Hello All,
I am currently modeling a water system with 3 storage tanks spread fairly far apart (3-5 mi). I keep getting extreme variations in flow, resulting in unsightly oscillations when I graph the discharge in a pipe. I've tried adjusting the time step, accuracy, no. of trials, controls and even how I supply water to the tanks. I have also noticed the sum of the flows don't add up at one particular juntion in my model. Any comments or suggestions would be greatly appreciated.
Thanks,
BLB
Hello,
I assume you're using WaterCAD or WaterGEMS.
In general, tanks that are hydraulically close in proximity should typically be modeled as a single, equivalent tank. What might be happening in your case is that in one timestep, one tank is at a higher HGL than the other, so the flow goes towards the lower one. In the next timestep, the low hgl tank is now at a higher HGL, so the flow reverses the other direction. This causes the oscillations. However, I can't tell for sure without examining the entire model setup. (problems can also occur in tanks become full/empty or if the model is unbalanced)
As for the flow not adding up - is that timestep unbalanced? If data input problems prevent the calculations from converging on a balanced solution, the results for that timestep will be compromised.
If this does not help (and for future questions of this nature), you should submit a service ticket (including your model is possible) using the online ticket manager .
Regards,
Jesse
Technical Support Group
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
That sounds like typical behaviour from hydraulically close tanks. As Jesse says, even if you have 2-3 physically different tanks, you should combine these into 1 big model tank, otherwise you'll always be battling calculation convergence problems, and have much, much longer calcuation run times and large results files.
What is unusual in your case is you say they are 3-5 miles apart? To be honest, you typically shouldn't see this problem in these cases but can occur if:
a) The diameter of the linking pipe(s) is too large
b) The length of the linking pipe(s) is too small (eg. Can happen if the drawing mode is set to "Schematic")
You might want to double check the modelled values for these to make sure neither of these are in error, as a typical water network with tanks that far apart wouldn't display that problem.
One strategy for battling tank flow/level convergence issues is to be really tight in modelling all entrance and exit losses to your tanks properly, and putting in the required diameters, lengths and especially minor loss coefficients for the inlets/outlets. This can significantly dampen, or even eliminate oscillating flows between tanks.
I concur with what Ben and Jesse have said.
Another factor to consider is the starting water levels in tanks that are in the same pressur zone. If their HGL values are very different, then water rushes from the higher to the lower tank (in terms of HGL) such that unless the time steps are sufficiently small, the lower tank then becomes the higher tank and water rushes back the other way.
Check your initial HGL values in the tanks and make sure that they are very similar.