Submersible Wastewater Pump Story
The Submersible Pump Evolution
Originally used in Europe, submersible wastewater pumps were first seen in the U. S. Market in the mid-1950s. They became popular in the early 1960s when a guide rail system was developed to lift pumps out of well pits for easy maintenance and repair.
Submersibles are now the dominant pump in the municipal lift station market. They offer these advantages:
- Low initial cost since they involve only one pit and less auxiliary equipment is required than for dry well/wet well installations.
- Low operating costs.
- Safe and quiet installations.
- A minimum of unsightly above-ground equipment; and, above all.
- Reliable operation over a long life.
A submersible lift station includes not only the pump-motor unit, but sophisticated electrical and mechanical controls, piping and wet well with access frame and cover.
Submersible pumps are also widely used to handle suspended solids effectively and efficiently.
Submersible Wastewater Pump Technology and Advantages
The submersible solids handling sewage pump operates under water. It is flood-proof. It is designed for single, wet-pit use. It can be removed easily for maintenance. It 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 stormwater systems.
Submersible pumps have been proven over the last half of a century, disproving those skeptics who originally asked how an electric motor-powered pump “could run under water”. Originally developed in Europe, they are now used throughout the world to pump clear water, raw water, and wastewater. Millions are in daily use.
The submersible wastewater pump came to the U.S. about 1955. It 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 has since been dramatic, as an increasing number of specifiers and users learned of its advantages.
There are two classes of submersibles. One is the smaller unit, used in home and light commercial applications; they normally handle up to 2-inch spherical solids and range from 1/3 to 2 horsepower. They are commonly called “sewage ejectors”.
Larger submersibles are discussed here. They handle 2-1/4 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 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 are also eliminated.
Another advantage is low operating cost. Compared to above-ground pumps, for example, submersibles are more efficient. 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. There is less chance for accidents from an exposed motor and there is a minimum of noise when the pump is operating.
There is a minimum of above-ground equipment, 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, there is little chance of odors.
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. Suction pipe clogging and net positive suction head (NPSH) problems are also eliminated. The water cools the motor naturally, adding to its life span. Field service is simple and sure. And submersible manufacturers report that fewer than one-half of one percent of the pumps they ship are returned for replacement.
Submersible wastewater pumps are the fastest-growing products in the fast-growing wastewater and solids handling field.
Here’s an introduction to selection and use of this fast-growing product.
Description
Submersible 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 are often installed, most applications require two pumps (called “duplex”) – to insure 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.
Discharge Size
Depending on the impeller design, a 4-inch discharge pump will normally 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.
Motors
Submersible pump motors can be sized to the application. They are normally 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 are also available with the use of variable frequency and voltage power supplies.
Again depending on the application, motors operating on 200/208-volt, 230-volt, 460-volt, 575-volt and higher are available. Motors may be single or three-phase, as specified; single phase units are usually limited to 10 horsepower. Thus, submersibles can be tailored to job requirements.
Capabilities
Like any pumps, submersibles can also 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 to 300-feet. Flow rates range from 10 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 (VSD) to further fit the performance to the application.
Control 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: (1) pump disconnects, (2) across-the-line starters with overload protection, (3) hand-off-automatic selectors, (4) elapsed time meters, and (5) 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 manufacture the control panel to this specification. All are built to NEMA 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 is usually 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. There is no need for wrenches or special tools, or 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.
