Learn the basics of centrifugal pumps, how they work, the different types and where we use them.
Learn the basics of centrifugal pumps, how they work, the different types, and where we use them.
Scroll down to see the YouTube tutorial.
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What is a centrifugal pump?
Centrifugal pumps come in many shapes, colors, and sizes, but they generally look like this.
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| Centrifugal pump |
Pumps consist of two main parts, the pump and the motor. The motor is an induction electric motor that converts electrical energy into mechanical energy. This mechanical energy is used to drive the pump and move the water. The pump sucks water from the inlet and pushes it towards the outlet.
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| Centrifugal pump electric motor and pump |
Inside a centrifugal pump
When we disassemble the equipment we can see that we have a fan and a protective box mounted on the rear of the electric motor. Then inside the motor we have the stator, mounted to the motor casing, which contains the copper coils, and we'll look at that in detail a bit later in this video. Concentric to this we have the rotor and the shaft. The rotor turns, and as it turns, so does the shaft. The shaft runs the entire length of the motor and enters the pump. This then connects to the impeller of the pump. Some centrifugal pump models, like this one, will have a separate shaft for the pump and motor. The separate shafts are joined by a connection known as a coupling. Coupled pumps usually have a bearing housing which, as the name suggests, houses the bearings.
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| Inside a centrifugal pump |
The shaft continues in the pump body. As it enters the cabinet, it passes through a gland, gasket, and gland that combine to form a seal. The axle then connects to the wheel.
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| Discharge outlet and suction inlet |
The paddle wheel imparts centrifugal force to the fluid that allows us to move liquids, such as water, through a pipe. The impeller is enclosed in the pump body. The casing contains and directs the flow of water as the turbine pulls it in and pushes it out. So we have a suction inlet and a discharge outlet.
How does the centrifugal pump work?
At the rear of the electric motor, we see that the fan is connected to the shaft. When the motor rotates the shaft, the fan also rotates. The fan is used to cool the electric motor and will blow ambient air over the case to dissipate unwanted heat. If the motor gets too hot, the insulation of the windings inside the motor will melt, causing a short circuit and destroying it. Fins on the outer perimeter of the shell increase the surface area of the shell, allowing us to remove more unwanted heat.
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| The fins increase the surface area. |
The electric motor is available in a three-phase or single-phase configuration, depending on the application.
We will see the three phases since this is the most common. Inside the three phase induction motor we have 3 separate coils that are wound around the stator. Each set of coils is connected to a different phase to produce a rotating magnetic field.
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| three phase induction motor |
When we pass alternating current, or alternating current, through each coil, the coil produces an electromagnetic field that changes both in strength and polarity as the electrons flowing through it switch directions between forward and backward.
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| Alternating current |
But, if we connect each coil to a different phase, the electrons will change direction back and forth at different times compared to other phases. This means that the magnetic field of each coil will change both in strength and polarity at different times compared to the other phases.
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| different phases |
To distribute this magnetic field, we rotated the coils 120 degrees from the previous phase and inserted them into the stator of the motor housing. This will create the effect of a rotating magnetic field. In the center of the stator we place the rotor and the shaft. The rotor will be affected by the rotating magnetic field and will also force it to rotate.
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| rotor and shaft |
The rotor is connected to the shaft, and the shaft runs from the fan, through the rotor, to the impeller. In this way, when the rotor turns, the wheel also turns. Now, by creating the rotating magnetic field in the motor, we turn the rotor which turns the shaft and which turns the wheel.
Looking at the pump body, there is a water flow channel called a volute. This volute wraps around the perimeter of the casing up to the pump outlet, this channel increasing in diameter as it heads towards the outlet.
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| Scroll |
The shaft passes through the seals and into the pump body where it connects to the impeller.
There are many types of turbines, but most will have these backward curved blades that will be open, half open, or closed with covers.
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| Closed, semi-open or open |
These backward curved paddles do not push the water. The curves rotate, with the outer edge moving in the direction of the expanding scroll. These vanes will provide the fluid with a smooth path for the water to follow. We will see that later in the video.
The wheel is submerged in water. When the wheel turns, the water inside the wheel also turns. As the water rotates, the liquid is pushed radially outward in all directions to the edge of the impeller and into the volute. As the water leaves the impeller, a region of lower pressure is created which draws more water through the suction inlet. The water enters the eye of the impeller and is trapped between the blades.
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| The liquid is radially pushed out. |
As the wheel spins, it transmits kinetic energy, or velocity, to the water. By the time the water reaches the edge of the wheel, it has reached a very high velocity. This high velocity water flows from the impeller into the volute, where it strikes the wall of the pump casing. This impact converts velocity into potential energy or pressure. More water follows behind this and thus a flow develops. The volute channel has an expanding diameter as it wraps around the circumference of the pump body. As it expands, the velocity of the water decreases, causing the pressure to increase. Therefore, this expanding channel allows more water to continue to join and become pressured.
Therefore, the discharge outlet is at a higher pressure than the suction inlet. The high pressure at the discharge allows us to force the fluid through the pipes and into a storage tank or around a piping system.
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| It forces the fluid through the pipes to a storage tank. |
The thickness of the impeller and the speed of rotation affect the volumetric flow rate of the pump, but the diameter of the impeller and the speed of rotation increase the pressure it can produce.
NPSH
One term you will hear is NPSH, which stands for Net Positive Suction Pressure. We will briefly explain what this means.
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| NPSHR |
There are two letters at the end of this acronym NPSHR and NPSHA. The R is the required NPSH. Each pump is tested to this value and can be obtained from the pump manufacturer via the pump performance chart. Don't worry about this confusing picture at this point, we'll break it down and cover it in detail in a dedicated article. The R value is essentially a point of warning or danger. As the water enters the pump and flows through the eye of the impeller, it experiences a loss of energy due to friction, giving us a pressure drop. Under certain conditions, the water flowing through this section can reach boiling point, when this happens we call it cavitation. We'll see more about that in a moment.
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| r-value |
The other letter was A and this is the available NPSH. This depends on the installation of the pump and must be calculated. It takes into account things like the type of installation and elevation, the temperature of the liquid, the boiling point of the liquid, etc.
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| NPSHA value |
The available value must be greater than the required value. (NPSHA > NPSHR)
For example, if we have a facility and we calculate the NPSHA to be 11 but the pump requires an NPSHR of 4, the pump should be fine. However, if we install a pump that requires an NPSHR of 13, the available NPSH is insufficient and cavitation will occur.
cavitation
So what is cavitation? As we know, water can change from a liquid to a vapor or a gaseous state. The point at which this occurs is known as the vapor pressure.
We know that water boils at about 100°C (212°F) and that is because it is at sea level which has an atmospheric pressure of 101.325 kPa (1 Bar) but if we climb to the top of Everest then the water boils at just 71 °C (160 °F) because the atmospheric pressure has dropped to 34 kPa (0.34 bar). As the pressure decreases, it becomes easier for the water to boil.
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| Atmospheric pressure |
So, at the suction inlet of the pump we know that there is going to be a pressure drop and if this pressure is lower than the vapor pressure of the pumped liquid, the water can reach boiling point. When this happens, cavitation occurs.
During cavitation, the air particles in the water expand when they reach boiling point, then collapse in on themselves very quickly. As they crumble they can damage the impeller and pump body, this removes small bits of metal from the surface and if this continues to happen it will eventually destroy the pump. Therefore, we must ensure that the available pressure is greater than the required pressure of the pump.
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| cavitation |
Where do we use centrifugal pumps?
We use centrifugal pumps everywhere. We use them to move liquids from one tank to another or around a system.
For example, we could use a small in-line centrifugal pump in our home's heating circuit to move hot water around the property.
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| Use of centrifugal pump |
We could use large centrifugal pumps to move condenser water from a chiller condenser to the rooftop cooling tower as part of the centralized cooling system.
We will look at the types of pumps and their applications in our next article in this series.
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