The overwhelming majority of contractor pumps use centrifugal force to move water. Centrifugal force is defined as the action that causes something, in this case water, to move away from its center of rotation.
All centrifugal pumps use an impeller and volute to create the partial vacuum and discharge pressure necessary to move water through the casing. The impeller and volute form the heart of the pump and help determine its flow, pressure, and solid handling capability.
An impeller is a rotating disk with a set of vanes coupled to the engine/motor shaft that produces centrifugal force within the pump casing. A volute is the stationary housing (in which the impeller rotates) that collects, discharges, and recirculates water entering the pump. A diffuser is used on high pressure pumps and is similar to a volute but more compact in design. Many types of material can be used in a centrifugal pump’s manufacture, but cast iron is most commonly used for construction applications.
For a centrifugal, or self priming, pump to attain its initial prime, the casing must first be manually primed or filled with water. Afterwards, unless it is run dry or drained, a sufficient amount of water should remain in the pump to ensure quick priming the next time it is needed.
As the impeller churns the water (see below figure), it purges air from the casing, creating an area of low pressure, or partial vacuum, at the eye (center) of the impeller. The weight of the atmosphere on the external body of water pushes water rapidly through the hose and pump casing toward the eye of the impeller.
Centrifugal force created by the rotating impeller pushes water away from the eye, where pressure is lowest, to the vane tips, where the pressure is highest. The velocity of the rotating vanes pressurizes the water forced through the volute and discharges it from the pump.
Water passing through the pump brings with it solids and other abrasive material that will gradually wear down the impeller or volute. This wear can increase the distance between the impeller and the volute, resulting in decreased flows, decreased heads, and longer priming times. Periodic inspection and maintenance is necessary to keep pumps running like new.
Another key component of the pump is its mechanical seal. This spring loaded component consists of two faces, one stationary and another rotating, and is located on the engine shaft between the impeller and the rear casing (see below figure). It is designed to prevent water from seeping into and damaging the engine. Pumps designed for work in harsh environments require a seal that is more abrasion resistant than pumps designed for regular household use.
Typically, seals are cooled by water as it passes through the pump. If the pump is dry, or has insufficient water for priming, it could damage the mechanical seal.
Oil-lubricated, and occasionally grease-lubricated, seals are available on some pumps that provide positive lubrication in the event that the pump is run without water. The seal is a common wear part that should be periodically inspected. Regardless of whether the application calls for a standard, high pressure, or trash, every centrifugal pump lifts and discharges water in the same way. The following section will point out design differences between these pumps.
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