Designing your Success
The key to any successful project is a good plan. This is especially true when it comes to a residential irrigation system. Keep this in mind and your customer's irrigation system will meet both their expectations and your own.
LOCAL CODES AND REQUIREMENTS
Checking with local and state rules and regulations is one of the most overlooked steps in designing an irrigation system. Each city has its own specific requirements when it comes to irrigation products, such as backflow preventers, valves, heads, etc. While some cities may have similar codes, it is important to check with the local water department to determine the requirements. For instance, certain cities have laws on the books against using anti-siphon valves and have made it illegal to not have a pressure vacuum breaker (PVB) mounted on the side of the house at your point of connection (POC). Checking with the city's water department can also make you aware of any permits you need to have or water regulations you'll need to address. This is an important first step because it can prevent fines and lost time during installation.
A second step that is often overlooked in designing an irrigation system is proper recording of site information. It's important that you treat each individual site differently and prepare a design based upon the particular information of each site. This includes recording static pressure, meter size, service line size and elevation changes. This site information will provide you with both the static pressure and the gallons per minute (GPM) — which are the two most pertinent numbers in designing a system. It is also important to know the material out of which the service line is made. The three most common materials for service lines are copper pipe, galvanized pipe and PVC pipe. Be careful of service lines that are galvanized — corrosion can build on the inside of the pipe over the years, affecting the flow of water.
Take an accurate static pressure reading at your water source and record it. Note the elevation change between the source of the water and the site where the POC will occur. Calculating GPM requires a water pressure reading at the source; however, for actual design pressure, use the following formula:
Source Static Pressure +/- (elevation in feet to POC × 0.433)
The two most effective ways to obtain the site's GPM is the five-gallon bucket test, or the P.V.V. equation. For the first method, use a five-gallon bucket and observe how long it takes to fill using a silcock. If it fills in 30 seconds, you have 10 gallons per minute available. The second method uses three concepts:
Pressure loss through the water meter is not to exceed 10 percent of the available PSI at the source.
Volume through the water meter should not exceed 75 percent of maximum safe flow of the meter.
Velocity through the service line should not exceed 7.5 feet per second (FPS) from the main to the meter.
The lowest GPM of the three concepts above becomes your design capacity. You can find charts outlining the information in the above three concepts in the back of most irrigation suppliers' and distributors' catalogues.
Proper sprinkler placement is another key to an effective irrigation system. Focus on placing all the heads in your design first, before thinking about pipe runs and sizing. When placing heads, always follow the manufacturer's recommendations on the charts provided to you in their catalogue. Simply match the head to about 80 percent of the static pressure reading in order to determine the throw of the sprinkler head. This estimation is your working pressure and will be tested during the pressure loss step. Typically, you would space rotors that are used for larger turf areas no farther than 30 feet apart, while spray heads range in spacing from 7 to 17 feet, depending on the particular nozzle. Another factor when choosing heads is noting any areas of depressed elevation where you plan to place a sprinkler head. Any sprinklers in a depressed area can cause the sprinkler head to seep, or bleed. In these cases, most irrigation suppliers offer sprinkler heads equipped with seals in them that help eliminate the drainage of water out of the lateral lines.
When marking the sprinklers on your design, you should use square or triangular spacing. In a square spacing pattern, the sprinklers in a row should be the same distance apart as the rows of sprinklers (see Figure 1, above). In a triangular spacing pattern, you should place the heads at the corners of an equilateral triangle (see Figure 2, above). It is important to use head-to-head coverage when placing your sprinklers. You can do this by using a compass to show the coverage of each head.
Matched precipitation is another concept that is based upon equal distribution of water throughout the lawn (see Figure 3, page C30).
The full-circle head has a nozzle in it that allows the flow of 4 GPM. Because the half-circle rotor only covers half of the area, you should install a 2-GPM nozzle. The quarter-circle rotor receives a 1-GPM nozzle. It is important to mark these on your design for the purpose of running pipe, and also as a guide for your employees to use when they are installing the sprinkler heads.
LATERAL AND MAINLINE PLACEMENT
When placing pipe in your design, start with the lateral lines first, thereby grouping sprinkler heads together into zones according to similar GPM. Keep rotors and spray heads separate, but try to group heads together in the same area. Making sure to use the most direct route, draw a line connecting all the sprinklers in each individual zone, one zone at a time. Try to choose a few select areas to place clusters of valves and run lateral leads from individual zones to these areas. These areas will serve as your manifold boxes. Irrigation valves are offered in a variety of options, including jar-tops, flow-control and anti-siphon, as well as a variety of fittings. Jar-top valves may make future repairs to the valve easier due to the fact that you can access the diaphragm of the valve with a simple twist. Flow-control valves have a handle on the top of the valve that allows adjustment to the flow. Finally, anti-siphon valves have a vacuum breaker built inside; however, keep in mind that this valve is against code in some cities. Always check local regulations when choosing a specific valve. It is also important to designate any large trees or other fixed objects in your design. This can change the location of a lateral line and can make installation of the project run smoother. PVC and polyethylene are the two products most commonly used for lateral lines. Choosing between the two comes down to personal preference and the usage of it in your area.
When laying out your mainline, first determine the location of your POC. Draw in your mainline, connecting all of the manifold boxes throughout the yard, leading it back to the backflow preventer and then to the point of connection. Keep in mind that the mainline will almost always be under constant pressure, and therefore may require PVC. Referencing PVC and poly charts, do not exceed the maximum flow when sizing your pipe. Typically, most residential designs require only 1-inch lateral and main lines because most residential sites have a GPM rate of 12 or under.
Quick coupler valves are often placed at the end of main line runs for the purpose of watering an area with a garden hose. This valve is located in a valve box, and activated when you insert a key attached to a garden hose into the valve. Because the mainline is under constant pressure, you can use the valve even when the irrigation system is off.
Calculating friction losses is the final step in designing your system. This step serves as a check to make sure that the friction losses will not be excessive. Because you based the design on readings at the POC, it is good to check each zone for friction loss, but is not a requirement. When deciding which zones are most important to check for friction losses, choose the zone with the most GPM — or highest flow rate — the zone farthest away from the POC and the zone containing heads at the highest elevation. Typically, these zones will represent the “worst case scenario” when it comes to friction losses.
You can find friction-loss tables in the back of irrigation distributors' or suppliers' catalogues. Using the chart, intersect the size of pipe with the GPM flow to get the PSI loss per 100 feet of pipe. Divide this number by 100 for PSI loss per foot. Multiply this number by the length of pipe supplying the last sprinkler on the zone (just to the next head on the zone). Record the loss for that section and repeat the process for all the lengths of pipe back to the zone.
From the valve, calculate the friction loss for the main line to the POC using the total GPM for that zone. Fittings also cause friction losses, but rather than figuring each fitting's individual effect, the general rule of thumb is to take 10 percent of the losses for the pipe. Also take into account pressure losses for elevation (elevation change in feet × 0.433). For valves, backflow preventers and other devices, refer to the individual manufacturer's catalogs for those pressure loss figures.
Once you have made all these calculations, add the losses together and subtract them from the PSI figure calculated for design capacity. Make sure this adjusted PSI is sufficient to operate the sprinklers chosen for this project.
THAT'S THE PLAN
Proper planning can take an irrigation design from paper to installation with ease. Checking these design capacity figures throughout the installation is an easy way to ensure that a system will operate properly upon completion.
Shaun Kanary is account manager for John Deere Landscapes in Avon, Ohio. You can contact Shaun at (440) 934-8400 or by e-mail at email@example.com
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