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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 centrifugal pump (above ground)
vertical multistage centrifugal pump (above ground)
Submersible/sump pump types
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
In this article, we will learn about multistage centrifugal pumps. We'll look at some of the main components of a multistage pump and how they work.
A multistage centrifugal pump is a rotating device that converts kinetic energy into a liquid head. This is one way of defining a centrifugal pump. There are many more, but for us, this is enough. In simple language, you can say that it is a machine used to transport fluids from one place to another. Multistage industrial pumps are used in all facilities such as refineries, oil production platforms, petrochemical plants, power plants, etc. Of course, multistage pumps are also used in other industries such as agriculture, food processing and residential construction as well as water supply.
Here is a cross-sectional view of a multistage centrifugal pump, let's take a look at some of the main components of the pump and some common terms
External power from an electric motor or diesel generator or even a turbine is used as the drive to provide kinetic energy to the pump impeller. The fluid enters the impeller and flows out from the tip of the impeller under the action of centrifugal force. Exhaust through the volute. The working principle of the pump is the conversion of energy
On the left side of the pump is the suction port of the centrifugal multistage pump, which descends and enters the eye of the impeller. The impeller has a wear ring, and the impeller rotates inside the diffuser.
We have a seal on the left. This is a mechanical seal. Mechanical seals are used to seal the space between the pump shaft and the pump casing. This prevents any leaks. We also have a ball bearing, also known as an anti-friction bearing
The liquid comes in from the suction port, enters the perforations of the impeller, exits through the impeller, and enters each impeller one by one, repeating the same process. Once we get to the final impeller, the liquid is drained and it goes out the top here, out of this drain.
This is the general layout of a multistage centrifugal pump, but now let's talk in more detail about how it works and what happens to a liquid as it flows through the pump.
We suck in the liquid at the inlet and the liquid goes into the eye of the impeller. The impeller rotates, and the impeller itself rotates within a stationary diffuser housing. We can see that around each impeller is a diffuser.
In this particular pump, we have 7 impellers and 7 diffusers. The liquid enters the impeller's orifice and is thrown radially outward into the diffuser housing. Once it enters the diffuser, it changes direction and leaves the diffuser.
The purpose of both the impeller and the diffuser is to reduce speed and increase pressure. Once the liquid comes out of the diffuser, it will be drained into the space in the housing. It will then be fed into the eye of the next impeller. Repeating this process the liquid is thrown radially. It will go through the impeller, through the diffuser, and then again we will get a reduction in speed and an increase in pressure. We repeat this process 7 times, and finally, the liquid will be discharged from the 7th impeller, and then the liquid will leave the pump through the outlet.
Each impeller and diffuser is classified as a stage. Our pump has 7 impellers and 7 diffusers, so it is classified as a 7-stage pump. It is important to realize that at each stage we are increasing the pressure but the flow rate is not changing.
Multistage centrifugal pumps have been one of the most popular types of pumps in recent years. Their ability to provide engineers with a flexible range of flow and head, along with their high degree of energy efficiency makes them a great choice for many applications. Could they be right for yours?
Multistage centrifugal pumps have multiple liquid chambers (or stages) that are connected in series. Fluid enters the first chamber at suction line pressure and leaves at some elevated pressure. Upon leaving the first stage, the fluid enters the second stage where the pressure is increased further.
The more stages the pump has, the higher the final discharge pressure. These pumps have the unique ability to produce higher and higher pressures with the addition of every stage, but flow range always remains constant for a given rpm.
There are a couple different types of multistage pumps. Those with a horizontal shaft, and those with a vertical shaft.
This type of pump has a segmented casing with modular interstage components. The rotating assembly is held between bearing housings, making the assembly more balanced for high pressures.
They typically handle higher flow rates than vertical multistage pumps (with the exception of vertical turbine pumps).
The horizontal configuration of multistage pumps is best applied in applications like these:
Reverse Osmosis
Boiler Feed
Shower
Spray
Cogeneration
Pressure Boosting
High Pressure Cleaning
Snow making
Condensate
Mine dewatering
This type of pump is pretty maintenance intensive, however. It takes a high skill set to work on these pumps and ensure they are put back together properly. They can also be quite costly upfront, depending on the degree of specification required and the materials of construction.
Vertical multistage centrifugal pumps have a vertical shaft, where stages are stacked one on top of the next. (Disclaimer: Although vertical turbines are technically multistage pumps, these are not the types of pumps we're discussing here. )
Clearances are small in these pumps, therefore, clean water applications are best for this pump:
High pressure shower systems
Boiler feedwater
Desuperheater feed
Vertical multistage pumps are great for areas that don't have much footprint to spare. Its ability to deliver high pressure output with a single pump body and motor combo is also a plus.
It is important to keep in mind, however, that this type of pump does not tolerate debris or significant solids, and is also vulnerable to deadhead conditions.
Multistage pumps provide many key benefits, from energy savings, to ability to operate in a wide range of flow/head scenarios. If you think it might be right for your application, be sure to discuss with a qualified engineer first.
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