A centrifugal pump is a simple hydraulic machine that transforms mechanical energy (rotational energy) into hydraulic energy (pressure energy) by utilizing the centrifugal force acting on the fluid. Mechanical energy is supplied with the help of an engine or an electric motor. The first machine that was based on the centrifugal working principle was designed by an Italian engineer, Francesco Di Giorgio Martini in 1475. This machine was used as a mud lifting device, while the true centrifugal pump was only developed in the 17th century when a French inventor, Denis Papin, developed straight blades centrifugal pump. In 1851, the curve vane centrifugal pump was designed by a British inventor, John Appold. Centrifugal pumps are the most preferred and widely used types of pumps to transfer the fluids to the high level from the low level as they offer great efficiency and can be used for both non-viscous and viscous fluids. In this article, we’ll understand the working principles and the other basics of the centrifugal pump.
Components of Centrifugal Pump
To understand the overall working of the centrifugal pump, let’s first understand the important components that are used for the construction of the centrifugal Pump.
1. Impeller
The Impeller is the main component of the centrifugal pump. It is the rotating part, which consists of several backward curved blades mounted on its circumference. It is connected to the external motor through the shaft, which is attached to the opposite side of the eye of the impeller. It is used to provide kinetic energy to the liquid by rotating at a high speed (500-5000 rpm). The liquid enters the impeller through its axis (the eye) and leaves along its circumference between the blades. The liquid is accelerated from the impeller blades into the casing of the pump by the rotational motion of the impeller.
2. Shaft and Shaft Sleeve
The shaft is the central part of the centrifugal pump, and it connects the impeller to the electric motor. The shaft sleeves is a hollow metallic tube that is mounted over the shaft to protect it from leakage and corrosion. The shaft sleeves are usually used in single-stage pumps. The single-stage pump is the simplest type of pump, which has only one suction impeller in the pump casing.
3. Casing
The casing of the centrifugal pump is a narrow air-tight passage around the impeller. The casing is designed in such a way so that the kinetic energy of the fluid discharged by the impeller gets converted into the pressure energy before the fluid leaves the casing and enters into the discharge tube. The main function of the casing is that it maintains the appropriate pressure inside the pump, and it also allows the easy removal or installation of the pump.
The casing of the centrifugal pump
4. Suction Pipe
The lower part of the suction pipe is immersed in the water that needs to lift up, and the other side is attached to the centre of the impeller or basically the inlet of the pump. The diameter of the suction pipe is always kept more than the diameter of the delivery pipe.
5. Delivery Pipe
The delivery pipe accumulates the water leaving the casing. Its one end is attached to the outlet of the centrifugal pump and the other end is attached to the height where the fluid needs to be transferred. Basically, the delivery pipe lifts the fluid to the required height. The delivery pipe is provided with a valve near the outlet of the centrifugal pump, which manages the flow of the fluid into the delivery pump from the centrifugal pump.
6. Foot Valve and Strainer
The foot valve and strainer are attached at the bottom of the delivery pipe. The foot valve opens only towards the upward direction, i.e., towards the suction pipe, hence it is a one-way valve. This valve ensures that the water flows in the upward direction only and does not get back to the lower surface from the casing. The function of the strainer is to filter the fluids and remove the debris such as wooden pieces, leaves, and sands from it, hence it prevents the pump from blockage.
Working Principle of Centrifugal Pump
The first step of the working of the centrifugal pump is to provide mechanical power to its impeller by using an electric motor. The impeller, which is connected to the electric motor through the shaft, starts rotating. Due to the rotation of the impeller, a vacuum creates inside the eye of the impeller, hence the liquid below the suction pipe begins to enter inside the impeller’s eye in the axial direction. After entering the impeller’s eye, the liquid strikes the vanes of the impeller. The high speed of the impeller rotates the liquid radially and in an outward direction by using centrifugal force. The impeller keeps on rotating until the liquid passes through every component of the impeller, the rotation accelerates the fluid, which increases the fluid’s energy and creates the fluid head. The blades of the impeller convert the kinetic energy of the liquid into its speed, which eventually increases the speed of the liquid. Now, the fluid enters the diffuser area after leaving the impeller. The speed of the fluid slows down at the diffuser area as the diffuser transforms the speed of the fluid into the pressure energy, this is based on Bernoulli’s principle, according to which, to produce a high head the fluid gets slow down. Finally, after gaining the desired pressure, the fluid discharges from the outlet of the centrifugal pump to the required area. Hence, we can conclude that the working of the centrifugal pump is mainly based on the principle that if liquid of a specific mass is allowed to rotate with the help of an external source, it gets thrown out from the centrifugal axis of rotation and the pressure head of the rotating fluid rises. The pressure head rises at any point is proportional to the velocity of the fluid at that point. Thus, the rise in pressure head is large at the outlet of the impeller because of the larger radius (due to the volute shape casing) towards the outlet, and the discharge fluid at the outlet have a high-pressure head. Hence, the fluid gets lifted to a high level from a low level.
Types of Centrifugal Pump
Centrifugal Pumps can be divided into various types based on the different categories, let’s discuss these categories.
1. Based on the Type of Impeller
The impeller of the centrifugal pump is mainly available in three types, i.e., open impeller, semi-enclosed impeller, and closed impeller.
1. Closed Impeller
2. Open Impeller
In an open impeller, the vanes are directly attached to the central hub, which is mounted to the shaft. The open impeller does not consist of any wall that surrounds the vanes due to which it is weaker than the semi-closed and the Closed Impeller. However, it is still used in centrifugal pumps as it is easy to repair and clean, moreover, they are faster than the closed and the semi-closed impellers. The open impellers are usually preferred in the small pumps for the fluids consisting of solid particles.
3. Semi-enclosed Impeller
The semi-enclosed impeller or semi-open impeller consists of one back wall that surrounds one side of the vanes. They are usually preferred to use in fluids that consist of solids as they allow the solids to pass through them. The major problem with this type of impellers is their reduced efficiency over time.
2. Based on the Number of Impellers
1. Single-stage Pump
The single-stage centrifugal pumps are the simplest types of pumps, consisting of only one impeller. These are primarily used in productions operations that require low to moderate total dynamic heads (TDHs) pumping services. The total dynamic head is the amount of pressure or head exerted on the suction side by also considering the losses due to friction in the suction pipe. They are widely used and are highly reliable; however, they offer unstable thrust and radial forces and have lower TDH capabilities than multi-stage pumps.
Single Stage Centrifugal Pump
2. Two-stage Pump
The two-stage centrifugal pumps have two impellers that operate side by side. The speed of any impeller can be changed according to the requirement that whether more volume is required or more pressure is required.
Two-Stage Centrifugal Pump
3. Multi-stage Pump
The centrifugal pump that contains more than two impellers is called the multi-stage pump. All the impellers are attached to the same shaft. Each impeller increases the head of the fluid by an equal amount. They are more reliable and efficient than single-stage and two-stage centrifugal pumps.
Multistage Centrifugal Pump
3. Based on the Type of Casing
The three main types of casing are,
1. Volute Casing/Spiral Casing
It is the most commonly used type of casing. It has a spiral shape, and there is a gradual increase in the area of flow of the casing. Due to the increase in the flow area, the speed of the fluid slows down, and the fluid pressure increases towards the discharge tube. Let’s understand this with the following equations.
According to the continuity equation,
AV = Constant
Where A is the cross-sectional area of the region, and V is the flow velocity in that region. Now, as in volute casing, the area increases gradually, hence as per the above continuity equation, the velocity (kinetic energy) of the fluid decreases to maintain the constant value. Now, let’s see why the pressure rises towards the discharge tube.
According to Bernoulli’s equation, the net mechanical energy of the flowing fluid, i.e., the sum of the kinetic energy of fluid motion, the potential energy due to elevation (neglected in this case, as the casing is assumed to be on the same level), and the pressure energy of the fluid, remains constant.
Mathematically,
P.E + 1/2 {mv}^{2} + ρgh= Constant
Where P.E is the pressure energy exerted by the fluid, 1/2{mv}^{2} is the kinetic energy, and ρgh is the potential energy.
Now clearly, from this equation, if the velocity of fluid decreases, the pressure energy of the water increases at the pressure head to maintain the constant value.
Volute Casing
2. Vortex Casing
In the vortex casing, a circular chamber is attached between the impeller and the casing. The vortex casing helps to improve the efficiency of the pump. It reduces the eddies formations, hence decreasing the loss of energy.
Vortex Casing
3. Guide Blades Casing
In the guide blade casing, a series of guide blades are provided around the impeller. The guided blades are attached to a ring called a diffuser. The guide vanes are designed in such a way so that the water leaving the impeller enters the diffuser tube without shock, and the areas of the vanes increase, which helps to lower the liquid velocity and eventually rise in the pressure. The fluid enters the surrounding casing after passing through the guided vanes.
Guided Blade Casing
4. Based on the Orientation of the Fluid
The centrifugal pumps are divided into the following type depending upon the orientation of the fluid flowing through the impeller.
1. Axial Split Pump
In the axial split pump, the fluid enters in the radial direction in the pumps and leaves in the axial direction, which is why they are named axial split pumps. These pumps offer a high flow, and they can be easily adjusted according to the various conditions. Their main disadvantages are that they provide low head and low discharge pressure, and they are also not suitable for the suction lift.
Axial Split Centrifugal Pump
2. Radial Split Pump
In these pumps, the fluid enters the pump in the axial direction and leaves in the radial direction. The main advantage of this type of pump is that they provide pulsation-free fluid flow. These pumps are available in different designs, which can be used for various applications. The disadvantage of these pumps is that they don’t have any suction power, hence instead of the suction process they use the impeller’s rotation to move the fluid.
Radial Split Centrifugal Pump
3. Mixed Flow Pump
In the mixed flow pumps, the fluid flows along the impeller’s axis till half of its distance, and it starts flowing vertically to the axis after that, i.e., the direction of the fluid is somewhat in between the radial and the axial. Its main drawback is that it requires a large area for its installation.
5. Based on the Type of Suction Pump
1. Single Suction Pump
Single suction consists of only one impeller eye, i.e., the whole (100%) amount of liquid that is being sucked by the pump flows into one side of the impeller’s inlet (eye). Then, the centrifugal force exerts pressure when the liquid leaves the impeller through the vanes.
2. Double Suction Pump
The impeller of the double suction pump is designed in such a way that it can draw liquid from both of its sides, i.e., the double suction pump splits the liquid flow into two equal halves and passes through both the eyes of the impeller. The splitting of the liquid flow reduces the axial forces exerting on the impeller that allows the higher flow of the liquid than the single suction pump.
6. Based on the Type of Volute
1. Single Volute Pump
As the name suggests the single volute pump consists of only one volute case around the impeller through which the water is discharged. These pumps only have one cutwater, which directs the fluid flow towards the discharge. The single volute pump is mostly used in the refinery industries.
2. Double Volute Pump
The double volute pump consists of two cutwaters, which are aligned 18o degrees from each other. It minimizes the displacement or bending of the shaft if the pump is operated over the “best efficiency point” (BEP); BEP is referred to as the point at which the centrifugal pump works at its best/peak efficiency, at this point the minimum level of turbulence, flow splits, and other losses are observed when the fluid enters or exits the pump.
7. Based on the Position of the Bearing Support
1. Overhung Impeller Pump
In the overhung impeller pump, the impeller is fixed at the end of the shaft; the shaft is overhung from its bearings, which is why it’s called an overhung impeller. This arrangement allows both the horizontal and the vertical attachment of the pump. The common examples of the overhung impeller pump are the ‘close-coupled pumps’ and the ‘separately-coupled pumps.’ In a close-coupled pump, the impeller is attached directly to the motor shaft, while in the case of separately coupled pumps, the impeller is fixed on the different pump shafts, which have their own bearing supports.
Overhung Impeller Pump
2. Between Bearing
In this type of centrifugal pump, the impeller is attached to the shaft and the shaft is suspended between the two bearings at both of its ends, which is why it is called a between bearing pump.
Between Bearing Centrifugal Pump
8. Based on the Shaft Orientation
The centrifugal pumps are divided into the following types based on the position of the pump.
1. Horizontal Centrifugal Pump
In a horizontal centrifugal pump, the shaft runs in the horizontal direction. These pumps can easily be attached with different accessories such as electric motors, or turbines. The working pressure and temperatures of the horizontal centrifugal pump are lower than the vertical centrifugal pump. These pumps are specially designed to deliver high-speed fluid. They are built of high-quality material, hence provides great productivity. Smooth operations, low noise, easy instalment and maintenance are some of the main features of horizontal centrifugal pumps. Their major drawback is that they require large installation space.
Horizontal Centrifugal Pump
2. Vertical Centrifugal Pump
In a vertical centrifugal pump, the shaft runs in the vertical direction. These are the negative displacement pumps that are preferred for the fast transfer of the various chemicals. They have relatively high thickness and operates at a high temperature than the horizontal centrifugal pumps. These pumps are mainly used for high-temperature and high-pressure fluids. They provide a smooth fluid flow rate and do not have priming problems; however, they generate high noise and do not provide high power.
Vertical Centrifugal Pump
3. Submersible Centrifugal Pump
These pumps are used for municipal, rainwater, household, industrial, and commercial purposes. They are used to supply the underground soil, rainwater, sewage, and chemicals. They are used for the low flow and high head fluids or high flow and low head fluid rate. These pumps do not require manual priming as they are already submerged in water. They also prevent the cavitation problem. These pumps often show leakage issues, which can damage the internal parts of the pump assembly.
Submersible Centrifugal Pump
9. Based on the Basis of Compliance with Industry Standards
Centrifugal pumps are also divided into the following types depending upon the basis of compliance with industry standards.
- ANSI pump (American National Standard Institute)
- API Pump (American Petroleum Institute)
- DIN Pump (DIN 24256 specifications)
- ISO Pump (ISO 2858, 5199 specifications)
- Nuclear Pump
Priming of the Centrifugal Pump
Priming is the process of filling the centrifugal pump with the liquid. Every centrifugal pump needs liquid in the liquid casing; however, if the liquid casing of the pump gets filled with the vapours or the gases (in the absence of the liquid) the pump impeller becomes incapable to function accurately. Hence, to make sure that the centrifugal pump does not get gas-bound, and remains primed, most of the centrifugal pumps are placed below the source level from which the centrifugal pump takes the suction. This can also be obtained by providing liquid to the centrifugal pump under the pressure given by the other pump located in the suction line.
Cavitation in Centrifugal Pump
The centrifugal pumps often suffer from the problem of cavitation. It is the phenomenon, during which the vapour bubbles forms in the part where the fluid pressure decreases below the vapour pressure, and these vapour bubbles collapse at the high-pressure region. A high-pressure region is generated at the area where the bubble collapse. Hence, the surface above which the bubble collapse takes place undergoes high pressure that can disturb the smooth functioning of the pump. This also results in unwanted noise and vibrations in the centrifugal pump.
Cavitation in Centrifugal Pump
Factors Influencing the Centrifugal Pump Performance
Here are some of the important factors that must be considered before selecting the type of centrifugal pump for a particular requirement.
1. Suction Pressure
The pressure with which the pump sucks the fluid is an important factor to be considered while buying the centrifugal pump. If the suction pressure is less than the needed pressure, it may result in cavitation. Cavitation can adversely impact the efficiency of the impeller.
2. Suction Flow
At the suction level, a constant flow of liquid is required during the operation of the centrifugal pump. If the liquid flow may reduce or there is no flow at the suction due to certain factors, this may result in severe damages to the components of the centrifugal pump. The centrifugal pump can not function in dry areas, hence constant priming is necessary for its proper working.
3. Fluid Density
If the density of the fluid is high, then a high amount of works needs to be done by the centrifugal pump to suck in the fluid. This increases the requirements for extra power. Hence, it should be noted that the centrifugal pumps do not work well with high-density fluids.
4. Fluid Viscosity
The pump should be decided according to the viscosity of the fluid that needs to pump, whether it is a highly viscous fluid or a low viscous fluid. Usually, the centrifugal pump works best for low viscosity fluids.
5. Temperature and Pressure
If the centrifugal pump is needed to operate in applications involving high temperatures or high pressure, some special type of centrifugal pump designs consisting of a special gasket, special seals, and specific mounting design is required.
6. Specific Density and Gravity of Working Fluid
Specific density is defined as the mass per unit volume of the fluid, and specific gravity is defined as the ratio of the density of the substance to that of the water’s density at a specific temperature (usually, {4}^{0}C). Different fluids have different specific densities and specific gravity, hence one must consider these factors before selecting any type of centrifugal pump.
Specifications of the Centrifugal Pump
1. Flow-rate
It refers to the rate at which the centrifugal pump can move the fluid into it. It is generally expressed as gallons per minute (GPM). The flow rates are mentioned on the pumps by the manufactures, hence the pump of the suitable flow rate should be chosen according to the flow rate requirement.
2. Head
It refers to the height above the inlet (suction level) of the pump up to which the centrifugal pump can lift the fluid. It is usually expressed in terms of meters (m) or feet (ft).
3. Net Positive Suction Head (NPSH)
It is an important parameter of the centrifugal pump that enables the users to prevent pump cavitation. It refers to the difference between the stagnation pressures head of the pump’s inlet and its vapour pressure head.
4. Output Power
It refers to the power transferred to the fluid by the centrifugal pump. It is measured in terms of horsepower (hp). It is also known as the water horse-power.
5. Input Power
It refers to the power that must be provided to the centrifugal pump for its operation. Like output power, it is also measured in terms of horsepower(hp). It is also known as the brake horsepower.
6. The efficiency of Centrifugal Pump
The efficiency of the centrifugal pump is the ratio of the output power to the input power. It tells about the total energy losses in the pump due to the friction and the fluid slip, and the amount of the input power utilized for the useful work. It is given by,
Ef=Pw/Ps
Where Ef is the efficiency, Pw is the power of the water, Ps is the power of the shaft.
Generally, the shaft power is the power provided to the shaft of the pump in terms of Brake horsepower (BHP), and the water power (Pw) is given by,
Pw=(Q x H) /3960
Where Q is the flow and H is the head.
In this equation, the constant value, i.e., 3960 is used to converts the flow of heads and products into BHP.
Applications of Centrifugal Pump
- Centrifugal pumps are used in multi-storey buildings for pumping the water.
- These are also used in fire protection services.
- Their applications can be seen in the pharmaceutical industries, mainly for supplying lactose and other drugs.
- They are used in refrigerators for coolant recirculations.
- Its application can be commonly observed in various sectors, such as agriculture (irrigation/sprinkling), power generation plants, mining, wastewater plants and many more.
Advantages of Centrifugal Pump
- They are cheaper than the positive displacement pumps.
- The prime reason for their wide applications is that they do not undergo power losses due to friction as the impeller is the only rotating part in the centrifugal pump.
- These pumps have a simple and easy to use design.
- They don’t have heat transfer problems between the chamber and the motor, and they are leakage proof.
- They can pump sensitive and hazardous fluids.
Limitations of Centrifugal Pump
- The centrifugal pump works efficiently only if the impeller of the pump rotates at a constant and high speed. However, the centrifugal pump becomes very inefficient due to the high viscosity feeds, this may result in the need for high pressure to maintain the definite flow rate. This shows that the centrifugal pumps are best suited for the liquids having a viscosity range between 0.1 and 200 cp.
- Some semi-liquid mixtures (slurries), such as highly viscous oils, or mud can cause excessive overheating, which can lead to the short shelf-life of the centrifugal pump. In this case, positive displacement pumps are preferred to overcome this problem.
- There may be energy losses in the centrifugal pump due to the magnetic resonance.
- The chance of clogging in the suction pipe of the centrifugal pump is very high.
- The centrifugal pump is not preferred for high head applications.
- The positive displacement pump provides suction when the pump gets dry; however, the centrifugal pump does not. The centrifugal pump needs to be primed with fluid through pumping. Hence, the centrifugal pumps can not be used in applications where the supply is not continuous and occurs at some intervals. Moreover, when the feed pressure is not continuous, the output given by the centrifugal pump will also not be constant. On the other hand, the positive displacement pump provides the constant output irrespective of the pressure changes. Hence, the centrifugal pump is not used where the liquid supply is not constant.