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Sprinkler System Lateral Pipes

 

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.

Three 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 a 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.

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.   In recent years new “push fittings” for poly tube that are universal and don’t require clamps has 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, but also doesn’t hold up as well under higher pressures.  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 is rated by a system of “SDR” ratios. The lower the SDR number, the stronger the tube is.  All poly tube of the same size will have the same INSIDE diameter.  As a general rule, poly tube should be avoided if pressures are  over 50 PSI.  For higher pressures use PEX tube.   PEX tube is a form of poly tube that is much stronger, but also more expensive.  It may be used for laterals but tends to be too expensive for most situations.  In higher pressure situations where the flexibility of poly is desired, PEX is a very good alternative.

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 don’t need clamps. See the installation tutorial for more info on this.)

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.  Do not use push fittings on PVC pipe, the teeth will not bite into the harder PVC material.

Maximum Operating Pressure

The industry standard is that the pressure rating of a pipe or tube should be TWICE the actual operating pressure.  So if the pressure in your sprinkler system laterals will be 50 PSI the pipe should be rated for 100 PSI of pressure.  This rule applies to all types of pipe and tubing.  The reason why the pressure rating has to be so high is because of “water hammer”.  Water hammer is surges in pressure that occur inside almost all plumbing systems, but is especially bad in automatic sprinkler systems.  When the automatic valves close they cause the flow of water to almost instantly stop.  This result in a huge pressure surge as the weight of all the water slamming into the closed valve is added to the already existing pressure.  Think of a truck slamming into the face of a cliff.

PVC Pipe Type Suggestions

I recommend that PVC lateral pipes be CL 200 PVC and buried at least 10″ deep (12″ deep is the industry standard for commercial irrigation systems.)  Many homeowners 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.  If you can’t find CL 200 PVC then use SCH 40.

Poly Tube Suggestions:

Most poly tube labeled as “irrigation tube” is rated for 80 PSI.  This is fine for a residential system with small sprinklers.  But per the 2x the pressure rule above, 80 pound tube should not be used for a water pressure over 40 PSI.  Most rotors need at least that much pressure.  I recommend that if you are using rotors which require higher pressures you use the 100 PSI poly tube, or PEX tube.

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 the short pipes that connect the sprinkler heads to the laterals.  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 the Installation Tutorial. 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 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 in areas where the ground freezes.  If water freezes solid in any pipe or tube, PVC, poly, PEX, copper, brass, even steel, the pipe/tube will split open!


This article is part of the Sprinkler Design Tutorial Series
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