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The relative simplicity of design has made centrifugal pumps the most commonly available and widely used pump type in the market. Based on the number of impellers, centrifugal pumps can be classified into two categories - single-stage and multistage.
Single-stage pumps have a single impeller. Hence the discharge pressure is a function of the size of the impeller. So, such pumps are a better-suited solution in higher flow, lower pressure installations.
Multistage pumps are capable of providing a flexible range of flow and head and a relatively higher degree of energy efficiency, which makes them an excellent choice for many applications.
A multistage pump is a pump in which the fluid flows through two or more impellers fitted in series. Hence, such a pump will have multiple liquid chambers or stages connected in series. Each stage consists of an impeller, a combination of a diffuser, and return guide vanes, all housed within the same stage casing.
These impellers are installed in series and rotate on a single shaft driven by a power source (usually a motor). The design of multistage pumps allows the fluid to flow through the chambers only in a linear fashion.
After priming the pump, the fluid enters the pump inlet at suction line pressure into the first chamber. The fluid then passes through various impellers sequentially from left to right (or vice versa, depending on the design of the pump). Finally, the fluid leaves at some elevated pressure. Each impeller feeds into the next impeller, and at every stage, the pressure increases further.
The more the number of stages or impellers in a pump, the higher is the final discharge pressure. Although the fluid pressure increases with every stage, the flow range always remains constant for a given RPM.
Multistage pumps are available in many types, as listed below, but the most commonly used ones are vertical and horizontal pumps.
Horizontal multistage (above ground)
Vertical multistage centrifugal pump (above ground)
Submersible/sump
Side channel pump
Horizontal split case pump
Vertical turbine pump
Sanitary multistage pump
Since multistage centrifugal pumps have multiple impellers, it is capable of increasing the water pressure in a series (i.e., from one stage to the next), thus delivering higher pressures than a similarly sized single impeller pump.
The head per stage is less, allowing for relatively smaller-sized impellers with tight tolerances, thus reducing leakage loss.
Multistage pumps also help reduce floor space. Additionally, due to smaller impeller diameters and tighter clearances, these pumps also require less motor horsepower resulting in higher performance and efficiency.
For pumps of the same discharge pressure output, an increase in the number of stages lowers noise levels than a single-stage pump.
Compared to a single-stage pump, the design of multistage pumps is complex and consists of more number of moving components. As a result, the repair and maintenance of these pumps are relatively expensive and demand a higher degree of technical proficiency.
Also, tighter tolerances do not permit any solids in the fluid flow, which is why multistage pumps are generally used for transporting water or other low viscosity fluids.
Due to multiple stages in such pumps, there is an increased sensitivity of the pump rotor to external or natural vibrations.
A multistage pump finds applications across a varied range of industry verticals requiring the movement of fluids. For example, High-rise buildings require higher pressure to deliver water to their overhead tanks. Multistage pumps are widely used for such applications. They are also often used to either boost the water system’s pressure or continuously circulate water in the system.
Other key applications include:
High-pressure cleaning
Irrigation
Reverse osmosis (RO)
Fuel delivery
Oil and gas production
Mining
Boiler feeder pumps in power plants
Pressurizing water to help with firefighting
Pressurizing water to make snow for use in sports and resorts
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.
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 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.
What are the benefits of multistage pumps?
Multistage pumps use multiple impellers plus diffusing element stages for developing higher head through the series addition of head from one stage to the next. Types of multistage pumps include the between bearing types, which consist of the axially split BB3 and the radially split BB4 and BB5.
These pumps are typically used in applications for boiler feed, reverse osmosis, and other high pressure and temperature applications. Overhung impeller multistage pumps such as the OH7j, the OH1j and the OH13j are useful in low-flow, high-pressure applications and control hydraulic radial load through the use of diffusers.
For multistage pumps, a low NPSH required (NPSHR) first stage can be added to supply the second stage—especially applicable when the second stage has a higher NPSHR than the first stage.
Vertical and horizontal multistage pumps behave similarly to multiple single-stage pumps operating in series. This should be considered when designing a pumping system that calls for higher head requirements.
Multistage pumps may also be useful for noise reduction. For pumps of the same power, an increase in the number of stages lowers noise levels compared to a single stage. Estimates for sound pressure reductions for increased stages at the same power level can be referenced.
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