How To Blow Out Underground Sprinkler Lines

blowing out underground sprinkler line air compressor
(Last Updated On: April 28, 2020)

Make Sure Your Sprinkler Is Empty!

It’s the time of the year when you have to make sure that all of your sprinkler lines are void of water.

Water increases in volume as it nears the freezing point, and this volume increase means burst lines, broken valves etc. if there is water in your sprinkler lines and it freezes.

Thanks to Mike Higgins of Grand Junction Pipe & Supply in Grand Junction Colorado who tells us…

“The following equation should be used to figure out the proper volume of air needed for your system. The size of the compressor needed should be based on this equation. This is based on Cubic Feet per Minute (CFM’s) and the Gallons Per Minute (GPM’s) for which your system is designed.”


Click on the image to check out the sprinkler line.
If you take the GPM of your system, and divide it by 7.5 it equals the CFM you’ll need to push the water out.

Mike goes on to say that if your system flows 20 GPM of water at 30 PSI, then you divide the 20 GPM by 7.5 to get the CFM necessary to adequately blow out your lines. In this case the answer is 2.66 CFM, so you’ll need a compressor able to provide that flow of 2.66 CFM at 30 PSI to get all the water out.

The rule of thumb for compressors is that for each HP of electric motor (over 10 HP) you’ll generate about 4 CFM of flow at 90 PSI. Under 10 HP I use 2.5 CFM @90 PSI for each HP of electric motor, as my rule of thumb.

Folks in the south don’t have to worry about this much, but folks in the north have to deal with sub-zero temperatures freezing sprinkler lines. Here’s a question from a reader about dealing with that very thing.

 

A Question About Air Sprinkler Lines

I have a underground irrigation system and every fall I clear the lines using an air compressor. How do I know the pressure I using is clearing the lines enough to prevent trapped water from freezing?

My lines are 3/4 inch diameter and my longest runs are in excess of 180 feet and the short runs are about twenty.

I have been using a 25 gallon air compressor at 90-125 psi.

What’s your recommendation?

An Answer:

In blowing out sprinkler lines, usually it’s the volume of air and not the pressure that does the trick.

If you’re using 90-125 PSI air, and it works, that’s OK. I’d be a bit concerned about the sprinkler pipe’s ability to handle that pressure?

If you’re blowing air into the sprinkler lines with a 25 HP compressor, you’re generating lots of volume for the application, and I might dial the pressure down to 30-50 PSI just to go easier on the sprinkler lines.
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air sprinker lines

While I’m not a physics major, it seems to me that damage to a sprinkler pipe occurs when that pipe is full of water and then the water freezes. Water expands as it freezes, and if the there’s no place for the forming ice to go, it’s powerful enough to rupture the sprinkler pipe.

If you’re blasting air into the sprinkler lines at the highest point in the sprinkler line, I would think that as long as there’s nothing but mist coming out the other end – after all the free water that had been filling the pipe had been blown down to and out of the outlet – then any small amount of water in the lines shouldn’t present a freezing hazard.

Again, I’m not a sprinkler guy, these are my thoughts.

 

Conclusion

If you don’t empty your sprinkler lines of water, when winter grabs the ground and freezes it solid, so too will the water in your sprinkler lines freeze. When that happens, odds are good that the sprinkler pipes will crack as the water inside the lines turns to ice and expands. A very powerful force, indeed.


BOSTITCH Pancake Air Compressor, Oil-Free, 6 Gallon, 150 PSI (BTFP02012)

Hello, I am Bill, the Compressed-Air-Man. I have years of experience in industrial and residential compressed air applications, air compressors and general pneumatics. I created this site to help professionals, students, and DIYers understand and properly implement and maintain compress3ed air systems.

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