Irrigation Tutorials

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Does the Pipe Size Impact Performance?

 

Q.  I was wondering if the size of the water pipe makes a difference in the performance of the sprinkler head — 3/4 inch or 1 inch ??    I have the option of supplying my sprinklers with either a 3/4 inch or a 1 inch water supply line.

A. Yes, the pipe size does make a difference.  The industry “rule of thumb” is that the pipe should never be smaller than the sprinkler inlet, although there is no technical reason for that.  There could be situations where a smaller pipe will work just fine.  But they are not common.  The truth is that in most cases the appropriate pipe size is one to two pipe sizes larger than the inlet.  I’ve seen some sprinkler systems where the pipe needed to be several times larger than the sprinkler inlet, although those situation are very rare.  To confuse things more, some sprinklers have two different size inlets.  In the case of two inlet sizes I would typically use a pipe the same size or LARGER than the largest inlet.

The Right Way:

Sprinklers require water pressure to operate.  Every sprinkler has an optimum water pressure that makes it work best.  Water pressure is lost as the water moves through pipes, valves, fittings etc.  This is called “friction loss” in the irrigation industry (although the pressure loss comes from more than just friction.)   So the size of pipe you use to supply the sprinkler head with water must not only supply the right amount of water, it must also not reduce the pressure below the amount needed to operate the sprinkler efficiently.
Unfortunately there is not a standard “use this size pipe with this size/brand/model sprinkler head” solution.  The right pipe size may vary with every irrigation system.  Figuring out what size to use is not an easy task.  My free tutorial on Sprinkler System Design takes you through the process step-by-step.  While it seems simple enough to stick a sprinkler on the end of a pipe, in truth, irrigation systems require a considerable amount of careful design calculations to work correctly and efficiently.  (That “efficient” part is really important, a sprinkler system designed wrong can easily use 2-3 times more water than one designed right.)  The first couple of pages of the tutorial give a more detailed explanation of what is involved in irrigation design and answer a lot of the “why?” questions that people have at this stage.  Then the tutorial will take you step-by-step through how to figure out what size pipe it should be.  If you are installing a new irrigation system, then the tutorial is the way to go.  Unfortunately once you have the system installed is not the best time to discover you should have had a better design up front!  I’m always telling people that delaying the process for 2-3 weeks to struggle through doing a good design may seem like too much work.  But when you have it installed and you realize you wasted hundreds, maybe thousands of dollars on a system that doesn’t work…  wow!   I get lots of email from people who are not happy when they realize that it often costs more to fix a poorly designed sprinkler system than they paid for it!!!  Please don’t be another one of them.  Better to spend another few weeks watering by hand and take the time to create a good design.

Method for Calculating the Size for a Pipe:

If you want to skip straight to the step of determining a sprinkler system pipe size, you can look at  How to Calculate a Lateral Pipe Size.  This page has a spreadsheet and instructions you can use that will figure out the pressure loss in a length of pipe.  You are jumping to the end of a tutorial when you do this, so you may need to go back a few pages in the tutorial to get the background information you need.  You are going to need to know the type of pipe you are using so you can get the right spreadsheet.  You will also need to find out the GPM of your sprinkler head (see the manufacturer’s website for this) and how much pressure loss (“PSI Loss” column on the spreadsheet) is allowable in your pipe.  Finding this PSI Loss value is going to be the hard part since you are jumping to the end of process.  You will probably need to make an educated guess (assuming you don’t want to just go back to the beginning of the tutorial and do it the right way.)  If you need to guess at the PSI Loss value, I would suggest a maximum value of 0.5 PSI Loss.  That’s 1/2 of a PSI in each pipe section.  Remember that is an educated guess and there is no way I guarantee it will work!  You are taking a risk to shortcut the process.

Risky but Expensive Simple Solution that Usually Works:

If you really don’t want to deal with looking stuff up and doing calculations, one solid rule in irrigation design is that a pipe can never be “too large”.  (For a detailed discussion of why, see why a smaller pipe will not increase pressure.)  So if you are in doubt, you always use a larger pipe size.   You could hook up a 1/2″ sprinkler to a 10″ pipe and the sprinkler would work just fine.  The 10″ pipe would not be kind to your budget, however.

You can use this “larger is never worse” rule to your advantage.  If you really don’t want to deal with calculating the right size, one way to solve the problem is to use a ridiculously over-size pipe.  Ie; if the sprinkler has a 1″ inlet use a 1.5″ pipe  and you will probably be safe.  Of course this may well be overkill as well, but if you don’t want to do all the calculations, then you pay the price of a larger pipe and go with over-kill.  Remember you do this solely at your own risk!

 



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