Why is the “lag pump count” field not available when selecting Fixed Flow as the “VSPB Type”?

Product(s): WaterGEMS, WaterCAD, HAMMER
Version(s): 08.11.XX.XX and 10.00.XX.XX
Area: Layout and Data Input

Problem

Why is the “lag pump count” field not available when selecting Target Flow as the “VSPB Type”?

How can the number of lag pumps of a fixed flow VSPB be specified?

Solution

Currently, lag pumps are not supported with the Fixed Flow option in the variable speed pump battery (VSPB) element.

When multiple regular pump node elements are in parallel and set to fixed flow, each pump will try to meet its defined "Flow (Target)" value.

For example, if pump 1 is set to be a variable speed pump (VSP) with a target fixed flow of 500 GPM, and pump 2 is set to be a VSP with a target fixed flow of 500 GPM, then there will be a sum of 1000 GPM flowing through both pumps (if they can provide it).

Lead/lag operation for fixed flow variable speed pumps in parallel is a very uncommon modeling situation.

See Also

Benefits of using variable speed pump batteries (VSPB) element and how it works

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