PVB backflow problem

Discussion in 'Irrigation' started by beocop, Jun 28, 2005.

  1. beocop

    beocop LawnSite Member
    from VA
    Posts: 10

    I have a problem with my NEW PVB device. After a zone is ran and shut off, the PVB starts gurgling at the bonnet and doesn't seal/pressurize. During this time, the inlet pipe is "thumping"/banging. In order to fix this problem, I have to close the inlet valve of the PVB and open it again. This will pressurize and seal the PVB.

    What is the cause of this problem? Could it be the height of the PVB compared to the risers, which might not be below the height of the PVB? Is it due to the zone using too much water that causes loss of pressure?

    Please help.

    Thanks
     
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,992

    Is it really a PVB? Sounds almost like an AVB, or an antisyphon valve. (which can gurgle and dribble, even if it's the final zone in the cycle) There is a world of difference between the two. A PVB can leak when its pressure drops way down, when cycling from zone to zone, while two zone valves may be open for a time. Maybe the controller has a delay between zones that could be employed, to give the 'earlier' zone valve the needed time to fully shut off.

    By the way, you must have at least a twelve inch elevation difference between the PVB and any pipe or sprinkler it feeds, in order for the device to comply with codes, and for it to have a useful protective function.
     
  3. beocop

    beocop LawnSite Member
    from VA
    Posts: 10

    It's the Febco PVB 765. I don't think it's the controller since it also does this when I open and close the zone manually. I know that the PVB is not 12"+ higher than all the heads but don't know if this will cause the said problem. If this is indeed the problem, I am guessing that the water is pushing back at the PVB, preventing the PVB to seal properly.

    Anyone else has other insight?

     
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,992

    A PVB is going to stay sealed, and you wouldn't know from elevation differences, unless the pressure drops so low that the spring-loaded poppet drops down. I see the phenomenom when opening systems for the season, when the extra-heavy flow into the empty pipelines can drop the pressure below the sealing point of the PVB. But during the season, I never want to see that happen. I saw it happen on a recent repair, on a system I did not install, that has flukey valves and low static water pressure. All I can suggest is to see if you can't program some valve-shutdown delay time between the zones, if the clock permits it.
     
  5. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    Is it only on one zone?

    I had a similiar problem on a repair. Turns out that if the valve has a seeping problem it makes the PVB do this.
     
  6. dozerboy

    dozerboy LawnSite Member
    Posts: 5

    Does it only happen when you are switching to another zone? I have seen this a lot on systems where the gpm thru the zone is too high. If it doesn't happen on the last zone on the system I would double check my flow thru the zones and split them if nec.
     
  7. beocop

    beocop LawnSite Member
    from VA
    Posts: 10

    The problem only happens on one zone. It happens when that zone is the last/only zone. I've discovered that I can circumvent this problem by NOT making this zone the last zone. The zone output/flow is ~13GPM on 1" lateral. So it's around the maximum output for that zone. I don't think there's a leak but will check later.

    I know the PVB elevation is not proper and will raise it up to regulation height. I also have water hammer using Rainbird DV valves and having a high input pressure of 80psi after the backflow, at the blow-out boiler drain. I will put a PRV to reduce this to ~60psi to reduce the hammer and misting. BTW, anyone got pics of what misting/fogging looks like. I am having a very fine mist from the 1804 heads that gets blown around in the wind.

    The RB 5004 works OK. I have 3 on a zone using 2(#2.5) and 1(#5) nozzles. The #5 nozzles works great and can get from head to head. However, the #2.5 nozzles are not shooting very far (maybe 60-70% coverage). If I use #3 nozzles, the zone is overwhelmed. Any input on this?

    Thanks guys
     
  8. bicmudpuppy

    bicmudpuppy LawnSite Silver Member
    Posts: 2,781

    It's going to sound backward, but try a #4 instead of the #5 and #2 instead of the #2.5...........It may not be perfect, but it should be closer than what you have.
     
  9. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,992

    Obviously, when diagnosing an unusual PVB situation, you can use a pressure gauge to see what's what during operation. Thread it into one of the test ports, if there isn't a blowout valve or drain plug handy.
     

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