How can a vertical turbine pump, or a multi-stage pump be modeled in HAMMER, WaterCAD, WaterGEMS, SewerCAD or SewerGEMS?
A multi-stage pump has multiple impellers and therefore just adds more head than a typical pump. The pump curve (head vs. flow) should represent all of the stage together and can be inputted as a "Multiple Point" pump definition type.
The pump element in HAMMER can be used to represent centrifugal, axial-flow (single and double-suction) or multistage (including vertical turbines) pumps. So, modeling a vertical turbine pump shouldn't be any different than modeling any other pump in HAMMER.
You would simply lay out the pump in the network, ensuring that the Pump curve represents the first quadrant of your vertical turbine pump (for initial conditions purposes) and that the appropriate selections are made in the Transient tab of the pump definition. If you have a certain rated operating point in mind, one option would be to use the single-point pump curve type. If unsure of the correct Specific Speed to select, consider estimating it with the equation seen in the help or in this article. Then, select the closest specific speed from the list of defaults. If you feel that you need to enter the exact four-quadrant characteristic curve for your vertical turbine pump, see the instructions in the Help topic entitled "Pump and Turbine Characteristics in Bentley HAMMER" or in this article: Defining custom four-quadrant curves (specific speed) for pumps and turbines in HAMMER
If the vertical turbine is submerged in a well (submersible), you would connect it to a reservoir on the upstream side, with a short, frictionless pipe (for example large diameter and smooth friction coefficient). The reservoir "Elevation" would represent the water surface elevation (hydraulic grade) of the well. You will need to decide on a hydraulic grade to use at the reservoir, perhaps based on an average drawdown or a conservative estimate for transient modeling purposes, such as the minimum level. The "elevation (inlet/outlet invert)" field for the reservoir does not impact the hydraulics, as the hydraulics are based on the hydraulic grade, which is the "elevation" field. The "elevation (inlet/outlet invert)" field is only used to calculate pressure at the reservoir element and to show an appropriate pipe elevation line on profiles. It represents the elevation of the actual invert of the pipe connected to the reservoir. In the case of a submerged pump, it could be set to the invert of the pipe leaving the pump, so as to look appropriate in profile view.
A multi-stage pump has multiple impellers. The pump curve (head vs. flow) should represent all of the stage together, but the Specific Speed that you select in the Transient tab of the pump definition should be calculated for a single pump only.
Modelling of VT pumps - in Hammer (forum discussion)
Source of the default Specific Speed for pumps and turbines
Estimating the Specific Speed of a Pump or Turbine