A steam turbine is one that is considered as the heart of the thermal power plant. This machine produces mechanical energy by converting the heat energy of the steam into kinetic energy. This heat energy is derived in various ways in various thermal power plants. In a coal-based thermal power plant, this heat energy is derived from a boiler, where the coal is burnt. To know how steam is produced in the coal-based thermal power plant, please visit my previous post.
https://sciencetopic03.blogspot.com/2021/12/how-thermal-power-plant-works-coal-based.html
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Credit (Steam Turbine): By MAN SE, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=7738609 |
There are generally two types of steam turbine, which produces mechanical energy through steam. They are,
- Impulse turbine
- Reaction turbine
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AirFoil shape |
Both types of turbines use steam as their working medium for producing mechanical energy. A steam turbine is a collection of several blades constructed around the stator and rotor of the turbine. These blades are generally made up of airfoil and bucket shape. The airfoil shape is similar to the shape of the airplane wings. When a forced air comes in contact with the wings a lift force is created, thus making the airplane to fly. The same method is used in the steam turbine to make it rotate. When forced air comes in contact with the turbine blades of airfoil shape, a pressure difference is created, thus making the turbine to rotate. The impulse turbine consists of bucket-shaped blades and the reaction turbine has airfoil shape blades. A typical thermal plant uses both the impulse and the reaction steam turbines for a better operation. Usually, the first part of the steam turbine in the power plant will be an impulse part while the second and third part will be the reaction part. The reason for the impulse turbine is to reduce the high-pressure steam to a low value at starting, which decreases the number of stages and the length of the turbine. The first part is called as the high-pressure turbine, while the second part is called as intermediate turbine and the third part is called as the low-pressure turbine. The reason for the various stages is due to the difference in the pressure-flow across the turbines. Now let’s look at the construction and working principle of the above two types of turbines.
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Two different turbine blades |
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Different stages of the turbine in a power plant |
Impulse Turbine
The impulse turbine is a uniform shape turbine that uses the direct jet of steam to hit the turbine blades, and giving an impulse force to the blades. This high impulse force makes the turbine blades to rotate, thus producing the mechanical energy. This high jet of steam is produced by the nozzle which is situated at the front of the turbine. There will be several nozzles in front of the turbine to inject the steam.
Steam usually carries three forms of energy namely velocity, temperature, and pressure. Before entering into the turbines the steam from the boiler will be in the order of high pressure and high temperature. The nozzle in the turbine compresses the steam thereby making low-velocity steam to high-velocity steam. This high-velocity jet of steam makes the impulse turbine rotates when it comes in contact with the blades. As the pressure of the steam, almost remains constant throughout the blades in this turbine, all of the turbine blades will be in the same size. That’s why an impulse turbine is a uniform shape turbine. Normally, a multistage impulse turbine is used in power plants, which have moving blades as well as fixed blades as many stages. The fixed blades are attached to the casing while the moving blades are attached to the rotor. This multistage is done as a method of compounding to absorb more power from the steam. Compounding is a method, done in steam turbine, in which the power of the steam is absorbed across several sections rather than absorbing in a single section. If only a single-stage turbine is used, then most power of the steam will cause the turbine to rotate at a higher speed which could cause heavy vibration and damage to the turbine. For this purpose, compounding is used. There are various types of compounding, in which the steam is made to flow in different forms, to absorb much energy from it. The various types of compounding used in impulse turbines are,
- Velocity compounding
- Pressure compounding
- Pressure velocity compounding
Velocity Compounding
Velocity compounding is the process, in which the turbine blades are protected from the high pressure and high temperature of the steam. This type of compounding causes the starting pressure of the steam to get decreased drastically and maintain it constantly throughout the turbine. As shown in the below figure, this type of compounding in impulse turbine incorporates fixed and moving blades, which are arranged in an alternative manner. All these blades will be in the shape of a bucket to capture the energy of the steam. At first, the nozzle produces a reduced pressure and high-velocity steam towards the blades. The moving blades absorb the force and start to rotate. Then the steam leaves the moving blades and hits the fixed blades which are placed in a flipped manner. The purpose of the fixed blades is to divert the steam without change in pressure to the next set of moving blades, which absorbs more power from the steam. This makes the pressure constant all over the period. The main disadvantage of velocity compounding is that high vibration is made due to high steam velocity during starting.
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Credit (Velocity compounding in Impulse turbine): By Subikkumar - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19425431 |
Pressure Compounding
Pressure compounding is done to reduce the velocity of the steam while entering into the first set of blades. Therefore the velocity is decreased, by partially expanding (decreasing) the pressure of the steam which can be observed from the below figure.
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Credit(Pressure compounding in Impulse Turbine): By Subikkumar - File:Fig3-Subik_Kumar-Schematic_Diagram_of_Pressure_compounded_Impulse_Turbine.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30462493 |
In this turbine also, the fixed and moving blades are arranged alternatively as in the above case. But here the fixed blades act as the nozzle, whereas in the previous case, the nozzles are present only at the front side and the fixed blades are used for diverging the steam to the moving blades. When the high-pressure steam from the boiler passes into the nozzle blades, the pressure of the steam is partially reduced. This in turn, increases the velocity partially. This partially increased velocity of the steam does not cause so much speed in the turbine when it hits the moving blades. So the high-speed vibration is avoided. When the steam hits the moving blades its velocity decreases gradually, but the pressure remains constant. The steam leaving the moving blades again goes through the nozzle and decreases in pressure, which in turn increases the velocity of the steam. Thus the nominal velocity is made to pass over, across all the turbine blades by decreasing the pressure partially step by step. Finally, the steam exits the turbine when the pressure of the steam is equal to the condenser pressure. The only disadvantage of this turbine is, it is bulkier, as the expansion takes in more number of stages.
Pressure - Velocity Compounding
Pressure-velocity compounding is the combination of the above two types of compounding. In this method the pressure is expanded across various stages, so the high blade velocity is avoided. Also, in this type of compounding, a large pressure drop occurs, due to which the size occupation is also less.
So in this compounding method, the steam from the boiler is brought and entered into the nozzle. As it enters into the nozzle, the pressure decreases and the velocity increases partially. This high-velocity steam is then passed to the moving blades which makes the turbine to rotate. During the contact with the moving blades, the velocity of the steam decreases while the pressure remains constant. The steam is then passed to the fixed blades which redirect the steam towards another set of moving blades to absorb more power of the steam. Here the steam never changes in pressure and remains constant over the period. Again the steam is passed through the nozzle and made to pass through the same set of blades to get absorbed more.
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Credit (Pressure-velocity compounding of Impulse turbine): By Subikkumar - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19425443 |
Reaction turbine
The reaction turbine is also the same as the impulse turbine which rotates due to the effect of the steam. The fixed and the moving blades here are in airfoil shape, which partially compresses the steam, as shown in the below figure. So the steam output from the nozzle will be at medium pressure and velocity. This steam is then passed over the moving blades, to cause them to rotate. As steam carries three forms of energy, these three parameters decrease, when it passes through a set of blades in a reaction turbine. As the velocity of the steam decreases when passing over the moving blades, a set of fixed blades is added in a vertically flipped manner next to each set of moving blades to increase the velocity of the steam. After the steam picks up speed it passes over to the next set of moving blades, to make it rotate. For a decrease in pressure, each fixed and moving blade before the previous one is slightly bigger in shape. So in this turbine, all the turbine blades will not be the same in size.
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Credit (Pressure compounding in Reaction Turbine): By Subikkumar - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19425451 |
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Credit (Reaction Turbine): By Pro-Per Energy Services - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37567941 |
The purpose of variation in the size of the reaction turbine blades is because, as the high-pressure steam flows over the steam turbine blades at first, it decreases in pressure over the period. As it decreases in pressure, its volume increases. So a bigger turbine is introduced at the second part to occupy such a large volume of steam. But in an impulse turbine, the pressure almost remains constant due to the structure of the blades. Thus a turbine according to the pressure of the steam is used, to capture as much of energy from the steam. This type of arrangement in the reaction turbine is called pressure compounding, as the pressure is made to pass all over stages. In a reaction turbine, only the pressure compounding is available and not any other compounding, because velocity compounding is used to protect the turbine from high pressure and high-temperature steam. A reaction turbine is a low-pressure turbine, so velocity compounding is not available. In a reaction turbine, sometimes the steam is directed in two directions from the inner small blade to the outer bigger blade for more power absorption. So by these successive arrangements of steam turbine, we can make a rotation and can generate electricity through the alternator.
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