Modeling Variable Speed Pumps (VSP) and Variable Speed Pump Batteries (VSPB) In Storm and Sewer Products

 Product(s): SewerGEMS, SewerCAD, CivilStorm Version(s): CONNECT Edition, V8i Area: Modeling

Problem

How to model a Variable Speed Pumps (VSP) and a Variable Speed Pump Battery (VSPB) in the storm and sewer products?

Background

Each solver can only use certain pump definitions and certain methods of modeling a VSP or VSPB.

VSP and VSPB (Storm and Sewer).zip

Solution

Variable Speed Pump (VSP)

Variable speed pumps are used in wastewater collection systems, usually to have pump outflow match wet well inflow to maintain a roughly constant wet well level.

Each of the Bentley solvers handles variable speed pumping somewhat differently. Be careful when switching solvers for models with variable speed pumps.

Because the different solvers use very different methods to handle variable speed pumps, the results will not exactly match between solvers but by adjusting input curves, some reasonable agreement can be achieved in most cases.

The different methods of modeling VSPs, for each solver in the storm and sewer products, are described below.

GVF-Convex Solver: VSP Pump Type

For the GVF-convex solver, variable speed pumps are solved with a true, pressure solver; the same solver used in WaterCAD and WaterGEMS. So, much of the content in this article also applies. You would create a pump definition corresponding to the full speed pump and assign that definition to a pump element. The definition could be any of those for pressure elements including 1-pont, 3-point, multipoint, standard extended and custom extended.

For the pump element, set the following properties:

• Is Variable Speed Pump? = True
• VSP Type = The appropriate type (usually Fixed head)
• Control Node = The node controlling the pump speed (usually the wet well if you want to maintain a fixed wetwell level)
• Hydraulic Grade (Target) = The level that will be maintained in the wet well
• When the control node is a wet well, the target HGL field is hidden and it assumes the target is the initial wet well level.
• Control on Suction Side? = Set to True if the control node is on the suction side (upstream side) of the pump.

The GVF-Convex solver will not only determine the flow from the pump but will also calculate the pump's speed, such that the target is met. It is also possible to set the speed as a function of time (VSP Type = Pattern Based) to calculate the flow and head given the speed, or alternatively specify a known flow (VSP Type = Fixed Flow) to calculate speed and head. Usually one pump in a station is run as a variable speed pump. When there are more than one running in parallel, the Variable Speed Pump Battery element should be used (see below).

Controls can also be used to turn a VSP on or off based on another condition. When the pump is on, it will continue trying to maintain the target by adjusting its speed.

A note on Wetwell as the control element: When selecting a wetwell as the "control node" for a VSP, the pump will attempt to adjust its speed such that the inflow equals the outflow, thereby maintaining a fixed level in the wetwell. It uses the current wetwell level as the target, ignoring the "Hydraulic Grade (Target)" field. For example if the initial elevation of the wetwell is 300 ft and the VSP is initially on, it will adjust its speed to maintain a constant 300 ft elevation (outflow matching inflow). If that VSP turns off any any point, the wetwell level may change and if the VSP turns back on, it will resume attempting to maintain the new constant level. For example if the wetwell increased from 300 ft to 302 ft and the VSP turned back on, it would start maintaining a constant 302 ft.

Implicit or Explicit Solvers: VSP with a Depth-Flow Pump Definition

With the Implicit and Explicit (SWMM) solvers, which solve the full St. Venant equations, the discharge from the pumps is represented as a function relating flow rate from the pump to the water level in the adjacent wet well. Usually the function should span the full range of water levels in the wet well such that when the wet well is empty, the flow should be zero while if the wet well is full, it should be the maximum flow that the pumps can deliver. When the wet well is at its typical level, the flow should roughly match the average inflow to the wet well. These solvers do not directly use the pump head characteristic curve, nor will they determine the actual pump speed. Under the Components > Pump Definitions, this type of pump is referred to as a "Depth flow (Simulated Variable Speed Pump)", which is sometimes called a "SWMM Type 4" pump.

In the GVF-Rational solver, pump curves are not used and flow into the wetwell equals flow out of the pump. This solver is not well-suited for pumps. See more here: How are pumps handled in each OpenFlows product?

Variable Speed Pump Battery (VSPB)

What is a Variable Speed Pump Battery?

Parallel VSPs represented by the VSPB element are operated as one group and led by a single VSP, the so-called lead VSP, while the other VSPs at the same battery are referred as to as lag VSPs. A lag VSP turns on and operates at the same speed as the lead VSP when the lead VSP is not able to meet the target head and turns off when the lead VSP is able to deliver the target head or flow.

From the standpoint of input data, VSPBs are treated exactly the same as single pump elements that are defined as variable speed pumps of the Fixed Head Type with one exception; the number of Lag Pumps must be defined in the Lag Pump Count field.

VSPB availability in the different solvers:

The Variable Speed Pump Battery (VSPB) element type is only available with the GVF-convex solver, for pressure networks.

The behavior of a VSPB cannot be captured with the GVF-rational solver if the pump is operated any way other than inflow matching outflow.

If you want to switch between the GVF-convex and either SWMM or DW with a VSPB, a different active topology must be used for the two types of models. Create an active topology which contains a VSPB element for GVF-convex scenarios and one which contains a pump that simulates a VSPB for SWMM and DW scenarios.

If there is an active VSPB element in a solver other than GVF-convex a fatal error message is issue and the run does not proceed.

More background information is available here: Benefits of using variable speed pump batteries (VSPB) element and how it works

GVF-Convex Solver: VSPB element

The VSPB element represents a set of identical parallel variable speed pumps which can be controlled to provide a fixed head or fixed flow or follow a time pattern of speeds.

The VSPB element represents multiple variable speed pumps that meet the following criteria:

• The VSPs are parallel with each other (not in-line)
• The VSPs are sharing common upstream (inflow) and downstream (outflow) nodes
• The VSPs are identical (have the same pump definition)
• The VSPs are controlled by the same target node and the same target head.

From the standpoint of input data, Variable Speed Pump Batteries are treated exactly the same as single pump elements that are defined as variable speed pumps of the Fixed Head Type with one exception; number of Lag Pumps must be defined in the Lag Pump Count field.

Relative Speed Factor
Reported for the lead pump. All operating lead and lag pumps will have the same Relative Speed Factor.

Number of Running Lag Pumps
Parallel variable speed pumps (VSPs) are operated as one group and led by a single VSP, the so-called lead VSP, while the other VSPs at the same battery are referred as to as lag VSPs. A lag VSP turns on and operates at the same speed as the lead VSP when the lead VSP is not able to meet the target head and turns off when the lead VSP is able to deliver the target head or flow.

As flow into the wet well increases, gradually additional flow is required from the pump. When the pump's relative speed reaches the value defined for the "Relative Speed Factor (Maximum)" (default is 1.0), an additional lag pump will turn on and the lead pump's relative speed drops. Similarly, when the inflow into the wet well drops low enough, a lag pump will be turned off and the remaining pumps will satisfy the required flow with a relative speed equal to the defined "Relative Speed Factor (Maximum)".

Controls
As of version 10.03.04.53, controls (such as on and off based on some condition) cannot be modeled with a VSPB and the VSPB will operate automatically based on the target and number of lag pumps. Reference #991158.

Pump Definition
Any that works with the GVF Convex Solver.
See: Which Pump Definition Types can be used by the GVF-Convex, Implicit and Explicit solvers?

Wet Well Depth
Is constant at the initial depth, which is set with the wet well property field "Elevation (Initial)".

Implicit and Explicit Solvers: Mimicking VSPB Behavior
VSPB behavior can be mimicked in the dynamic wave implicit (DW) and explicit (SWMM) solvers using the “Depth-Flow (Variable Speed)" (Type 4) pump definition.

This method would be modeled as explained in the above section "GVF-Convex Solver: VSP Pump Type" with one difference as explained below.

For example, suppose that a variable speed pump station with several pumps is operated to maintain a wet well depth of 5 ft. (outflow matching inflow). The inflow varies, but it is initially 0.4 cfs. and the maximum the station can pump is 3 cfs. The table of flow vs. depth in the pump definition might have values like the ones in the image below.

The final point should correspond to the maximum pump battery flow at the maximum wet well depth. It is advisable to cover the full range of wet well depths.

As soon as the inflow deviated from the previous value, the depth would change and the outflow would respond.