Figure 2. Impact Drive and Gear Drive Rotary Sprinkler Heads
Selecting sprinkler heads for a system will require investigating various sprinkler models to find one with the characteristics best suited for the site and watering needs. The important information to the selection includes the operating pressure, flow range, precipitation rate, and the radius and arc of coverage. This information is found in the product catalogs provided by the sprinkler manufacturer. In addition, many sprinklers have special features (such as angles of trajectory and nozzle options), which may be important in the selection process. While no single head will work best in all wastewater applications, this bulletin will provide a step-by-step method for selecting a head that will provide proper application for the site.
| Recommended for Wastewater Irrigation | Not Recommended for Wastewater Irrigation |
| Rotary Heads | Bubblers |
| Spray Heads |
Three general types of sprinklers used for small residential irrigation projects are:
All three types are suitable for traditional irrigation of potable water, however for irrigation with treated wastewater the first two should be avoided and the system should be designed using rotary heads (Figure 2).
Rotary heads provide a large spray radius (15-50 ft) and a relatively low gpm flow rate, which reduces the probability of puddling and runoff. Rotors also spray with more force, preventing clogging of the heads with particles or icing up in the winter. The heads should be installed on stationary risers (a minimum of 18 inches) to allow for continued application through the Ohio winter season. Higher risers may be needed in the high snowfall area of northeast Ohio.
Selecting the proper sprinkler for the project is a matter of analyzing the amount of water to be distributed and the area to be watered. Sprinkler selection will determine the pressure requirement, pump size, and irrigation run times. This may be an iterative process requiring several attempts before the right combination of these factors merge. The following guidelines will provide a general rule for sprinkler selection.
Guideline 1: Sprinkler Radius
Based on the application area geometry, how far does the sprinkler need to throw water? The system design should include head to head coverage, which means that the spray from one head reaches the next head (+/- 10%). As a general rule, using the largest radius sprinkler will result in less initial system cost. However, while rotors have the ability to spray up to 75-100 feet, a pump that can create the required pressure may be cost prohibitive for most projects. A radius of 35-50 feet is a reasonable starting point. For distances greater than 40 feet additional heads will be required. To avoid over-spray (spraying outside the application area) in areas with irregular perimeters, the shortest dimension should be considered when selecting the sprinkler spray radius.
Guideline 2: Flow Rate
The type and number of sprinklers on a zone will determine the required capacity of the pump used for the system. If a zone contains four sprinkler heads with 3 gpm flow rates, the pump will need to have a minimum capacity of 12 gpm. The flow rate of a system is controlled by selecting heads with specific flow rates and by separating heads into different zones. It is important to keep pump costs in mind and not oversize the zones of the system.
Guideline 3: Pressure Requirements
Operating pressure is the pressure that provides optimum sprinkler performance. Manufacturers establish the optimal pressures for their sprinklers and publish them in catalogs and product literature. As a general rule, greater throw radius requires greater pressure, which in turn requires a more powerful pump. Proper functioning of the sprinkler head requires that the dynamic pressure of the system meets or slightly exceeds the pressure specifications given for the sprinkler head. In addition to the pressure requirement of the sprinkler head, account for pressure losses due to elevation changes and friction in the piping system. Pressure loss calculations are discussed further in Step 8.
Guideline 4: Sprinkler Spray Pattern
Most sprinklers distribute water in a circular pattern. The “pattern” describes the portion of the full circle that is covered by the sprinkler. The pattern of some sprinklers is predetermined or “fixed,” while the pattern of others is adjustable.
Guideline 5: Angles of Trajectory
The standard trajectory of most sprinklers ranges from 18° to 28°. The trajectory affects the throw radius and the precipitation rate of the sprinkler. For irrigating with treated wastewater consider using low trajectory sprinklers with angles of 13o to 15o. The lower trajectory shortens the throw distance radius of the head and increases the precipitation rate, but it also reduces the wind drift, which may be a concern when irrigating areas close to homes or other structures. Low angle heads also allow for spray under low hanging tree branches or large shrub foliage.
Notes:Sprinkler head manufacturers publish specifications for sprinkler head optimal pressure, throw radius, and discharge flow rate (see Example: Step 5). Use this information to select sprinkler heads for the system.
| Sprinkler Head Performance | ||
|---|---|---|
| Pressure(psi) | Radius(ft) | Flow Rate(gpm) |
| 30 | 38 | 2.4 |
| 40 | 40 | 3.1 |
| 50 | 41 | 3.4 |
| 60 | 42 | 3.7 |
Sprinkler type: rotor
Pressure: 30 psi
Spray radius: 38 ft
Flow rate: 2.4 gpm
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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