SPRINKLER SYSTEMS - 5
Planning the system
There is nothing complicated about the design or the installation of a lawn irrigation system, but there are certain principles which must be understood and followed if the system is to operate satisfactorily. A poorly designed system fails to achieve its full purpose and also wastes water. If there are substantial areas not reached by the sprinkling heads when the system is in normal operation, there will be patches of grass damaged by drought even though the sprinkling system is used. Almost as bad is too much overlapping, as this defect produces sizable soggy areas in the lawn. Where water is metered and the rates are high, or where there are some limitations on its use; it is essential that the system be laid out so that it is efficient. There are two ways of obtaining a lawn sprinkling system. The first is to select a contractor who has a reputation for making good installations and to give the work to him. He will look over your grounds and probably discuss with you what special features you want such as special locations for the valves; but after you have stated your views on these points, you will not be concerned with the design or installation problems at all. You will only have to pay the bill when it comes in. The other method is to buy the supplies and install the system yourself. The discussion from this point on will assume you are going to follow the latter procedure. The first step in designing and installing a system of your own is to select a source of supply, especially to decide which manufac- turer's heads you are going to use. Once you have made this deci sion, obtain from this manufacturer whatever information he has available for homeowners about the installation of his system. This information should include not only a description of the heads, but also data on the water pressures required and the areas covered by the heads when they operate at different pressures. Most manu- facturers also furnish some information on how to figure the all- important subject of pressure drop due to friction in the supply pipe and branches.
Drawing the Plan
The type of ruled paper known as graph paper provides a convenient way of drawing the necessary plan without pain or too much effort. First measure the lawn and outline it on the graph paper according to the scale you have adopted. Next mark on the plan the locations of the sprinkling heads, giving them the spacing recommended by the manufacturer for the water pressure you know will be available. Typical spacings for stationary heads are 10, 12, 14, or 16 ft., but don't guess at the spacing and especially important don't overestimate it. It is well to use symbols to indicate the different styles of heads so that you will not make a mistake in ordering them. Selection of the water pressure to be used as the basis for the spacing is a critical point. City water pressures are often stated in terms of the maximum rather than the minimum, whereas lawn irrigation systems are often used at times when the pressures are considerably below maximum. It is good procedure to obtain from the offices of the utility which supplies the water a statement of the minimum pressure that can be expected under summer conditions in your locality. The value given should then be used as the design pressure for the system. If sprinkling is done at times when the water pressure is considerably in excess of the minimum pressure, the situation can be controlled by only partly opening the supply or branch valves. In spotting the heads, locate them so that overlapping of the spray will theoretically not occur with square heads and will be held to a minimum with circular heads. However, in the case of circular heads, do not make the mistake of spotting them so that there are even small unwatered areas between the areas covered by adjoining heads. Observe the correct relationship between adjoining circular heads. If the area where the system is to be installed is very irregular, it sometimes helps to cut templates which represent, in the scale you are using, the areas covered by full, half, and quarter heads. The templates can then be shoved around on the plan until the best possible arrangement of the heads is arrived at. Once the location of the heads is determined, the next step in design is to locate the valves. On small lawns, the only valve needed is one in the supply line; but on lawns of moderate and large size, it is usually advisable to plan the system so that it can be operated in sections. If the system is so large that all of it cannot be operated at one time from the available water supply, such an arrangement is, of course, essential. It is a convenience when you wish to water part of the lawn while you use another part for recreation. Very often it is discovered after a lawn sprinkling system is in- stalled that the water supply pipe which runs from the water main in the street to the house is too small to handle both the sprinkling system and the house plumbing system simultaneously. Such a con- dition can be expected if, before the sprinkling system is installed, the opening of several faucets on the lower floor of the house causes a considerable diminution of the flow from faucets on the floor above. Robbing of the house plumbing by the sprinkling system sometimes develops even when this test does not foreshadow it. The condition can be remedied by installing a new supply pipe.
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