Water hammer causing backflow to dump?

Discussion in 'Irrigation' started by justgeorge, Aug 17, 2006.

  1. justgeorge

    justgeorge LawnSite Senior Member
    Posts: 352

    Me again with another backflow dumping problem. I have a new install that I just finished. It's at a new house, still under construction, but the landscaping is installed and the sod is being installed tomorrow so the irrigation system HAS to run.

    Anyway, there is no plumbing hooked up yet in the basement other than one outside hose bib and the line to my irrigation system. When the painters showed up there this morning the backflow was dumping. I got called on it this afternoon, and when I got there it wasn't dumping but all the valves were still on. I asked the painters how they got it to stop. They said it was dumping in a pulsing dump, not a constant stream, and when they turned on the outside faucet to run some water to clean their brushes it stopped dumping and didn't restart. I took the backflow apart (Febco 860 RP 3/4") and found no debris in either of the check valves or in the relief valve. After I put it back together, when I turned on the valves everything was fine; I shut off the output valve and it started dumping in that same "pulsing" dump. Again, turning on the faucet stopped it from dumping. Looking at the Febco website I found this tidbit in the troubleshooting section:

    Symptom: Intermittent discharge from relief valve during NO FLOW condition.

    Cause: Pressure surges (water hammer) causing relief valve to discharge as pressure wave passes thru the zone.

    Solution: Eliminate or reduce pressure surges.

    I'm guessing that since none of the plumbing is hooked up to the rest of the house that when my system ran this morning and then shut off, the sudden pressure from the Hunter PGV valve closing (master valve installed about 1' beyond backflow) is causing the water hammer and with no other plumbing hooked up in the house to help absorb the pressure there is no where for that pressure to go and so it causes my backflow to dump. There is 80 lbs of pressure at the one functioning hose bib, which is also right by my backflow.

    I've got a call in to my plumber but haven't heard back from him yet. Would installing a pressure regulator solve the problem? Or do I need a pressure regulator AND some sort of water hammer arresting device?

    Sorry to be so lengthy but better to provide more details than not enough.....

    Thanks,
    George
     
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,041

    I've had those 'pressure wave' RPZ dumps. Definitely water-hammer related. On one system with a Febco 825Y, and static pressure over 100 psi, the highest-flow zone would hammer, and a dump cycle could start. The cure was a simple one - evacuate all of the air from the RPZ. The presense of compressible air made the situation much worse. I also made sure that the last zone on the cycle was a lower-flow one, which didn't hammer when it closed. Lucky for me, this is a system I personally open each Spring, so the homeowner doesn't see the dumping.

    A more extreme cure would be to install another check valve upstream of the RPZ, which is supposed to interrupt the 'pressure wave' that causes the surging.
     
  3. justgeorge

    justgeorge LawnSite Senior Member
    Posts: 352

    Thanks for the quick feedback. What do you mean by "another check valve"? A pressure regulator? A simple check valve like I see installed in the normal plumbing lines (which aren't there yet)?

    The last zone to run changes each day depending on the cycle; some days it may just be the landscape zone.

    George
     
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,041

    Just a plain check valve. I'd use a resilient-seated check valve, like the ones I use on the suction side of a pump, although that might not be important. The idea is to stop the back-and-forth water-pressure motion. The quick-closing sprinkler valves will still make for a relief-valve dump on the RPZ, but the surging would be interrupted.

    I've never had to go to this extreme. Clearing out the air has always been enough.

    Wouldn't it be nice if one could get those zone (or master) valves to shut off a bit slower? I only know of one (discontinued) brass valve that had a special 'slow-closing' option.
     
  5. justgeorge

    justgeorge LawnSite Senior Member
    Posts: 352

    It's also strange that it never happened while I was testing the system, turning zones on and off multiple times. I was using the controller to operate the zones also, not the manual on/off at the valve. The other difference is this morning the last zone to run was operating two soaker hoses (don't ask about why it's running soaker hoses, long story) and it turns out one of the hoses had a big hole in it so the GPM while it was running was probably pretty high.
     
  6. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,794

    I've found really bad water hammer problems when automating regular perferorated soaker houses.

    Turning down the flow control on those valves helped a bit.
     
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,041

    Also, connecting a pressure regulator to the line, before the soaker hoses, might help out.
     

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