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A centrifugal pump uses centrifugal force to move water. A Shipco® Model P pump uses a two-stage impeller design to achieve a low required NPSH pump (which allow higher temperature water to be pumped). The pressure differential, that is necessary to force water through the pump case and out the discharge volute of the pump, is created by the first-stage impeller (often referred to as a propeller or inducer) and the second-stage impeller (the typical impeller used on the Shipco® Model D pumps).
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 and the bleed line.
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.
As water flows through the suction housing of the pump, the first-stage propeller (a rotating fan-like mechanism with blades) pushes it into the center (or eye) of the second-stage impeller (a rotating disc with a set of vanes). Both propeller and impeller are coupled to the motor shaft.
The second-stage impeller creates the pressure differential, between the eye of the impeller 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.