Problem

How can a screw pump be modeled in SewerGEMS or SewerCAD?

Solution

A screw pump is a positive displacement pump so it will pump whatever flow comes in to the head necessary to reach the discharge. Theoretically, you really have two systems: the one upstream of the wet well and the one downstream. Typically screw pumps are used as lift from the wet well to the first treatment process at treatment plants or pumping over levees in a flood control situation. There are a few possible options for modeling these:

1) If you're using SewerCAD or the GVF-Convex solver in SewerGEMS, use the pump element with a fairly flat pump curve up to it's maximum capacity. This can be done using the multi-point pump curve type, in the pump definition. It is best not to make the curve perfectly flat, in the interest of numerical stability. In the calculation options for the GVF-Convex solver, set the "Use linear interpolation for multipoint pumps?" option to "True".

You may also need to determine if you are using an open screw or a closed screw as this will make a difference in how to determine the capacity after which the performance drops off. Contact the manufacturer for specifications (which will not consist of the same type of information typically seen with a centrifugal pump)

2) If you're using the Implicit or Explicit numerical solver in SewerGEMS, use the pump element either with a very flat pump curve using the multi-point type, or use the Depth-flow pump definition type.

3) Since you have two separate systems in theory, you could model the upstream wet well as an outfall and model the discharge of the screw pump as an inflow into the downstream network (with no physical connection between the two). That may be easier and more accurate than trying to force a positive displacement pump to fit the description of a centrifugal pump.

Note: a progressive cavity pump is not a true screw pump and can actually pressurize a pressure pipe.

See Also

Modeling positive displacement pumps in HAMMER