Centrifugal Pump.

This pump generally is used where a large volume of flow is required at relatively low pressures. It can be connected in series by feeding an outlet of one pump into an inlet of another. With this arrangement, the pumps can develop flow against high pressures. A centrifugal pump is a nonpositive-displacement pump, and the two most common types are the volute and the diffuser.

The centrifugal pump is the most used pump type in the world. The principle is simple, well-described and thoroughly tested, and the pump is robust, effective and relatively inexpensive to produce. There is a wide range of variations based on the principle of the centrifugal pump and consisting of the same basic hydraulic parts. The majority of pumps produced by Grundfos are centrifugal pumps. Principle of the centrifugal pump An increase in the fluid pressure from the pump inlet to its outlet is created when the pump is in operation. This pressure difference drives the fluid through the system or plant. The centrifugal pump creates an increase in pressure by transferring mechanical energy from the motor to the fluid through the rotating impeller. The fluid flows from the inlet to the impeller centre and out along its blades. The centrifugal force hereby increases the fluid velocity and consequently also the kinetic energy is transformed to pressure. Figure shows an example of the fluid path through the centrifugal pump.

 

Hydraulic components

The principles of the hydraulic components are common for most centrifugal pumps. The hydraulic components are the parts in contact with the fluid. Figure shows the hydraulic components in a single-stage inline pump. The subsequent sections describe the components from the inlet flange to the outlet flange.

 

Inlet flange and inlet

The pump is connected to the piping system through its inlet and outlet flanges. The design of the flanges depends on the pump application. Some pump types have no inlet flange because the inlet is not mounted on a pipe but submerged directly in the fluid. The inlet guides the fluid to the impeller eye. The design of the inlet depends on the pump type. The four most common types of inlets are inline, endsuction, doublesuction and inlet for submersible pumps. Inline pumps are constructed to be mounted on a straight pipe – hence the name inline. The inlet section leads the fluid into the impeller eye.

Endsuction pumps have a very short and straight inlet section because the impeller eye is placed in continuation of the inlet flange. The impeller in doublesuction pumps has two impeller eyes. The inlet splits in two and leads the fluid from the inlet flange to both impeller eyes. This design minimises the axial force. In submersible pumps, the motor is often placed below the hydraulic parts with the inlet placed in the mid section of the pump. The design prevents hydraulic losses related to leading the fluid along the motor. In addition, the motor is cooled due to submersion in the fluid.

The design of the inlet aims at creating a uniform velocity profile into the impeller since this leads to the best performance. The velocity distribution at different cross-sections in the inlet.

 

Impeller

The blades of the rotating impeller transfer energy to the fluid there by increasing pressure and velocity. The fluid is sucked into the impeller at the impeller eye and flows through the impeller channels formed by the blades between the shroud and hub. The design of the impeller depends on the requirements for pressure, flow and application. The impeller is the primary component determining the pump performance. Pumps variants are often created only by modifying the impeller.

The impeller’s ability to increase pressure and create flow depends mainly on whether the fluid runs radially or axially through the impeller.

In a radial impeller, there is a significant difference between the inlet diameter and the outlet diameter and also between the outlet diameter and the outlet width, which is the channel height at the impeller exit. In this construction, the centrifugal forces result in high pressure and low flow. Relatively low pressure and high flow are, on the contrary, found in an axial impeller with a no change in radial direction and large outlet width. Semiaxial impellers are used when a trade-off between pressure rise and flow is required.

The impeller has a number of impeller blades. The number mainly depends on the desired performance and noise constraints as well as the amount and size of solid particles in the fluid. Impellers with 5-10 channels has proven to give the best efficiency and is used for fluid without solid particles. One, two or three channel impellers are used for fluids with particles such as wastewater. The leading edge of such impellers is designed to minimise the risk of particles blocking the impeller. One, two and three channel impellers can handle particles of a certain size passing through the impeller.

Impellers without a shroud are called open impellers. Open impellers are used where it is necessary to clean the impeller and where there is risk of blocking. A vortex pump with an open impeller is used in waste water application. In this type of pump, the impeller creates a flow resembling the vortex in a tornado. The vortex pump has a low efficiency compared to pumps with a shroud and impeller seal.

After the basic shape of the impeller has been decided, the design of the impeller is a question of finding a compromise between friction loss and loss as a concequence of non-uniform velocity profiles. Generally, uniform velocity profiles can be achieved by extending the impeller blades but this results in increased wall friction.