Troubleshooting Pressure Dependent Demands with a network unbalanced user notification

   
  Applies To 
   
  Product(s):
Bentley WaterGEMS, Bentley HAMMER, Bentley WaterCAD
  Version(s): 08.11.XX.XX
  Environment:  N/A
  Area: 
  Original Author: Mark Pachlhofer, Bentley Technical Support Group
   

Problem

I'm getting the user notification "Cannot solve network hydraulic equations" and/or "Network Unbalanced"  when attempting to compute a model with pressure dependent demands. How do I resolve this user notification? 

Solution

First start by making sure the parent scenario that runs the steady state or EPS is running without user notifications or a condition with no red user notifications and few yellow user notifications. This is to ensure that the pressure dependent demands (PDD) scenario will have a chance to run well because once you introduce PDD solving the network hydraulic equations become more complicated for the calculation engine. 

Secondly assess the zone or areas you have PDD assigned to an determine if there are junctions that need have local pressure dependent demands applied to them because the pressure vs. demand relationship is different than its neighboring junctions. Apply as many unique local pressure dependent demands as necessary. Also, keep in mind that pressure dependent demands are probably not going to be applied to all the junctions in your model and should just be applied to junctions whose demands you know are going to be pressure dependent based on some event that produces low pressure in your system. It's more likely the PDD will be junctions in a certain zone or area of the model.

Thirdly examine your junctions and note if you have an junctions where the pressure is close to zero. After doing this check your PDD functions to see if there are sharp changes in demand when pressure is gets close to zero and try to adjust the function or values if possible to avoid the sharp change. This quick change could be the difference between the model computing successfully or getting a network unbalanced user notification.  

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