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Monday, May 25, 2009

Submersible Motor Driven Wet Pit Pump

The installation of conventional vertical wet-pit pumps with the motor located above the liquid level may require a considerable length of drive shafting, particularly in the case of deep settings. The addition of this shafting, of the many line bearings, and possibly of an external lubrication system many represent a major portion of the total installed cost of the pumping unit. Furthermore, shaft alignment become more critical, and shaft elongation and power loses increase rapidly as the setting is increased, especially for deep well pump

A great variety of submersible motors have been developed to deviate these shortcoming. Submersible wet-pit pumps eliminate the need of extended shafting, shaft couplings, a mechanical seal or stuffing box, a subsurface motor stand, and to some cases, an expensive pump house. Both vertical turbine and volute type wet-pit pumps may be so driven.

There is of course, no shafting above the pump and the pump and motor unit is supported by the discharge pipe only. No external lubrication is required. The motor is completely enclosed and oil filled and is provided with a thrust bearing to carry the pump down thrust. A mechanical seal is provided at the motor shaft extension, which is connected to the pump shaft with a right coupling. Only a discharge elbow and the electric cable connection are seen above the surface support plate. On occasion, this type of pump is used horizontally as booster pump in a pipeline, and in such cases the elbow at the discharge is eliminated.

Vertical wet-pit volute type sump pumps can be obtained with close coupled submersible motors for drainage sewage, process, and slurry service. The pumps are supported by guide rails that make it possible to lower and raise the pumps by means of a chain hoist. During this operation, the discharge pipe is connected and disconnected without dewatering the tank. Other arrangements use foot supported pumps with rigid discharge piping.

Motor used for this type of construction are usually hermetically sealed, employing a double mechanically sealed oil chamber with a moisture sensing probe to detect any influx of conductive liquid past the outer seal. Controls to start and stop the pump motors can be either an air compressor bubbler system or level sensing switches that tilt when floated.

Small portable pumps are available with flexible discharge hoses and built-in water level motor control switches activated by trapped air pressure. Motor for this pumps are usually oil filled and have a single mechanical shaft seal but are also available in a hermetically sealed design. The submersible motors are cooled by the liquid in which they are immersed and therefore should be run dewatered, although some motors can operate for short periods (10 to 15 minutes) this way.

In pump-motor combination, the motor is cooled by the pumped liquid as it moves through a passage around the sides of the motor. This design also uses a pressurized oil-sealed chamber to assure positive sealing.