About blowing out underground sprinkler lines…

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.

The question:

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.

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.

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