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Best Practice for fixing Negative Pressure at Pump

System: 

WaterGems Connect Edition Update 3

Not sure how to check which version but I checked and all my applications are the most up to date. 

Alright so I set up an existing system for a city that I help with. I have everything as accurate as possible. I have this problem on other projects as well. I have a negative pressure because of the suction on  between the reservoir. The pipes are the right size and roughness. I even tried using check valves to correct the issue. 

I uploaded the model through a secure file upload if you want to take a look at it. 

I tried increasing the elevation of the reservoir but then I have to change the pump curve to match. 

Is there another way?

  • Hi Mark,

    Thanks for providing a copy of the model. I see that you're using the latest version of WaterGEM and that the reservoirs are connected directly to the pumps. If the elevations are equal, you will always see a negative pressure at the pump with any amount of headloss in the pipe between. 

    What do these pumps represent in the real system? Meaning, how can the water level be equal to the pump elevation without cavitation/NPSH problems? Are these submersible pumps? If you simply want to not see the negative pressure messages, you could simply lower the pump elevation (this will not impact the hydraulics since your reservoir boundary condition does not change), but it might be best to address the root issue if the real model does not actually experience negative pressures at the pump.

    You can find even more information about this situation in the section "Why do I get a user notification about negative pressure at a pump?" in this article: 

    You had also sent me a private message about the impact of this to automated fireflow. So, you might be wanting to remove the negative pressure in order to avoid any impact on the automated fireflow results related to the pressure constraints. In this case, the following article explains how the "nodes to exclude" selection set option that was added in the most recent version of WaterGEMS (which you appear to be using) can help with this: Fire flow nodes are failing because of low pressures on the suction side of a pump

    However in the model you sent, this problem does not happen because you do not have any nodes (junctions) between the reservoir and pump. In your model, automated fireflow runs successfully, with nodes failing because of low pressure at other nodes unrelated to the pumps.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • This sounds like an existing pump. The first thing I would do is put a vacuum/pressure gage on the suction side of the pump and check the pressures when the Pump(s) are running.

    The cause of negative pressure is having the pump at too high of an elevation or too much head loss in the suction pipe. 

  • First look at the suction piping output results.  Is the velocity in the pipes high?

    Double check your pipe sizes and elevations, then do what Jesse suggested - lower your pumps and see what happens.  I suspect that if everything checks out ok then the model is accurate and that you are in fact having negative suction pressures.  I had this exact same situation with a water pumping station I was modeling for a client.  After double and triple checking all piping and elevations, then looking at the detailed results, I concluded that the model was 100% spot on.

    Most water system pumps have fairly small suctions which can introduce a LOT of headloss right at the pump.  You should have reducers at the suction (and most likely discharge, too) to lower the velocities to reasonable values to limit suction pipe losses.

    As Dr. Walski suggested, if you can, put a compound gauge on the suction side of one of the pumps and see what the pressure/vacuum readings are.

    Good luck.

  • Actually, I am not to sure if they are submersible pumps. I haven't been able to get the specs for the Wells. So I did what you suggested. I moved the pumps down a 5 feet and the negative pressure went away. Thanks. 

    I am still getting the message for my booster pumps and PSV which are directly connected to my Tank. I can upload my updated model. I've tried dropping the pumps 10 feet  in elevation. Again, I don't have the specs for the area. I used an old model to put these settings in. For some reason even though I dropped the pumps down 10 ft they are still creating a negative pressure. 

  • Hi Mark, it may be best to start a new thread for a separate issue. Also, uploading the model would indeed be useful: Sharing Hydraulic Model Files on the OpenFlows Forum

    Please be sure to reply with the file name and steps needed to reproduce and understand the issue.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.