The Submersible Wastewater Pump Story
The submersible solids-handling sewage pump operates under water and is flood-proof. It is designed for single, wet-pit use and can be removed easily for maintenance. The pump is efficient, quiet in operation, safe to install and performs long and reliably.
The submersible wastewater pump is used primarily for wet-pit sewage lift stations and for industrial sump or process effluent applications. A common use for small pumps is to move effluent from tank to disposal in septic tank systems. Large and small units are used in a variety of ways in the home, farm, motel, school, marine, commercial building, industrial plant and municipal sewage and storm water systems.
Submersible pumps have been proven over the last half of a century, disproving those skeptics who originally questioned how an electric motor-powered pump could run under water. Originally developed in Europe, millions now are used daily throughout the world to pump clear water, raw water and wastewater.
The submersible wastewater pump became popular in the early 1960s, when a guide-rail system was developed to lift the pump out of the pit for repair. This ended the dirty and sometimes dangerous task of sending people into the sewage or wet pit. Growth of the submersible for wastewater pumping since has been dramatic, as an increasing number of specifiers and users learned of its advantages.
Two classes of submersibles exist. The smaller units, commonly called sewage ejectors, are used in home and light commercial applications; they normally handle up to 2-inch spherical solids and range from 1?3 Hp to 2 Hp.
Larger submersibles handle 21?4-inch and larger solids and have a minimum of 3-inch discharge. They are used in municipal and industrial applications for pumping sewage and all types of industrial wastewater.
Submersible AdvantagesSubmersible wastewater pumps have a number of advantages to users. A major one is low initial cost. In sewage pumping applications, only one pit is necessary, which reduces initial investment. There is no need, in most installations, for ventilation, lighting or other equipment, which is normal for dry pits. Flooding problems also are eliminated.
Another advantage is low operating cost. They have the obvious hydraulic advantage of working in the water and not some distance above it. They never lose prime.
Submersibles have safety and noise reduction benefits, too, since the working installation is well below ground level. A lesser chance for accidents from an exposed motor exists, and noise is minimal when the pump is operating.
Above-ground equipment is minimal -- usually only the control box and a frame for use in attaching the hoist for removal of the pumps. There is no unsightly pump or any need for a pump house. Moreover, because wet-wells are designed so incoming wastewater scours the bottom, little chance of odors occurs.
But the most striking advantage of submersibles is reliability and long life. The pump is designed to operate efficiently under water. It runs only when needed, reducing wear and power bills. The water cools the motor naturally, adding to its life span.
Pump FeaturesSubmersible wastewater pumps are vertical, direct-coupled, extra-heavy duty units, which operate under water and have a solids-handling, non-clog capability. While single pumps often are installed, most applications require two pumps (called duplex) -- to ensure continued operation if one pump fails -- to minimize wear on one pump and equalize it between two -- and to provide extra capacity in times of extraordinary loads.
A submersible pumping system consists of the motor-pump unit together with automatic electrical controls. Controls can be simple or complex, depending on the application. The latter may consist of an entire factory-packaged station enclosed in a steel or fiberglass tank and ready for installation and pipe-electrical hookup.
Submersibles are being specified increasingly in applications where self-priming, dry pit, straight centrifugal, vertical extended-shaft and pneumatic ejector pumps once dominated.
Depending on the impeller design, a 4-inch discharge pump normally will handle spherical solids from 2 inches to 3 inches. Each manufacturer's literature specifies the maximum solids size, which can be handled by a particular pump. Normal discharge sizes for larger submersibles range from 3-inch to 14-inch and larger. The pump selected should be sized to the application.
Submersible pump motors can be sized to the application. They normally are available in 850, 1150, 1750 and even 3450 designs, on 60-cycle power. Horsepower ratings range up to 100 Hp or larger. Variable speed units also are available with the use of variable frequency and voltage power supplies.
Again, depending on the application, motors operating on 200/208-volts, 230-volts, 460-volts and 575-volts and higher are available and may be single- or three-phase. Single-phase units usually are limited to 10 Hp. Thus, submersibles can be tailored to job requirements.
Like any pumps, submersibles also can be tailored to the capacity requirements of the particular installation. A specifier can ask for a high dynamic head, or can accept a lower head and obtain a higher gallons-per-minute flow rate or can get both a higher head and a higher flow rate by increasing the horsepower rating.
Typically, dynamic heads range from 15 feet to 300 feet. Flow rates range from 10 gpm to 2,500 gpm, and larger pumps produce 10,000 gpm or more. The pump-motor unit can be tailored to installation needs. Many larger pumps can be used in conjunction with a variable speed drive to further fit the performance to the application.
System ControlControl panels are engineered for the particular installation. The heart of the control system for submersibles is the liquid level control, which activates and deactivates the pump(s) at specified levels within the wet-well. The simplest control system would contain an on-off magnetic contactor and disconnect. Systems normally have three sets of controls -- one for turn-off of the first pump, one for turn-on of the pump and one for the high liquid alarm. Duplex systems usually alternate pumps on each successive cycle. Duplex systems also usually include an override control, which brings in the second pump when in-flow is unusually heavy or in case of failure of the first pump.
Control panels are installed above ground and usually contain:
- pump disconnects
- across-the-line starters with overload protection
- hand-off-automatic selectors
- elapsed time meters
- alarm systems for indicating high level conditions in the wet-well
In addition, duplex systems provide for automatic sequencing and alternating of pumps. Alternation allows for equal run-time and wear of the pumps.
Alarm systems vary but can be visual, audible or remote monitoring by telemetry devices or telephone lines.
The manufacturer will help determine what controls are needed for a particular application, and then design the control panel to this specification. All are built to National Electrical Manufacturers Association standards and in accordance with the National Electrical Code.
Wet-pit installation is a major advantage of submersible wastewater pumps, since only one pit is required. The pump usually is installed on guide-rails. If field service or replacement is needed, the pump is easily lifted to the surface. When lowered into position, the pump outlet flange automatically seats with the discharge piping. Wrenches or special tools are not needed, nor is there need for anyone to enter the pit.
Most submersibles can be serviced in the field without disturbing the piping; this represents a major cost saving to the user.
Complete-package sewage lift stations are available from some manufacturers. The complete package -- from the tank to the pump-motor unit, guide-rails, piping and valves, all controls, etc. -- is shipped ready for installation.
Access frames and covers are available from manufacturers for either wet-pit or sump allocations. They are designed so the cover can be locked safely in the open or closed position. Traffic-bearing covers are available.
This article is provided through the courtesy of the Submersible Wastewater Pump Association (www.swpa.org).