| Product(s): | WaterGEMS, WaterCAD |
| Version(s): | All |
| Area: | Calculations |
When attempting to compute a model with pressure dependent demands (PDD), the user notification "Cannot solve network hydraulic equations" and/or "Network Unbalanced" occurs. Why? How can this be resolved?
Applying PDD adds more complexity to the calculations, as more iterations are typically required to converge.
First, confirm that the model converges without applying PDD, and resolve any other stability issues per the guidance here:
Troubleshooting the Network Unbalanced or Cannot solve network hydraulic equations
Second, assess the selection set you are using (in the calculation options) for PDD. Attempting to apply PDD to the entire network may not be realistic as typically only certain demands may be pressure dependent. Limit your PDD selection set to those elements.
Also, assess the PDD function and apply local PDD functions to certain zones or types of demands, to fine tune the PDD relationship (instead of applying a blanket PDD function to all elements)
Next, examine your model results (without PDD if needed) and determine if you have many junctions where the pressure is close to zero or very low. After doing this check your PDD functions to see if there are sharp changes in demand when pressure is gets close to zero or low and try to adjust the function or values if possible to avoid the sharp change. Check if if the reference pressure is set to be equal to the threshold pressure or not, try changing it. This quick change could be the difference between the model computing successfully or getting a network unbalanced user notification.
Lastly, ensure that you are using the most up to date version. Notably, major improvements to PDD convergence were introduced in a patch for version 26.00.00.514 (and are included in newer versions). Check your version number under File > Help > About and upgrade if needed. If using 26.00.00.514, contact Technical Support for the PDD patch. reference # 2034661