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

Background

This article covers how variable speed pumps are modeled in WaterGEMS and WaterCAD, as well as how the different types of variable speed pumps operate. Further details can be found in the Help documentation and the Advanced Water Distribution Modeling and Management book.

The characteristic curve for a pump is fixed for a given motor speed and impeller diameter. However, this characteristic curve can be determined for any speed and any impeller diameter by applying the affinity laws. For variable speed pumps, these affinity laws are presented as:

where Q = Pump flow rate (m3/s, cfs), h = Pump head (m, ft), and n = Pump speed (rpm)

The application of the affinity law is the property field Relative Speed Factor. The relative speed factor is the ratio of the pump's actual speed to some reference speed. Typically, the reference speed is the full speed of the motor. For example, if the pump speed is 1558 rpm while the motor is a 1750-rpm motor, the relative speed factor is 0.89. This relative speed is used with the pump affinity laws to adjust the pump head characteristic curve to model the pump, making it easier for you to model changes in speed for given conditions, like constant flow or constant head.

Note: If you are using a steady state run and you know the relative speed factor, the speed of the variable speed pump can be set in the pump properties. However, if the conditions that control the pump are not known at the start or an EPS run is being made, then variable speed behavior must be described in more detail.

Variable Speed Pump Types

There are few different variable speed pump types: Fixed Flow, Target Head, and Pattern Based. To set a pump as a variable speed pump, open the pump properties and set "Is Variable Speed Pump?" to True. When you set this, new property fields will be available based on the type of variable speed pump you are modeling.

Fixed Flow

A fixed flow VSP will adjust the pump curve, changing the relative speed factor, so that a given flow is generated by the pump. After setting the VSP Type to "Fixed Flow", you can enter a Flow (Target). You can also adjust the maximum relative speed factor value. A value of 1.0 means full speed.

It is not possible to change the target flow value as a logical control. See this link for details and workarounds: Is it possible to a model a fixed flow VSP where the flow changes over time?

Note : Sometimes fixed flow VSP pump(s) provides more flow than specified as the flow(Target) in the properties, in that case look at the hydraulic grade of the pump(s) vs. the hydraulic grade of the downstream elements.

If the hydraulic grade at the pumps is higher than the downstream hydraulic grade then it means the pumps need to operate at a higher flow rate because the flow could really be transferred by gravity. In many cases the pumps may be showing operating at the zero head point or a near zero head point.

Target Head

If you need the variable speed pump to attain a target hydraulic grade or pressure value at some point in the model, set the VSP Type to "Target Head." There are a few new fields that will be generated.

First, you will need to select the control node. This will be the node associated with your target head value. This node can be on the discharge or suction side of the pump. There is a field where you denote if the control node is on the suction side of the pump. Tanks can be used as the control node; the pump will try to maintain a given elevation in the tank if a tank is used as the control node. To select this, click the Select option. This will open a Select tool bar so you can select the element.

You can choose between using hydraulic grade or pressure as the Target Head Type. Depending on which field you choose, you would then enter a target hydraulic grade or pressure value for your control node. You can also enter a maximum relative speed factor; a value of 1.0 is full speed.

It is possible to adjust the target pressure or hydraulic grade using controls. See this link for details: Using controls to change the target pressure / HGL setting of a variable speed pump.

With this VSP type, the pump will attempt to adjust its speed in order to maintain the target head or pressure at the target element (or a constant water level in a tank, matching inflow with outflow, when a tank is set as the control node).

Ensure that the pump has a direct influence over the hydraulic grade at the target element. If for example other things happening in the network can interfere with the pump's influence on the target node, or if the target node is in a separate pressure zone (for example a tank in between), then the pump may not be able to meet the target HGL and will produce unpredictable results.

Pattern-Based

If you know how the relative speed factor for the pump will change with time, you can use the VSP Type "Pattern Based." You will be able to create a pattern that will apply a multiplier to the initial relative speed factor.

To create the pattern click the Pattern (Relative Speeds) field and click Edit. You can also go to View > Patterns. In the Patterns manager, right-click on Pump and choose New. Create a pattern based on time. This will be a multiplier based on the initial relative speed factor and not the relative speed factor itself.

Once the pattern is created, you should see it available in the pull down menu for the Pattern (Relative Speed) field.

Direct Pumping

"Direct pumping" refers to the use of a variable speed pump in a closed system that does not have any storage (only demands downstream). Fixed head would be used as the variable speed pump type, and the affinity laws will scale the pump curve such that the sum of the downstream demand is provided while adding a head that results in the desired target head at the target node.

Note that with a closed system, pressure-dependent demands would need to be used in order to produce a valid system head curve, as described here: System head curves with no downstream storage (closed system)

Known issue with 2.2 solver version

Please note that there is a known issue with fixed flow VSPs and the WaterGEMS Engine Compatibility 2.2.0 solver version in certain versions of WaterGEMS and WaterCAD. In this issue, the Relative speed factor is not calculated and it causes the 'Pump cannot deliver....' User Notification error. In this case, the pump delivers water but the calculated speed is zero. This is happening only in WaterGEMS 2.2.0 engine. The issue has been addressed in the 2023 release (version 23.00.00.16 and higher) Reference #1213133. To resolve this issue, either upgrade or use the WaterGEMS 2.00.12 engine compatibility setting as a workaround.

Using Variable Speed Pumps in WaterGEMS and WaterCAD