Cast Iron Multistage Centrifugal Pumps

Multistage pumps are centrifugal pumps (often vertically mounted) consisting of a series of impellers and casings mounted one after the other in series so that the discharge of one casing enters directly into the suction of the next. As each stage will only elevate the pressure by a set amount, a multistage pump will increase the pressure in each stage so that a final high pressure comes from the last stage. 

For instance, let's take an example of a 10-stage vertical pump, and each stage can increase the pressure by 0.5 bar. Liquid enters the first stage at atmospheric pressure and is elevated by 0.5 bar to enter the second stage, this then increased the pressure by 0.5 bar to enter the third stage a 1 bar. This is repeated until the liquid exits the final stage at 5 bar pressure. 

It's important to realise that only the pressure is increased and not the flow rate as the stages is mounted in series. To increase the flow rate, you would need to mount the pumps in parallel.