Modeling a variable speed pump that changes speed during a transient simulation

  Product(s): Bentley HAMMER
  Version(s): 08.11.XX.XX, 10.00.00.40
  Environment: N\A
  Area: Modeling

Problem

How can I model a variable speed pump that automatically changes speed to meet a target head or flow, during a transient simulation?

OR:

Variable speed pump not changing speed automatically to meet target head or flow during a transient simulation.

Solution

The dynamic calculation of relative speed factor to maintain a target head or flow only applies to the initial conditions (steady state or EPS). During the transient simulation, the pump will not dynamically change its speed to maintain the target head or flow. This is mainly because a pump usually won't react fast enough during a fast-changing transient condition (and it would require information on how fast the reaction can occur).

Consider the below options.

Option 1 - Assume Constant Speed

During a transient simulation, the variable speed pump may not react quickly enough to shift speed. Also, such a shift in speed may not have a significant impact on the transient results.

Option 2 - Manual Operating Rule

The first option would be to observe the starting and final speed and configure a transient pattern of manually shift between those two speeds during the transient simulation.

1) Compute initial conditions (steady state) and note the calculated Relative Speed Factor
2) Compute another steady state run with the system set to model the final conditions (and the VSP still set as a VSP). In other words, if the transient event in question is a valve closure, then set the initial status of the valve to closed. If the event is an emergency pump shutdown, set the pump to off (this only works for pumps if there is a downstream boundary, else demands will not be satisfied).
3) Note the new calculated relative speed factor, then calculate the multiplier necessary to shift between them. For example if the first Relative Speed Factor was 0.6 (step 1) and the second was 0.786, the multiplier would be 1.31.
4) Configure a transient pump pattern that starts with a multiplier of 1.0 and shifts to a multiplier of 1.31 by the end of the simulation. The duration over which this transition occurs depends on exactly when the VSP's target head/flow starts to change during the transient simulation, and based on your own judgment of how fast the VSP will be able to react.
5) In the properties of the pump, set the transient pump type to variable speed/torque with the control variable set to Speed and select the pattern you create in step 4. Also set the Initial Relative Speed Factor property equal to the value observed in step 1 (for example 0.6) Note that this initial relative speed factor multiplies against the "full speed" entered in the Transient tab of the pump definition, to determine the initial rotational speed of the pump during the transient simulation.
6) Observe the transient results of the flow or head at the target element and adjust the final pump multiplier in the pattern as needed. Also check the pump Speed result in the Extended Node Data tab of the Transient results Viewer.

Note that if there is a tank level change or other change occurring in the simulation during that period, it can skew the results (in that the final steady state check that you did in step 2 would not have accounted for that), so the multiplier required may be different.

Option 3 - Assume a boundary Head of Flow

If the speed change that would normally occur in the VSP during the transient event happens slowly enough that a constant head (in the case of Target Head) or constant flow (in the case of Target Flow) can be assumed downstream of the pump, then consider replacing the pump and upstream network with a head or flow boundary.

Target Head VSP: In the case of a Constant Head (target hydraulic grade) VSP, morph the pump node into a reservoir and set the reservoir elevation such that the target head is maintained at the target node. Typically the target node is directly downstream of the pump, in which case the reservoir boundary HGL can be used at the pump location to maintain it.

Target Flow VSP: In the case of a Constant Flow (target flow) VSP, morph the pump node into a Periodic Head-flow element. In the properties of the Periodic Head-flow element, choose "false" for "Sinusoidal?", select Flow as the Transient Parameter, then open the Flow Pattern collection. For a constant flow, define two rows, one for time zero and the other for a time greater than the simulation duration. Enter the desired constant flow for both rows, with a negative sign (inflow).

    

You'll need to use your own judgment to decide of the hydraulics of the pump and upstream network may still have an effect on the transient results (in other words, if it's OK to assume a boundary element).

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