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A centrifugal pump uses centrifugal force to move water. An impeller (a rotating disk with a set of vanes coupled to the motor shaft) creates the pressure differential, between the center (or eye) of the impeller and its outer edge, that is necessary to force water through the pump casing and out the volute of the pump. Both the diameter size and vane design of the impeller determine the operating conditions -- flow capacity (gpm) and discharge pressure (psig) -- of a particular pump.
A Shipco® Model U pump is based upon a two-stage, patented design to move water from a basin depth ranging from 6 inches to 27 feet. The first-stage impeller (located at the bottom of the column assembly) is typically referred to as a propeller or inducer. The 2nd-stage (located near the motor) is a bronze impeller.
Water first passes through the suction strainer (standard on most underground models) at the bottom of the column assembly before it enters the suction of the pump. The strainer helps protect the column assembly from debris that typically ends up in basins.
As the water enters the suction of the column assembly, it passes through the propeller (a rotating disk with fan-like blades) that pushes the water up the column into the center (or eye) of the second-stage impeller. The propeller is designed to increase the pressure on the water at the eye of the second-stage impeller and lift water up the column.
The water is drawn up the suction column assembly and into the eye of the impeller by the low pressure created in the eye by centrifugal force. It is important that the suction pressure on the water exceeds the vapor pressure of the water to prevent steam bubbles from forming (discussed later) which will damage the pump over time.
Bearing assemblies are spaced at regular intervals (typically 6-9 inches apart) within the column assembly to keep the propeller stem that is driven by the motor shaft rotating true. The rubber, silicone tubing of the bearing assembly that rests against the propeller shaft is water lubricated.
On initial startup, the chambers inside the pump must be filled with water before the motor is turned on. These chambers include the suction and discharge cavities of the pump, along with the area in and around the impeller up to the mechanical seal. As water fills the pump suction housing, the air inside the pump is pushed out through the discharge.
As water fills the pump suction housing, the air inside the pump is pushed out through the bleed line. The petcock (on the bleed line) should be opened to allow air to escape. The petcock remains open until a steady stream of water is running out. At that point, the petcock should be closed. The bleed line runs down the side of the assembly column so that it empties below the water line of the basin. Emptying below the water line prevents air from leaking back into the pump.
After the pump is primed, Shipco®'s bleed line serves several functions:
An electrical signal is sent by a pump controller to turn on the pump motor. The type of pump controller varies depending on the particular application. For example, on condensate units the pump controller is typically a float switch or mechanical alternator.
The mechanical seal is designed to prevent water seeping into and damaging the motor. The water slinger provides additional protection to the motor if some moisture should leak through the pump head (perhaps due to a damaged mechanical seal). When the moisture comes in contact with the rotating water slinger, it is slung from the motor shaft onto the slinger and into the pump head chamber, thus preventing it from seeping into the motor.
When the small amounts of moisture comes in contact with the rotating water slinger, it is slung into the pump head chamber. The slinger dissipates the moisture and helps prevents it from seeping into the motor. However, if the mechanical seal fails, the water slinger will not prevent motor failure. Mechanical seals should be replaced immediately if leakage is noticed.
A good quality, spring load check valve must be included in the pump discharge to ensure the pump head chamber and suction column assembly remains flooded. Otherwise both the O-ring seal in both the bearing assembly and the mechanical seal might dry out.
WARNING! It is critical that the O-ring seals remain lubricated to prevent the propeller stem from binding and consequently damaging the both the propeller stem and seal of the bearing assembly.
As water flows through the suction housing of the pump, it enters the center (or eye) of the impeller (a rotating disc with a set of vanes). The impeller is coupled to the motor shaft.
The impeller creates the pressure differential, between its center (or eye) and its outer edge, that is necessary to force water through the pump casing and out the discharge. The diameter size and design of impellers depend on the operating conditions such as flow capacity and discharge pressure.
The suction pressure created by the weight of the water (typically in a receiver attached to the pump) pushes the water through the pump suction housing and into the eye of the impeller. As the impeller rotates, the water flows from the eye of the impeller out along the vanes. The pressure on the water increases (Low to High) as it flows from the eye to the outer edge and then out the discharge.
Different designs of impellers are used on the pump to create the required discharge pressure for the specified flow capacity (i.e., expressed in gallons per minute).
The pressure on water in the suction housing must exceed water vapor pressure to prevent cavitation (or steam bubbles) from forming within the eye of the impeller, the point were pressure is lowest.
If steam bubbles form, they are swept along the vanes of the impeller. At some point the pressure upon the water once will again exceed the water vapor pressure causing the steam bubbles to implode (or collapse) which is known as cavitation. These implosions sound like marbles or rocks rattling within the pump. Also commonly known as water hammer. The force of shockwaves created by implosions WILL DAMAGE THE PUMP!
After the pump has satisfied the operating conditions of the pump controller, the controller sends an electrical signal to turn off the pump motor.
When the motor is turned off, the pull (or flow) of water generated by the impeller stops. The pump case should remain full of water and therefore is primed for its next cycle. The cycle will then repeat.