Sprinklers driven by offset nozzles

On some types of sprinkler, the spreader nozzle sprays water at an angle to the main delivery nozzle. This offset nozzle provides the drive to cause the sprinkler to rotate and distribute water over the irrigated circle. The nozzles of these sprinklers are normally plastic and clip onto the sprinkler.

These plastic nozzles are resistant to wear and seldom need to be replaced. However, a fine balance exists between the driving force provided by the offset nozzle and the frictional resistance offered by the bearing washers.

These sprinklers wet the full circle covered by rotating at a regular and even speed. The distribution pattern measured will often differ from those of rotating impact sprinklers, and these sprinklers are usually best suited to triangular spacing. The bearing washers sometimes become blocked by fine silt particles, preventing the sprinkler from rotating properly. If this type of sprinkler stops rotating, or rotates at an irregular speed, the sprinkler will normally have to be discarded and replaced.

At low pressures, the distribution pattern is likely to become very erratic. A double doughnut will be formed with most water falling in one of two wetted circles described by the nozzles.

At the correct pressure, the distribution pattern will tend to be trapezoidal
in shape. This makes the sprinkler suited to fixed triangular spacings, but even distribution of water on the more popular square spacings will be more difficult to obtain. At high pressure, the effects of increased rotational speeds, combined with an excessive breakup of the jet of water, will cause water to fall even closer to the sprinkler. Pressure control is of utmost importance.

Simple ball or gear driven sprayers

A wide variety of simple ball or gear-driven sprayers is used to irrigate agricultural crops (normally in orchards). Although most suppliers of gear-driven sprayers do not stipulate filtration, they are prone to blockage when dirty water is used. Few of the ball-driven sprinklers on the market is ever submitted to one of the major irrigation laboratories for testing. For many of these sprinklers, accurate distribution test results are not available.

These sprinklers are often more expensive than rotary impact sprinklers and because of a smaller spray radius, more sprinklers must be used to cover an area. Water distribution can, however, be very efficient and ball-driven sprinklers are well suited for use in solid or semi-solid systems for high-value crops. Ball-driven sprinklers are typically used on 12 x 15 m triangular spacings, but can be used on spacings of up to 15 x 15 m (square).

Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.