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PVB question

Discussion in 'Irrigation' started by Dirt Boy, Aug 29, 2011.

  1. Dirt Boy

    Dirt Boy LawnSite Senior Member
    Posts: 487

    Have an installation with an variable speed pump - capacity 25 gpm with an 1 1/2" Watts PVB.
    Problem is that due to low/no back pressure at pump startup, PVB will due a short flush until valve seats. Hope that makes sense.
    Is there a solution to this?

    Thanks
     
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,429

    It's supposed to vent when there's low pressure.
     
  3. Dirt Boy

    Dirt Boy LawnSite Senior Member
    Posts: 487

    Yes, and it does exactly that.

    But, is this normal for system to behave like this? I would have to surmise that I am losing the back pressure in the mainline. IOW's system is running, valve shuts off, all is OK, system sits for a period of time, controller tells it to start again, valve opens, system (main line) loses pressure ... i.e. starts to run, there is a small delay while the pressure tank pressure decays, VSP starts running, but that lag time, opens up the PVB, thus the "flush" action until pressure builds up and seals PVB.

    It doesn't do this between stations, just at startup of new cycle, since the lag time is small.
     
  4. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,298

    You need a controller that has the option to delay valve opening. It typically is called a master valve/pump to zone delay or something similar. Many modern controllers have this capability, but some hide the feature.
     
  5. Dirt Boy

    Dirt Boy LawnSite Senior Member
    Posts: 487

    OK, so the valve delays opening, I guess I don't see that making any difference. The system does not have a pump relay switch. It's one of these VSP (not sure if thats the correct term) electronic units that are common now adays.
     
  6. AI Inc

    AI Inc LawnSite Fanatic
    Posts: 25,151

    I think your talking about a constant pressure switch. It dosnt wait for pressure to drop before turning on but instead works off a flo senser.
     
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,429

    Is there a pressure tank in the supply?
     
  8. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,298

    As long as you can introduce a delay between the pump start and your first zone valve opening it should help. If that requires adding a master valve, then that is what you should do.
     
  9. AI Inc

    AI Inc LawnSite Fanatic
    Posts: 25,151

    or swap the pvb to a dcva and call it done.
     
  10. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,298

    If you are going to recommend replacing the backflow, then at least recommend one that is actually rated for high hazard backflow protection please ..... and in that case .... it won't solve the problem.
     

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