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Thread: How to reduce pressure in a network without PRV

  1. #1

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    How to reduce pressure in a network without PRV

    Hi, I have modeled the attached network in InfoWater and I have high pressures at all service nodes. The demands into the services are 1.25 gpm average daily flow and the Flow Anaysis at "Pump" location is


    Flow Rate, gpm Residual Pressure (psi)
    0 120
    1500 106
    1800 100
    2650 80
    3350 60
    3900 40
    4404 20

    I now the energy in the mains is high but how do I reduce the pressure at the supply nodes to below 100 psimodel.jpg
    Last edited by D.Assan; December 21, 2016 at 11:01 AM.

  2. #2
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    David,

    The pressure at any location is going to calculated from the Hydraulic Grade line (HGL) at that point (Pressure head_ft/2.31 ft/psi) = HGL_ft - Elevation _ft) so if your pressures are high then it could only be one of 2 things generally :
    1) An error in the node elevations
    2) An error in the head value

    If the error is in the head value (make sure you check elevations first) , then look at what is controlling the HGL. In your case it is both the pump and the head on the suction side of the pump. If the pump curve or pump definition is not accurate or too many pumps are on the pump can be adding more energy than expected to the system at low demands causing the HGL to be high and thus the pressure to be high. Perhaps check to see if you are using too many pumps or a fire pump at normal system demands as this could cause this and make pump and or control adjustments as necessary. NOTE: if a pump has no controls or initial status it will run at time = zero. If your suction head that is feeding the pump is also too high this would also cause the pump to add its normal energy (head) to a high value which would also elevate the discharge pressures. Find your "head" problem and you will generally fix the pressure problem.

    Patrick Moore

    One other note: If your system demands are below 1800 gpm from your pump curve (likely for normal demands) the pump would only operate far to its left on the curve and would likely show pressures above 100 psi (as 1800 gpm was 100 psi). In that case you would generally need a smaller pump for normal operation that would suit the normal operating demands better.

  3. #3

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    Thanks Patrick,

    The pump I have on the system was generated from the Fire Flow Test conducted by the Fire Dept. on the Fire Hydrant located on the City Main Pipe to determine expected flow. That's how the pump curves were generated. The residual pressure in the mains is 120 psi and since each service is connected directly to the main, installing PRV's will be expensive.

  4. #4
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    David,

    If you know the head is already high and is correct, the thing that many engineers consider in this situation is to install individual PRV's on each users meter. These are capped at a max pressure of 80 psi to conform to standard plumbing codes. My knowledge of actual costs is a bit dated, so please verify the actual costs yourself, but I would expect these for a standard house to generally be around $300-$500 each (installed) and are used when there are not enough users for a system PRV which can run in the $50-$100K range or more. You generally determine the cost of a PRV and vault and divide that by the cost of the individual home PRV to determine the point at which a system vault is more cost effective. This is the only way to protect users if the system head would cause pressures above 80 psi at any time in the home and would be required generally by a local plumbing code.

    In the model you generally would not model these small PRVs, but would just know any location where the pressure was above 80 psi would generally require one. The only way to control the HGL of a service line is to use these or to use a full PRV vault as you can’t simply rely on pipeline headloss to keep a reasonable HGL at lower flows.

    Patrick Moore

    One other note: Those are some long service lines shown that will have a high head loss. You may wish to consider just tapping the service lines directly off the fire line and using individual PRV's as you would in a typical system. Those long service lines would likely be expensive and not ideal for the users or for the utility.

    Quote Originally Posted by D.Assan View Post
    Thanks Patrick,

    The pump I have on the system was generated from the Fire Flow Test conducted by the Fire Dept. on the Fire Hydrant located on the City Main Pipe to determine expected flow. That's how the pump curves were generated. The residual pressure in the mains is 120 psi and since each service is connected directly to the main, installing PRV's will be expensive.
    Last edited by Patrick Moore; December 29, 2016 at 11:34 AM.

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