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negative pressures at junctions

Hi,

I have a water supply network and I'm trying to solve this using WaterCAD V8i. In this network, there is a pump and also a ground level tank (GLT) with a volume of 5000 m3. Pump serves the zone (Zone-2) which have higher elevation than GLT and the other zone (Zone-1) receives water by gravity. Here are the pump properties:

Shutoff head: 60 m    Design head: 50 m    Max.Operating head: 40 m    Design flow: 13.1 L/s    Max.Operating flow: 22 L/s

I have input all junction, valve and pipe data, and then run the system all for average flow, max.daily flow and max. hourly flow under steady state condition. There is no problem here. However, I create a fire flow (50 L/s) scenario for max daily and max hourly flow and then run it as a batch, I get green light in Calculation Summary. It's OK but there are some messages all with blue light "negative pressures at junctions" in User Notifications. When I look at Junction Table, I see that hydraulic grade is lower than the elevation. For example, J-223 with 625.732 m elevation has 558.019 m hydraulic grade and -662.7 kPa pressure. How can I handle this negative pressure problem?   

(Junction demands are between 0.001-0.614 L/s for Qmax daily and  0.001-1.025 L/s for Qmax hourly.)

Note: All of the negative pressure junctions are in gravity zone (Zone-1) and fire flow scenario is also in this zone.

Best regards,

Seylan

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  • If WaterCAD says the pressures will be negative, then in all likelyhood you will have problems. There are usually two general causes of negative pressure;

    1. Trying to serve a customer at too high of an elevation. This will show up as low/negative pressure at any demand. You need to increase pump head or adjust pressure zone boundaries.

    2. Some restriction in the system. This will show up as good pressure during low demand and poor pressure at high demand. You need to look at the model results and see if the pipes are too small causing excess head loss or the pumps are inadequate such that they are running far off to the right of the curve. You need to upsize the pipes or pumps accordingly.

    Trying different solutions to see what works is the beauty of using the model as opposed to building with the wrong design.

     Best wishes,

    Tom

  • Junctions having negative pressures are fed by gravity, not by pump. Still, do I need to increase pump head? By the way,  I have upsized the pipes coming to these junctions but the pressures are still negative. Is there anyting else I can try?

    Best regards,

    Seylan

  • If the junctions are gravity fed, Tom's suggestions are still good but you need to change hte wording a bit.

    If the pressures are always negative, no matter what the demand, then check the HGL of the tank that feeds the zone, and check the elevation of the junctions.  Is the HGL higher than the elevation?  If it is not, then you will never have positive pressure at these junctions.  You can test this by raising the HGL of the tank to something above the elevation of the junctions and seeing if that fixes the problem.

    If the pressures are positive at low flows but negative at high flows, then the problem is loss of head due to friction somewhere between the tank and the junctions.  Or, looked at another way, trying to satisfy demands that are too high for the size of the piping network.  You can fix this by lowering the demands, or by increasing the size of the pipes, or making them smoother so that there is less energy lost to friction.

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  • If the junctions are gravity fed, Tom's suggestions are still good but you need to change hte wording a bit.

    If the pressures are always negative, no matter what the demand, then check the HGL of the tank that feeds the zone, and check the elevation of the junctions.  Is the HGL higher than the elevation?  If it is not, then you will never have positive pressure at these junctions.  You can test this by raising the HGL of the tank to something above the elevation of the junctions and seeing if that fixes the problem.

    If the pressures are positive at low flows but negative at high flows, then the problem is loss of head due to friction somewhere between the tank and the junctions.  Or, looked at another way, trying to satisfy demands that are too high for the size of the piping network.  You can fix this by lowering the demands, or by increasing the size of the pipes, or making them smoother so that there is less energy lost to friction.

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