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User notification is generated after computing a model with parallel variable speed pumps (VSPs) about the pumps or "Network Unbalanced" or "Cannot compute network hydraulics"

 

  Applies To 
  Product(s):
Bentley WaterGEMS, Bentley HAMMER, Bentley WaterCAD
  Version(s): 08.11.XX.XX
  Original Author: Mark Pachlhofer and Scott Kampa, Bentley Technical Support Group

Problem

User notification is generated after computing a model with parallel variable speed pumps (VSPs) about the pumps or "Network Unbalanced" or "Cannot compute network hydraulics"

Background

Variable speed pumps have special rules because of the way they operate which make user notifications with them common. Please be assured we work on improving there functionality with future versions of the software whenever possible.

Solution

These are the VSPs rules that must be followed, which are outlined in the help document that comes with the WaterCAD, WaterGEMs, and HAMMER software titled "Parallel VSPs":

  1. Parallel VSPs must be controlled by the same target node.
  2. Parallel VSPs must be controlled by the same target head.
  3. Parallel VSPs must have the same maximum relative speed factors.
  4. Parallel VSPs must be identical, namely the same pump curve.
  5. Parallel VSPs must share common upstream and downstream junctions within 3 nodes (inclusive) of the pumps in order for them to be recognized as parallel VSPs.
  6. All upstream pipes should have the same diameter, roughness, length and minor loss coefficient, the same for all downstream pipes within the parallel VSP group. As opposed to the first five criteria a difference in these attribute values will not stop the calculation run. Only a warning user notification is generated for each attribute with at least one deviation. Note that the results within the suction and the discharge junction of the parallel VSP group will not be completely correct in this case.

-- Parallel VSPs need to be modeled with the same pump curve and the same control node. If this cannot be done in a model, there are two workarounds you can try:

a. Instead of modeling them as parallel VSPs, model one VSP at one station and then set the relative speed factor of other pumps by using logical controls.

b. Set the relative speed factor of all the pumps by using the logical controls.

Examples

This setup in a model may cause problems because the pumps are not all connected to the same upstream and downstream node. I this case you might get a "Network Unbalanced",  "Cannot compute network hydraulics", or "Variable speed pumps not in parallel cannot be controlled by the same control node." user notification.

The setup for the VSPs that will compute looks like this:

See Also

How do I model parallel fixed head (target head) variable speed pumps that are controlled by flow in a downstream pipe?

  • Created by
  • When: Mon, Mar 21 2016 3:25 PM
  • Last revision by
  • When: Mon, Jul 31 2017 12:28 PM
  • Revisions: 5
  • Comments: 0
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