How to Design a Looped Irrigation Mainline

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.