The multistage pump is a type of centrifugal pump which contains 2 or more impellers. The multistage centrifugal pump operates by one impeller feeding into the next impeller and the number of impellers required depends on the discharge pressure requirement. Each stage provides a pressure boost to the fluid before it moves to the next stage. This allows multistage pumps to achieve very high outlet pressures that a single stage pump cannot reach.
Multistage pumps can be horizontally or vertically oriented. In a horizontal configuration, the impellers and diffusers are stacked in a series along the length of the pump shaft. In a vertical orientation, they are stacked vertically at different heights within the pump casing. Despite the different configurations, they operate on the same principle of increasing pressure over multiple stages.
How Does a Multistage Pump Work?
A multistage pump works by directing fluid into the first stage suction inlet, where the first impeller rotates and imparts velocity to the fluid. The fluid then enters the first stage diffuser, which converts the velocity into pressure.
The fluid is now at a higher pressure and enters the second stage suction inlet. The second impeller increases the velocity again, and the second stage diffuser converts it to more pressure increase. This continues for as many stages as the pump has.
With each stage, the pressure rises incrementally until the fluid reaches the final discharge outlet at the desired high pressure. The number of stages can vary from 2 to over 12 in some large industrial pumps. More stages allow reaching higher discharge pressures.
Key Components of a Multistage Pump
Impellers: Impellers are rotating components with vanes that impart velocity to the fluid. Multiple impellers mounted along the pump shaft provide the staged velocity increase.
Diffusers: Diffusers are stationary components that convert the velocity imparted by the impellers into pressure. They have an expanding passage that slows the fluid.
Shaft: The shaft connects and drives all the impellers. It rotates at high speeds to turn the impellers.
Casing: The casing houses all the internal components. It has separate chambers for the different stages.
Bearings: Bearings support the shaft and allow smooth rotation. Thrust bearings handle axial forces in the shaft.
Seals: Seals prevent leakage along the shaft and casing. They ensure efficiency and safety.
Suction and Discharge Nozzles: Nozzles are inlet and outlet points for the fluid to enter and exit the pump.
Advantages of Multistage Pumps
High discharge pressures - Can reach over 1000 psi which single stage pumps cannot.
High efficiency - Convert velocity to pressure efficiently at multiple stages.
Compact size - Multiple stages in one casing gives high pressure in small footprint.
Lower maintenance - Only one shaft seal for all stages reduces maintenance.
Smooth operation - Produce less noise and vibration than single stage pumps.
Lower NPSH required - Handling liquids with entrained gases efficiently.
Longer life - Bearing loads spread over multiple bearings give longer service.