Reducing energy demands in municipal wastewater infrastructure continues to grow in importance, including wastewater collection systems. In high-head pumping applications that result from long force mains or high static heads from hilly terrains, the designer and municipality must evaluate numerous factors to efficiently overcome friction losses and achieve installation with reasonable capital costs.
Competitive bidding induces quoting the smallest horsepower pump at the highest speed. When the smaller pump is deficient in head, a larger pump is called for nearer to shut off. However, a dramatic reduction in efficiency likely occurs as it moves away from its best efficiency point. In these situations, use of vacuum-primed series pump arrangements help obtain higher pumping efficiencies—resulting in lower power costs—and potentially smaller force mains.
Series pump arrangements differ from the parallel arrangements seen in a typical duplex lift station, which comprise the duty pump and the standby pump. Pumps in series connect two pumps, where the outlet of the first pump leads to the inlet of the second pump. Working in concert, the flow rate still remains the same, but the heads produced by the two pumps are added.
Unlike typical submersible pumps, this unique series pump construction can be accomplished because the vertically constructed, non-clog pumps are housed outside the wet well—typically above-grade—and so each pump already comes designed with a suction flange. This enables the entire lift station to be above the wet well, thereby eliminating any need for extra valve vaults and the associated confined space concerns.
The bottom-line advantage gained in the series arrangement comes from selecting two smaller and more efficient pumps working in tandem to achieve the higher head. Even by adding the second pump, the total connected horsepower and/or resulting power consumption would still be less than what the larger single pump can achieve at the higher head.
Consider a real example where a medium-sized Midwest town with a large residential area was served by septic tanks. Increasing problems with improper drainage and increasing nitrate levels in the groundwater prompted the city to plan sewers. Wastewater must be pumped up and over a gradually rising ridge to the sewage treatment plant in an adjacent valley. The city wanted to minimize capital costs because of limitation in their bonding capacity.
The static head is only 70 feet, but the length of the force main to the top of the hill is 6,200 feet. The flow was 410 gallons per minute and was not expected to increase much. A six-inch internal diameter ductile iron force main was selected with 4.65 feet per second velocity. A long-term Williams and Hazen coefficient of friction, C, of 120 was chosen based on the expected relative roughness of the pipe when coated with residual sewage. The resulting hydraulic conditions, including manifold losses, were thus established as 410 gallons per minute at 174 feet total dynamic head (TDH).
Typical parallel arrangement lift station pumps were considered, including a leading submersible pump and a vacuum-primed pump. The designer’s submersible selection required an 8-inch pump with an 88-horsepower motor and a pump speed of 1,770 rpm. With a six-inch discharge nozzle and manifold pumping, the pump’s efficiency measured 37% at the stated design conditions. The resulting brake horsepower draw would be 48.7.
By contrast, the vacuum primed pump was 4-inch in size with a similar pump speed of 1,760 rpm but only 50 horsepower. Also with 6-inch manifold piping, the pump’s efficiency measured at 40.7%. Brake horsepower draw would be 44.3.
The difference between the two pump scenarios would favor the vacuum-primed selection by a differential of roughly 9,585 kWh, which translates to more than $766 annually in power savings.
Yet, because of the high head application, a series pump arrangement was also considered. It was determined that smaller vacuum primed pumps in series would produce higher pump efficiency with lower connected horsepower than the single pump selections. At the same head, two 15-horsepower vacuum primed pumps in series with a similar pump speed of 1,760 rpm would best meet the conditions—at 68.9% efficiency. The series arrangement dictates two pumps serving in the duty role and two more in standby. Although the total connected horsepower was 60, as opposed to 176 or 10 horsepower above, the 68.9% efficiency yields a brake horsepower draw of only 26.2 at design point.
When compared to the submersible selection, the series pump configuration was almost half of the brake horsepower draw (48.7–26.9). Assuming a typical eight hours running time, 365 days a year, the differential in kWh favors the series configuration by 49,012 kWh per year. At a typical rate of $.08 per kWh, that differential translates to $3,921 annually in power savings for just one lift station!
Other savings can be accomplished as well. Series connected pumps provide for step starting, which can reduce demand power and, thus, require smaller standby power generators. In variable speed applications using the first stage pump connected to a variable speed device, a smaller and less expensive variable frequency drive may be utilized because the series vacuum-primed pump does not need the higher degree of horsepower required by a single-stage pump.
Therefore, in high-head collection system applications, especially those with lower flows, it’s important for designers to review the pump curves for both single stage and series connected lift station pumps. Higher efficiency and less total connected horsepower can be achieved than is possible using larger and higher head pumping units. Quite often, the initial cost of the two more efficient series sets of pumps will be less than the larger horsepower, less efficient duplex set, especially when startingequipment and standby power requirements are considered.