high uniformity of application, but that is the trade off for having the edge (walkway, etc.), that is protected from overspray.

The Right Pressure/Nozzle Combination “fits” the Spacing

Manufacturers specify pressure and nozzle combinations at specific spacing criteria.  Always follow these criteria, and never – never “stretch” spacing!  It is important that an overlap of coverage occurs per the manufacturers recommendations.  Also, if you are routinely irrigating in a windy environment, “shrink” the spacing to ensure a greater chance of even application.  All manufacturers provide guidelines on spacing their product in windy conditions. 

Delivery Systems Articles • Posted on 02/22/2009 • Permalink

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