How can I model variable speed pumps (VSP’s) in parallel?
How do parallel VSPs work?
For general information on Variable Speed Pumps, see: Using Variable Speed Pumps in WaterGEMS and WaterCAD
VSP’s in parallel work on the principle of “lead” and “lag”. For a certain number of VSP’s in parallel, there will be one pump which functions as the “lead” VSP which first turns on to attempt to meet the target pressure or flow. If the lead VSP cannot satisfy the target head or flow, despite running on the user entered maximum relative speed (which defaults to 1.0 meaning full speed), then the next VSP will be triggered to assist the “lead” VSP. This second pump is known as a “lag” VSP. The VSP calculation determines the common speed for both VSP’s. When both the VSP’s cannot deliver the target head within the maximum relative speed factor, the third VSP (if present) is triggered to turn on. This will continue for all the VSP’s in parallel until the target is satisfied.
It is not necessary that all the VSP’s will be triggered. If the target head is satisfied then only the required number of VSP’s will run. In case all the VSP’s are running at maximum speed and still not able to deliver the target head, then they are considered as fixed speed pumps. This is referred to as “fixed speed override”.
The above logic occurs automatically when multiple VSPs are detected as being in parallel, with the same pump definition and target.
The following demonstrates how to model variable speed pumps (VSP’s) in parallel.
VSP’s can be modeled in parallel like ordinary pumps. In the pump properties there is a field “Is Variable Speed Pump?”. By setting this field to “True” the pump would be treated as a variable speed type.
Once the status is changed to “True” some additional properties appear. Here, the type of VSP should be specified in the “VSP Type” field. There are three types of VSP’s
This would be the typical setup of parallel VSP’s;
Notice that the start and the end node are the same for all VSP’s. This is important as the software recognizes this configuration as parallel VSP’s.
Sometimes there are multiple junctions between the individual VSP’s and the common node for all VSP’s. A user notification is generated when such parallel VSP’s are run; “Variable speed pumps not in parallel”.
In such a case a common node must be defined so that all VSP’s are identified in parallel. The following article explains how to resolve such an error.
How do I model parallel fixed head (target head) variable speed pumps that are controlled by flow in a downstream pipe?
Once the pumps are setup in parallel, each pump needs to be defined as a variable speed type pump with a target head. It should be noted that, the pump definition (pump curve), maximum relative speed factor, target head/flow and target node for all the VSP’s in parallel should be the same, otherwise they would not function as parallel VSP’s.
Criteria for parallel VSP’s
When configuring the VSP’s in parallel you have the option to assign them to a Pump Station like any other pump setup. However, you can use the variable speed pump battery element (VSPB) to model VSP’s in parallel. The benefits of using this are outlined here.
Fixed Flow VSP's
A fixed flow VSP can be defined in the VSP type field of pump properties.
The Relative Speed Factor is typically kept as 1 because the pump typically cannot operate beyond its maximum speed. However, it can be changed as per operating conditions.
The “Flow (Target)” field takes the input of the fixed flow at which the VSP is desired to operate. The relative speed is then adjusted to meet the fixed flow specified.
However, the lead and lag principle of operation of parallel VSP’s will not function with Fixed Flow VSP’s or Variable Speed Pump Battery (VSPB) because each VSP will try to maintain its own “Fixed Flow”. More information about this can be found here.
Help Section of WaterGEMS. Modeling Variable Speed Pumps > Parallel VSP’s
Using Variable Speed Pumps in WaterGEMS and WaterCAD
Modeling variable speed pumps in parallel with different pump curves
Modeling fixed head parallel VSPs that have different head values
Modeling multiple VSPs where only one turns on at a time
The headloss in a pipe downstream of a parallel VSP is higher than expected
Why is the “lag pump count” field not available when selecting Fixed Flow as the “VSPB Type”?