What is a multistage pump?
We're often asked about why we've chosen an end-suction centrifugal pump instead of a multistage pump, and vice versa, so this post aims to give a brief outline of the specific advantages of multistage pumps, where they work, and where they don't work so well.
A multistage pump consists of two or more impellers stacked on the same shaft and driven by the same motor. Each impeller boosts the pressure of the water by a given amount, so the more impellers the greater the pressure discharged. The flow stays the same, however many impellers are added. See the previous post about pump pressure and flow for more details (a multistage pump is like connecting pumps in series).
This gives the output a higher pressure:flow ratio than other types of pump, so it's ideal in applications where a smaller amount of water is needed but a larger amount of pressure is required to get it there. For example: if you turn the tap on at the top of a tall block of flats, you only need a small amount of water but it takes a lot of pressure to get it there from the ground floor. An end suction centrifugal would generate too much flow at that pressure, and the energy consumption would be far higher.
7.5 kW 2" end suction pump curve showing a duty point of 25 m head and 65.9 m3/h flow:
2.2 kW 2" vertical multistage pump curve showing a duty point of 25 m head and 17.1 m3/h flow:
Check out our previous post about pump curves to see how to read these.
Multistage pumps are really efficient, for two main reasons. First, by having multiple smaller impellers instead of one larger impeller the tolerances are smaller, keeping wasted energy as low as possible. Second, as there's only one motor and one shaft the energy lost through transmission, heat, sound, etc is hardly affected by each impeller added. Another reason you can get great efficiency out of multistage pumps is that the curve is steeper, so you can control the flow more precisely with a variable speed drive.
To illustrate this with the above pump curves, if the pressure is reduced by 1 m on the multistage pump, the flow increases by just 1.5 m3/h. If the pressure is reduced by 1 m on the end suction pump, the flow increases by 5 times as much - 7.5 m3/h. It's much harder for a variable speed drive to get it right when it varies this much.
Applications that multistage pumps are ideal for are water pressure boosting (eg. for the block of flats mentioned earlier), borehole pumps (usually extracting water from deep wells and bringing it up to the surface), boiler feed pumps, irrigation, and wash down facilities, to name a few.
The main drawback to multistage pumps is that because the tolerances are very small, multistage pumps can really only handle fluids with little or no solids content. Of course, if you're wanting a low pressure but relatively high flow, for example to circulate hot water for a heating system, then they won't be any good for you either. The pump type for that application is in-line circulation pumps...but that's a different subject.