A centrifugal pump depends on a rotating impeller to move water through a piping system. The rotating impeller increases water pressure through the effective transformation of kinetic energy. As water is drawn into the pump, the rotating impeller releases kinetic energy into the water. As the water exits outward through the impeller vanes, the kinetic energy (also called velocity) is transformed into pressure as the water moves toward the periphery. Depending on the location and design of the impeller vanes, the water will attain varying amounts of pressure.
Because centrifugal pumps can be used to perform different functions in a piping system and different water pressure levels may be required, there are two general kinds of pumps that control and maintain water pressure differently. The two typical centrifugal pump categories are turbine pumps and volute pumps.
In a turbine pump, multiple diffusion vanes surround the rotating impeller. As water is released from the center (or eye) of the impeller, it spins outwards as the impeller rotates. Around the impeller’s circumference are various diffusion vanes, passages that gradually widen and open into a circular or spiral casing. The main role of diffusion vanes is to reduce the velocity of water slowly, thus transforming velocity into pressure. Once in the outer casing, the water then circles or spirals around toward the nozzle, where it exits the pump.
The primary difference between turbine and volute pumps is the presence or lack of diffusion vanes. In volute pumps there are no diffusion vanes, but rather the outer casing is a spiral. The form of the outer casing reduces water velocity (and eventual pressure) by creating an equal flow of water as it moves around the spiral toward the nozzle. The spiral is sometimes called a volute, hence the name for this particular type of pump.
Centrifugal Pump Components: Impeller Considerations
Within each centrifugal pump category, component traits can vary according to the pump’s desired function. An impeller, typically made of brass, bronze or iron, may either be open or enclosed. An enclosed impeller has a front and back protective shroud, whereas an open impeller does not.
Open impellers are often used in low-power applications and the majority of the impeller weight is supported by the hub. When compared to an enclosed impeller operating at the same speed, an open impeller is more efficient because enclosed impellers suffer from disc friction as a result of the close relation of the rotating impeller shrouds to the fixed casing walls. However, open impellers are more prone to abrasion. Because an enclosed impeller maintains a small relative velocity between itself and the fluid, abrasion is lessened. Additionally, enclosed impellers can handle moderate wear without compromising the general performance of the centrifugal pump.
In addition to open or enclosed specifications, an impeller can be single-suction or double-suction. In a single-suction impeller, fluid enters the impeller from one side. In a double-suction impeller, fluid enters the impeller from two sides. Single-suction impellers can be open or enclosed, but double-suction impellers are always closed. Single-suction impellers are the most common of the two.
Double-suction impellers, on the other hand, are often found in horizontally split case pumps or in the early stage of vertical turbine pump system. Additionally, a double-volute casing may be used to reduce radial bearing loads, which are considerably higher in higher-flow pumps.
In applications where thrust is a considerable concern, impellers can be fashioned with pump-out vanes that help move water from out behind the impeller to improve the stability of flow in high-flow applications. As the impeller rotates, the pump-out vanes push the fluid out, lowering axial thrust on the backside of the impeller and reducing pressure. Pump-out vanes can also be useful in removing abrasive agents.