Variable speed pumps (VSPs) can be run in parallel. This allows you to model multiple VSPs operated at the same speed at one pump station. To model this, one VSP is chosen as a "lead VSP", which will be the primary pump to deliver the target head. If the lead VSP cannot deliver the target head while operating at maximum speed, then the second VSP will be triggered on and the VSP calculation will determine the common speed for both VSPs. If the target head cannot be delivered while operating both VSPs at the maximum speed, then another VSP will be triggered on until the target head is met with all the available VSPs.

All VSPs that are turned on are operated at the same speed. VSPs are to be turned off if they are not required due to a change in demand. If all standby VSPs are running at the maximum speed but still cannot deliver the target head, the VSPs are translated into fixed speed pumps.

The number of available parallel VSPs at a certain time step may vary depending on the status (either initially or set by a control) of the VSPs and their discharge/suction pipes. For example an initially closed VSP cannot not be used until the VSP is turned on by a control. In addition, when a lag pump is turned on by a control, this doesn't necessary mean that the lag pump will run. It will only run if needed. An initially closed suction/discharge pipe also prevents the related VSP from turning on.

The main difference between a VSPB and a group of parallel VSPs is the possibility to control the number of available parallel VSPs over time using controls. It's possible to limit the usage of a specified pump for a certain time range or a tank level.

To correctly apply the VSP feature to multiple variable speed pumps in parallel, the following criteria must be met:

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.

Below is a list of user notification messages related to parallel VSPs with an explanation how to correct the incorrect model data: