Troubleshooting stability problems with PRVs or PSVs in parallel

  Applies To 
  Product(s): WaterGEMS, WaterCAD
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Mark Pachlhofer, Bentley Technical Support Group


When computing a model with multiple PRVs or PSVs in parallel, a user notification indicates that the network is unbalanced at one or more timesteps. When only one PRV/PSV is active, the model computes without error.


If the PRVs or PSVs that are in parallel have pressure or HGL settings that are close together, the numerical solver may have trouble converging on a balanced solution. When trying to meet the pressure setpoint of one valve, it changes the pressure sensed at the other valve, which tries to adjust itself to meet its setpoint, which can then effect the pressure at the other valve, which then tries to adjust itself again - this process can oscillate between trials and cause the numerical solver to become unbalanced when it reaches the maximum trials specified in the calculation options.


Option 1

Combine the valves into a single valve that uses a hydraulic grade or pressure setting that is an average of the two valve settings. Then you could consider increasing the size of the pipe  directly connecting to the upstream and downstream side of the valve (for example double the diameter).

Option 2

If you'd still like to keep both valves in your model you could choose either the higher initial setting, lower initial setting, or a setting that is an average of both (as seen in option 1) and just assign that initial setting to both valves. This is similar to option 1 because we are assuming that downstream of these valves where the pipes converge there is some average of the hydraulic grade (pressure).  

Option 3

In recent versions, the ability to use the WaterGEMS version 2.0.12 numerical solver version is available, which has some enhancement specific to this type of situation. If you are not already using it, set the Engine Compatibility calculation option to version 2.0.12 and try adjusting the new additional calculation options that are exposed. See the link below for more.

See Also

Engine Compatibility Modes and related Calculation options

Troubleshooting the Network Unbalanced or Cannot solve network hydraulic equations user notification