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.

Shaft Elongation in Vertical Pumps

The elongation of a vertical pump shaft is caused by three separate phenomena:

1. The tensile stress caused by the weight of the rotor.
2. Tensile stress caused by axial thrust
3. The thermal expansion of the thrust
   

In most cases the tensile stress created by the axial thrust is several times greater than that created by the weight. In typical example of a 16,000 gpm pump designed for a 175-ft head and 5o ft long, the elongation caused by the weight of a 1600 lb (726 kg) impeller will be of the other of 0.0033 in. The elongation caused by the axial thrust will be approximately 0.0315 in.

The elongation caused by thermal expansion has to be considered from two angles. Firs, if the shaft and the stationary parts are built of materials that have essentially the same coefficient of expansion, both will expand equally and no significantly the same range of temperature in which they were assembled, no significant relative expansion will take place, even if dissimilar coefficients expansion are involved. Whatever the case, the pump manufacturer take these factors into consideration by providing the necessary vertical and play between the stationary and rotating pump components.

Load of foundations of Vertical Pumps

It the motor support is integral with the pump discharge column, and if the hydraulic thrust is carried by the motor thrust bearing, this thrust is not additive to the deadweight of the pump and of its motor plus the weight of the water contained in the pump, insofar as the load on the foundations is concerned. This is because the pump and motor mounted in this fashion form a self contained entity and all internal forces and stresses are balanced within the entity. If the pump and motor is supported separately, however, and joined by rigid coupling that transmits the pump hydraulic thrust to the motor thrust bearing, the foundation will carry the following heads when the pump is running.

Axial Thrust in Vertical Pumps

Axial Thrust in Vertical Pump With Single Suction Impeller . The subject of axial thrust in horizontal pumps has already discussed in my article before. When pumps are installed in a vertical position, additional factors need be taken into account when determining the amount and direction of the thrust to be absorbed in the thrust bearings.

The first and most significant of these factors is the weight of the rotating parts, which is a constant downward force for any given pump, completely independent of the pump operating capacity or total head. Since in most cases the single suction impellers of vertical pumps are mounted with the suction eye facing downward, the normal hydraulic axial thrust is exerted downward and the weight of the rotating parts is additive to this axial thrust.

The second factor involves the dynamic force (or change in momentum) caused by the change in the direction of flow, from vertical to either horizontal or partly horizontal, as the pumped liquid flows through the impeller. This force acts upward and balances a small for water having a specific weight (force) of 62.34 lb/ft2.