A multistage pump has a design that is similar to a single stage centrifugal pump, only that it comes with a series of impellers and volutes. This design was invented as an economic solution to increasing the pressure of water rather than installing a series of single stage pumps.
There are limitations in the pressure which can be generated by a single impeller, as once an impeller reaches a particular diameter, the impeller side friction increases that consequently causes a reduction in pump efficiency. This means it is necessary to add several impellers along a single shaft to generate high pressures while maintaining a high level of efficiency. And multistage pumps operate exactly that way.
The first impeller draws fluid in through the suction casing, to the outside of the impeller before being discharged through the discharge casing, which will then enter another ring section where the second, third, or fourth impeller is located. The fluid is passed on through stages repeatedly until the fluid discharges through the outlet.
When stages of impellers are added, the flow rate is not altered but the total head and shaft power increases proportionally to the number of stages. Each stage consists of an impeller and diffuser.