Flow required to properly blow out sprinkler system

Discussion in 'Irrigation / Sprinkler Forum' started by cpo1, Oct 22, 2016.

  1. cpo1

    cpo1 New Member

    Joined:
    Oct 22, 2016
    Location:
    Indiana
    I've been investigating the winterizing requirements for a home sprinkler system. There are many articles referencing CFM and PSI. What seems to be often missing is how much CFM flow is required and what are the maximum and minimum PSI requirements. For 1" polyethylene main pipe 50 PSI seems to be the maximum pressure that should be used and multiple short times should be used at that PSI, rather than one long episode. I read one article from a university extension service that the flow should keep the sprinkler heads up after the water is expelled. That said, here are my observations. I used a 6 gallon 2.6 CFM @ 90 in series with a 6.5 gallon 5.2 CFM @ 90 system (total 12.5 gallon tank) on a 75 foot 3/8" air hose. I regulate the pressure to 50 PSI, although the tank pressure builds to about 75 PSI prior to releasing the air. When released in the open zone, the pressure immediately drops to 50 PSI. With a single 3 GPM sprinkler head zone that is about 10' from the pipe connection source, I can keep the head up at 50 PSI without issue. For a single 3 GPM head that is about 75' away from the air source, I can keep the head up for about 5-6 minutes before it wants to start slowing receding. It seems that once the tank PSI gets somewhere below 30, (maybe 20-25), the heads will not stay up. When blowing out a 2 head 6 GPM zone at about 150', the current system can keep the heads up for about 2 minutes. With a 3 head 9 GPM zone, I can keep the heads up for about 40 seconds. So it seems to me that the horsepower or tank volume needed to maintain a minimum flow of approximately 40 PSI for a period of about 3-4 minutes is required to ensure that sprinkler winterizing is properly accomplished. In my case, I believe my system will not properly handle a 9 GPM or 12 GPM zone. It is also apparent that the longer the head distance is from the air source and also the air connection point of the system, the more horsepower is required. I can see where a 30 gallon compressor with CFM of about 5.5-6.0 at 40 PSI might be able to handle a 12 GPM zone for 3 minutes or so (keeping the heads up during that time), but I am not sure about that. That might be a stretch. But coupled in series to another 12 Gallon of tank space, I believe it may be adequate. I am combining an older system with a new one and the old system has 12 GPM zones in some areas. I have had 5 and 10 HP compressors before, but don't want that bulk at home, so I am trying to keep things portable, if possible, and still accomplish the winterizing task for both my sprinkler system and in-ground swimming pool here in Indianapolis, IN. Prior to purchasing a new and larger compressor, I thought I would see what the experts have to say about it. Thanks for your comments.
     
  2. DonL

    DonL Jack of all trades Master of one

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    I think you are over thinking things.

    Once water stops coming out more CFM of air will not help anything.
     
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  4. cpo1

    cpo1 New Member

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  5. cpo1

    cpo1 New Member

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  6. cpo1

    cpo1 New Member

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    Indiana
    Thanks for your reply DonL. Here are a few quotes from my investigation of this. " If an insufficient volume of air is used, after forcing some water out, the air will ride over the top of the water. This will result in the remaining water draining into low spots and subjecting the system to freeze damage. Ideal pressures are in the range of 40 to 80 pounds per square inch (psi) for the air compressor with 80 psi being the maximum for rigid PVC pipe and 50 psi for polyethylene pipe (flexible black pipe)..........Rule of thumb: If the sprinkler heads stay up after the water is blown out and compressed air continues to flow through the system, you are using the right size compressor." Accordingly, you can blow water out of a zone head with less than ideal pressure and CFM flow. Taking that information, I experimented with my own system to see how what it took to keep the heads up at the recommended range (I used 50 PSI and let it go down to 40 PSI). I also found that water was remaining in some zones which were taxed by my under-powered equipment. Each time I ran air through some of the longer distance zones, water would come out for a short period of time and then the heads would retract back into the ground. This indicated to me that the fellows who wrote the information above were providing some good information. In the worst case longer distance higher GPM zones, I could generate 50 PSI degrading to 40 PSI for no more than 40 seconds and this was telling me that I was providing the proper CFM, at least for those periods of time. When the PSI dropped to 20-30 PSI, the heads retracted and this told me that I was not providing the full CFM flow that is required and very little water came after that. Once in a while, there might be a short burst of a little more water, but not much. Those zones required more than one encounter with air and I am not positive that I have all of the water out of some of these zones; hence my search for what tank volume and CFM may be necessary. I believe I likely have enough water evacuated to keep freeze damage from occurring until I can get a little stronger compressor system, but it would be nice to know for sure that you have completed the task in a proper manner. I fully understand that the pro's have mobile equipment that can easily evacuate a typical residential system, but I would still like to be able to do this myself since I am now semi-retired. As I said, I have owned 5 HP and 10 HP air compressors in the past, but do not want that bulk and maneuverability problem in my garage at this time. I am not trying to make a mountain out of a mole hill; just trying to get my head around the logic and process requirements for winterizing success. Do you agree with the article quotes above? Just curious. Thanks again.
     
  7. DonL

    DonL Jack of all trades Master of one

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    Some of it makes sense, Some of it does not.

    For one thing the heads are held up because of hydraulic water pressure, Not air pressure. Big difference.

    Once the water is gone it would be normal for them to drop down. Enough air pressure to keep them up could damage your control valves.

    Just to check that theory, You can blow the water out, Let the compressor come back up to pressure and try it again.

    I am no sprinkler pro, I just know some theory, And theory works best in a vacuum. ;)

    You could also use a wet vacuum cleaner and suck the water out. Or put antifreeze in the lines would work too.

    Sorry I can not be of more help. Maybe a sprinkler pro will come by the forum soon.

    Good Luck.
     
    Last edited: Oct 23, 2016
  8. cpo1

    cpo1 New Member

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    Indiana
     
  9. DonL

    DonL Jack of all trades Master of one

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    Location:
    Houston, TX
    Did I miss something ?

    Your last quote had no new text.
     
  10. Gary Swart

    Gary Swart In the Trades

    Joined:
    Sep 1, 2004
    Location:
    Yakima, WA
    You are wasting your time. Hire one of your local yard service companies to blow the system with the big industrial compressors they use. Will have your system cleared of water in a few minutes and you will have the assurance that it is done right.
     
    DonL likes this.
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