Multistage Pumps

A multistage pump is a type of centrifugal pump designed to generate significantly higher pressure compared to a single-stage pump. This is achieved by incorporating two or more impellers within a single casing, arranged in a series.

Design and Components

The fundamental design of a multistage pump is characterized by its multiple stages. Each stage typically consists of an impeller and a diffuser (or guide vanes), all mounted on a shared shaft.

• Impellers: These are the rotating components with vanes or blades that impart kinetic energy to the fluid. In a multistage pump, the impellers are stacked in a series configuration.

• Diffusers/Guide Vanes: These stationary components are crucial in multistage pumps. Located around each impeller, they convert the kinetic energy (velocity) of the fluid exiting the impeller into pressure energy and guide the flow smoothly into the eye of the subsequent impeller.

• Casing: The casing houses the impellers and diffusers, containing the fluid and directing its flow through the various stages.

• Shaft and Bearings: The shaft transmits power from the motor to the impellers. Bearings support the shaft and manage the axial forces generated by the impellers.

• Balance Devices: Due to the high axial thrust generated by the impellers facing the same direction, multistage pumps often incorporate balance devices, such as a balance disc or drum, to minimize this thrust and protect the bearings.

Operating Principle

The defining characteristic of a multistage pump is the sequential increase in pressure. As the fluid enters the pump, it passes through the first impeller, which increases the fluid's velocity and pressure. The fluid then moves into the diffuser of the first stage, where the velocity is reduced, and the pressure is further increased.

From the first stage, the fluid is channeled to the inlet (eye) of the second impeller, and the process is repeated. Each successive stage adds to the pressure generated by the previous stage. While the pressure (head) increases with each stage, the flow rate remains constant. The final discharge pressure is the cumulative total of the pressure gained at each stage.

Key Differences from Single-Stage Pumps

The primary distinction between single-stage and multistage pumps lies in the number of impellers and the resulting performance capabilities.

• Number of Impellers: Single-stage pumps have only one impeller, whereas multistage pumps have two or more.

• Pressure (Head) Capabilities: Multistage pumps are capable of achieving significantly higher pressures (heads) than single-stage pumps.

• Efficiency: Multistage pumps can often be more energy-efficient for high-pressure applications.

• Complexity: Multistage pumps have a more complex design, which can lead to higher initial costs and more involved maintenance.

Applications

Multistage pumps are essential in applications requiring high head or pressure, where a single-stage pump would be insufficient. Common applications include:

• Water Supply to High-Rise Buildings: Delivering water to elevated floors requires high pressure, making multistage pumps ideal for this purpose.

• Boiler Feed Water: In power plants and industrial facilities, these pumps are used to feed water into boilers at high pressure.

• Reverse Osmosis (RO) and Water Treatment: Multistage pumps are used to provide the high pressure necessary for filtration processes.

• Oil and Gas Industry: They are used for high-pressure fluid transfer and other demanding operations.

• Irrigation and Agriculture: Multistage pumps are utilized in large-scale irrigation systems to distribute water efficiently over long distances.

• High-Pressure Cleaning and Spraying.