Planning or Renovating an Institutional Irrigation System
by Don Turner, Professional Education Manager, Hunter Industries

Installed Irrigation Systems
The popularity of automatic irrigation systems is on the upswing at municipal and institutional sites. That is because the price of installing an irrigation system has substantially decreased, while the reliability of operation has dramatically improved. Automatic systems save maintenance and labor costs when compared to portable alternatives, and the even distribution (and the resulting savings) of water is commonplace on well designed and maintained systems.

Design
As with the layout of the field itself, proper design of the irrigation system is a critical step; a poorly planned system cannot be made to work properly by remedial steps later. The system design should be driven by player and user safety, as well as the need for dependability, efficiency and easy maintenance.

It may be tempting to employ the services of a volunteer designer for an irrigation project. But an experienced institutional or sports field irrigation design professional or consultant will usually allow the turf manager to save both time and money in the long run. Designing an effective system requires knowledge of pipe hydraulics, zoning techniques, head spacing, wire sizing, and the efficiency, features, and reliability of various manufacturers rotary sprinklers, valves, and controllers. The cost to repair a single error in any of these areas can easily be higher that the fee charged by an experienced designer.

In planning a system, the designer will give a great deal of consideration to choosing the most appropriate equipment, since each sports field project has its own specific product and budget requirements. Placement of the heads and valves will also be a major consideration, and the designer will space the heads so they are not in heavy traffic areas and the valves are well off the field of play.

Pipe
The pipe running from the irrigation system's point of connection (POC) into the service line to the zone control valves is called the "continuous pressure main line" or, simply "main line". It is a common practice to use solvent welded PVC pipe at a depth of 18 inches below the surface for pipe sizes up to 4 inches, and 24 inches below the surface for pipes larger than 4 inches. Gasketed pipe is sometimes specified and should be placed deeper;36 inches under the surface for pipes larger than 4 inches is usually considered reasonable. However, higher system pressures may lead the designer to specify deeper installation to hold the pipe securely in place.

The non-pressure pipes which connect the control valves to the sprinkler heads are referred to as "lateral lines." These lines can be installed at a depth of 12 inches, which is standard for the industry.

The backfill surrounding the pipe should be rock free and compacted to the same degree as the neighboring soil. The rock free backfill will help to prevent pipe breakage, and the consistent compaction will prevent ruts in the turf that are caused by the uneven settling of the soil over the pipe trench.

Poured-in-place concrete "thrust blocks" are sometimes specified where pipe connections must be especially solid. For instance, thrust blocks are typically specified at all changes in direction on all gasketed pipe, for main line pipes over 2" in diameter, and on long runs when the system will have higher-than-normal pressure.

Specifying the proper pipe sizes helps to maintain the correct water velocity and minimizes friction losses throughout the system. Water flowing through pipes experiences considerable drag or friction from the pipe itself; when the velocity of the water increases, the pressure loss from friction increases. If the pipe used for the system is too small, the operating pressure will be much lower for the heads at the end of the zone than for the heads closest to the valve serving that zone. Irrigation designers agree that there should be no more than a 10 per cent variation in pressure among all heads on a zone.

Most institutional and sports field irrigation system designs show a looped main line. Designers also like to maintain about the same pressure from one zone to another. The reduced pressure loss experienced when the water is flowing from two directions in a looped main line helps them to achieve a relatively balanced pressure throughout the main line at a reduced cost. The designer would have to size the pipe larger with a single connection main line than is necessary with the looped system. This smaller pipe size saves money while reducing the pressure loss to the furthest valve.

PVC pipe is also used as sleeves for the irrigation system's pipes and wires where they pass under walkways, driveways and roads. A good rule of thumb for sleeve sizing is two times the size of the pipe being sleeved.

Sprinklers, Nozzles and Swing Joints
The sprinkler head consists of three major components: the main body of the device, the nozzles through which water flows out of the body, and the swing joint at the bottom of the body which maintains the sprinkler's connection to the lateral lines.

Sprinkler heads have been gradually downsized over the years, primarily for safety purposes. Newer heads have a small surface diameter and a protective thick rubber cover which makes them a very safe alternative to the older-style sprinklers.

Additionally, many of the newer heads have a strong spring for positive retraction so the head will not endanger the public by staying in the up position after the watering is completed. Many of today's safer heads also have a heavy-duty body cap to stand up to the large equipment now being used in routine maintenance. Most newer gear-driven rotary sprinkler heads should be installed right at the finish grade of the turf.

Designers also prefer a rotary sprinkler with a large nozzle selection. The nozzle is chosen to fine-tune the flow of water out of the system. The experienced irrigation system designer will use correct nozzle sizes to obtain "matched precipitation." (The precipitation rate for sprinkler systems is the rate, expressed in inches per hour, at which water is applied over the surface of the turf. "Matched precipitation" means the entire field is receiving about the same amount of water.)

Swing joints can be fabricated on-site by the installers of the system, or can be manufactured parts provided by the supplier. A three-elbow double swing joint will perform efficiently if fabricated correctly. Manufactured double swing joints with O-Ring seals cut down on installation time and are often more dependable. Correctly installed, a double-swing joint provides flexibility and resists breakage when large mowers or other heavy equipment rolls over the sprinkler.

Valves
In an irrigation system, there are two basic types of valves: shut-off valves and sprinkler valves. Shut-off valves ("gate valves" or "ball valves") are used in a continuous pressure main line to temporarily turn the water off to the entire system or a section of the system. Sprinkler valves (manual or automatic) are used to deliver the water from the continuous pressure main line to the sprinklers. In the discussion of sprinkler systems, when the term valve is used alone, it usually refers to an automatic control valve.

Gate valves have a wheel type handle, and several turns of the handle are required to turn off the flow of water. Gate valves are most commonly used on mainline pipes with high water pressure or high water flow. Because several turns are required, they are easier to turn off and the potential for damage caused by water hammer is reduced. Gate valves have a brass-to-brass seat, which means that they may eventually lose their secure seal. For this reason, gate valves are not recommended for frequent use.

Ball valves have a single arm or lever that requires one-quarter turn to turn on or off. This single action is convenient, but caution should be used and the valve should not be turned on or off too quickly, because damage to the system from water hammer could result. Ball valves have resilient seats which maintain their ability to seal longer than brass-to-brass seats, making ball valves a better choice than gate valves where frequent use is expected.

Among sprinkler valves, manual control valves are not as common as they once were. The manually controlled sprinkler system requires an operator to time the irrigation of each zone, and to turn valves on and off at the proper time. The operator must be present the entire time the system is operating. Obviously, this type of system does not have the convenience of the automatic system.

Automatic control valves are used in conjunction with automatic controllers or timers, and are a much more convenient way to deliver water to the turf. Because these valves can be more precisely controlled, an automatic system allows the sports turf manager to efficiently schedule the delivery of the right amount of water to each zone.

There are many types of automatic control valves. On systems not connected directly to city water systems, choose a valve with an inexpensive self cleaning filtering mechanism. Because of the possibility of higher surge pressure in a large turf system, it is recommended that the valve be able to withstand pressures up to 200 PSI. A pressure regulator can be installed at the beginning of the system, but with the long main line and the potential for high friction loss, it is generally better to have an automatic control valve with a built-in pressure regulator, and control the pressure right at the valve.

Placement of the valves should be planned with maintenance and safety in mind. Automatic control valves are generally installed in groups, or manifolds, and placed underground in a plastic or concrete valve box. If the valves will be near the field of play, use plastic. The manifolds should be built with plenty of room between valves. The distance between valves is dependent on the size of the valve box; for most standard size boxes, two or three 1" valves can be installed in a valve box, while valves larger than 1" are usually installed one valve per box.

Controllers
The controller (or "timer") is the part of an automatic sprinkler system that determines when a valve will turn on and how long the valve will operate. The controller sends a low voltage signal through buried wires to the automatic control valve, which then opens for a predetermined amount of time, allowing water to flow to the sprinklers. When the predetermined watering time is completed, the controller turns off the valve.

A controller should be chosen for its wide range of programming capabilities as well as other convenience features. A non-volatile memory means the controller will not lose the program due to power outages or surges. Surge protection will protect the controller from serious damage due to nearby lightning strikes or power surges.

Controllers should be placed in an easily accessible location, in a lockable room or waterproof enclosure. In considering the controller, it is important to remember that some systems require a pump to maintain adequate water pressure. Be sure the controller chosen for this type of system includes a pump start feature. If a pump is needed, the two pieces of equipment are typically placed in the same enclosure. However, the controller should use a separate electrical circuit to reduce the possibility of damage to the controller due to power surges.

As with any electrical equipment, to prevent injury from high voltage electrical shock, it is essential to specify that a proper ground wire be installed to the controller. Consult local codes for the grounding requirements in a specific area.

Wires
In an automatic sprinkler system, low voltage direct burial wire is used to carry the signal In an automatic sprinkler system, low voltage direct burial wire is used to carry the signal from the controller to the automatic control valves. The most frequently used wire for commercial applications is single strand, heavy gauge direct burial copper wire. A lighter gauge wire can be used when installing valves close to the controller, but heavier gauge wire is necessary for longer distances, and may be necessary even for short distances on higher pressure systems. An irrigation designer or consultant will specify the necessary wire size when designing the system.

Many specifiers recommend that the wire be buried in the same trench as the main line, taped to the under side of the pipe. This will help to protect the wire, and make it easier to locate. Furthermore, a wire taped to the pipe can serve as a tracer wire to locate the main line should it become necessary to do so.

Wherever control wire will be exposed, it should be installed in a protective conduit. Waterproof wire connectors should be used to connect the solenoid wires to the low voltage irrigation wire. Where permitted by local code, wire nuts can be used for connections not exposed to moisture.

Backflow Prevention
In order to protect the potable (drinking) water supply from contamination, most states and municipalities require the installation of backflow prevention devices on all plumbing systems that are connected to the public water supply. (These devices prevent water from the system from flowing backward into the supply lines.) In planning or installing an institutional or sports field irrigation system, the designer should consult with the local water company to determine the backflow devices required and their proper installation.

Don Turner is the author of "A Professional Turf Manager's Guide to Efficient Irrigation Practices and Equipment" from which this article was extracted.