Product(s):	WaterGEMS, WaterCAD, HAMMER
Version(s):	CONNECT Edition, V8i
Area:	Modeling

Problem

How can I model variable speed pumps (VSP’s) in parallel?

How do parallel VSPs work?

Background

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.

Solution

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

Target Head – The most commonly used VSP type. Specify the target pressure/hydraulic grade to be maintained at a junction downstream of the parallel VSP’s. If a tank is specified, it will attempt to maintain a constant hydraulic grade in the tank.
Fixed Flow – Specify the desired fixed flow from the pump
Pattern Based – Specify a specific pattern of speed over time. Enter relative speed factors/multipliers over time (typically not greater than 1 since the speed of the motor typically cannot exceed maximum speed.)

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.

Note:

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

Parallel VSP’s must be controlled by the same target node
Parallel VSP’s must be controlled by the same target head
Parallel VSP’s must have the same maximum relative speed factors
Parallel VSP’s must be identical, namely the same pump curve
Parallel VSP’s must share common upstream and downstream junctions within 3 nodes (inclusive) of the pumps for them to be recognized as parallel VSP’s.
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.

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.

Modeling variable speed pumps (VSP’s) in parallel

Problem

Background

Solution

See Also