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. Users need to be careful switching solvers for models with variable speed pumps.

With the implicit and explicit solvers, which solve the full St. Venant equations, the discharge from the pumps is represented as a function relating flow rate from the pump(s) 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.

For the GVF-convex solver, variable speed pumps are solved with a true pressure solver. The user 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. In the pump element, the user would set "Is Variable Speed Pump" to True, "VSP Type" to the appropriate type (usually Fixed head), "Control Node" to the node controlling the pump speed (usually the wet well), "Hydraulic Grade (Target)" to the level that will maintained in the wet well and "Control on Suction Side" to True if the control node is on the suction side of the pump. When the target element of a VSP is a wet well, the target HGL field is hidden and it assumes the target is the initial wet well level. The convex solver will not only determine the flow from the pump but will also calculate the pump's speed. 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) t 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.

In the GVF-Rational solver, the pump uses the pump curve provided in the pump definition without any adjustment for speed.

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