Modeling a Complex Manifolded Pump Station Sewer System with Parallel Pressure Pipes with Variable Vacuum and Pressurized Conditions

Hydraulic models are used throughout the United States and worldwide to plan sewer infrastructure improvements and evaluate system operations. Increasingly, utilities include force mains and pump stations in the hydraulic model and use them to evaluate upgrades for existing pump stations and development of new pump stations. Modeling force mains that can be intermittently under pressure, half full, or in vacuum in the sewer system is a challenging aspect of the model build process with various modeling applications. Orange County Sanitation District (OC San) provides wastewater collection, treatment, and recycling for approximately 2.6 million people in central and northwest Orange County, CA. OC San owns, operates, and maintains 4 wastewater pump stations which are interconnected via 2 parallel force mains (Newport Force Main, NFM) along the Pacific Coast Highway (PCH) in the City of Newport Beach, CA. The NFM network, shown in Figure 1, consists of three-manifold pump stations, Bay bridge Pump Station, Rocky Point Pump Station, Lido Pump station, and parallel force mains that convey wastewater to the Bitter Point Pump Station. The Bay Bridge Pump Station (BBPS), the farthest upstream pump station of the NFM network, is currently in design for its replacement. As part of the Bay Bridge Pump Station replacement project, OC San and Arcadis (OC San's engineering consultant for this project) developed a new hydraulic model of the NFM network using InfoWorks ICM (10.0) software to select new pumps at the pump station. The Infoworks ICM modeling application has two modeling options for modeling sewer force mains. Both of these options, listed below, can result in different hydraulic profiles, which can affect the pump selection for the BBPS. - Pressure Solution: The 'Pressure' solution model uses pressurized pipe equations when the pipe is fully pressurized. The model reverts to the gravity solution whenever the pipe is not full during low flow conditions and subsequently uses gravity solution for the rest of the simulation. - Force Main Solution: The 'Force Main' model is an advanced solution to model pressurized pipes, which assumes the pipe is always full and uses the pressurized pipe equations for all flow regimes during the simulation. Thus, the 'Force Main' solution model allows suction pressures to develop in the pipes and accounts for siphoning effect during the simulation. The NFM network is relatively flat with local high points and minimal air release. Therefore the total dynamic head for pumps at BBPS is primarily correlated to headloss and can be subject to localized siphons and changes to gravity flow. To determine the potential affects of siphons and flow regime changes, OC San analyzed the NFM hydraulics using both the Pressure Solution and Force Main Solution under low flow and high flow conditions to determine most accurate representation of the NFM network. The results are shown in Figures 2 and 3. - The Force Main solution results in the lowest hydraulic grade line (HGL) during the low flow conditions (Figure 2) because Force Main Solution accounts for a siphoning effect that can happen when pipelines with local high points have HGL below the crown of the pipe. The siphon pulls the water over the high point, resulting in a lower grade line for points upstream of the high point. - The Pressure Solution results in a low HGL compared to the ForceMain solution during the high flow conditions. The Pressure Solution reverted to a gravity solution during the simulation that can happen when the pipe is not fully pressurized. The gravity solution underpredicted flow attenuation and a low flow velocity, resulting in lower head-loss and low HGL. OS San selected the Force Main Solution to model the NFM network for the Bay bridge Pump Station design. The Force Main Solution is expected to provide the most accurate model of the NFM network and is the most conservative, requiring a lower total dynamic head (TDH) for consideration during pump selection.