Consider a typical sewage collection system. At the initial point of discharge, water first flows (by gravity) into a network of (sloped down) drain pipes, which gradually intercept a larger main pipe. Eventually, all this water needs to be lifted to a sewage wastewater processing plant.
The lift varies from several feet to hundreds of feet in some cases. To accomplish this eventual lift, there are several options:
1. Archimedes Screw
Archimedes screw is the oldest known pumping method, and it is rarely used today. The obvious benefit is its simple design, with no seals or packing to worry about. However, it requires significant space to accommodate the necessary low angle of incline, which also increases weight substantially. Typically, only one bearing near the discharge (drive) side is provided, so the entire rotor sits cantilever at a tight clearance required to keep water from flowing back.
Eventually, the rotor sags through the clearance and wears the bottom trough, requiring frequent-and rather expensive-repairs. When a second (lower) bearing is provided, it is lubricated by grease, which-due to submergence-eventually washes out and is difficult to reduplicate. Removal of the unit for maintenance is also difficult due to weight and inaccessibility.
Figure 1. Archimedes Screw
2. Self-Priming Centrifugal Pumps
Self-priming centrifugal pumps are common, mostly for relatively low flow applications (under 1,000-gpm or so). Like any other pump, it has pluses and minuses. Maintenance is easy since the pump sits on the surface, and trash can be removed by the access ports at the side of the casing. Priming is achieved by:
a. Foot valve
b. Check valve
c. Auxiliary vacuum pump evacuation of the inlet air
Both (a) and (b) options are the Achilles' heel of the method, and a vacuum pump adds complexity to the system. However, if these issues are understood and pumps regularly and properly maintained, a reliable operation results. If an install-and-forget maintenance is practiced, these applications become a problem after three to five years, when wear opens clearances and priming no longer looks or works as good as promised in the glossy paper brochure.
3. Vertical Sump
Vertical sump design solves priming problems by submersing the pumping element under water. A long shaft connects it to a surface-mounted electric motor, which keeps it from getting wet. The maintenance of packings or seals is easy since the pump does not need to be pulled for repacking service. However, with a long shaft comes an alignment problem, unbalance or wear of the line bushings. Lubrication of bushings can be problematic, due to plugging or breaking of the long (and often flimsy) grease tubing. When tubing, bushings or impeller require maintenance, the entire unit needs to be pulled, which can be an issue for hard-to-access places.
Figure 2. Typical example of a cantilever sump pump. Courtesy of ITT Goulds.
Vertical sump pump
4. Submersible (Wet) Pumps
Submersible (wet) pumps have an electric motor directly coupled to a pump. The entire unit is compact and relatively light, which makes it acceptable for relatively low depth wet wells (10-ft to 20-ft) at horsepower typically under 30-hp. Submergence of the motor, while a benefit, can also be trouble. To ensure water does not get to the motor windings, a double mechanical seal, filled with oil, is installed. As the impeller wears out (wastewater applications can be nasty!), unbalance and vibrations eventually tend to deflect the cantilever shaft and fail the seal.
Given this issue, submersible pump motors are typically made with better quality stator windings as compared to dry, surface application motors. Even after being flooded, the windings continue to function without shorting for some time. Moisture sensors are provided to detect, warn and alarm, but unfortunately, many operators do not have these connected-or disconnect them on purpose-to avoid nuisance alarms, and thus set the units on a road towards eventual undetected failure.
Even for a perfectly maintained submersible pump, with no impeller wear and resultant unbalance to consider, a mechanical seal life has a finite lifespan. While the secondary seal sees a better environment (clean oil), the primary seal is in direct contact with dirty sewage and eventually wears. While the exact value of such seal life depends on the application, it will likely not significantly exceed five years on average, so the pump cannot be viewed as install-and-forget.
5. Submersible (Dry) Pumps
Submersible (dry) pumps are similar to the wet submersible except that they are installed in the dry well, and connected to the wet well via suction piping. Servicing and pulling such a pump for maintenance is easier with simpler, obviously cleaner access. The issue of mechanical seal life, however, remains the same as for the wet submersibles. Since cooling of the motor is no longer done by submersion, dry submersibles require circulation of a portion of the pumpage through the cooling passages of the motor housing, which can clog these passages with dirty pumpage and overheat the motor.