Reducing energy costs and operation requirements in water infrastructure continues to be a concern, especially in wastewater and stormwater pumping. Packaged pump stations are increasingly popular among project designers and end users because of the low capital costs they bring to a pumping project, along with their space-efficient, preassembled and pretested arrangements.
This article will examine two wastewater pumping challenges in which vacuum-primed packaged systems offer solutions to optimize efficiency and performance. The first application will evaluate the merits of applying a specific pump scheme to meet the challenges of high total dynamic head (TDH) from a long force main. The second will discuss ways to optimize multiple pump stations operating on a common force main.
High-Head Pumping for Low Flows
In high-head wastewater pumping applications that result from long force mains or high static heads from hilly terrains, the design engineer and end user must evaluate numerous factors to efficiently overcome friction losses and achieve installation with reasonable capital costs.
Competitive bidding induces quoting the smallest horsepower (hp) pump at the highest speed. When the smaller pump is deficient in head, a larger pump is needed close to the shut off. However, a dramatic reduction in efficiency likely occurs as it moves away from its best efficiency point (BEP). In these situations, use of vacuum-primed series pump arrangements can help obtain higher efficiencies, often resulting in lower power costs and potentially smaller force mains.
Series pump arrangements differ from the parallel pumping arrangements seen in a typical duplex pump 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.
This series construction can be accomplished because the vertically constructed, nonclog pumps are housed outside the wet well—typically above-grade—and so each pump is designed with a suction flange. This enables the entire pump station to be above the wet well, eliminating 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.
Consider a real example in which a collection system in 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 had to 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 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 (gpm) and was not expected to increase much. A 6-inch internal diameter ductile iron force main was selected with 4.65 feet per second (fps) 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 established as 410 gpm at 174 feet TDH.
Typical parallel arrangement lift station pumps were considered, including a submersible pump and a vacuum-primed pump. The submersible selection required an 8-inch pump with an 88-hp motor and a pump speed of 1,770 rpm. With a 6-inch discharge nozzle and manifold pumping, the efficiency measured 37 percent at the stated design conditions. The resulting brake horsepower draw (BHP) would be 48.7.
The vacuum-primed pump was 4 inches in size with a similar pump speed of 1,760 rpm and 50 hp. With 6-inch manifold piping, the pump’s efficiency measured at 40.7 percent. BHP draw would be 44.3. The vacuum-primed selection provided efficiencies that translated to hundreds of dollars annually in power savings for that one pump station.
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 hp than single pumps. At the same head, two 15-hp vacuum-primed pumps in series with a similar pump speed of 1,760 rpm would best meet the conditions—at 68.9 percent efficiency. The series arrangement dictates two pumps serving in the duty role and two more in standby. The total connected horsepower was 60 hp, with the 68.9 percent efficiency yielding a BHP draw of 26.2 at design point.
Assuming a typical eight-hour running time, 365 days a year, the differential in kilowatt hours (kWh), and with a typical rate of $.13 per kWh, the differential translates to $6,372 annually in power savings for the one lift station.
Series connected pumps provide for step starting, which can reduce demand power and, thus, require smaller standby power generators. In variable speed applications where the first stage pump connects to a variable speed device, a smaller variable frequency drive (VFD) may be used.
Multiple Pump Stations Operating on Common Force Main
Industrial end users and municipalities can be faced with problems installing multiple wastewater pump stations on a common force main. Given that the variable pressure that occurs in the force main is dependent on how many pump stations are operating at one time, designers must incorporate solutions that can be costly from a capital and operational standpoint.
Challenges occur when the force main pressure is low, when only one pump station is operating, or when the pressure rises as multiple pump stations operate at the same time. The individual pump station should be designed to handle the worst conditions.
However, if this is the only condition it is designed for, the pump can overpump when fewer stations are operating. This can result in pumps running out on the pump curve, exceeding the maximum motor amp draw.
Yet, if the additional pressure is not incorporated in the design, the pumps can go to shutoff at the high-pressure condition and wet well or industrial
sump overflows can occur. Operating a pump outside of its normal operating range runs the risk of low and high flow cavitation. These phenomena result in impeller and volute erosion, reduced bearing and seal life, excessive pump noise and vibration.
There may be a simple solution. By integrating a VFD and a force main sensor, the programmable logic controller (PLC) can receive a signal from the sensor indicating changes in the force main
when additional pump stations come on or shut off.
From this information, an algorithm embedded in the PLC calculates the frequency for the VFD to adjust the pump speed to maintain a constant flow rate no matter how many stations are online.