By Allan R. Budris
Hurricane Sandy demonstrated how vulnerable some wastewater lift stations and other facilities are to flood damage, especially those with standard air-cooled pump motors mounted in dry pits (near the bottom of the adjoining wet well). One of the best ways to prevent potential water damage to these pump motors is to replace the standard dry pit pump with a submersible pump/motor, which is designed to operate in a dry and/or submerged environment. Such motors are usually either equipped with a water jacket or are filled with a high dielectric mineral oil for heat transfer and cooling.
However, the efficiency of the typical dry pit submersible motor is measurably less than that of a typical dry pit pump powered by a standard air-cooled motor. This means that the typical dry pit submersible sewage pump station will consume more power than its air-cooled motor counterpart. But there are ways to prevent or mitigate this efficiency penalty with submersible dry pit pumps.
Submersible Dry Pit Pump Motors
The two issues that can cause the efficiency of a submersible dry pit motor to be lower than that of a typical standard air cooled motor are: the power consumed by the motor cooling system; and the typically smaller (non-optimum) outer diameter-to-length ratio.
For maximum efficiency, submersible motors with jacketed water cooling should be chosen over those cooled by the rotor being immersed in a high dielectric mineral oil, even though internal oil cooling may have some minor advantages. Operating the motor rotor in a viscous (oil) environment adds power consuming drag to the motor rotor (especially for larger rotor diameters).
Also, the more elongated and smaller the motor outside diameter, the lower the motor efficiency. There is an optimum diameter-to-length ratio for maximum motor efficiency. A recent comparison between the efficiency of a submersible well pump motor (which probably has the smallest rotor O.D.) and an air-cooled vertical hollow shaft motor showed a 5-percentage-point lower efficiency for the submersible motor.
Fortunately, there are water-jacketed, larger diameter submersible motors that even meet "premium motor efficiencies," such as those used on some of the Xylem Flygt N-Impeller pumps (see Fig. 1). This Flygt pump/motor uses an internal water (with glycol) jacketed cooling system, circulated by an integrated propeller. The pump user, or his consultant, should obtain and compare the actual pump motor efficiencies for the viable pump vendors under consideration.
|Fig. 1. Flygt N-Impeller Submersible Wastewater Pump with Water Jacket Motor Cooling (Courtesy Xylem Inc.)|
Wastewater Submersible Pump Efficiency
Generally, sewage pumps are substantially less efficient than the typical water pump for the same head and flow conditions. The Hydraulic Institute's "General Obtainable Pump Efficiency" by pump type (for an optimum specific speed of around 2,800 and a BEP flow rate of 1,000 gpm) lists the sewage pump efficiency at only 72% compared to 81% for the equivalent ANSI B73 end suction pump. This lower efficiency is due the compromises in hydraulic design generally required to handle the solids found in sewage. This often limits the impeller to only one vane.
However, there are some more recent sewage pump designs that demonstrate much better efficiencies, such as the Grundfos "S Tube" single vane closed impeller, and the Xylem Flygt self-cleaning two vane, semi-open, N-impeller submersible pumps (see Figure 2). Do not assume that all wastewater/sewage pumps are equal with regards to efficiency.
|Fig. 2. Flygt Submersible Wastewater N-Impeller Pump (Courtesy Xylem Inc.)|
Further improving Sewage Pump System Efficiency
The following additional actions, covered in many prior Pump Tips columns, can further improve or optimize the efficiency of a sewage pumping system (in addition to most other pump system types):
1. Conduct a field test of the existing pump(s) to determine the actual (current) pump and system Head-Capacity performance curves. This allows the selection (or modification) of the pump(s) that best match the system requirements without wasting energy across control valves or from pump operation far off the best efficiency point (see WW, Sept. 2007 & Jan. 2009).
2. Select and implement the most efficient pump control method (see WW, Sept. 2011 & Aug. 2012), for the required investment payback. Variable frequency (speed) drives typically have the potential to save the most energy, however, it may be hard to justify VFDs under certain conditions, such as with very flat system H-Q curves, or when there is very little change in flow rate or system head during normal operation. The resulting slower VFD pump speeds not only save energy but also can reduce maintenance costs due to less wear on the pump and other system components.
3. When more than one pump is installed in parallel there are typically opportunities to save energy by operating the pumps in the most efficient manner (see WW, Nov. 2008). Typically, the minimum number of pumps should be operated for any specific system flow condition. If the pumps are dissimilar, the pumps with the highest "Energy Effectiveness" should be selected first. The highest energy effectiveness of a pump normally occurs at its maximum flow rate. Energy Effectiveness (GPM/kW) is a function of the pump head, pump efficiency and drive efficiency.
4. Finally, for older pumps where the performance has fallen off, reconditioning the pump wearing ring clearance(s), and/or coating worn impeller and/or volute waterways can result in a marked improvement in the efficiency of the pump (see WW, May 2008).
Now that municipal water and wastewater systems are being repaired and/or upgraded in the wake of Hurricane Sandy, and funded by some of the available federal and state grants, it is a good time to not only upgrade these facilities to be less vulnerable to flooding, but also to make the pumping systems more efficient at the same time using some of the recommendations spelled out above. Users can also contact the Submersible Wastewater Pump Association for further guidance.
About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, NJ, he can be contacted via e-mail at email@example.com.
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