Designing an Irrigation System (Part II)

In this two-part series, we’ve been taking the mystery out of designing functional and efficient irrigation systems. It’s boiled down to understanding pressure and flow, and how they affect your site. Now that we’re past some of the trickier aspects of hydraulics as covered in Part 1, it’s time to select some sprinklers, take a look at how to space them, and determine how many heads should be on one zone. Finally, we’ll decide which sprinklers turn on and off together, and how to size pipe.

As mentioned before, there is truly a great deal to know about sprinkler-system design. The purpose of this two-part series
is to cover the basics and motivate you to get started. So, let’s pick up where we left off . . .

Step Four:
Select sprinklers
As we saw in Part 1, choosing a sprinkler is primarily based on the available pressure after pressure losses are calculated.
Let’s take a quick look at the two main types of sprinkler heads: spray heads and rotors. Each uses a different method of distributing the water and requires different pressure, spacing and nozzles.

Spray heads
Fixed spray heads are sprinklers that distribute water in a fan-shaped spray pattern. Most of these small heads use different nozzles for different patterns, such as quarter, half and full circle, as well as for different distances of throw. Spray heads require 20 to 30psi (30psi is optimum) of water pressure and can be spaced up to 16 feet apart. A good rule of thumb is if the area to be irrigated is less than 16 feet wide, use spray heads. They have a high precipitation rate compared to rotors. Use 4" or 6" pop-ups for turf areas, and 6" and 12" for shrub and high-growing ground cover.

Rotors are sprinklers that distribute water in rotating single or multiple streams rather than in a spray. Rotors can be spaced farther apart than spray heads, from 16 feet to 70-plus feet. While this means fewer heads are required to cover an area, rotors cost more per sprinkler than spray heads.

Rotors generally require at least 40psi minimum to operate, though some can handle lower pressures. For areas larger than 16 feet, rotors may be the better choice over spray heads. Also, rotors have a low precipitation rate compared to spray heads. Use 4" pop-ups for turf, 12" for shrubs and high-growing ground cover.

Match the right sprinkler to the site conditions
While pressure is key to selecting the appropriate sprinkler, it isn’t the only determining factor. Once you know what kind of sprinkler you can use (i.e. spray head, rotor, drip emitter, etc.), you’ll want to choose sprinklers with features appropriate for the specific site conditions. Read the manufacturer’s catalog and specification information carefully to make the best choices. Here are a few common scenarios:

  • Windy sites: low angle discharge, high precipitation rate and large water droplets to fight wind.
  • Slopes/berms: built-in check valves to prevent low-head drainage and built-in pressure regulation to maintain even precipitation rate.
  • Low pressure/low flow: spray heads or rotors that operate on minimal pressure and flow.
  • High pressure: spray heads or rotors with built-in pressure regulation to prevent misting.
  • Irregularly shaped areas: adjustable arc/pattern.
  • Areas with varying widths: adjustable radius/throw.

Step Five:
Space sprinklers evenly

Once you’ve chosen the sprinklers, you can begin laying them out. When possible, lay sprinklers out in a grid pattern for the most even coverage.

The rule of thumb for spacing sprinklers is to space them no further apart than their radius of throw. Check the manufacturer’s specs for radius, pressure and flow. For example, if the radius of a spray head is 15 feet, space them 15 feet apart. This is called head-to-head spacing or 100 percent coverage. Spacing sprinklers head-to-head will add to the flow rate and eat up valuable pressure, but the resulting even coverage will require shorter watering cycles, and ultimately save water. Conversely, stretching the spacing will require longer watering times and more water use to even out the dry spots caused by gaps in the coverage.

Head-to-head spacing is even more critical with rotors because each head needs to spray as far as the next to avoid dry spots right around the head.

Let the width of the planted area also help determine the spacing. If an area is 20-feet-wide but pressure is too low to use rotors, you could use three rows of 10-foot-radius spray heads spaced ten feet apart. If the area is only 18 feet wide, use three rows of 10-foot spray heads but space them at nine feet and adjust the radius down slightly.

Be sure to use the appropriate nozzle for the width of the area. A common practice is to use the same nozzle and ratchet it down with the radius adjustment. Doing this can reduce uniformity.

Step Six:
Group sprinklers into zones

Now that sprinklers are laid out and spaced properly, you can group them into zones. A zone is a group of sprinklers connected together by a system of lateral lines and controlled by a single control valve. A zone is determined primarily by available pressure and maximum flow and further determined by solar exposures, topography, plants’ water requirements and precipitation rates.

Sprinklers should first be zoned according to the available pressure and maximum flow. As discussed in Part 1, the maximum flow rate of a typical residential water service is around 10-12 gpm to minimize pressure loss. If you had a turf zone with 24 1-gpm spray heads, it would have to be separated into two zones of 12 spray heads each, so that the flow rate of each zone doesn’t exceed the available 10-12 gpm.

Sprinklers should also be zoned according to different solar exposures such as shade versus sun, i.e., hot south and west exposures versus cooler north and east exposures, etc. Sprinklers at different elevations should also be on separate zones to better control the application of water and runoff. For example, the top, middle and toe of slopes should all be on separate zones. Separate zones should be established for plants with different water requirements such as lawn versus shrubs.

Different precipitation rates require different watering times, so they also need separate zones. For example, full-circle rotors may have to be separate from half-circle rotors. Also, rotors should be separate from spray heads, and spray heads should be separate from bubblers and drip. Combine any of these together and you’ll have to overwater some areas to avoid dry spots in others. However, with matched-precipitation-rate sprinklers, you can mix and match different nozzles and radii within the same family while maintaining even coverage.

Step Seven:
Connect sprinklers with pipes and valves

Once all the zones are established, the next task is to connect each sprinkler in each zone together with lateral lines and control valves. The best lateral piping design is like a plant’s root system, with pipes branching out from the control valve, decreasing in size and then terminating.

Sizing pipe depends on our good pals, pressure and flow, again. In other words, pipe sizing depends on how many heads are on that lateral line and their flow rates. If you look at any manufacturer’s technical data for friction loss (which leads to pressure loss), you’ll see that there are recommended flow limits for different size pipes. Anything past the limits results in excessive velocity or force, which can damage pipes and valves.

Unless you really know what you’re doing, size pipe by the book. For example, with class 200 pipe — probably the most typical for lateral lines — the maximum recommended flow for ¾" is 10 gpm with a pressure loss of 4.31psi per every 100 feet of pipe. The question is, how many heads can you get on ¾" class 200 pipe? The answer: as many as you want as long as you don’t exceed 10 gpm.

After zones are piped and valved, the control valve should be connected to the mainline. The mainline, which is under constant pressure, supplies water to all zones. Mainline pipe should be class 315 for sizes 1½" and larger because it’s thicker walled and stronger than schedule 40 at these sizes; it should be schedule 40 for sizes 1¼" and smaller. Size the mainline for the capacity to handle the largest flow-rate zone. For example, a 50-gpm zone will require a 2-inch main to stay within the recommended velocity limits.

Step Eight:
Shut-off valves, backflow preventers and pressure regulators
Now back to where we started when we began this whole design process: at the source of water. The mainline needs to be connected to a water source, such as a water meter from the city water system. And if we’ve done everything right, we already know our pressure and maximum flow, and have based our design on these factors — so everything should work!

There may be some additional valves required at the point of connection (POC) such as a shut-off valve, backflow preventer and pressure regulator, so let’s take a look at each.

The first additional valve placed after the water meter may be a shut-off valve. This valve, which can be a gate or ball valve, allows the entire system to be shut down from a single point without the need to turn off the water meter.

A backflow preventer, which would be placed immediately after the shut-off valve, is required by plumbing code for most installations. It keeps irrigation water from flowing back into the drinking-water system. For large sites or where there are sprinklers higher in elevation than the water source, you will need a reduced-pressure type of backflow preventer. For medium to small projects, and residential sites where sprinklers are below the level of (but not higher than) the water source, you can use a pressure vacuum breaker or an atmospheric vacuum breaker. A single pressure vacuum breaker can be used for multiple zones while an atmospheric breaker is used one per zone. Each must be at least 12 inches higher than the highest sprinkler on that system.

A pressure regulator is usually required for pressures over 80psi to protect heads, pipes and valves from excessive pressure and surges. It can be located just after the shut-off valve or immediately after the backflow preventer.

Well, there you have it: the tried and true ways to design an excellent irrigation system. I’ll leave you with one last and perhaps most-important thought: design your system to operate under the worst-case scenario, i.e., the peak ET rate, the zone with the highest pressure loss, the highest possible flow rate, etc. Do this and you can be assured you’ll have a great system that will work under all other conditions!

January 2003