Severe water hammer when irrigation starts up

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Jarniscipus, May 9, 2018.

  1. Jarniscipus

    Jarniscipus Member

    Joined:
    Oct 16, 2016
    Location:
    Prior Lake, Minnesota
    Hi everyone,

    I have severe water hammer in my indoor copper pipes when my irrigation system starts up on the first zone. I am on a well, 1 ¼” feed to expansion tank at approximately 25 GPM well pump, 65 PSI set if I recall, 1” irrigation line, 20 feet is copper and then 200 feet of poly to the first irrigation valve. After the first irrigation valve is about 1000 feet of poly pipe to the first zone head. I’m thinking that when my irrigation valve opens, the pressure drops momentarily and the slug of water from my expansion tank or well pump starting up hammers the pipe. There is a big pressure drop on my gauge when the irrigation valve opens up. Any thoughts on how to fix this problem? Thoughts on is there an irrigation valve that opens extremely slowly? Perhaps a 1” arrestor or larger at multiple points? I should note this is the only hammer I ever get in my house, it wakes us all up and it shakes the house and there is never hammer on valve closing. Thanks!
     
  2. DonL

    DonL Jack of all trades Master of one

    Joined:
    Mar 30, 2011
    Occupation:
    Rocket Scientist
    Location:
    Houston, TX
    May be air in the line. The use of a check valve near the head or a new head should help.

    Good Luck.
     
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  4. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    It is important to figure out exactly when the water hammer is happening. Is it happening just before the pump starts, or the same time as the pump starts? My guess is filling the 1000' of line is taking 30+ GPM and your pressure tank bladder is bottoming out just before the pump starts. When the bladder hits the bottom of the tank it makes a hammer. You may simply need to let a little air out of the tank. You want less air pressure in the tank than the pressure that the pump starts.

    You also may have an additional check valve above ground that is not needed. If the hammer happens at the exact time the pump starts, you should remove the above ground check valve.
     
    THE DIRTY TIGER likes this.
  5. Jarniscipus

    Jarniscipus Member

    Joined:
    Oct 16, 2016
    Location:
    Prior Lake, Minnesota
    Hi, it happens right before the pump starts up. I think the 1000 feet of empty line is like opening a valve immediatly. The air pressure is 20 PSI less than the pump. Someone suggested I try a dole valve before the irrigation valve to prevent the rapid 30+ GPM from occurring. I think a dole valve is worth a shot, the only issue is I don't know what size dole valve to try on the system. It's 1" pipe pumping at 65PSI, I have no idea what size to try in GMP. I have no idea how many GPM my irrigation zones are running. I have 5 hunter heads per zone and they are adjustable from 2.5 to 14GPM each, so that's 14.5 to 70 GPM which is not possible and not helpful I know. If I try a dole valve any thoughts on a size that would not restrict the 1" 60 PSI copper pipe?.
     
  6. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Elaborate on "the pump". You are saying that the pump is 65 psi and the air precharge is 45?

    The thing is, a pump is usually controlled by a pressure switch. That pressure switch has a cut-on and a cut-off pressure. So if the pump cuts on at 45 and off at 65, that would typically be called 45/65. If you have such a system with a submersible pump, you set the air precharge to 43 (2 psi less than the cut-on pressure). If the pump is a jet pump, the precharge is set lower still.
     
  7. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    I'll bet the pump is already on when the water hammer happens, you just don't know it. With a 45/65 pressure switch setting the pump will start at 45. But the empty line is taking 30+ GPM and a 25 GPM pump cannot keep up. So the pressure drops to 20, and since you have 25 PSI air in the tank, when it passes through 25 the bladder on the tank bottoms out and causes a water hammer.

    You can make a variable Dole valve with just a ball valve. When the sprinklers are running restrict this ball valve until you see the sprinkler pressure dip. Then barely open the ball valve until the sprinklers have enough pressure to work. This will limit the line fill rate to the same amount the sprinklers use, so the pump should be able to keep up while filling the line and the water hammer will go away.
     
    DonL likes this.
  8. Jarniscipus

    Jarniscipus Member

    Joined:
    Oct 16, 2016
    Location:
    Prior Lake, Minnesota
    Valveman you are the man! I closed the valve until the point where the sprinklers just started chocking out and it solved the problem!!!!
     
    valveman likes this.
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