One Forcemain – Multiple Sewage Pumping Stations
The sewage collection system for the community of Port Perry Ontario includes several sewage pumping stations conveying wastewater to the existing aerated sewage lagoons for treatment. Initially the system originally included two sewage pumping stations pumping from the Community directly to the lagoons; the Water Street Pumping Sewage Station and the Cawker's Creek Sewage Pumping Station. Sewage from the stations were conveyed using two twinned 450 mm diameter polyethylene forcemains with each station pumping into a separate forcemain.
During the review of sewersheds in the area and servicing of new development, a third pumping station (Reach Street Sewage Pumping Station) was proposed. The Reach Street Sewage Pumping Station would have an initial capacity of 30 L/s with an ultimate capacity of 120 L/s. In order to ensure proper flushing velocities were obtained within the forcemain without excessive headosses, two forcemains were proposed for the ultimate pumping station capacity. Initially one forcemain would be constructed and then as the station capacity was upgraded beyond 60 L/s a second forcemain would be constructed. This approach would have resulted in 4 separate forcemains.
In order to reduce costs of the Reach Street Sewage Pumping Station construction and to manage velocities within the forcemains, an analysis was undertaken to determine if the three pumping stations could be serviced using the existing forcemains during both normal operation (two forcemains in operation) and under emergencies (one forcemain in operation).
Forcemain And Sewage Pump Design
In Ontario good design practice is to select a forcemain diameter which will ensure that the sewage velocity within the forcemain Analysis is between 0.9 m/s (3 ft/s) and 2.6 m/s (8.5 ft/s). Maintaining minimum velocities ensures that solids in the raw sewage do not settle within the forcemain. Solids settling within a forcemain can be a source of odours and also reduce the forcemain capacity due to a reduction in the effective volume. Limiting the velocity to a maximum velocity ensures excessive headlosses are not developed within the forcemain reducing pumping costs. As well, the higher the velocity, the greater the potential that exists for forcemain transient pressures.
Sewage pumps are designed to pass solids within the sewage and in order to have this functionality require impellers with relatively large passages. These large open passages work well is pumping solids at high flows and low Totally Dynamic Head (TDH). However, they are relatively inefficient in pumping at higher head applications.
Fixed speed centrifugal sewage pumps have performance curves in which as the TDH of the system increases, the output of the pump decreases. As the backpressure in a forcemain increases, the output of the sewage pumping station will decrease. The headloss in a forcemain is proportional to the square of the flow, thus if the flow increases by a factor of two, the headloss increases by a factor of four for a given forcemain size. Thus, multiple sewage pumping stations discharging into a common forcemain at the same time can substantially reduce the output of the individual stations due to the headloss resulting from the combined flow and resultant headloss.
Due to the nature of a centrifugal pump curve if the TDH that the pump is pumping against is low there is a risk to the pump operation due to vibration and/or the pump running out.
Based on the above, the design of a forcemain and the resultant pump selection must be balanced to ensure the sewage pumping station will operate acceptably under all potential flow scenarios.
The development of a“system curve”for a single pumping station discharging into a single pipeline is relatively straight forward. This system curve is overlaid by the specific pump curve and the flow that the station will operate at can be determined.
With multiple pumping stations each with multiple pumps discharging to a single forcemain, the analysis becomes dynamic. The flow from each pump effects the overall system headloss, which then impacts the pump curve and the actual pump output. The system analysis is based on balancing TDH and flows for each of the stations based upon each flow scenario analyzed.
In order to review the operation of the three pumping stations discharging to both single and multiple forcemains, water distribution system modeling techniques were used. In this technique, a hydraulic model of the pumping stations and forcemains was developed using the program EPANET Ver 2.0. In the model the actual pump curves for the stations along with wet well operating elevations were used along with proposed Reach Street pump curves. The forcemains were added including the discharge elevations of the forcemains at the lagoons with ground elevations incorporated at key points. A series of valves were included in the model which allowed pumping stations and forcemains to be turned on or off depending upon the scenarios to be analyzed.
When the hydraulic model was then run for various scenarios and provided the dynamic analysis of the system identifying the actual TDH experienced by each station and the corresponding station output.
The results of the analysis showed that with multiple pumping stations discharging into a single forcemain, the actual output of the stations would be reduced. In order to manage this reduction in flow higher capacity pumps could be installed or larger wet wells used to retain peaks flows for short periods of time.
The dynamic analysis allowed the pump curve for the new Reach Street Pumping station to be selected so that the station would operate effectively when discharging by it's self into a dedicated forcemain, or in tandem with both of the two existing stations operating at the same time.
With one of the stations, the resultant combined system headloss was predicted to be higher than the actual head capacity of the pumps and would have resulted in the station not being able to convey flow. In this case, an interlock was proposed to not allow this station pump while the other two stations were on and pumping.
Hydraulic sewage pumping station analysis is complex when several stations discharge to a common forcemain. In the analysis, the headloss and flow for the system must be balanced for all pumping stations. Using dynamic water distribution system modeling is an effective tool to analyze sewage pumping station networks to complete the calculations required for these systems.
A conservative approach to pumping station and forcemain design is to have each pumping station discharge to its own dedicated forcemain. Combining pumping stations into single forcemains can reduce forcemain costs. As well, redundancies can be provided if multiple forcemains are configured so that multiple pumping stations can discharge into a common forcemain if necessary.
Document Type: Research Article
Publication date: 2007-01-01
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