In the fall here the standard procedure is to hook up an air compressor to your irrigation lines to blow the water out of the lines, to prevent freeze damage. I have a very simple drip irrigation system with one zone, a total of maybe 200 feet of 3/4" PVC line, with small diameter drip lines running off it. Flow is about 4 GPM when it’s running.
I’m thinking of just buying a portable compressor rather than paying someone $100 for a 15 minute job each year. But I’m finding conflicting information online about what specs for the compressor will be adequate. In principle you want high volume at low pressure. Some source say you want 20 CFM - but that’s way beyond what any portable compressor will deliver. Yet other sources say a small portable compressor that delivers a fraction of that will do the job fine. The specs of a reasonably priced portable compressor are something like: 6 gallon tank, max pressure 150psi; 4 CFM at 40psi.
Is any of that really necessary? I’ve got considerably more drip irrigation system than that, all on the surface in the upstate New York climate, and all I ever do is drain it by opening all the valves, opening lines at the bottom, and disconnecting from the filter at the top of the system (the filter also gets drained, and spends the winter in the barn, though it freezes in there too.)
Is your system set up so that air can’t get in to allow it to drain unless you inject the air with a compressor?
If the lowest point’s not out at the end of the lines but somewhere in the middle of the headers, can’t you just open the header at that low point (and wherever the high point is, of course) and splice it back together in the spring (ideally inserting a drain valve at that point?)
Or is this whole thing buried underground where you can’t get at any of it?
I clear my own irrigation system every fall, but I’ve got a 60-gallon compressor that delivers close to 20 CFM. My irrigation system has six zones covering about 0.2 acres of lawn; it’s a bit of a judgment call as to when any given zone seems adequately cleared, but I feel like each one takes maybe a total of 3 minutes of gurgling/spitting/spraying after the bulk of the water has been cleared out, with the compressor running continuously. (I still need to take care of this for this season; if I can remember, I’ll time the whole deal and post results here).
Portable compressors are generally single-stage and oil-free. They’re fine for things like tires and impact wrenches, but they don’t like running continuously for very long periods of time. That said, 200 feet of 3/4" line is a volume of about 4.5 gallons, so if you buy the 6-gallon portable and prefill the tank to 150 psi, you might be OK for your single-zone system. You could do it in stages: blast a tankload of air through the irrigation system, then valve-off and refill the tank for a second go. With only one zone, this seems doable - besides which, then you’d have a compressor in the garage for things like tires and impact wrenches.
That’s helpful - for some reason it hadn’t occurred to me to just do the volume calculation. The recommendation is no more than 50psi in these PVC lines, so a fully charged cylinder should give me 18 gallons at 50psi, should easily be enough to do it in one shot, maybe two to be safe. I’m going for it. I can buy this compressor and the adapter for about $150 vs the irrigation service company charging me $100 for just doing it once. And this way I have a compressor in my garage for “free”.
I did this for years at my old place with a little $100 compressor from Wal-mart and it worked great. The one I used had a max pressure setting to avoid letting the pressure get too high.
You don’t have to get all the water out BTW. A little left over won’t hurt anything even if it expands and freezes solid. It’s when the pipe is full that freezing destroys everything.
If you’ve never done this yourself before, the backflow preventer can throw you for a loop. If you lose system pressure, the device opens to atmosphere, and your compressor probably won’t be able to move enough air to get it to close again; you’ll have to open the main water valve again inside your house to get the backflow preventer to shut.
So here’s the sequence to follow to avoid trouble:
-Connect your air line to the irrigation system, using one of the small fittings seen at top right in this image. The little valve on that fitting should be closed. Before you connect your air line, make sure mud dauber wasps haven’t blocked the fitting with a nest.
-Pressurize your air line, but don’t open that little valve on the backflow preventer just yet.
-Close the upstream water valve (somewhere inside your house) for the irrigation system’s branch.
-Open the little air valve on the anti-siphon valve (1/4 turn on that flathead screw). Now the irrigation system pressure is being maintained by your air supply.
-Turn on your irrigation system to blow the water out.
If your air pressure starts to get low, turn off the irrigation system to keep it pressurized so the backflow preventer doesn’t open, and give your compressor a few minutes to fill up again. If the backflow preventer does open, you’ll probably need to use the water supply to get it to close - but there should be a valve just downstream of it that you can close so you don’t have to refill your entire irrigation system to make this happen.
I purged my irrigation system yesterday and wrote down some notes on duration. I made one pass through all six zones, spending 2-3 minutes on each zone (some cleared faster than others). Then I made a second pass through all six zones, and some took longer than others before I was satisfied. For certain zones, there was one sprinkler than kept spitting spray for a good five minutes (on that second pass) before I felt comfortable moving onto the next zone. Don’t know what the deal is, maybe a low spot in the underground line that gets water dragged out of it bit by bit with the passing air. As noted upthread, you don’t need to get absolutely all of the water out; you just need to ensure that there’s not enough remaining to block a fitting/hose and then freeze solid.
My compressor was able to maintain about 80 psi running full time during all of this. Your proposed 4CFM unit surely won’t, but if you start at 150 psi, and maybe take a break when the tank pressure drops below ~40 PSI, you can probably take care of your single-zone system yourself.
You’re talking about quick-disconnect fittings, right? You’ll want one of these on the backflow preventer, one of these on the compressor regulator outlet, and a suitable hose with QD fittings on it (if the compressor doesn’t already come with it).
If your compressor doesn’t come with a tire chuck and an air blow gun, I highly recommend getting one of each (with their own QD fittings).
A leaf blower will only make a fraction of a PSI in pressure. That’s not enough to blow water up and out of the sprinkler heads. Moreover, it’s not enough to keep the irrigation system’s backflow preventer from opening to atmosphere.
Done. As we knew, this cheap compressor could not come close to pushing enough air to maintain pressure, so I did not leave it running continuously. But the fully charged 150psi 6-gallon cylinder held several multiples of the total volume of the system. I set the regulator to ~30psi and the first charge maintained that for about 2.5 minutes. As you’d expect, I got diminishing returns as the nearer drip heads cleared leaving more points for air to escape. I think 3/4 of the heads cleared the very first time, but it took 8 charges to fully clear the most distant drip head. I’m confident it worked, because even the final charge with air escaping from all the dripheads was still able to maintain 30 psi for about 1 minute.
So I think this 6-gallon cylinder was the sweet spot for my system, very portable for other uses and only about a 30 minute job.
