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This is important, read it carefully!
Sprinkler Coverage:
The area watered by each sprinkler must overlap substantially the
area watered by the adjacent sprinkler. This overlap may seem like a
waste at first, but it is a very important necessity. Without this
overlap it would be impossible to design sprinkler systems that provided
uniform water coverage.
Have Doubts? See for yourself, it only takes a couple of minutes
to prove! Grab a piece of paper and draw circles on it so that all
areas of the paper are inside a circle, but no circles overlap. You
can't do it, can you?
Important!
Sprinklers are intentionally designed to require 100% overlap of watered
areas. That means each sprinkler throws water ALL the way to the next
sprinkler in each direction. READ THAT AGAIN!
That's right, 100% overlap of watered areas is REQUIRED or you
will get dry spots! This is known in the industry as "head-to-head
coverage or head-to-head spacing". A lot of those free design
brochures you find in stores get this wrong. They don't show enough
overlap! The writers of those brochures think you are going to look at
the overlap and buy the brand of sprinkler that shows the least heads.
So they try to make it look like you can use less heads with their
sprinklers. After you've bought the sprinklers if you have dry spots,
well hey, it's YOUR problem now! You'll probably just buy a few more
sprinklers to fill in the gaps. $$$ Ching, ching!
Rule: Sprinkler Radius = distance between sprinklers
One more time: The water from any single sprinkler should actually
get the sprinklers on each side of it wet!
Now that I've told you that you SHOULD use head
to head spacing I'm going to backtrack a bit and tell you that you can
space a few
of the sprinklers slightly farther apart as needed to work around odd
shaped areas. I still recommend that you keep at least 80% of the
sprinklers at head-to-head spacing! Take the sprinkler head watering
DIAMETER and multiply it by 0.6 to get the absolute maximum distance
that should ever occur between any two adjacent sprinklers. (Remember
most manufacturer's give you the radius of the sprinkler, you need to
multiply by 2 to get the diameter.) For example, 15' radius spray heads
should never be more than 18' apart (30' diameter x 0.6 = 18'). Note
that we rounded to the nearest foot. If the sprinkler system is in a
windy area I suggest the majority of the sprinklers be spaced at 45% of
the diameter (that's closer than head to head!), as winds over 10 mph
really mess up the sprinkler patterns.
Back when I designed my first sprinkler system in High School I
wondered why they wanted so much overlap of the sprinklers. It seemed
to me to be nothing more than a ploy to sell more sprinkler heads! I
was smarter than that, so I stretched them out to save my folks some
money! The result was big dry spots, and my parents wound up replacing
the sprinkler system a few years later. (They never said anything
about it to me, I just noticed the new sprinklers a few years later on
a visit home from college.) Ouch! Not a good start for a future
irrigation expert! Now that I'm a bit wiser and more knowledgeable I
realize there is a good reason behind the head-to-head coverage.
Unfortunately, it's rather hard to explain. The perfect sprinkler
would put out a pattern of water that is heaviest right next to the
sprinkler, then uniformly declines out to the radius. So the farther
you move away from the sprinkler, the less water falls on any given
patch of ground. When we test sprinklers for water coverage we set up
a series of cups between the sprinklers to collect the water that
falls. That way we can see how much water falls at various distances
from the sprinkler. In the diagram below you can see what happens when
there are various distances between the sprinklers.
In example "A" the sprinklers are just barely
overlapping and much more water is falling in the cups next to the
sprinkler heads. But the middle 3 cups are only getting ½ the water of
the cups next to the sprinkler. If you watered long enough to keep the
middle green, the areas around the sprinklers would turn to mud! In
example "B" we see that moving the sprinklers closer together has
evened up the amount of water a bit more. However the areas near the
heads are still getting 25% more water than the other areas. Not
enough to cause mud, but you would definitely see rings of greener
grass around the sprinklers! Example "C" shows almost head-to-head
spacing. The cups are almost all uniformly full! So don't stretch the
distance between sprinklers.
What if you need a smaller radius than the sprinklers available?
Almost all sprinklers have a radius adjustment device on them so
that you can reduce the radius of the water throw. This is one way you
can adjust for narrower areas. Keep in mind that for most sprinklers
you can't reduce the radius by more than 50% without causing problems.
The other solution for smaller areas is to use nozzles made to spray
less far, or that spray a special pattern. An example of a special
pattern would be the nozzles that spray a 4' x 30' rectangular
pattern. These are commonly used in long, narrow areas.
Remember
if you reduce the radius of the sprinkler you must reduce the distance
between sprinklers by the same distance! Keep the coverage
head-to-head!
Calculating the GPM for sprinklers when you reduce the radius
is easy:
For spray heads you just use the manufacturer's chart.
When you use the radius adjustment on a spray you are simply
reducing the water pressure by closing a small valve in the nozzle.
As the pressure drops, so does the radius. Just look at the
manufacturer's chart for the radius you plan to reduce the sprinkler
down to. Then read the GPM for that radius! For example, your
designing for 30 PSI. The radius at 30 PSI of the sprinkler you
selected is 15 feet with 1.85 GPM according to the manufacturer's
chart. But you want the radius to be 14 feet. Looking at the
manufacturer's chart you see that the radius of the same sprinkler
is 14' at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you
reduce the radius to 14' will be 1.65 GPM. That's because when turn
the radius adjustment screw to reduce the radius to 14' what you
REALLY did was reduce the pressure to 25 PSI!
For rotor heads the GPM stays the same no matter how much
you reduce the radius! That's because reducing the radius on a
rotor doesn't change the amount of water coming out of the nozzle.
To change the radius a small screw extends into the stream of water
coming out of the nozzle. The tip of the screw deflects the water
which "screws it up" (pun intended) so it doesn't go as far. This
creates another problem, however, which is that it really messes up
the uniformity of the water. So when you use the radius adjustment
on rotors, you tend to get dry spots. This is one reason I strongly
suggest that you use a smaller nozzle if possible rather than using
the radius adjustment screw on the sprinkler. The other reason is
that when you reduce the radius you really should also reduce the
GPM of the sprinkler. Otherwise there will be a lot more water
under the sprinkler with the reduced radius. Bottom line- use the
radius adjustment screw on rotors only when nothing else will work.
Warning
for rotors only:
When designing systems with rotors do NOT rely on the manufacturer's
stated radius for design. They get those distances by testing the rotors
inside a building with no wind. The real world is harsher! If the
gallonage of the rotor is less than 6 GPM the maximum spacing should
never be more than 35' between rotor type sprinklers.
Stryker's Rule: the spacing in feet between rotors can never
exceed the operating pressure in PSI at the sprinkler inlet (30 PSI
operating pressure = 30 foot maximum spacing). Ignore this rule and you
will be very sorry!
Sprinkler Precipitation Rate and GPM
The precipitation rate is the amount of water the sprinkler throws
onto the area it waters, measured in inches per hour. (Inches per hour
is how deep, in inches, the water would be after one hour if it didn't
soak into the ground or run-off.) Precipitation rate must be considered
when selecting your sprinkler heads to eliminate water application
uniformity problems (dry spots).
Spray Heads: Almost all sprinkler manufacturers make their
spray heads so that you can mix and match nozzle patterns and the
precipitation rates will still match for all the heads. This is
referred to as "matched precipitation rates". Look for this feature
when selecting your sprinklers. Important: do not mix different brands
of spray heads and nozzles together on the same valve circuit without
checking to see that they have the same performance specifications.
Just because the nozzle will screw into the sprinkler body doesn't
mean it's designed to work with that sprinkler!
Rotors: Rotor-type heads aren't quite as easy. You must
select the appropriate nozzle size for each rotor in order to match
the precipitation rates. A simple illustration will help explain.
Rotor heads move back and forth across the area to be watered. The
rotation speed is the same regardless of whether the rotor is adjusted
to water a 1/4 circle or a full circle. So the stream from a 1/4
circle head will pass over the same area 4 times in the same amount of
time that it takes for a full circle head to make one pass over the
area it waters. With the same size nozzle in both, a 1/4 circle rotor
will put down 4 times as much water on the area under the pattern as a
full circle rotor will. (Remember that after every quarter turn the
1/4 circle rotor reverses direction and covers the same area again!)
To match the precipitation rates between these sprinklers, the quarter
circle rotor must have a nozzle that puts out 1/4 the amount of water
that the full circle nozzle puts out! A half circle rotor must have a
nozzle that puts out 1/2 the water of a full circle. This is why when
you buy a rotor-type sprinkler head they often include a handful of
different size nozzles with it. Wait, there's more (don't panic yet,
there is a simple solution forthcoming)!
If you have rotors that are adjusted for different radii you will
need to adjust the nozzle size to compensate for the radius change
also! For example if most of the rotors are set for a 30 foot radius,
but one is adjusted down to 20 ft., the 20 ft. one will need a nozzle
1/2 the size. (Remember: when you reduce the RADIUS by 1/3 you reduce
the AREA by a little more than half.)
Avoid
using rotors with nozzle flows that are less than 2.5 GPM, except
in corners (quarter circle patterns). Flows under 2.5 GPM give very
poor coverage due to the tiny water stream. Even a slight breeze will
distort the watering pattern and give you dry spots. I strongly
suggest that you stick to using nozzles as close as possible to the
GPM of those in the cheat chart below.
O.K. Now that you understand the principles, let's simplify this a
bit by using a cheat chart...
Unless you really know what you're doing (in which case you
wouldn't be reading this tutorial), you should stick with the nozzles
on this chart:
Jess Stryker's
Quick & Dirty Guide for Rotor Nozzle
Selection
1. Find the section of the chart with your desired
spacing.
2. Find the pattern (1/2, full circle,etc.) of the sprinkler.
3. The chart tells you the GPM the nozzle must have.
4. Use a nozzle size that comes close to matching both the PSI - GPM
combination.
5. Ignore the radius given by the manufacturer.
6. Be sure to read the notes below the chart!
For 20-29' spacing between sprinklers-
1/4 circle . . . 30 PSI - 0.8 GPM
1/2 circle . . . 30 PSI - 1.6 GPM
3/4 circle . . . 30 PSI - 2.4 GPM
full circle . . 30 PSI at 3.2 GPM
Important: see notes below!
For 30-39' spacing between sprinklers-
1/4 circle . . . 40 PSI - 1.5 GPM
1/2 circle . . . 40 PSI - 3.0 GPM
3/4 circle . . . 40 PSI - 4.5 GPM
full circle . . 40 PSI - 6.0 GPM
For 40-55' spacing between sprinklers-
1/4 circle . . .55 PSI - 3.0 GPM
1/2 circle . . . 55 PSI - 5.5 GPM
3/4 circle . . . 55 PSI - 8.0 GPM
full circle . . 55 PSI - 11.0 GPM
Important Notes:
It is critical that the water pressure (PSI) at the sprinkler be as
high, or higher, than the distance between the sprinklers in feet (per
Stryker's Rule). For example, if you space the sprinklers 45' apart,
you must have at least 45 PSI of pressure at the sprinkler inlet.
That's the pressure at the sprinkler inlet, not the total
pressure available. Remember, you will lose pressure in the pipes and
valves, so the pressure at the sprinkler inlet will be lower than your
available pressure! Go back to the tutorial
pressure
loss pages to figure out how much pressure will be lost in your
sprinkler system.
Select the nozzle size closest to these GPMs without regard to the
radius the manufacturer gives. For example, if you are looking at a
25' radius, the chart above says to use a 1.6 GPM nozzle for a
half-circle rotor. But you happen to notice that the rotor
manufacturer's literature says that at 25 PSI, a 1.6 GPM nozzle has a
radius of 32 feet. So why am I telling you to space it at 25'? When
the manufacturer tested the rotor on their test range (inside a large
building with no wind) they measured a few drops of water 32' from the
rotor. When you install it out in your yard it will not perform as
well. You may still get a few drops of water 30' or even 32' from the
head, but not enough to grow anything. You need to trust me on this
one! Remember, if the sprinkler sprays too far, most rotors have a
radius reduction screw that will allow you to very easily fix the
problem. But, if the rotor does not spray far enough there is nothing
you can do about it without a major expense! Best to play it safe.
You may want to make additional adjustments to nozzle sizes after
installation to compensate for your specific conditions. Most rotors
now come with a "nozzle tree" that contains most of the different
nozzles for the rotor, so you can change the nozzle sizes if you need
to. Some manufacturer's don't offer nozzles sizes larger than 3.0 GPM
for their economy-priced heads (providing those extra nozzles would
probably cost them at least another penny in costs!). You may need
to upgrade to the next better model line if you have a large yard! The
larger size nozzles for 40' spacing are not available with most of the
"mini-rotor" models sold for residential use. You will need to upgrade
to the next model. Also, sometimes other nozzle sizes are available
separately from the manufacturer, for example low angle nozzles. You
will probably need to get these from a store that specializes in
irrigation sales, rather than a hardware or home store. Look in the
yellow pages under "Irrigation" or "Sprinklers", or try one of the
online stores listed in the tutorial
links pages.
There is a conflict between the nozzles recommended for the 20-29'
spacing range of the chart and my previous advice to "avoid using
rotors with nozzle flows that are less than 2.5 GPM". This is because
the Nozzle Selection Guide assumes you will be mixing 20-29' radius
rotors together on the same valve with 30' plus radius rotors. To keep
from having enormous nozzles on the larger radius rotors I am
recommending that you use smaller nozzles than I would otherwise
consider for the smaller radius rotors. This is essentially a
compromise. Sometimes it is not practical to obtain perfection! If all
or a majority of your rotors will be spaced at 20-29' apart, then you
should probably use larger nozzles than I recommend in the chart. In
other words, use those listed in the chart for 30-39' spacing for the
20-29' spacing. This will help avoid problems caused by the wind
blowing the spray out of the irrigated area. However, if your
sprinkler system will be located in an area with little or no wind you
can go ahead and use the smaller nozzles in the chart. What is little
or no wind? Go outside in the evening or early morning when you will
likely be irrigating. If you can feel the wind blowing even gently
against your face, I would consider that enough wind to need the
larger nozzles.
If you calculate the precipitation rates you will notice that the
shorter spacings result in a higher precipitation rate than the larger
spacings. This is because the smaller heads with lower GPM rates are
more susceptible to wind and evaporation, and thus it is assumed less
of the water is actually reaching the ground. The higher precipitation
rate compensates for this.
Select Your Sprinklers
If you haven't started shopping for sprinklers yet, now's the time to
start checking out what's available. Check out which sprinklers are
available and look them over. Write down a list of the heads you think
will work well for your irrigation system on your Design Data Form. Be
sure to list the PSI and GPM for each head as given in the
manufacturer's literature, along with the maximum spacing between heads.
One last
warning!!!
Do not blow-off my advice on sprinkler spacing in order to save a few
bucks on sprinkler heads! Right now you may be feeling pretty smug about
how much money you saved by stretching the sprinkler spacing. But next
summer you're going to look pretty stupid to the neighbors, standing out
there with a hose watering the yellow spots your new sprinklers don't
cover! I have a collection of "wish I'd listened to you" letters from
people who didn't take this advice. I get a few more of these every
year, and these are just the brave folks willing to confess they messed
up. They all say you should listen to me on this!
 Later on you will
need to know the flow rate for each sprinkler you use, so it might be
helpful to make some notes on the back of your Design Data Form showing
the nozzle size and GPM you will need for each different sprinkler you
plan to use. Otherwise you'll wind up having to look the information up
over, and over, and over...
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