Off the Curve - Using Hydraulic Modeling Software to Identify Problems in Pump Stations and Force Mains

Purpose and Findings: When designing a pump station, most engineers follow state and local standards, many of which relay upon guidelines set forth in the 10 States Standards Recommended Standards for Wastewater Facilities. In fact, the successful design of a pump station includes the detailed analysis of several different elements of the pump station facility including the wet well, force main and pumps. Occasionally, these facilities are designed as part of master plan developments, and sometimes are designed to account for future flows. Traditional design methodology involves identifying a pumping rate and system curve, then selecting a pump with a curve that shows the pump can meet the flow demands as shown in the figure below. Figure 1 The graphs are used to select a pump that meets the required design operating duty point. In most cases this method is successful and these facilities go into operation with little to no problems. We now are tasked with a new host of challenges while rehabilitating existing and designing new pump stations. This is where things can go 'off the curve'. Rehabilitating existing stations: - Inadequate wet well or piping sizing - Underperforming or mis-sized pumps - Changes to operation upstream and/or downstream of the pump station Designing new pump stations: - Connections to existing pressure systems - Extreme topographical changes and high pressures - Potential for mixed flow and negative pressures in the line This presentation will focus on the benefits of using Pipe-flo hydraulic modeling software to help with these challenges the traditional methodology struggles to address. Case Study 1 – Bay Street Pump Station The Bay Street Pump Station (BSPS) is in the City of Pontiac and is owned and operated by the Oakland County Water Resources Commissioner (OCWRC). The pump station lifted flow to a gravity sewer that was beginning to corrode and was in an easement through private property. A conceptual project was developed to redirect the 20- inch BSPS force main to the recently constructed 36- inch Perry Street Pump Station (PSPS) force main. Preliminary hydraulic analysis determined that the existing pumps at the BSPS would not be capable of pumping the flow thru the redirected 20-inch BSPS force main and the 36-inch PSPS station. New pumps were needed at the BSPS, Fishbeck used Pipe-flo software to model the system, because the new pumps had to also be capable of pumping when PSPS was operating. The impacts to the hydraulics of PSPS also needed to be understood before making the connection in the proposed location. See below for the Pipe-flo model developed for BSPS. Figure 2 The Pipe-flo software allows for the development of 'lineups' which are different steady-state conditions calculated in the system at any given point. Like the traditional methodology, where we calculate for a peak flow rate, Pipe-flo allowed for the evaluation of the system when one or both pumps were on and with either facility operating. Multiple lineups under dry- and wet-weather conditions were run in Pipe-flo and both the proposed BSPS and existing PSPS pumps still met the design flow requirements for each facility. This output data was used to demonstrate to the Michigan State Department of Environment, Great Lakes and Energy (EGLE) that both facilities could satisfactorily utilize the same force main in our proposed configuration. This project has been fully permitted and is awaiting easements before bidding for construction fall 2021. Case Study 2 – 8 Mile Pump Station The 8 Mile Pump Station (8MPS) was built in 1965 as a lift station, which brought flow from an incoming 60-inch gravity sewer 50+ feet deep up through a recirculation chamber to a shallow 60-inch outlet. There are 5 pumps of various sizes to accommodate different levels of flow, and all 5 pumps run during wet weather events. When the 60-inch gravity outlet is at capacity, the flow overtops a weir in the recirculation chamber back into the PS wet well. This has historically led to rising water upstream of the PS and overflows to local waterways. A 30-inch relief force main was added later due to help reduce overflows, with 2 of the existing pumps piped to be able to discharge to it. Overflows continue even with the relief force main and additional improvements are needed at 8MPS. The current proposed solution is to convey more flow downstream to the system outlet and bypass the existing bottleneck at the 8MPS by eliminating the recirculation chamber and slip-lining the existing gravity sewer for 4,000 feet so it could act as a force main. The original concept as proposed intended to utilize existing pumps, but during the basis of design stage, the existing pumps were found to be underperforming. Fishbeck modeled the existing station using Pipe-flo and conducted flow verification testing in the field and found many of the five pumps to be operating well below capacity. The pump manufacturer assisted in developing degraded pump curves. These new curves were imported into Pipe-flo and the results showed that the existing pumps would not be acceptable and would fail to meet the goals of the project. Fishbeck then modified the Pipe-flo model to reflect a new 4-pump configuration. Pipe-flo was used to quickly assess the characteristics of the system using each set of pumps under dry- and wet- weather conditions, as well as how the pump station will operate under interim conditions during construction with different pumps or discharge outlets out of service. Pipe-flo was also used to assess the pressures and velocities in the existing piping where flow was being increased to verify that existing air and surge relief valves would meet the new service requirements. Had the existing methodology of using the manufacturer's pump curves and a new system curve been used, the existing pumps may have been assumed capable of meeting the project requirements only to realize the shortcomings during construction. Completing the modeling allowed for several configurations to be easily evaluated and potentially significant design decisions made earlier in the project schedule. This project is under design and moving forward with anticipated construction in 2022-24. Figure 3 Table 1 In each case presented above, using traditional methods of calculating the system curve and selecting pumps may have produced a satisfactory pump selection, but did not allow for deeper evaluation of some of the fringe operational conditions. It is during those unusual conditions when pumps are most susceptible to running off their curve and potentially incurring damage. if these conditions are identified, they can be mitigated through design by the selection of different pumps, addition of air/vacuum relief or throttling valves or even adjustments to wet well operating levels.