Irrigation Tutorials

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General Information About Irrigation System Laterals

 

Quick & Dirty Summary*

  • Laterals are the pipes or tubes located downstream, or after the zone control on/off valve.  Laterals are not pressurized all the time, only when irrigation is on.
  • In temperate zones it is typical to use Cl 200 PVC pipe for laterals.  125 PSI polyethylene is becoming increasingly popular in temperate zones also, due to ease of installation.
  • In cold winter areas it is typical to use 125 PSI polyethylene tube for laterals.
  • Pressure loss in the laterals is typically 4 PSI, with 6 being the maximum advisable if you have a high static water pressure, and economy is more important than quality.
  • Lateral pipes should be buried at least 10″ deep to protect them from damage from lawn aerators and cars driving over them.

*Quick & Dirty Summary information is based on Industry Standards for a typical home irrigation system and will not work for everyone, everywhere.  The typical solutions in the Quick & Dirty Summary may not give you the least expensive or most efficient irrigation system.  If you want excellent water savings, lowest cost for top quality, or in-depth explanations please keep reading...

Types of Commonly used Irrigation Pipe

Definition: Lateral Pipe or tube.  In irrigation, the pipes/tubes between the zone control valves and the sprinkler heads or emitters are called “laterals.”  These lateral pipes or tubes are not pressurized unless the valve is open and the sprinklers are operating.  Lateral pipes are generally subject to less water pressure, surges, and stress.  So a less durable and less expensive pipe material may be used for them.  If you visit your local warehouse hardware store you may find that they use the name “branch” pipe for laterals.

Two types of pipe/tube are commonly used for sprinkler system laterals, polyvinyl chloride pipe (PVC), and Polyethylene tube (Poly). Both are types of plastic. PVC is usually white or gray color and semi-rigid. Polyethylene is usually black and is flexible.

PVC is the type most commonly used in warm winter climates. PVC pipe is rated by two different systems, the first is the “class” system (Cl) the other is the “schedule” system (SCH). It is not possible to say that one is always better than the other. Schedule pipe is rated by the pipe’s wall thickness, while class pipe is rated by the pipe’s operating pressure. All PVC pipe has uniform outside diameter sizes.  So pipe of the same size has the same outside diameter, regardless of which type or rating they are.  This allows the fittings that join them together to be a universal size, the same fittings will fit all of them.

All PVC should be protected from sunlight.  Some types of PVC are marketed as “sunlight resistant” my experience is that they last longer, but still degrade in sunlight.  The UV rays in sunlight causes the PVC to become brittle, it will eventually become so brittle that just tapping it will cause it to fall apart.  How long it takes depends on how intense the UV in the sunlight is.  So you would guess that in Arizona, with intense sunshine, it would be a huge problem.  But did you know dim winter sunshine reflected off snow can give even more UV exposure?  That’s why you get sunburned so easily when skiing.  Same with high altitude areas (less atmosphere allows more UV through.) The damage is permanent and not reversible, so if the store you buy it from keeps it outside, you might want to question how long it has been sitting out in the sun!  A few months stored in the sun in most climates is not a problem.  UV damaged PVC tends to display a noticeably brownish “sunburn”.  If you use PVC above ground you should protect it from sunlight.  3 coats of exterior latex paint seems to do a reasonable job.  Better yet, put foam pipe insulation around above ground PVC pipe and “kill two birds” with one stone.

Most PVC pipe is connected together using “PVC fittings” which are glued in place. The fittings are typically rated as SCH 40 (standard white PVC fittings), some are available as SCH 80 (stronger and normally gray color.) Sometimes PVC pipe has threaded ends just like steel pipe. PVC pipe and steel pipe have the same outside diameters, and are interchangeable and steel fittings will fit onto threaded PVC pipe & vice versa.

As of 2014 when I am writing this page update they have just introduced slip together “push fittings” for pvc that do not use glue.  These fitting use o-rings or flexible seals to prevent leakage, and a locking device with teeth that grips the pipe to hold it inside the fitting.  These slip fittings are removable using a special tool.  I have no idea how well these will hold up long term.  As with all connectors that use flexible seals, the seal material quality is key to how long the life of these fittings will be.

Poly tube is commonly used in areas with cold winters (where the soil freezes,) and is also used in special situations that require a more flexible tube be used, such as very rocky soil.  High density polyethylene (HDPE) with high pressure ratings is much more widely stocked in home and irrigation stores than previously.  New “push fittings” for poly tube that are universal and don’t require clamps make installation much easier and faster.  Both of these developments have resulted in increased use of poly tube in all areas, regardless of climate.  Poly tube doesn’t break as easily if water freezes in it.  Poly tube is more forgiving in rocky soils (big rocks, like granite boulders) as it is less likely to crack if it is installed against the side of a large rock.

Poly tube is rated by a system of “SDR” ratios, sometimes labeled just “DR”.  The lower the SDR number, the stronger the tube is.  HDPE poly is commonly rated for 160, 200, and 250 PSI.   A cheaper form of poly is sometimes sold as “irrigation pipe” and is rated for 80 PSI, I personally would avoid the 80 psi stuff for all but the most “economically challenged” irrigation systems.     All poly tube of the same size will have the same INSIDE diameter.   

PEX tube is a form of poly tube that is much stronger, but also more expensive.  It is being used more and more as a replacement for copper plumbing inside homes.  It is constructed to the same outside diameter as copper tube.  However it has a thicker wall than copper tube, so has less flow capacity in a given size of tube.  It may be used for irrigation laterals, but tends to be too expensive for most situations.

Traditional “insert fittings” used for poly tube are often called “barbed fittings“. Insert fittings shove into the pipe and have barbs to help hold them in place. Do not rely on the barbs to hold the tube on the fitting! You need to also install a stainless steel clamp around the tube where it fits over the barb in order to securely hold the tube on the barb. (Exception: sprinkler risers using special “twist on” barb fittings are designed to not require clamps.)

A newer fitting type used with both poly tube and PEX are “Push fittings”.   These fittings do not have a barb, the tube “pushes” into the fitting and locks in place.  These fittings use a flexible seal that stretches around the tube to create a water tight seal even with the less than uniform outside diameters found with poly tube.  The “locking” feature of these fittings is the result of a set of stainless steel teeth that “bite” into the soft poly tube to hold it in place.  Thus these push fittings allow quick and easy tube assembly without the need to install separate clamps on each connection.  Push fittings are also used with PEX tube.  Warning: some push fittings made for irrigation use are NOT recommended for use on mainlines and are only for irrigation system laterals.  I know this page is about laterals, but I wanted to warn you to be sure to check this if you want to use them on a mainline!

Saddles are often used on poly tube laterals (most are not made for use on mainlines!)  Saddles make a quick and easy way to attach the sprinkler risers to the lateral pipe.  They work well and are very popular due to the ease of installation, especially when connecting a smaller diameter riser to a larger diameter tube. If you use a saddle that requires drilling through the tube be sure to flush out the tubes extremely well before installing any sprinklers as there will be lots of small pieces of the tube left inside.  Those little fragments of polyethylene will clog up your sprinklers and nozzles!  You don’t want to spend hours cleaning brand new sprinklers.  If you have a drip system the fragments will ruin the emitters making them trash.

Maximum Operating Pressure

Background:  The pressure ratings for pipes and tubes are based on the maximum occasional surge pressure allowable in the pipe or tube.  So this means a pipe rated for 200 PSI should not be subjected to 200 PSI pressures on a regular basis.  Almost all pipe/tube manufacturers recommend that the pressure rating of the tube be double the normal operating pressure.  So to restate what this means, if the system pressure is 100 PSI, the pipe or tube manufacturer requires that you use 200 PSI rated pipe.  The rating is NOT the normal pressure for the pipe or tube!  (“Burst pressure” is not the same thing as the pressure rating.  If a burst pressure is given, assume that is 3x the normal system pressure.  So 300 PSI burst pressure = 100 PSI normal pressure.)  The reason behind this weird labeling system is that water pressure surges that double the pressure in a pipe or tube are fairly common.  For example, a automatic valve closing can easily cause a pressure spike that doubles the pressure.  Thus a valve closing would be an “occasional surge”.

Pressure ratings for lateral pipes:  In the case of laterals, the lateral pipe/tube is located after, or downstream of, the valve.  Fortunately the pressure surges caused by valves closing will be confined to the mainline, which is upstream of the valve.  This is why strong, high pressure pipe is needed for mainlines (see the page on mainlines.) Pressure surges are much less likely in laterals, and when they do occur most of the surge pressure is released through the sprinkler head nozzle or drip emitter.  For this reason the lateral pipe can have a lower pressure rating, often 125% of the operating pressure is used.  I still recommend not using any pipe or tube rated for less than 160 PSI for laterals.  I’ve just seen too many of those lower pressure pipes and tubes split over the years.  The thin pipe and tubing walls of those low pressure rated pipes and tubes are too susceptible to physical damage.

Using PVC Pipe:

The industry standard for PVC lateral pipes is to use CL 200 PVC.  Many homeowners are tempted to use the CL 125 PVC pipe because it is cheap, but it breaks easily and they often regret using it later.  I’ve noticed that starting around the year 2000 most hardware stores started stocking Cl 200 pipe and stopped selling CL 125.  I believe they were losing too much money on returns of split pipe.  There is a good chance you will split open Cl 125 just transporting it home!  If you can’t find CL 200 PVC then use SCH 40 PVC, which is slightly stronger. (For pipe under 6″ diameter.  Starting with 6″ and larger pipe, Cl 200 is actually stronger than SCH 40.  As SCH 40 pipe sizes get larger the pressure rating is less.  This is because the SCH 40 standard is based on pipe made from steel rather than plastic.)

Most homeowner systems have the pipe buried too shallow, often 6″ or less.  If you install it less than 10″ deep you are asking for trouble.  Remember PVC is brittle and will easily split with even a slight nick from a shovel while gardening.  Likewise a lawn aerator will make Swiss Cheese out of a 6″ deep pipe.  While PVC doesn’t float as bad as poly, a 6″ deep pipe will often float up during the first winter, especially if the soil is saturated with water.  Thus a pipe installed at 6″ might be 2″ deep the next spring.  I’ve seen many actually float all the way up to the surface!  10″ of dirt over the pipe helps keep it down where it belongs.

Most pros, myself included, avoid using 1/2″ size pvc pipe.  It has very low flow capacity, is often not available in Cl 200, and has a very small inside diameter, making it plug up easily.  It is very easy to partially block the flow through 1/2″ pvc pipe by using too much glue on the fittings.  This can severely impact the performance of your sprinklers!  3/4″ is worth the extra price.

Using Poly Tube:

Industry standard is to use 160 PSI, HDPE,  SDR 13.5 poly tube for laterals.  You may see “irrigation tube” sold, this is usually the 80 PSI thin wall stuff.  Best to avoid it if you don’t want to be digging up and repairing tubes a lot.  Often poly tube is plowed into the soil with a “vibratory plow” machine, typically 6″ deep because that is fast and cheap.  Unfortunately if you install the tube 6″ deep and then aerate the lawn you will poke holes in it.  Plus 6″ deep tube is easy to stick a shovel through when gardening.  Finally shallow tube tends to “float” to the surface in the winter when the water is blown out of it, especially if the ground gets saturated with water.  You might plow a tube in at 6″ and discover it is 1″ or 2″ deep the next spring when you cut it open digging out weeds!  I recommend poly tubes be at least 10″ deep.  All but the cheapest plows can go 10″ deep, it just goes in a bit slower.

 

pencilDecide which type(s) of pipe best fits your needs and make a note of it on your Design Data Form.


Lateral Pressure Loss:

We will determine the actual sizes of these pipes later using the pressure loss value that we establish here.  So this is an important value!

For most residential sprinkler systems on City size lots a lateral pressure loss value of 4 PSI will work great. It gives a nice balance between cost savings and performance and is a safe figure to use with any type of sprinkler. I would start with 4 PSI for this value. If you find later that you need to raise it, come back and read the rest of this page before you do. If you decide you want to use a lower value you can do so without problems.

Note: You can skip to down “pencil” logo near the bottom of this page if you are going to use 4 PSI for your lateral pressure loss value.

At this point in the design we need to make somewhat of an “educated guess” for the lateral pressure loss. However there is a guideline for maximum allowable pressure loss, so we can use that as a starting point. This rule is: The lateral pressure loss may never be greater than 20% of the sprinkler head or drip emitter’s operating pressure. The sprinkler or emitter operating pressure is established for us by the manufacturer, and you should have entered it in your Pressure Loss Table already.

Sprinkler Head Pressure  x  0.20 = Maximum Lateral Pressure Loss

Example:
Say the sprinkler heads we want to use have an operating pressure of 30 PSI. Then the lateral loss may not be more than 6 PSI (30 x 0.20 = 6 PSI). Therefore we make an educated guess that a lateral pressure loss of 6 PSI will work.

whyNo doubt some are wondering why the lateral pressure loss is limited to 20% of the sprinkler or emitter operating pressure. This is an industry standard for limiting the variation in performance between the sprinkler heads or emitters controlled by the same valve. We know that the first sprinkler after the control valve will most likely (but not always, see last paragraph below) have more water pressure than the furthest sprinkler from the control valve. After all, the water has to pass through a lot more pipe and fittings to reach that last sprinkler, so a lot of energy is going to be lost getting there! Since both sprinkler and emitter performance is directly related to water pressure it is necessary to limit the pressure difference between the first head and the last. Otherwise the first head might flood the area around it with water before the last head even got the area around it wet. I’ve seen poorly designed sprinkler systems where the grass is dark green by the valve, and gets yellower and yellower as you move toward the last sprinkler! This is even more critical with drip systems. I remember a Eucalyptus tree farm I visited a few years back,  where they had planted long rows of trees for firewood production.  The first trees (next to the valve) in each row were twice the size of the trees at the other end of the row! All because the first trees were getting much more water.

Now I guess I need to explain why the last sprinkler head on a line might not have the lowest operating pressure. There’s only one situation I can think of where this might happen, and that is where the sprinkler system is installed down the side of a steep hill. If the valve and first head were at the top of the hill and the last head was at the bottom, then the added pressure that results from the elevation change (gravity adds energy) might be great enough to cancel out all of the pressure loss in the pipes! More on elevation changes and pressure loss in the next page of the sprinkler design tutorial!

pencil Enter the Lateral Pressure Loss on the line labeled “Laterals” of your Pressure Loss Table.

Sprinkler Risers:

Sprinkler risers are what connects the sprinkler heads to the lateral pipes or tubes.  Some people consider them part of the lateral, others consider them a separate part of the sprinkler system.  They are usually flexible to allow the sprinkler head to move without breaking the lateral pipe.  That’s a nice feature to have when you drive over a sprinkler with the car or hit a sprinkler with the mower.  The better quality sprinkler risers, known as “swing risers”,  have jointed arms that allow the sprinkler to be moved up-and-down as well as side-to-side, which allows you to adjust the height and position of the sprinkler.  To simplify things a default pressure loss value for the sprinkler risers is built into this Tutorial.  (Hurray! Finally, something you don’t need to worry about!) So as long as you use this tutorial for your design and a reasonably standard riser for your sprinkler heads, you don’t need to worry about the pressure losses.

What’s a reasonably standard riser you ask?  A standard riser would be a PVC swing joint using pipe and fittings the same size as the sprinkler inlet, any of the poly tube risers commonly know as “Funny Pipe®”, “swing pipe” or equal (keep the riser tube less than 18″ long), or Cobra Connector® (keep them less than 12″ long). Don’t worry about those riser names for now, more information on risers will come later. In fact, now would be a good time not to worry about risers at all! I only mention them here because people get to this point and send me emails telling me I have made a grievous mistake by leaving them out.

Winterization:

In most places above ground pipes should have insulation around them.  Even here where I am in sunny temperate Southern California we sometimes get enough of a hard frost to freeze our above ground pipes, so I put insulation around mine just to be safe.  One year we had a really hard frost here, and it split open thousands of backflow preventers that weren’t insulated.  It was a great year for backflow preventer manufacturers.  Prices skyrocketed all over the USA due to the demand!

I’ve written a separate tutorial on ways to winterize your sprinkler system that you should read if your irrigation system will be in an area where the ground freezes in winter.  You cold weather folks need to provide for some method of removing the water from the pipes or tubes.  While poly and PEX are more resistant to freezing damage, they still need to be drained or blown out with air in areas where the ground freezes.  If water freezes solid in any pipe or tube, PVC, poly, PEX, copper, brass, even steel, the pipe or tube will split open!


This article is part of the Sprinkler Design Tutorial Series
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