The web's mother lode of free irrigation information!
Types of Sprinklers:
Sprinklers heads are divided into two types based on the method they use to distribute the water.
More properly called "fixed spray heads" these are the small heads that spray a fan-shaped pattern of water. Think of a shower nozzle. Most use interchangeable nozzles installed on the sprinkler which determine the pattern (1/2 circle, full circle, etc.) and the radius of the water throw. Some specialty patterns are available for long, narrow areas. Spray heads are spaced up to 18 feet apart. They need between 20 and 30 PSI of water pressure to operate properly.
Rotor is the term used to describe the various sprinklers which operate by rotating streams of water back and forth over the landscape. The example which most people are familiar with is the "impact" rotor sprinkler (often improperly called a "rainbird*") which moves back and forth firing bursts of water. You probably know this sprinkler best for the distinct sound it makes when operating-- tooka, tooka, tooka, tic, tic, tic, tic, tic, tooka, tooka, tooka, etc... The impact rotors are rapidly being replaced now by gear driven rotors which are very quiet, lower maintenance, and much smaller in size. These gear drive rotors have one or more fingers of water which move silently across the landscape. The prettiest of these are the "multi-stream rotors" where multiple streams of water rotate over the landscape one after the other. Multi-stream rotors are fascinating to watch. Rotors can be spaced from 15 feet to 65 feet apart. There are rotors available that can be spaced farther apart than 65 feet but I don't advise using them in most situations, even golf courses are moving away from using them due to problems. Most rotors require a lot more water pressure to operate than spray heads. Here's a rule of thumb, "The water pressure at the rotor head in (PSI) must exceed the distance (feet) between the heads." (Known as Stryker's Rule, admittedly that's a little ego stroking on my part, but I did create the rule!) Thus if you want to space rotors 35 feet apart you will need 35 PSI of pressure at the rotor. More on this later. The small rotors sold for residential use work best at 25 to 35 foot spacings.
* Rain Bird is the name of a sprinkler company and is a registered trademark. The Rain Bird company makes many different types of sprinkler heads, including impact rotors. They also make a full line of other irrigation products.
Rotary Nozzles: A new type of miniature rotor has been introduced in recent years. These are often called rotary nozzles, the first brand on the market was called the "MP Rotator", and several other similar products quickly became available from other companies. These are called rotary nozzles because they are a very small turbine-driven rotor mechanism that is the same size as the standard nozzle on a spray-type sprinkler. Thus they are made to fit onto the smaller, and less expensive, spray head pop-up bodies. These rotary nozzles have a radius generally between 15 and 30 feet. The exact distance depends on the model. They all use multiple streams of water that rotate around the nozzle and look like rotating spider legs. Some research indicates these rotary nozzles are more efficient than standard spray heads and result in lower water use. So far they have performed well overall, but beware of recently introduced models as this is tricky technology and new products may have a lot of "bugs".
Guide to Selecting the Right Sprinkler Type:
Which to use, sprays, rotary nozzles, or rotors? Here are some questions to guide your selection.
- Is your water pressure less than 40 PSI static? If so you should consider using sprays or rotary nozzles.
- Is the area long and narrow, between 15-28' wide? Then you should look into rotary nozzles.
- Is the area you want to water greater than 25' x 30' in dimensions? If so, rotors may be the best solution.
- Is the edge of the area to be watered curved? If the edge has sharp curves (less than 20' radius) then rotors will have difficulty watering the edges without over spraying them. This may not be an issue depending on what is beyond the edge. If the area beyond the edge should not get water on it you might want to consider a smaller rotary nozzle or spray-type sprinkler.
Installation Issues related to Head Selection:
Rotors are spaced farther apart and require less trenching, but they also cost much more per sprinkler. If the area to be watered is less than 15 feet wide then spray heads will likely be the best, most economical choice. For most normal-size city residential yards spray heads or rotary nozzles are usually the better choice. For areas less than 22 feet wide spray heads are generally less expensive. In areas larger than that it comes out about the same cost for rotors or spray heads.
Cost Issues in Selecting Type of Sprinkler:
Surprisingly regardless of the type of sprinkler you use, the cost per square foot of area irrigated comes out about the same if they are done right. When using rotors there is less pipe and trenches, but the rotors themselves cost more. Spray heads are less expensive to buy, but they require more pipe, trenches and valves to install. In the end, the price really comes out pretty close either way.
Note: If your "design pressure" is less than 40 PSI rotors will not work properly, DO NOT USE THEM. See the section "Measure Your Water Supply". If you have a well and pump you must have your pump-on setting adjusted to no less than 40 PSI if you plan to use rotors. A "40-60" setting is typical. Contact your pump company for assistance.
If you are unsure, try using rotors in your design. If they don't work out well, then erase them from your plan and try rotary nozzles. In may situations the best option maybe to use rotors in the large areas, and spray heads or rotary nozzles in smaller or more narrow spaces. So you may have a mixture. This is OK, but there are some things you need to be careful of when mixing different types of sprinklers. The first is that each type must have a separate valve. More on this later in the tutorial. The second is determining how to space the heads where the meet each other. For example, if you have a 30' radius rotor next to a 15' radius spray head, how far apart should they be from each other? There are many different schools of thought on this, but my general recommendation is to split the difference. In this example put them 22' apart. Yes, the rotor would overspray the spray head by a considerable distance. But if you put them 30' apart you will get a distinct dry spot between them.
Basic Body Styles:
Pop-Up Style Sprinklers:
Pop-up style sprinklers are installed below ground. A portion of the sprinkler rises up out of the ground when the sprinkler is operating and then retracts back below ground when not in use.
Shrub Style Sprinklers:
Shrub style sprinklers are installed above ground on top of a pipe called a "riser". Read the warning below!
In most cases you will want to use pop-up style heads, even in shrub areas. Pop-up sprinklers often don't cost any more than shrub sprinklers when you include the cost of the riser (the upright pipe the sprinkler is mounted on). Two major advantages of pop-up sprinklers are safety and appearance. Many people are injured each year when they trip over, or fall on, shrub style sprinklers. For shrub and ground cover areas pop-up sprinklers are available with pop-up heights of 3", 4", 6" and 12" above ground. A few brands are available with lower pop-up heights, but be warned that lower heights often cause problems. I recommend that you not use any sprinkler that pops up less than 3". As a general rule it is best to also avoid shrub style sprinklers unless a very tall riser is needed to raise the sprinkler spray over the tops of tall shrubs. When needed, shrub style sprinklers should only be used in areas well away from sidewalks, patios, and areas where children play.
Metal or Plastic?
At the grocery store it's "paper or plastic?" but with sprinklers the question becomes "metal or plastic?". The conventional wisdom is that metal is more durable than plastic, and therefore is better. Up until the late 1970's metal (usually brass, sometimes zinc) was the standard material from which almost all sprinklers were made. However, times have changed and now plastic is the most common material for sprinklers. Very few manufacturers even bother to make an all-metal sprinkler anymore. The primary reason for this change in materials is cost; machined metal parts are enormously expensive in comparison to injection molded plastic. Fortunately, most of today's plastic sprinkler heads are very well engineered and will perform as well as, if not better than, the old metal sprinklers.
Hybrids: A few companies manufacture plastic sprinkler bodies which accept brass nozzles, which they claim results in a better water pattern. Other manufacturers claim that plastic nozzles perform as well as brass. The research tends to indicate that a really well-machined brass nozzle has better water distribution. But that's laboratory tests, and in the real world a lot of other factors come into play. I personally haven't noticed any significant difference in performance between most brass and plastic nozzles in well-designed, sprinkler systems, although brass nozzles will no doubt last longer. More importantly, there are a few nozzles, both brass and plastic, which don't seem to perform as well as others. Fortunately, they are easily identified by comparing prices (as in "you get what you pay for"?). Typically these bad nozzles come pre-installed on sprinklers that don't have the features I list below, so if you stick to sprinklers with my recommended features you will get acceptable quality nozzles.
Features to Look For:
The following features are common to all good-quality sprinkler heads (for both rotors and spray type heads). Choosing a sprinkler without these features is asking for trouble.
- Spring Retraction: Make sure a spring is used to pull the pop-up riser (sometimes called a "piston") down into the case when the sprinkler isn't on. As a general rule the stronger the spring, the less likely the riser is to "stick up". Stay away from sprinklers that rely only on gravity to retract the pop-up riser.
- Wiper Seal: This is a soft plastic seal around the pop-up riser stem that seals the riser so it won't leak . The wiper seal also is responsible for keeping dirt out of the sprinkler body, and is the most important part in determining how long the sprinkler will last. Make sure the sprinkler model you select has a wiper seal. Note: on some sprinklers you must remove the sprinkler's cap and look inside the bottom of it to see the seal. Be careful when removing the cap, on some models the spring will shoot out!
- 3 Inch Pop-Up Height (or higher): Unless you just like to trim grass around sprinkler heads, make sure the pop-up height is 3" or more. This way the spray nozzle will clear the top of the grass. Most professionals use 4" pop-up sprinklers in lawn areas, and 6" or 12" pop-ups in shrub areas.
Sprinkler Make and Model Recommendations:
The most common question I get from users of this tutorial is "what do you think of the ABC model sprinkler made by XYZ sprinkler company". You don't really think I would risk making the major sprinkler manufacturer's mad by publishing that kind of information do you? You do? Well... why not? See my irrigation product reviews.
More on selecting your sprinklers is coming later on in the tutorial. For now lets just get an operating pressure. The first thing you may have noticed is that I used the term "operating pressure" here rather than "pressure loss" as previously. While pressure loss is a perfectly accurate term for the pressure used by sprinkler heads and emitters, operating pressure is more commonly used. Operating pressure is simply the pressure that needs to be present at the sprinkler or emitter inlet for it to perform as intended.
Manufacturers of sprinklers and emitters provide specifications for each of their products that list the various acceptable operating pressures for the units and how they will perform at that pressure. You will need to obtain the specifications for the products you intend to use. You may find this information printed on the sprinkler box or you may need to request it from your supplier. Most manufacturers also make specifications available on their web sites. Typically for a sprinkler this specification will list an inlet pressure as pounds per square inch (PSI) and then give a watering radius (feet) and flow rate in gallons per minute (GPM) that will occur at that pressure.
For an emitter the information would include only operating pressures (PSI) and a flow rate in gallons per hour (GPH) for each of those pressures. (Radius of throw isn't applicable to drip emitters.)
Pressure Requirements for Sprinklers
Spray Type Sprinklers:
For spray type sprinklers most designers use an operating pressure of 30 PSI, unless a lack of available pressure forces a lower level. Remember that if you use a lower pressure the sprinklers will need to be spaced closer together, because the water won't spray as far. Sprinkler manufacturers provide charts that tell you how much pressure is required for the sprinkler and how far it will spray with that pressure. Look on the package for the chart. Additionally, almost all spray type heads have a radius adjustment screw that allows you to adjust the watering radius down for smaller areas. (When you adjust the radius using the adjustment screw on a spray head, you are actually reducing the pressure at the nozzle by means of a small valve under the nozzle. The reduced pressure results in a decreased radius of throw.) At pressures above 45 PSI most spray heads start to create lots of mist, which results in poor irrigation. This can be controlled by using the radius adjustment feature to reduce the pressure, partially closing the valve to reduce the pressure, installing a pressure regulator on the mainline to reduce the pressure, or by using special pressure regulating nozzles made by some sprinkler manufacturers (which, you guessed it, reduce the pressure!
Rotor Type Sprinklers:
For rotor type sprinklers the higher the operating pressure the better. (O.K., within reason. We don't want to blow the sprinkler apart with high pressure-- and rotors can cause mist too under extreme pressures.) But as a general rule, most rotor type sprinklers do not work well with less than 30 PSI operating pressure. Keep reading!
"Stryker's Rotor Spacing Rule" states that the spacing in feet between rotor-type sprinklers can't exceed the pressure in PSI at the rotor. There is a lot of competition in the sprinkler business to see who can get the most radius from a rotor-type sprinkler. Manufacturer's literature and packaging tends to wildly exaggerate the maximum spacing of rotors. They get those distances by testing the rotors inside a big building with no wind. Even the most gentle breeze will shorten the real-world watering radius (water droplets are very light). If the package says the rotor has a radius of 35 feet at 30 PSI-- DO NOT BELIEVE IT! In the real world you will not get that distance. If you have 30 PSI do not space the rotors more than 30 feet apart. If you ignore this rule, 9 chances out of 10, you will have dry spots in your lawn! (Yep, over-size ego alert, it's my rule, thus the name.)
Rotor Spacing Example: If you want to space the rotors 30 feet apart then you will need to use a pressure of at least 30 PSI for the rotor. If you want to space rotors 40' apart you will need 40 PSI for the sprinkler head pressure. I don't recommend spacing sprinklers farther than 55 feet apart unless you have an experienced professional design the sprinkler system. Many tricky problems occur with sprinklers when they are spaced greater than 55 feet apart.
Most emitters operate best at around 20 PSI. Some emitters are "pressure compensating" which means they should put out approximately the same amount of water over a wide range of inlet pressures. (I've found that many pressure compensating emitters are not a whole lot more "pressure compensating" than standard emitters are. Keep in mind that at pressures over 45 PSI emitters may blow apart. Barbed emitters in poly tubing may pop out of the tubing at 30 PSI.
Mix and Match:
Sometimes you need to use sprinklers that require high pressure such as rotors, with sprinklers that use low pressure on the same irrigation system. To do this the system is designed using the pressure requirements of the high pressure sprinklers. The low pressure sprinklers (or emitters) are installed so that a separate valve turns them on and off, and a special pressure reducing valve is used. This valve reduces the pressure down to the correct amount for the low pressure sprinklers. Almost all irrigation manufacturer's now make pressure reducing valves, although you may have to go to a specialty irrigation store to get them.
Enter the sprinkler head operating pressure (or the drip emitter pressure if no sprinklers) on the "Sprinkler Heads" line of the Pressure Loss Table.
Remember- the pressure you enter in your table is the pressure for a single sprinkler head. So if you will have 10 sprinklers and they each require 30 PSI you still only write"30 PSI" on your pressure loss table. Also the value you enter should be the highest sprinkler head pressure requirement. So if you plan to use a spray head that will need 20 PSI and also a rotor that will need 35 PSI, you will enter the higher value- which in this case would be 35 PSI. Finally, remember why pencils have erasers. You can always come back and change this value later if you want to! So don't agonize over it.
A lot of people ask why we only write down the pressure for a single sprinkler. This is a bit difficult to understand but I will try to explain. I think the easiest way to understand is with a mental image. Think of the water moving through your sprinkler system as millions of water droplets, rather than a single mass of water. On it's journey through your sprinkler system a single drop of water will loose pressure along the way. Each place where it will lose pressure is one of the items on your pressure loss table. Let's go along for the ride. First our water droplet will travel through a pipe from the water company to your water meter. Then it will proceed through the meter into the house supply pipe and on to the irrigation system connection. From there our drop goes into the irrigation system and may pass through a backflow preventer. Onward it travels to the valve and through the valve into the lateral pipes leading to the sprinkler heads. Finally the drop goes into one of the sprinkler heads and is propelled out onto the lawn. Note that our droplet only passes through one sprinkler head on the way to the lawn. I'll bet you've never seen water on the lawn jumping back into the sprinkler head so it can go back and try going out through another sprinkler! So it can only pass through one sprinkler head. Thus we only consider the pressure needed for a single sprinkler head. (O.K. smart guy, yes I have seen water sucked back into a sprinkler head. But that's not supposed to happen, it means something is wrong with the sprinkler system.) At any rate, even if you still don't understand why you use the pressure loss for only a single sprinkler, please trust me, it's correct!
Much more information on sprinkler selection is coming later in the tutorial, such as spacing and nozzle selection. If you want to jump ahead and check it out, click here. Just don't forget to use your "back" button to return here!
By using this tutorial you agree to be bound by the conditions and limitations listed on the disclaimer page.
Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2011. All rights reserved.