Before you begin, here are some guidelines to keep in mind about drip systems:
Filtration may be required. From an equipment catalog you can look up the specifications for the type of emitter you choose. In most cases the filtration requirements of an emitter will be stated in either mesh count or micron size. "Mesh count" refers to the number of openings per linear inch on the screening surface of a filter element. A sixty mesh screen has sixty openings per linear inch forming over 3,000 little openings per square inch of filtering surface.
Always choose a filter with a mesh count equivalent to or slightly higher than the mesh count requirements of the emitter you select. Typical mesh counts for filters and emitters include 60, 100, 140, and 200 mesh among many others. The actual size of openings may vary between filters with identical counts depending on how large the filaments are in diameter. Quite often a metal filament is thicker than nylon, for instance.
" Micron" refers to the actual size of the particle that can pass through a filter element and therefore the actual size of the openings. A micron is equal to 1/1000 of a millimeter. Most drip equipment however is rated by mesh count.
Pressure regulation may be necessary for your system. Most drip emitters operate from between 10 to 30 psi water pressure. Sprinklers, on the other hand, may operate with pressures as low as 15 psi for some spray heads and up to 100 psi for some impact driven sprinklers. On today's modern landscape irrigation systems, circuits of sprinklers and circuits of drip emitters are often on the same project. Smaller pressure regulators for individual circuit control have been developed to accommodate these varying pressure requirements.
Be aware of pressure variations caused by elevation changes on the job site. Ordinarily pressure losses due to friction (flow losses) are very slight in drip circuits. Flow loss for a two or three hundred foot long drip lateral may often be less than 2 or 3 psi. Yet a 10 to 15 foot rise in the elevation along that lateral tube or pipe could double the pressure loss.
When designing drip systems, always plan on wetting a minimum of 50% of the plant's area. In agriculture, 60% is often the minimum design spec for plant area irrigating. The precise small amounts of water being applied make it critical to plant health and survival that sufficient portions of the plants root zone be irrigated. This manual will cover plant area calculating and calculating the proper number of emitters to meet the wet area requirement.
Design the system for mature plant material. Make sure your drip system can carry the flow rate required to maintain the fully mature plantings. In most cases, as the plants mature, no one returns to the job site to add emitters or tubing.
Design your systems to avoid salt build up in the plant root zones and for salt leaching in areas prone to such build-up.
Use 300, as a rule of thumb for maximum tubing lengths (400', for 1/2" PVC). This greatly reduces and controls flow losses of pressure and assures more uniform flow rates from emitters. A 600' length of .580" tubing can be supplied from a valve at the center with the same desirable results. In the hydraulics section of this booklet, short cut pressure loss charts are provided for tubing lengths of 300' and less.
Keep the actual water emission points of the drip irrigation system above the soil surface.This accomplishes two objectives. It aids visual checking of the system for proper operation and it reduces system clogging that can be caused by root intrusion.
Special backflow prevention may be required. Know you local backflow prevention codes as to what specific types of devices are approved. A trend that seems to be developing, is the requirement for the most sophisticated backflow prevention devices for drip systems where even the remote possibility of future addition of fertilizer injection exists. The backflow of fertilizers into the potable water lines would be distasteful to say the least, if not dangerous.
Keeping the above mentioned rules in mind during the design process will help avoid problems that otherwise might crop up after installation.
Field Data Requirements
In addition to the typical site information you would need for a sprinkler system such as water pressure, service line size, water meter size and a site plot plan showing all landscape, elevation and structural features, there are a few special pieces of data needed for drip design.
The climate type where your project is located will have an effect on water use. Be aware, at the very least, of the climate type; hot dry, cool dry, warm humid, etc. An even better piece of information is the actual P.E.T. (Potential evapotranspiration) rate for the area during the hottest and driest part of the year. In case there is an absence of such data this manual provides some rough guidelines (another short cut).
The location and plant type for each plant you plan to irrigate. For the water use calculating in the site "the plant type" need only be broken down into "tree", "vine" or "shrub".
The size of each plant is important in calculating the plant area.
Soil texture (course, medium or fine) is important to know for spacing emitters.
When the above information has been gathered, you are ready to answer one of the most often asked questions about designing drip irrigation systems.