Problem

Can you run a VSP with a Control node and Logical Controls at the same time?

Solution

VSP Interactions with Simple and Logical Controls

The VSP model and its APEX (Automatic Parameter Estimation eXtension) algorithm have been designed to fully integrate with the simple and rule based control framework within WaterGEMS and WaterCAD. The VSP APEX algorithms override what would "normally" happen, and creates its own artificial "water balance" and pipe network. For more details on the lead and lag pump operation with APEX, see: Using Variable Speed Pumps in WaterGEMS and WaterCAD

You must keep in mind that the definition of controls requires that the state (On, Off, Fixed Speed Override) and speed setting of a VSP be properly managed during the simulation. Therefore, the interactions between VSPs and controls can be rather complex. We have tried to every extent possible to simplify these interactions while maintaining the power and flexibility to model real world behaviors. The paragraphs that follow describe guidelines for defining simple and logical controls with VSPs.

Pattern Based VSPs

The pattern of relative speed factors specified for a VSP takes precedence over all simple and logical control commands. Therefore, the use of controls with pattern based VSPs is not recommended. Rather, the pattern of relative speed factors should be defined such that control objectives are implicitly met.

VSPs with APEX (target head or pressure)

A VSP can be switched into any one of three different states. When the VSP is On, the APEX algorithm will estimate the relative speed sufficient to maintain a constant pressure head at the control node. When the VSP is Off, the relative speed factor and flow through the pump are set to zero, and the pressure head at the control node is a function of the prevailing network boundary and demand conditions. When the control state of a VSP is Fixed Speed Override, the pump will operate at the maximum speed setting and the target head will no longer be maintained. The Temporarily Closed state for a VSP indicates that the check valve (CV) within the pump has closed in response to prevailing hydraulic conditions, and that the target head cannot be maintained. The VSP control node can be specified at any junction node or tank in a network model. As described below, however, the behavior of simple and logical controls depends on the type of control node selected.

Junction Nodes

When the VSP control node type selected is a junction node, the VSP will behave according to some automatic behaviors in addition to the controls defined for the pump. If the head at the control node is above the target head, the pump state will automatically switch to Off. If the head at the control node is less then the target head, the pump state will automatically switch to On. The VSP will automatically switch into and out of the Fixed Speed Override and Temporarily Closed states in order to maintain the fixed head at the control node and prevent reverse flow through the pump. Additional controls can be added to model more complex use cases.

Tanks

When the VSP control node is a tank, you must manage the state of the pump through control definitions, allowing for flexible modeling of the complex control behaviors that may be desired for tanks. If a VSP has a state of On, the pump will maintain the current level of the tank. For example, at the beginning of a simulation, if a VSP has status of on it will maintain the initial level of the tank (i.e the VSP’s target elevation/head will now be level of tank when the VSP turns on). For example if the initial elevation of the tank is 300 ft and the VSP is initially on, it will adjust its speed to maintain a constant 300 ft elevation (outflow matching inflow). As the simulation progresses and the pump happens to turn off, temporarily close, or go into fixed speed override, the level in the tank will be determined in response to the hydraulic conditions prevailing in the network. When the VSP turns on again, it will maintain the current level of the tank, not the initial level. Thus control statements must be written that dictate what state the pump should switch to depending on the level in the tank. A pump station with a VSP and a fixed-speed pump operating in a coordinated fashion can be used to model tank drain and fill operations.

See Also

Using controls to change the target pressure / HGL setting of a variable speed pump