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Pressure Spiking in WaterCAD system model

Hello All,

I am modeling an existing system in WaterCAD version 10.03.02.75, where there are currently low-pressure problems in the far southern area because all the wells are in the northern area. When I run my model, the pressure bounces between low pressures (in the 10-30 range) and extremely low pressures (below -100) at the southern side. The expected pressure in that area is between 20-40 psi. 

It appears that this pressure drop happens when the booster pumps that fill the high elevation tank(as shown in the pic below) turn on. This high elevation tank supplies so much head to the system that when it is filling the system appears to have extreme pressure drops. However, this is not the case, as the client is not seeing these kinds of problems when that tank is filling. 

Is there some way to help balance out these pressure spikes? Or are there ways to tell the other wells/pumps to maintain a certain pressure throughout the system? I am not sure what other ways I have to tune the system. 

Any help is greatly appreciated, and thanks in advance. 

  • WaterCad is demand-driven, which means it will supply whatever demands are placed on the nodes, regardless of how low the pressure drops.

    I'm not a WC expert by any means, but I have seen the same type of problem you are experiencing.

    I would try a couple of things:

    1. Reduce your node demands to try and get a stable system.  You can apply a system-wide demand multiplier (say 0.25 or so) - easy and quick.

    2. What is the length of your EPS time step?  If your time step is too great you could see huge fluctuations in system pressure.  I also set my pump start/stop water level elevations between the elevated tank high and low levels (stop pumps at about 95% full, start pumps at about 5-10% full).  This way the tanks are never either completely empty or completely full and are always connected to the system. I've had models where I had to reduce the time step to a few minutes in order to get it to behave properly.

    Your first priority should be to get a stable model that yields realistic results.  Once you get that then you can start adjusting other parameters (demands, tank water levels, pump start/stop elevations/pressures, etc).  You probably already know this, but only change one thing at a time, run the model and see what happens.

    Good luck.

  • Hello Noah,

    in addition to the suggestions Gary provided, it may be worth exploring any errors that may be occurring that are causing these low pressures. The results from your screenshot show pressures far below physically possible pressure results.

    If you are getting any red user notifications, like network unbalanced or disconnected demand nodes, resolving these errors would be first step to take. If there are not any errors I would review the model to find why the results are so low. Information on negative pressures can be found at this link.

    If that doesn't help, it would be useful to see a copy of the model files.

    Regards,

    Scott

  • Hey Scott,

    Thanks for your reply. I do not have any red notifications, they are all either yellow or green. Please see the attached file, i hope it helps offer some context. 

    2020-04-13_GENMODEL.wtg

  • Hey Gary,

    Thanks for your suggestions. When I reduce the demands I do not get a more functional system. 

    I have varied my time step from .25 hrs to .01 hrs to all time steps, and that only increases the resolution of the graphs. I have tried only changing one thing at a time, and this is the best iterative solution I have been able to come to. 

  • What I would do at this point is make a copy of my model, then in the copy, delete all demands, then add back just a couple of demands at a couple of nodes that are giving problems.

    Hopefully then you could start adding back demands until you find out where the problem is coming from.

    Good luck!

    And FYI - just the .wtg file is not enough.  The sqlite file is also required as a minimum to run the model (or use the WC utility to make a package).