Sprinkler water hammer aggravated by small feed pipe?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by lifespeed, May 8, 2009.

  1. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Hi,

    3 years ago I installed an Accurain sprinkler system (3 sprinklers) using 160' of 3/4" schedule K copper buried 2 feet deep. It worked great until about a month ago when I replaced the sprinklers with an updated model utilizing a different electronic feedback valve.

    Not surprisingly, the new valve characteristics now generate water hammer. It was helped somewhat by the installation of a 3/4" arrestor where the house plumbing connects to the sprinkler. (I know ideally I should have an arrestor at each valve, but this is fairly impractical) But the problem hasn't been eliminated. The arrestor has a fairly small hole in the brass pipe fitting leading to the diaphragm and standpipe, which does not seem like such a brilliant idea. Perhaps it damps the resonance through flow restriction?

    When I put in the system I utilized an existing sprinkler connection to the house installed by an idiot. It is 1/2" (undersized for any sprinkler system but the unique Accurain) galvanized pipe that runs the length of the house, a good 60'. I suspect one of the primary problems is this long, skinny run of pipe feeding a 3/4" pipe terminated with three solenoid valves.

    So my question is: Would replacing the long 1/2" pipe with a shorter 3/4" pipe to the nearby bathroom plumbing fed by 3/4" galvanized pipe cure the problem?
    Last edited: May 8, 2009
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,491
    Location:
    Cave Creek, Arizona
    valve

    When does the hammer occur? When the valves open, or when they close? Two different conditions with two different cures. Arrestors will do nothing for a hammer caused when the valves open. And 3/4" K copper for an irrigation system? Even 3 years ago that would have been overkill and outrageously expensive.
  3. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    They hammer intermittently the whole time the sprinklers are running. The 3 sprinkler valves continuously adjust the pressure and distance of the water stream, these are not popups with on/off valves, but an electronically-controlled variable-pressure valve.

    As for the K-copper, these sprinklers are pressurized full-time with the "valve" in the head on top of a 4 foot post. Yes, it was expensive, but I expect a long life and zero maintenance to the pipes. I am not a fan of plastic. Each to their own.

    This is not a 'normal' sprinkler system, check the link.

    I hired a plumber yesterday to replace the long 1/2" feed with a short 3/4" connection to the sprinklers. The water hammer disappeared almost entirely. Instead of dozens of times in a 4-hour sprinkler session, I only got slight hammering once. A huge improvement, but I would like to see it disappear entirely.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,491
    Location:
    Cave Creek, Arizona
    connection

    I cannot visualize how a "long 1/2" section" could be replaced by a "short 3/4" section", but the velocity in the 1/2" pipe would be much greater than a 3/4" one, and if it is flowing at, or near, the maximum for that 1/2" then it would create a hammer as adjustments occurred. IF the 3/4" is also flowing at maximum velocity at some time during the cycle it could happen then also.
  5. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Suffice it to say the previous owners made the 1/2" pipe much longer than it needed to be.

    The average flow for each sprinkler is 1 gal/min. However, I could envision a coincidence where all three sprinklers went from high flow to low or no flow and setup water hammer. There is no synchronization of the individual sprinklers other than start time.

    Anyway, the problem is 99% fixed with the 3/4" pipe. I think any water hammer with the 3/4" pipe is when I get those lucky coincidences as the sprinklers cycle through their low/high flow settings every 10 seconds or so.
    Last edited: May 9, 2009
  6. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Just an update: A 100% fix to the water hammer problem required placing a hammer arrestor directly under one of the three sprinkler heads, a fix I was avoiding for esthetic reasons. I bought three arrestors in total and will place them under the remaining two if for no other reason than to isolate pressure pulses that cause the water stream to move erratically rather than smoothly.

    It is clear to me now that this is a feedback loop problem related to the distance (time) between sprinklers that allows the pressure pulses to set up a control loop oscillation within the electronic valve.

    Great sprinklers, but their sophistication can also incur some unusual problems under the right circumstances.
    Last edited: May 28, 2009
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