THE GROUND RULES
First a tip that just may save your behind!
It will not harm anything to use a larger pipe size. Period. If you are uncertain whether to use a 3/4″ or 1″ pipe, then you should use the 1″. Using a larger size pipe is ALWAYS the safest choice.
No, I don’t own stock in an irrigation pipe manufacturer and I’m not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be “too large”. Contrary to what might appear to be true, forcing water into a smaller pipe REDUCES the water pressure, and hurts sprinkler performance. This is because the smaller pipe creates more pressure loss due to friction and turbulence as the water flows through it. It’s another of those hard to grasp hydraulic principles! Just remember that when it comes to pipe, bigger is better! I’m always amazed at how many irrigation equipment sales people don’t know this most basic of irrigation rules. I’ve had clients tell me they were told to use a smaller pipe to keep the pressure up by tech support people at some of the major sprinkler manufacturer’s. That’s an industry disgrace!
So one more time to drill it into your head– You don’t decrease the pipe size to keep the pressure up- or down for that matter. That is totally, completely, wrong. The reason we use smaller pipe is to save money. Which of course, is a good reason! For those who want more specifics on this, there is a very boring scientific explanation at the bottom of this page.
Is it Pipe or Tube? For the most part I use the term “pipe” rather than “tube” on this page and elsewhere. Bad habit of mine (note that by reading carefully, you have found one of my faults!) The difference is the material they are made from. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually referred to as tube. I often screw up and call tube pipe!
TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE USING A SPREADSHEET
This method involves trying various pipe sizes until a good combination is found.
Definitions you need to know:
Lateral pipe: all the pipes between the control valve and the sprinkler heads.
Mainline: The pipe that goes from the water source to the control valves.
Control Valve: The valve that turns on and off a group of sprinklers. Most often it is an electric valve operated by a timer.
Valve circuit: a single valve, and all the pipe, fittings and sprinkler heads downstream from it. In other words, all the sprinkler heads that start working when you turn on the valve are part of the same valve circuit.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
You will need a spreadsheet Friction Loss Calculator.
Here’s a page with calculators for almost every type of pipe: Friction Loss Calculator Spreadsheets
Grab the appropriate spreadsheet for the type pipe you plan to use.
Now you just enter the appropriate data for each section of pipe into the calculator and then read the total pressure loss at the bottom of the spreadsheet. If the pressure loss is too high, then try making one of the lengths of pipe larger. The calculator will also give you the water velocity in each section of pipe, and warn you if the velocity is too high.
The best way to teach this is probably to walk you through a couple of examples.
If I may make a suggestion, download the spreadsheet for Cl200 PVC now, and open it up in a separate window. Then think about each step, enter the values I show into the spreadsheet, and actually try to duplicate what I do in the examples below. Something about actually doing this helps engage people’s brains. People tell me they read it twice and still don’t get it when they just read it, but as soon as they actually TRY it, then it suddenly makes sense. It’s called learning by doing, and it is considered the best teaching method. This process is simple, HOWEVER, it is not obvious and sounds illogical to those not trained in hydraulics.
A Simple Example:
The sketch above is an example of a very simple valve circuit with 5 sprinkler heads. In this example the sprinklers are 15′ apart and each sprinkler uses 3.7 GPM of water. The red numbers on the sketch are the total water flow for each pipe section in GPM.
Let’s assume we want to use Cl200 PVC pipe and we want a maximum total of 4 PSI of pressure loss in our lateral pipes.
If you are working through the Sprinkler Design Tutorial the maximum total pressure loss is entered on your Design Data Form in the Pressure Loss Table section. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.
If you don’t understand how to calculate the water flow in each section (the red numbers) you should take a look at the Sprinkler Pipe Layout page.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
If you don’t understand pressure losses in irrigation, see Pressure Loss & Selecting Your Sprinkler Equipment.
For advice on types of pipe (Cl200, poly, etc.) see Irrigation System Lateral Pipes.
To use the spreadsheet friction loss calculator to determine the pressure loss:
- Download and open the Friction Loss Calculator.
- There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
- Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
- Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
- Enter the GPM for the section of pipe.
- Enter the length of the section of pipe.
- Use an error factor of 1.1
- Go to the next line down and repeat steps 4-7 for the next pipe section.
- The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
- At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Here’s what the spreadsheet calculator looks like after we enter the data requested for each of the pipe sections using the example in the sketch above.
Note that the “Total of all Sections” shown at the bottom exceeds the 4 PSI maximum limit we set for pressure loss. Also notice that the velocity in two of the sections (highlighted in red) exceeds the safe level. The marginally high velocity highlighted in yellow is considered acceptable by most experts, since these are lateral pipes. (The marginal velocity level would not be as acceptable in mainlines.) Start by fixing the velocity problems. To decrease the velocity in those sections we will need to increase the pipe size. So let’s increase the pipe size for the two sections highlighted with red to 1″. Here’s what it looks like after the change:
Now the velocities are all within acceptable levels. Also note that increasing the pipe sizes reduced the pressure loss “Total of All Sections” shown at the bottom to 2.9 PSI, which is well below our maximum level of 4 PSI. That’s good, no more changes are needed. It is not possible for the pressure loss to be “too low.” As long as it is under the maximum it is fantastic. So what would happen if the pressure loss was still too high? If there was still too much pressure loss we would need to try increasing the size of some of the pipes to lower the friction loss.
So we now have pipe sizes that will work for each section of pipe in our lateral. I’m often asked at this point if it would be OK to make some of the pipes 1/2″ since the pressure loss is so low? The answer is yes, but you might not want to do it. See my explanation of the problems associated with the use of 1/2″ pipe.
A More Complex Example:
Now lets look at a more complex valve circuit. (Please note that this circuit is much larger than that found on a typical residential irrigation system. It would require much more water than most residences have available and is just used to show you an example of a much more complex layout.) As with the previous example we will assume that our maximum pressure loss value for the valve circuit is 4 PSI.
This valve circuit involves numerous paths the water may take. This makes the calculation a bit more complex, as a separate calculation is needed for each possible route that the water might take through the laterals on it’s way to the last sprinkler at the end of a pipe. If you look at the example above you will notice there are 3 sprinklers that are at the end of pipes, each sprinkler at the end of a pipe represents a different route the water can take. So this circuit has 3 and will therefore require 3 separate pressure loss calculations. The next drawing shows the possible water routes in magenta, blue, and red colors.
It may help to think of each path as the shortest route that a single drop of water could take to go from the valve to the last sprinkler on a pipe branch. For some people it helps to think of it as a road map and your looking for the shortest route to each of the dead ends at the end of the roads.
Start your calculations with the water route that is the longest. In this case that would be the route highlighted in red. There are 9 pipe sections in this route, I have labeled them 1-9 for clarity. Just as before, enter the data from this route into the calculator, and make all the pipe 3/4″ size. Here’s what the spreadsheet looks like:
As you can see there are a number of pipe sections highlighted red due to unsafe velocity. Change those pipe sections to larger pipe sizes until all the velocities are within safe levels.
Here’s the resulting spreadsheet calculator with the smallest possible pipe sizes. However, notice the Total of All Sections is 4.4 PSI, which is more than our 4 PSI maximum:
So we need to make some of the pipe sections larger in order to reduce the pressure loss (or friction loss.) Start by increasing the size of one of the the smaller pipe sections. Changing a 3/4″ pipe to a 1″ size is a lot less expensive than changing a 1″ pipe to 1 1/4″. So for the example lets change section 7 from 3/4″ to 1″. Doing that drops the Total of all Sections value to 3.77 in our example, below the 4 PSI maximum we set earlier. So now everything is good, these sizes will work for the “red” highlighted water route.
Now we add the pipe sizes from the spreadsheet to our circuit drawing (note that the pipe sizes for the red highlighted sections have been added on the next drawing below.)
Now we relabel our sections to follow the blue highlighted route.
Using the blue highlighted water route, repeat the same process used for the red one. Enter the GPM and pipe lengths for each section in the spreadsheet. This time we already know the sizes for sections 1-6, they were entered into the spreadsheet when we did the red section. So we just enter those for the new blue sections 7 and 8, again using 3/4″ size pipe. And it looks like this on the spreadsheet calculator:
Using 3/4″ pipe size for our two new sections works good. The velocity is safe and the Total of All Sections is 3.29 PSI, so the pressure loss for this route is also within the 4 PSI maximum we set. Write the size of the two new sections on the drawing and the blue water route is done.
Now all that remains is to do are the calculations for the magenta highlighted water route. That is done the same way, entering the data into the spreadsheet calculator for each pipe section. Start with 3/4″ then change to larger sizes until the velocity is safe. Then check that the total of all Sections is less than 4 PSI as before. Here’s the data entered into the spreadsheet calculator:
Now all that remains is to insert our lateral pipe sizes from the spreadsheet calculators into the drawing of the valve circuit.
All done! So the pressure loss for the entire circuit is the same as that for the highest water route. In this case the red route was highest at 3.77 PSI. So the pressure loss for the lateral circuit shown here is 3.77 PSI.
Often I get asked at this point why the “Total of all Sections” pressure losses for all 3 routes wasn’t added together? The pressure loss for the red route was 3.77 PSI, for the blue section it was 3.29 PSI, and for the magenta route it was 3.02 PSI. So the confusion is that it seems like there should be a total loss of 10.08 PSI! Nope, the pressure loss for the entire lateral is 3.77 PSI, the loss of the highest route. To understand this think of a single drop of water again. It can only travel on one route from the valve to the farthest sprinkler. It is not going to go backwards and try another route! So the pressure loss for the entire valve circuit is equal to the pressure loss from the valve to the farthest sprinkler.
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Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2011. All rights reserved.