A single-stage pump is defined as having one impeller and its related discharge collector that together produce fluid flow when powered by a driver, but due to having just one impeller, there are limits to the performance of a single-stage pump. On the other hand, a multistage pump is one where fluid flows through multiple impellers that are in series, which increases the total head (pressure) generated by the pump. Single volutes, double volutes and diffuser casing designs can all be configured into multistage pumps.
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As the fluid flows through each stage, pressure becomes higher than it was in the previous stage. In multistage pumps, the impellers sometimes differ in design but should always be designed to have nearly identical flow characteristics. An example where the impellers will differ is to avoid significant cavitation in low net positive suction head (NPSH) systems. In this case, or others requiring a pressure boost prior to the subsequent stages, the first stage will be designed for low NPSH operation (such as double suction impeller) and effectively operate while increasing the pressure, prior to entering the subsequent stages.
The main purpose for selecting a multistage pump is to efficiently operate in systems that require a high total head. There is no clear-cut delineation when to move to a multistage pump, but Image 3 can serve as a guide, which shows that multistage pumps are generally selected above 1,000 feet of total head.
While the fact that multistage pumps produce higher head is the most important application consideration, there are other reasons why a multistage pump would be used. All rotodynamic pumps produce noise, and this noise is contributed to by each of the individual components of the pump. Generally, the noise generated by these pumps is by hydraulic effects that are transmitted to the pump case.
Data has shown that multistage pumps will exhibit lower noise levels when compared to single-stage pumps of the same power levels. This is due to the energy being spread out over multiple stages rather than a single stage. Because of this, multistage pumps may be a better fit when high noise level is
a concern.
For more information on multistage pumping, refer to “ANSI/HI 14.3 Rotodynamic Pumps for Design and Application” at pumps.org.
A multistage centrifugal pump is a type of pump that features two or more impellers stacked together on the same shaft with a shared motor, as if connected in a series.
Each impeller and volute (or stage) that the water flows through will boost the pressure of the water, so the more impellers and the more stages, the greater the pressure discharged. No matter how many impellers you add, the flow is constant; should flow change then you’ll need a variable frequency drive.
The multistage centrifugal pump’s high pressure: flow ratio is useful for applications that require high pressure to get a small amount of liquid. For example, when you need to pump water up to reach the top apartment in a tall block of flats. Learn more about the different types of multistage pumps and how to choose between horizontal or vertical multistage pumps.
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Advantages of multistage pumps
Multistage pumps are very efficient as they have several smaller impellers to allow smaller tolerances. With just one motor and one shaft, every impeller added has minimal energy loss for each increase in stages. There will also be lower noise levels at each additional stage than that of a single stage pump. In general, any application that requires high output or high pressure would benefit from a multistage pump, whether horizontal or vertical.
When choosing between setting up multiple centrifugal pumps in a series or installing a multistage pump, there are five great reasons why you should choose a multistage centrifugal pump:
1. Greater efficiencies
A multistage centrifugal pump has small impeller diameters and clearances that allow improved performance and efficiencies at less horsepower. With just one motor, energy usage is lower than most alternatives.
2. Less space
When using a vertical multistage centrifugal pump, you can save on floor space as the pump has a shaft that runs vertically, with stages stacked on top of each other.
3. Higher pressure
A multistage pump has a small motor size (and uses less energy) while allowing increased pressure at each stage. However, you may need a variable frequency drive to adjust pressure build should the application require constant flow.
4. Lower head for each volute or stage
Lower head can be achieved despite smaller impeller size, which results in less leakage. This means a multistage centrifugal pump can pump a fluid to greater heights than another alternative.
5. Cost savings
Multistage centrifugal pumps may cost a little more upfront than other options, but their running costs are less.
The main disadvantage of multistage pumps is that while their small tolerances ensure hydraulic efficiencies, this makes them unsuitable for pumping solids or abrasive materials.
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