Model Pump Station Force Mains in SWMM

A previous post about how to model a pump station can be found here and this post is about how to model a force main on the discharge side of a pump station.

A force main is a pressurized pipe that conveys sanitary sewer or storm water under pressure from the discharge side of pump stations (Figure 1). In SWMM modeling, a pump is a type of link which requires an inlet node and an outlet node. Most of the time, modelers assign the pump station wet well as the inlet node (model it as a storage node) and choose the highest discharge side manhole as the outlet node. The force main in between is usually not modeled explicitly and its friction loss is to be incorporated into the pump curve (refer to Figure 3 of the previous post about pump station). For a storm sewer network shown in Figure 1, the inlet node is the wet well while the outlet node is the manhole on the right, and the force main, the pipe with a negative slope, is usually not modeled in SWMM.

Some study or design tasks may require a force main to be modeled explicitly in SWMM. A conduit needs to be defined as force main (Figure 2) and two equations are available to choose from to calculate its friction loss for pressurized flows: Hazen-Williams and Darcy-Weisbach, which are found under the settings of Simulation Options –> Dynamic Wave. The inlet and outlet nodes of a force main require a large surcharge depth value so pressurized flow can happen inside the pipe.

It is worth noting that a pipe under pressurized flows does not necessarily need to be a force main in SWMM, and it can just be a normal circular pipe – under this case, Manning’s Equation is used for friction loss calculation for pressurized or free surface flows.

To demonstrate the two different ways to model a force main, namely model it explicitly or incorporate its impact into the pump curve, the sewer network shown in Figure 1 is modeled in PCSWMM (Figure 3).

The two networks in Figure 3 are exactly the same except the way how the Force Main C11 with a negative slope is modeled. The pump curve of Pump1 is modified by subtracting the C11 friction losses calculated by Hazen-Williams equation and saved as a new pump curve for Pump2 (Figure 4). In the two modeled networks, the first (C10/C20) and last conduits (C13/C23) are defined as normal circular pipes and Junction J11, J12, J13, J22, and J23 are supplied with a very large surcharge depth .

The two models behave similarly and their results are almost identical regarding HGL, flow rates, and the depths in wet wells, which is not a surprise (Figure 5 to Figure 8).