Tag Archives: mainline

How to Install Underground Pipe For Irrigation System

This page outlines the minimum requirements,  along with tips and tricks, for installing the pipes, tubes, and wires for your new irrigation system.  Summary of major points:

  • Wet areas to be trenched a couple days in advance.
  • Consider hiring someone to dig the trenches.
  • Install mainline pipes 18 inches deep.
  • Install lateral pipes 8 to 12 inches deep.
  • Make provisions for winterizing the system in cold winter climates.
  • Use proper methods for gluing PVC fittings to prevent future leaks.
  • Use good quality fittings for PEX.
  • Don’t forget to install wires in the trenches!
  • Sleeve the wires if you have aggressive critters.

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Compression Tees and Couplings

A compression coupling or tee is a special fitting designed for joining existing metal and PVC pipes or tubes.  They are sometimes sold as “pipe repair couplings”.  They are primarily used for underground connections.  Other types of compression fittings, such as threaded adapters, are also available for situations where something other than a coupling or tee is needed. Compression couplings are primarily used for pipe repairs, compression tees are used to tap a new pipe/tube into an existing one. A compression fitting may be constructed of PVC or metal.  PVC should only be used for connections that will be hidden from sunlight, like underground or in a box.

A PVC Compression Coupling
A PVC Compression Coupling

Continue reading Compression Tees and Couplings

Pump Cycles On Briefly When Irrigation is Off

Q.  My irrigation pump runs fine when the system is operating, but after it turns off it cycles on for 5 seconds every 10 minutes or so.

A.  If you are using a pressure switch and pressure tank to turn the pump on and off my first guess would be that you have a water leak in your irrigation system.  The water leaks out, which cause the water pressure to drop, then the pump kicks on and recharges the pressure.  Then the pump shuts off again.  That would cause exactly this symptom.

Knowing the problem is the easy part.  Finding the leak, that could be harder to do.  It could be a zone valve that isn’t turning off all the way or it could be a leaky pipe.  You can narrow the search area a little,  the leak will be someplace in the pressurized part of the system, that is, between the pump and the zone control valves.  Start by looking for obvious dripping, then look for someplace that seems wetter than it should.  If it is a leaky zone valve then the water will be leaking through the valve into the sprinkler zone pipes and will dribble out at the lowest sprinkler head.  So look at the sprinkler heads.  There will be a small “swampy” area around the lowest sprinkler head that is controlled by that valve.

PVC Pipe: SCH vs. Class

SCH Rated Pipe

PVC pipe types labeled “schedule” (abbreviated “SCH“) are made based on the traditional dimensions used for steel pipe.  Unfortunately steel has very different strength characteristics from plastic, so it is a system that isn’t very logical for use with PVC pipe.  But when plastic first came along it was made to the same size standards that were already in use for steel.  The common PVC pipe schedules you will see in stores are SCH 40 and SCH 80.  As the pipe sizes rated SCH increase, the strength and pressure rating of SCH pipe decreases.  So 1/2″ SCH 40 PVC pipe is very strong, while 2″ SCH 40 PVC has comparatively a low pressure rating, and is more easily damaged.  In sizes 1/2″ to 1 1/2” SCH 40 is a thick wall pipe with a reasonably high pressure rating and good resistance to physical damage.  It is often used for mainlines and other situations where a tough high pressure pipe is needed.  Sch 80 is generally used for making threaded plastic nipples because the plastic walls are thick enough to have threads cut into them (although most now have molded threads rather than threads “cut” with a die.)

Pressure ratings of SCH 40 PVC pipe:

1/2″  =  600 PSI
3/4″ = 480 PSI
1″ = 450 PSI
1 1/4″ = 370 PSI
1 1/2″ = 330 PSI
2″ = 280 PSI
2 1/2″ = 300 (not a typo, 2.5″ pressure is an oddity)
3″ = 260 PSI
4″ = 220 PSI

As you can see, the pressure ratings drop as the pipe size increases.  Note that the industry standard rule is that your normal operating pressure should not exceed 1/2 of the rated pipe pressure.  In other words, you shouldn’t use 1 1/2″ pipe for pressures higher than 165 PSI (330 x 0.5 = 165 PSI).  This is because pressure surges created by closing valves can easily double the water pressure in the pipe.  This rule applies to all PVC pipe, including that labeled SCH and CL.

Class rated pipe

PVC pipe types labeled “Class” (abbreviated “CL“) are based on the pipe’s pressure rating.  So Cl 200 PVC pipe is rated for 200 PSI of water pressure.  Cl 315 PVC pipe is rated for 315 PSI of water pressure.  The strength of CL labeled pipe is directly related to the pressure rating.  The standard “Cl” pipes are Cl 125, Cl 160, Cl 200 and Cl 315.  Of these Cl 200 and Cl 315 are most common.  Cl 125 is sold as a low cost pipe for use in sprinkler laterals for those for whom low price is everything.  It has a very thin wall and breaks easily if not handled carefully or nicked with a digging tool.

1/2″ size pipe is generally only available in SCH 40.  This is because of the thin wall of 1/2″ pipe makes it very easy to break.  I don’t recommend using 1/2″ PVC pipe at all, however if you must, you should use SCH 40.  Sometimes you will find 1/2″ Cl 125 PVC pipe at discount stores due to the very low price.

The Class system is obviously a more logical system for labeling pipe as you know immediately how strong the pipe is based on the label.  Unfortunately the more confusing “SCH” system became entrenched in the industry and remains.

What Pipe Type to Use

All PVC pipe labeled for a given size in the USA has the same outside diameter.  So any pipe labeled as 3/4″ will be the same diameter, whether it is SCH 40 or Cl 200 or any other type.  That allows the same fittings to be used to join the various pipe types together.  Most fittings are made to SCH 40 standards, although SCH 80 fittings are available, typically only at specialty plumbing and irrigation stores.  Technically most codes require SCH 80 fittings for pipe sizes 2″ or over.  In practice I’ve noticed that  SCH 40 fittings are often used up to 3″ size.  When dealing with sizes 4″ and above the use of non-glued “rubber ring-joint” fittings is recommended and usually required by code as well.  Glueing joints on 3″ and larger PVC pipe is very, very difficult.

“Mainlines” are all of the pipes that are under constant pressure, that is, the pipes that are before the sprinkler zone valves.   In most of the industry SCH 40 PVC pipe is used for irrigation mainlines up to 1 1/2″ size.  For 2″ size and larger Cl 315 PVC is used.  Most building codes prohibit the use of 2″ and larger SCH 40 PVC pipe for pressurized water lines.  Depending on the jurisdiction, this rule may or may not be applied to irrigation systems.  Those same codes generally require that all pressurized PVC pipes (mainlines) be buried at least 18″ deep to protect them from accidental damage, regardless of the type or size of pipe used.

“Lateral” pipes are the pipes after the sprinkler zone valve.  These pipes are only pressurized when the sprinklers are operating.   For lateral pipes the standard is to use Cl 200 PVC pipe.  Where budget is a concern and you can find it, sometimes Cl 160 is used.  As previously mentioned I recommend you avoid Cl 125 PVC pipe.  Laterals can be buried any depth, but I generally recommend at least 10″ deep to avoid a lot of maintenance problems with broken pipes.

Spreadsheets for Calculating Pipe Pressure Loss

Here are some spreadsheets I have created to help you calculate the capacity and water pressure loss through pipes and tubes of various types and sizes. These should be useful for both figuring pressure loss in mainlines and laterals. Each spreadsheet allows for multiple sections of pipe of various sizes and flows. All you do is select the proper spreadsheet for the type of pipe you are going to use, select the pipe size from a drop down list, enter the flow through the pipe in GPM, then enter the length of the pipe in feet. The spreadsheet calculator will then do the math to give you the water velocity in the pipe along with the pressure loss in PSI for that section of pipe. If there are multiple sections of pipe the spreadsheet will also total all of them for the total pressure loss.

Full instructions for using the spreadsheets are included on the spreadsheets.


Please read this paragraph before you try to use the spreadsheets.

These spreadsheets use Apache Open Office.   That means you need to have the Open Office program installed on your computer for them to work.  I use Open Office because it is a free, safe program that is available for just about every desktop and laptop computer make and model.  That means just about everyone can use it, and nobody has to make a major software investment just to use the spreadsheets.  If you don’t already have Open Office you will need to install it on your computer before you can use the spreadsheets. You can uninstall it when you are done using it if you want.  Download it free from http://www.openoffice.org .  It does take a while to download Open  Office.  It is a full office productivity suite.  (Check it out, there are some cool apps in it!)

Your browser may try to “open” the spreadsheets if you left click on the links, even if you don’t have Open Office installed.  This is because most computers have some type of minimal spreadsheet reader installed on them, so the reader will try to open these spreadsheets.  If they were just simple spreadsheets they would probably work with the readers.  But they aren’t.   In most cases you will get a corrupted version of the spreadsheet that does not work.  This is because these friction loss calculators use very complex formulas that the “stripped down” spreadsheet readers can’t handle.  You need to install Open Office.   I may have already mentioned this. 🙂

Remember, you can un-install Open Office when you are done if you don’t like it.

If you try to open the spreadsheets directly using your Internet browser they will probably open as Read Only and the spreadsheet won’t work.  This is because the browser will not open an executable file directly.  (It is trying to protect you from possible viruses.) 
Solution:  Right click on the link and “Save” the file to your hard drive.  The actual wording varies, so depending on your browser you may select “Save link as..”, “Save Target as…”, etc.   This should save the spreadsheet file to your hard drive.  Then open it directly from your computer without using the browser plug-in.  Tablet and phone users:  you may need to get to a real computer to use the spreadsheets.

If the spreadsheets don’t work for you..

  1. Do you have Open Office installed?  If not, install it.
  2. Are you using Open Office to read the spreadsheet?  Sometimes another spreadsheet program will try to open it instead of Open Office.
  3. Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
  4. If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in.  Install & use Open Office.
  5. Just dragging the spreadsheet link to your desktop may not actually save the file.  You need to right click on the link and select Save as…
  6. If you have an older version of Open Office you may need to upgrade it.
  7. Try rebooting.  I’ve experienced a problem where if I try to open one of these spreadsheets in a browser the computer gets messed up and won’t start Open Office.  Rebooting fixed the issue.


Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Cl 125 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 160 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 200 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 315 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 40 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 80 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.



Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Polyethylene Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.



Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

PEX Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.



Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

Type K Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type L Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type M Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.



Do not try to open these spreadsheets by left clicking on the links.  Save the spreadsheets to your hard-drive first.  See the explanation above.

SCH 40 Steel Pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.  (also use for galvanized steel)




Using a Looped Mainline for Irrigation

Looping your mainline often allows you to use a smaller pipe size for it, so using a loop system can be financially advantageous on a large irrigated area.  A looped mainline also provides maintenance advantages on larger sprinkler systems, and almost all large landscape irrigation systems, like parks and golf courses, utilize a loop mainline layout.  For smaller sprinkler irrigation systems they often provide little or no advantages.   As a general rule if you have less than 10 valve zones you are not going to get much of an advantage from a looped mainline.

What is a Looped Mainline?

The irrigation mainline is the pipe that runs from your water source to the individual valves that turn on a group of sprinklers or a drip irrigation circuit.   When the mainline is looped that simply means that all or part of it creates a continuous loop.  Typically a looped mainline starts with a single pipe coming out from the water source (pump, water meter, etc.) then the single pipe splits into two pipes.  the two pipes loop around the irrigated area and then rejoin each other to create the “loop”.   Zone valves would be located at various points along the loop to supply groups of sprinklers.   Normally you would put a isolation ball valve on each leg of the loop at the “split”, the location where the pipes separate into the loop.  A third isolation ball valve is placed on the far side of the loop, allowing the loop to be divided into sections.  (see the sketch of a looped mainline below)   Additional isolation valves may be added anywhere along the loop if desired, to divide it into more segments.  The isolation valves allow you to shut down sections of the mainline for repairs while the rest of it may still be operational.  For a large irrigation system being able to shut down only a portion of the system for repairs can be very advantageous.   Very large irrigation systems may have multiple loops, sometimes one loop will even be inside of another loop.

Plan View Sketch of a Looped Mainline
Plan View Sketch of a Looped Mainline

How to Design a Looped Mainline

When using a loop you should use the same size pipe for the entire looped portion of the mainline.  (This is not a hard and fast rule, just a strong suggestion unless you really understand hydraulics!)  The pipe leading from the water source to the loop may need to be a larger size than the loop pipe.  It is also OK to have mainline “spurs” off of the loop leading to other valves or faucets.  While unusual, the pipe size of the spurs may also be larger than the size of the loop pipe if they need to be.  Valves for sprinkler zones, faucets, quick coupler valves or any other equipment may be placed anywhere along the loop, as well as on the mainline leading to the loop or even on spurs off of the loop. Normally drinking fountains would be on a separate pipe and not connected to the irrigation system due to the possibility of water contamination from the sprinklers.  See the article on use of backflow preventers for more information on contamination.

Multiple Loops

You can have multiple loops, but I suggest that if you do, you size the pipe using the outside perimeter (largest) loop, as if there were not any cross pipes within the loop (using the method that follows below.)  Then after you have determined what size the outside loop pipes need to be,  use the same pipe size for any smaller loops or cross pipes inside the perimeter loop.  (Technical note: The pipe sizes of inside loops and cross pipes do not necessarily need to be the same size as the outside loop, it is just that using the same pipe size will almost always work.  Using a smaller size pipe may work, but it may not.  So it is  advisable for non-experts to stick with the same size!)

Loop Mainline Calculations

You must calculate both the friction loss AND the velocity for a looped mainline.  We’ll go through the process step-by-step.  You calculate the pressure loss for the non-looped mainline section from the water source to the beginning of the loop in the normal way (as described in the Irrigation Mainline Tutorial), using the pipe size, flow rate, and length of the pipe section.  I suggest that you use one of the  the Friction Loss Calculator Spreadsheets I’ve created, they are easier for non-mathematically oriented folks to use than the old manual calculations using charts.  Choose the proper spreadsheet for the type of pipe or tube.   Then enter the  size, the GPM, and the length of the loop.  Pressure loss for spurs off of the loop are also calculated using this same method.

Pressure Loss in the Loop:

To calculate pressure loss for the looped section we simply will assume that the water flow splits, and 1/2 of the water is going around 1/2 of the loop and the rest of the water is going the other way around.  To do this start by determining the “highest flow GPM” that is found on the looped section.  If you are planning to operate only one valve at a time that would be the flow for the largest zone valve to be installed on the loop section.  If you will be running more than one valve at a time then the “highest flow GPM” will be the combined flow rate for the largest group of valves that will operate at the same time. Once you have the “highest flow GPM” you calculate pressure loss in the pipe using the calculator.  But for the looped portion you will enter 1/2 of that “highest flow GPM” for the flow in the looped pipe and 1/2 of the total loop length (all the way around and back to the beginning) as the length of the pipe.  The calculator result is the pressure loss in PSI for the entire looped section.

Total Pressure Loss for the Mainline:

Just total up the pressure loss for the mainline leading to the loop, the pressure loss for the loop, and if you have any spurs add to your total the pressure loss for the single spur having the largest loss value.  The total of those is the pressure loss for your entire mainline network.

Velocity Problems:

As mentioned, this method of calculating pressure loss uses an averaging system that assumes that half the water goes one direction to the valve, and the other half goes the other direction.  While this is not a perfect method, it works good enough for figuring out the pressure loss.  However, the flow doesn’t really split evenly in both directions.  In reality the flow balances in each direction based on pressure loss, with most of the flow in the loop going the shortest distance to a valve.  So if one of the zone valves is just a few feet from the split point on the loop, almost all the flow will go through that short distance rather than going the long way around and back to the valve.  Also in the event you make a repair you may close those isolation valves mentioned earlier.  This will force all of the flow in a singe direction through one side of the loop.  So we come to the second rule of loops, the pipe size for the looped section must be large enough to handle ALL of the flow in one direction.  If it is not, you may create excessive velocity of flow in a section of pipe which can cause major, and very expensive, problems.  This means you must check the velocity of the flow while assuming all the flow may go in a single direction around the loop.  If the velocity is over 7 feet per second you may create excessive pipe wear and water hammer.  Pipe wear can seriously shorten the life of your system, water hammer is much worse, let’s just say you don’t want it.  (Look it up if you really are curious.)

Calculate the Velocity:

Using the Friction Loss Calculator Spreadsheets mentioned above enter the pipe size, the “highest flow GPM”, and any random value for the length of the loop.    Ignore the pressure loss it gives you, just check the ft/sec velocity result it gives you.  The velocity MUST be less than 7 ft/sec.  If it is not, you will need to use a larger size pipe for the loop.

That’s it!  You should now know how to create a looped mainline.  If it seems confusing try rereading it, it is admittedly a bit confusing after the first reading!  It really is simpler than it first sounds, be patient, grab some coffee, take your time.  Go back and actually work through it one sentence at a time, study the sample sketch of a looped mainline and try sketching your own on paper.    Practice a bit with the Pressure Loss Calculator to see what happens when you change the input values.  It should start to make more sense.