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Centrifugal water pumps are hydraulic machines that are used to transport fluids by converting rotational kinetic energy to hydrodynamic energy of fluid flow.
They were first invented in 1689 and built on a very simple principle: fluid is sent to the hub of the impeller and driven towards the periphery of the impellers through centrifugal force. Later on in the 1850s, the design of this pump was further improved and now, decades later, these are the most popular and commonly used types of pumps for fluid transferring purposes in both commercial and residential settings.
In this article, we cover the basics of centrifugal pumps, including what to look for if selecting one.
What is a Centrifugal Pump?
A centrifugal water pump is a mechanical device that’s specially designed to move fluid by transferring rotational energy from a motor to energy in a moving fluid from one or multiple impellers.
These pumps are available in many forms. The construction of the device is inexpensive, simple and robust and its ability to work at a very high speed makes it possible to connect it directly to an induction motor which is not possible with some other types of pumps.
Centrifugal pumps are used for numerous applications across many industries. Some of the most common applications are boosting pressure, hot water circulation, regulating boiling water and sewage drainage, to name a few. Whatever the purpose, there is a centrifugal pump that will take care of it.
How Does It Work?
The most important component of a centrifugal pump is its impeller which consists of several curved vanes (the blades which rotate with the shaft of the pump). When the pump is operational, the impeller begins to rotate rapidly in its volute casing.
The fluid enters the impeller along its axis (also called the ‘eye’ of the impeller). As it rotates through the liquid, it creates pressure and this pressure differential (the difference between the suction side of the pump and the discharge side of it) is what causes the liquid to flow through.
The impeller continues to rotate fast from a center axis of rotation, while increasing the velocity and pressure of the fluid.
Then, the liquid that’s drawn into the center is driven out using centrifugal force through the vane tips towards the side radially and away from the eye of the impeller.
These pumps come with specially designed casings that constrict the fluid from the inlet of the pump and direct it right into the impeller. Finally, it slows and controls the fluid before it’s discharged.
Features of a Centrifugal Pump
The centrifugal pump has many features. Here are some of the main ones:
- Impeller – this is the most important part of the machine, with its backward curved vanes. It’s placed on a shaft that’s connected to the electric motor.
- Casing – the volute casing is a passage placed around the impeller of the pump. It’s airtight, and designed in a way that it converts kinetic energy of the fluid that’s discharged at the impeller outlet into pressure energy before the fluid exits the casing and is driven into the delivery pipe. There are three types of commonly used casings for centrifugal pumps.
- Volute casings: a spiral type casing surrounding the impeller. It gradually increases the flow area while decreasing the velocity of flow. At the same time, it increases the pressure of the water that flows through the casing. The efficiency of the pump is creased due to volute casings.
- Vortex casings: this type of casing is a circular chamber that’s placed between the impeller and the pump casing. Pumps with vortex casings are more efficient than those with volute casings.
- Casings with guide blades: with this casing, there is a series of guide blades which are mounted on a ring that is known as the diffuser that surround the impeller. The vanes of the impeller are designed so that water can enter the guide vanes from the impeller without any trouble. As the area of the guide vanes is increased, the velocity of flow through the vanes is reduced.
- Suction pipe with foot valve and strainer – one end of the suction pipe is connected to the pump inlet whereas the other end is dipped into the water. The foot valve is fitted to the end of the suction pipe that’s in the water. It’s a one-way valve that prevents water from draining back into the water source. There is also a strainer at this end of the pipe for filtration purposes.
- Delivery pipe – the delivery pipe is connected to the pump outlet and its other end delivers water at the required height.
Types of Centrifugal Pumps
There are many types of centrifugal pumps used for transferring fuel as well as other types of liquid. These can be categorised into three different groups: axial flow pumps, mixed flow pumps and radial flow pumps.
From these three, the most commonly used types are the semi-axial flow and radial flow pumps. Let’s take a closer look at these types in a little more detail.
- Axial Flow Pumps
Axial flow centrifugal pumps consist of an impeller with 3 or 4 vanes that are oriented in a way that allows the pumped fluid to exit axially instead of radially like other centrifugal pumps. These pumps generate only about 10 to 20 feet of head which is much lower than most other types of centrifugal pumps. They product high flow rates which can go as high as several hundred thousand GPM which is the highest flow rate of any centrifugal pump.
Axial flow pumps are used in instances where large quantities of water need to be moved a short distance, such as in flood dewatering. They’re also used to circulate large quantities of fluids in evaporators in the chemical industry and also as circulating water pump systems in power plants.
- Radial Flow Pumps
Radial Flow Pumps are centrifugal machines which pump water or fluid perpendicularly to the shaft of the pump. The water is driven out from the impeller in an outward radial direction. The radial movement of the water flow in the impeller increases the centrifugal force which then causes higher discharge pressure, but small flow rates of volume.
These pumps are widely used for numerous purposes including boiler feed systems, irrigation systems, dairy plants, re-cooling, shipbuilding, water treatment and booster systems.
- Mixed Flow Pumps
Mixed flow pumps are a cross between radial flow pumps and axial flow pumps. These are used primarily in applications that require a very high level of flow mixed with relatively low levels of discharge pressure. They are used for industrial applications as well as for jet skis and watercrafts.
Pros and Cons of Centrifugal Pumps
Centrifugal pumps are seen today as the most important and widely used types of pumps in the hydraulic world. Here are the pros and cons.
Pros of Centrifugal Pumps
- Centrifugal pumps don’t leak since they don’t have a drive seal.
- They have the capacity to draw hazardous liquids without any risk of the pump deteriorating.
- Their design makes them highly energy-efficient
- They are durable and built to last
- They operate quietly
- Because there’s a gap between the chamber and the motor of these pumps, there is no transfer of heat between the motor and chamber, enhancing workability and durability.
- Fluids being pumped cannot leak into the motor.
- These pumps can handle high loads due to the magnetic coupling system which can break up when the load is high.
Cons of Centrifugal Pumps
- It is possible that the coupling of this pump can slip when there is a sudden, heavy load.
- If the fluid that’s being pumped contains ion particles, they can eventually clog up the pump by accumulating at the impeller, attracted by its magnetic drive.
- A tiny amount of energy can be lost because of the magnetic resistant in these pumps.
How to Choose a Centrifugal Pump
Considering the countless options available on the market, choosing the right centrifugal pump can be rather daunting. However, having a basic understanding of what to look out for will be helpful when the time comes to make a decision.
To decide on the right pump, it’s important to consider the cost of ownership which includes several other costs: operating costs, maintenance costs and capital cost. A low cost pump would be of no use if it’s costly to maintain or highly inefficient.
When choosing the right centrifugal pump, remember to match it to the pumping system as well as the application hydraulically and mechanically. If there is a mismatch, it could result in performance issues until the imbalance issue is corrected.
The following hydraulic factors should be taken into consideration when selecting a centrifugal pump:
- System design: the pump needs to be the correct size for the system. If it’s the wrong size, it won’t operate the way it should and can cause excessive noise, cavitation, vibration or issues with the circulation.
- Flow rate: consider the flow rate which refers to the volume of useful flow that’s delivered by the pump through its outlet.
- System resistance: this is the head that’s necessary to overcome both the friction and the static head in the pumping system.
- Duty point: when the head created by the pump and the system resistance head are both equal, it’s known as the ‘duty point’. Identifying the precise duty point is necessary when deciding on a centrifugal pump. If the rate of duty point is between 50-110% of the best flow rate, the pump can be considered ‘acceptable’.
- Net Positive Suction Head Available (NPSHA): this is the quantity that’s used to determine whether the pressure of the liquid that’s pumped is sufficient to avoid cavitation from occuring. To get the NPSH, the vapour head of the fluid that is pumped should be subtracted from the total absolute head at the suction of the pump. Being able to determine the NPSH that’s made by the pump is essential when choosing a pump to perform well in its given application.
- Pump sealing: pumps are sealed at the point where the shaft passes through the casing. The seals are mechanical seals, centrifugal seals and packed gland seals. Nowadays, pumps without seals are becoming increasingly common especially in applications where corrosive or toxic materials are being pumped and leakage shouldn’t be allowed. The type of pump seals depends on the application and the system so make sure the model you’re buying is well sealed for your purpose.
The Bottom Line
Selecting the correct centrifugal pump isn’t an easy task. Since these pumps are quite complicated and need to be chosen carefully, the best option is to seek the advice of a professional. When choosing the pump, remember to avoid going for low cost products made of cheap materials as these can result in performance issues and decreased efficiency in the long run.