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Hydraulic Problem

Discussion in 'Irrigation' started by Planter, Jul 23, 2005.

  1. Planter

    Planter LawnSite Member
    from Utah
    Posts: 214

    Here’s a little hydraulic problem I am having and I would like some opinions.

    POC, downstream of the double check pegs my 100-PSI gauge in static pressure. 120 feet downstream are the 5 or 6 valves for the system. I have about a 10-foot elevation change upward here so I should lose about 2 or 3 PSI. Static pressure at the manifold is 72 PSI. I realize that the static pressure at the POC should be the same as at the manifold, plus or minus the elevation change. When I turn on the large rotor zone I drop to about 26 PSI and I get 22 PSI at the rotor. When I turn on the large field of 15-foot full heads my pressure at the manifold drops to 22, which fits with the 18 PSI I get at the first head of the zone.

    I have a zone 50 feet away that had a lot less heads on it than the 15-foot full zone and they are half heads with varying radiuses. With this zone my pressure drops at the manifold to 10 PSI and they spray poorly.

    When I shut off the valves the pressure goes to around 50 PSI and slowly creeps upward to 72. I have more investigation to do, but wonder what the collective wisdom here is?

    I may have a main leak and/or lateral leaks (there is no evidence of this) or I may have an obstruction in the mainline (rocks, crushed line, etc.)

    What are your thoughts and any guidance would help.
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,632

    Check the pressure at the upstream test port on the double check valve assembly when you operate the zones. When was the last time it was tested for proper operation?
  3. jerryrwm

    jerryrwm LawnSite Bronze Member
    Posts: 1,274

    Also, check to see if there are any other devices in the mainline between the DCA and the manifold. A pressure regulating valve that has started to go south. If you have a "static" pressure reading of 100 psi and you move 120' downstream you will still have a "static" reading of 100 psi (plus or minus any elevation change). This is true even if you go from 1" pipe to 1/2" pipe. The static reading will be the same. There is no flow, so there is no pressure loss due to friction. If it is as you say down to about 72 psi, then there is some sort of regulating device in the line. Also, when you shut the valves off and the pressure goes to 50 psi and then slowly up to 72 gives an indication that there is a PRV in the line somewhere. If it were a partially closed valve, or a kinked line, the static pressure would still be the same, but there would be a drastic drop in dynamic pressure.

    Also, is the POC that you are referring to a hose bibb/sill ****? Is the entire water supply on the DCA or just the irrigation system? Check for the PRV between the meter and the DCA, and also downstream of the DCA. Check just before the manifold.

    You have some digging to do my friend unless you get lucky and they put it in a long forgotten valve box.

    Good luck,

    Jerry R
  4. Planter

    Planter LawnSite Member
    from Utah
    Posts: 214

    The system is pretty new. The POC that I took the first pressure reading from is a quick coupling a few feet down stream from the double check valve. The irrigation system in the only thing on the double check. There is NO sign of a pressure reducing valve and, unless it is under the asphalt, there is no place to put one. It goes 1) stop and waste, 2) double check, 3) quick connect and straight to asphault to the valves.
  5. bicmudpuppy

    bicmudpuppy LawnSite Silver Member
    Posts: 2,781

    Static Pressure is Static Pressure, unless you have a PRV or another check that is creating a PRV type activity either with a fowled spring trash on top of the check, or something along those lines.
  6. jerryrwm

    jerryrwm LawnSite Bronze Member
    Posts: 1,274

    It may be in the line on the other side of the asphalt. Unless I am missing something, there is no way for static pressure to change other than elevation changes. If you have a 500' line that is on flat ground, with an outlet every 100' and a guage at each outlet, If the static pressure at the start is 100.0 psi, then the static pressure reading is going to be 100.0 psi at every gauge on down the line.

    So, in order for you to have a 100+ reading at the quick coupler and then 120' further it is 72.0 psi that is evidence that there is a regulating valve of some kind in that supply line.

    Was there another quick coupler or bibb at the manifold?

    I agree that some restriction in the line would contribute to the 50 psi loss when the zone is turned on, but still feel that a PRV is the culprit.

    Good luck

    Jerry R
  7. Planter

    Planter LawnSite Member
    from Utah
    Posts: 214

    I agree, this one has me baffled. I need to do some more testing at the check valve.

  8. HBFOXJr

    HBFOXJr LawnSite Bronze Member
    Posts: 1,712

    stop and waste is a scary word. I've seen them installed backwards and also seen the rubber disc peel up from it's base and block the flow. Either is bad and makes for HUGE pressure loss.

    What was the working pressure before the stop and waste. Y ou need to knwo what is happening on the supply side while working in addition to the lateral line pressure to figure where the problem is.

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