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D.Assan
December 21, 2016, 09:57 AM
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 psi354

Patrick Moore
December 29, 2016, 08:22 AM
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

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.

D.Assan
December 29, 2016, 08:47 AM
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.

Patrick Moore
December 29, 2016, 09:00 AM
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 canâ€™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.

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.