Steam Turbines: Classification and Advantages | Mechanical Engineering

In this article we will discuss about:- 1. Introduction to Steam Turbines 2. Parts of Steam Turbines 3. Classification 4. Advantages.

Introduction to Steam Turbines:

A steam turbine is a form of prime mover in which the energy available with high-pressure steam is converted into mechanical power directly in the form of rotary motion. The operation of steam turbine depends on the dynamic action of steam expanded through nozzle. The steam turbine develops power from the change of momentum of steam jet while passing over a smooth vane or blade (Fig. 4.3).

High-pressure steam from boiler is expanded in a nozzle (Fig. 4.4). It is well known that nozzles are used to convert pressure energy into kinetic energy. The enthalpy of steam is first converted into kinetic energy in the nozzle or blade passage. The high-velocity steam produced by the nozzle flows over the curved vane or curved blade, thereby gaining a change in momentum.

Figure 4.5 is the projected view of a set of two nozzles mounted by the side of a rotor mounted with a number of blades. They are separated by a gap of 5 mm. The flow of steam and the motion of blades have been shown in Fig. 4.5.

The force produced by the change in momentum is known as impulsive force which tries to push the blade in forward direction, but the blades are not free to move. They are fixed on wheel or rotor. If a certain number of blades are fixed on the periphery of the rotor and the steam jet is allowed to pass through it, the rotor will start rotating.

The rotor of the turbine is mounted on the shaft and supported by bearing. Thus, the shaft starts rotating (Fig. 4.6). A steam turbine is mainly used to drive electrical generator in thermal power plants to generate electrical energy.

Parts of Steam Turbine:

The main parts of steam turbines are – nozzles, wheel or rotor, vanes or blades, and casing.

i. Nozzle:

A high-pressure steam from the boiler enters into the convergent divergent nozzle attached with the casing of the turbine. The expansion of steam takes place through the nozzle. A high-pressure and relatively low-velocity steam is available at the entrance of the nozzle. The pressure at the exit of the nozzle is found very less due to the expansion of steam. A high-velocity jet of steam is produced at the exit section.

ii. Rotor:

It is a wheel mounted on a shaft. The rotor is mounted on the bearing to have smooth and effective rotation. The blades are erected on the entire circumference of the rotor. A rotor fitted with blades has been shown in Fig. 4.7. In actual practice the blades are shrouded from the top which has not been shown in the figure. It is provided to keep the blades in their position without deflecting them by the thrush given by high-pressure steam and also the steam flowing in blade channel will not be allowed to leave from the top due to centrifugal fujal action.

iii. Blades:

On the entire periphery of the rotor, a large number of blades are erected. Two conjugative blades form a blade passage through which a jet of steam at high kinetic energy impinges on the blade surface into the blade passage.

A change in the momentum is experienced by the flow of steam, which results in tangential force on the blade. The blade starts moving, but it is not free to move because it is fixed on the rotor. So, the rotor starts rotating. The surface of the blades is made very smooth to minimize the blade friction loss (Fig. 4.8).

iv. Casing of Turbine:

It is an enclosure which covers the rotor as well as blades assembly. The nozzle is attached with casing. The casing is kept separate from the rotor by placing some kind of packing or oil seal to run the shaft freely and to prevent leakage.

Classification of Steam Turbine:

The classification of steam turbine is given in Fig. 4.9. The most important common divisions are with respect to the dynamic action of the steam. A steam turbine is mostly a case of axial flow.

I. Impulse Turbine:

i. Simple Impulse Turbine (De Laval Turbine):

A simple impulse turbine or single-stage impulse turbine is suitable for low-pressure steam. An impulse turbine is a type of turbine in which the pressure drop takes place only in the nozzle. The pressure remains constant throughout while flowing over the moving blades. In this case, the high-pressure steam is initially expanded in a nozzle or a set of nozzles which is placed by the side of rotor leaving some gap of about 5-6 mm.

As the steam flows through the nozzle, pressure falls from steam chest pressure to condenser pressure in the case of condensing type or atmospheric pressure in the case of non-condensing type of plant. Due to pressure drop, a high velocity is obtained at the outlet of the nozzle. The steam at this high kinetic energy flows over the blade channel and then it is discharged from the turbine.

It is evident that if the initial pressure of steam is very high, the kinetic energy obtained at the exit of the nozzle is very high and hence the velocity of steam leaving the moving blade is high in proportion to the maximum velocity of steam when leaving nozzle. The loss of kinetic energy at the exit is known as leaving loss. The velocity of steam at the exit is known as lost velocity.

In the beginning, the simplest form of turbine was developed which was known as De Laval turbine. In this turbine, the exit velocity or leaving velocity or lost velocity was about 4-5% of the nozzle outlet velocity. In addition to this, since all the kinetic energy was to be utilized by one row of moving blade only, the revolution of the rotor could be expected about 20,000 rpm.

Such a high revolution was rather unmanageable with balancing point of view. However, the rotor speed could be reduced by the method of compounding. The compounding of turbine was done to reduce the rotor speed. Figure 4.10(a) shows the diagrammatic representation of a simple impulse turbine in which large number of blades is fitted on the circumference of a wheel, called rotor.

Figures 4.10(a) and 4.10(b) show the representation of an impulse turbine. The top portion shows the nozzle and blades, and the lower portion shows the variation of the pressure and velocity of steam as it flows over the nozzle as well as blade channel. Since the expansion of steam takes place only in the nozzle, the pressure drop is represented by the curve PQ by dotted line.

As there is no change in the pressure of steam while passing over the row of blade, the pressure is shown by the horizontal line QR. Since the velocity of steam increases due to expansion in the nozzle, an increase in velocity is represented by AB by a full line. As some of the kinetic energy is utilized as it flows over the moving blade, the steam comes out at less velocity represented by the line BC.

ii. Compounded Steam Turbines:

Due to advancement in technological ability, the trend is to generate steam at high pressure and temperature, as high as 100-150 bar pressure and about 550°C super heat. For obtaining maximum thermal efficiency, the total pressure drop from boiler to condenser pressure must be completely converted into kinetic energy.

If the entire pressure drop from high to low pressure takes place only in one set of nozzles, then the turbine rotor rotates at a very high speed in order of about 20,000 rpm. Such a high revolution of turbine rotor is not useful for practical purposes. It poses a number of technical problems such as structural failure due to high centrifugal stress developed, increase in vibration, excessive noise produced, and overheating of bearings.

The high speed will require a reduction gear to couple with generator. In addition to this, the lost velocity at the discharge point is very high. It is about 5-6% of the initial velocity. This gives rise to a great loss. Therefore, the expansion of steam is carried out in several stages instead of a single stage. The successful utilization of total available energy can be done by compounding.

The different types of compounded steam turbines are as follows:

(a) Velocity-compounded turbines

(b) Pressure-compounded turbines

(c) Pressure-and-velocity-compounded turbines

a. Velocity-Compounded Turbines (Curtis):

A velocity-compounded turbine is shown in Fig. 4.11. In this case there is one set of nozzles and two or more rows of moving blades arranged in series. In between two rows of moving blades, one set of guide blades which is fixed and hung from the casing of turbine is arranged in a suitable manner.

The placement of the blades in fixed row is just the reverse of moving blades rows. The steam is expanded in nozzle from boiler pressure down to condenser pressure. The high-velocity jet of steam coming out from nozzle is passed onto the first row of moving blades where it produces a change of momentum.

The kinetic energy gained in nozzle is utilized in the stages of moving rows and finally the steam is exhausted from the first row at comparatively less kinetic energy but in reverse direction. The steam then enters the first row of fixed blades and is redirected by changing direction to the second row of moving. This way the expansion is continued till total kinetic energy is fully absorbed. A turbine working on this principle is known as velocity-compounded steam turbine.

b. Pressure-Compounded Turbines (Rateau):

Pressure-compounded impulse turbine is one in which a number of simple impulse turbines are arranged in series and placed on a common shaft. In this case, a row of fixed nozzles is placed at the entry of each row of moving blades, i.e., this comprises alternate rows of fixed nozzles and moving row of blades in series.

The total pressure drop from high pressure to exhaust pressure is split into series of smaller pressure drop to take place in different stages in series. Each set combining one row of fixed and one row of moving blade is known as one stage. High-pressure steam is expanded in the first row of fixed nozzles of the first stage with a small pressure drop.

Due to small pressure drop, less kinetic energy is available at the exit of first row of fixed nozzles. The steam with small kinetic energy enters to the first row of moving blades where it undergoes a change of momentum and the kinetic energy is absorbed. The pressure remains the same while flowing in the first row of moving blades.

The steam from first stage then enters to the second stage where there is further small pressure drop and increase in the kinetic energy which is again absorbed. This will continue till the steam pressure becomes equal to the exhaust pressure.

c. Pressure-and-Velocity-Compounded Turbines:

Such turbines are the combination of pressure and velocity compounding. If the pressure range is very large, this arrangement is very much suitable. As we know, a two-row velocity-compounded turbine is more efficient than a three-stage velocity-compounded turbine. But the construction of two-row velocity-compounded turbine possesses some difficulties as it increases the velocity per blade.

Hence, the total pressure drop of steam from boiler pressure to exhaust pressure would split up in two and three steps, as done in pressure compounding, and the kinetic energy gained in each step is absorbed completely in two moving wheels before the next pressure drop occurs.

In a two-step pressure-and- velocity-compounded turbine, the first pressure drop occurs in the first set of nozzles, the gain in kinetic energy is utilized completely in two rows of moving blades before the second pressure drop occurs in second set of nozzles, and the kinetic energy gained is again utilized in two rows of moving blades successively.

II. Reaction Turbine:

In principle, pure reaction turbine has no meaning with steam as a working fluid. It is actually with a joint application of impulse and reaction. In this type of turbine, the high-pressure steam is not expanded initially in the nozzle only, as in the case of impulse turbine, but the expansion of steam takes place in moving blade row also.

The moving blade also acts as nozzle. The shape of the blade is so well-designed (known as aerofoil section blade) that the blade channel formed between two consecutive blades acts as a nozzle and forms a narrow passage at the exit. At the exit of the nozzle, steam will possess both pressure as well as kinetic energy.

The steam which possesses some pressure energy gets further expanded while passing over in blade channel and hence further increase of kinetic energy is obtained. Hence, a gradual pressure drop takes place continuously over the nozzle and moving blades. The increase in the velocity of steam flowing over the blade passage develops an opposite reaction.

The opposite reaction force acting on the blade forms a propulsive force. In addition to this propulsive force, there is also a change in momentum due to change in velocity. This causes an impulsive force on the moving blade. Thus, the net force on moving blade in reaction turbine is the resultant of reaction force and the impulsive force as shown by the force diagram shown in Fig. 4.12.

Figure 4.13 shows a four-stage reaction turbine. The actual reaction turbine (Fig. 4.13), also called impulse reaction turbine, consists of a number of rows of moving blades fitted on a single rotor. The fixed blade ring is hung from the casing of the turbine between the two rows of moving blade. The blade of fixed row is placed just reverse to the blade of moving row. The high-pressure steam passing in the first row of fixed blade undergoes a small drop in pressure causing the increase of velocity of the steam.

It then enters the first row of moving blades where it suffers further drop in pressure and the kinetic energy is converted into mechanical energy in terms of the rotation of rotors. Thus, the velocity of steam decreases. This continues in the further rows of moving and fixed blades till the pressure of steam is completely reduced. The variation of pressure and velocity has been shown in Fig. 4.14.

Comparison of Impulse and Reaction Turbine:

Impulse Turbine:

i. Pressure drop takes place only in the nozzle and not in the moving blade. The pressure remains constant in the moving blade row.

ii. The area of blade channel at entrance and exit is same.

iii. Symmetrical profile type blades are used in impulse turbine which provides a uniform section. Impulse turbine may be either partial admission or full admission.

iv. This occupies less space for the same power.

v. Not much power can be developed.

vi. Efficiency is found low.

vii. Suitable for small power generation.

viii. High rotor speed requires compounding of turbine.

Reaction Turbine:

i. Pressure drop takes place in the fixed blade (nozzle) as well as in the moving blade row also.

ii. The area of blade channel at the exit is made narrow. It is of convergent type.

iii. Aerofoil types of blades are used in reaction turbine. Reaction turbine is always full admission turbine.

iv. This occupies more space for the same power.

v. Much power can be developed.

vi. High efficiency can be achieved.

Vii. Suitable for medium and high power generation.

viii. The speed is relatively low and hence no compounding is required.

Advantages of Steam Turbine over Steam Engines:

(a) The thermal efficiency of a steam turbine plant is higher.

(b) Steam turbines are highly simplified in construction and operation.

(c) Condensate can be directly used in boilers.

(d) Vibration and noise are minimized.

(e) Balancing of rotor can be done accurately.

(f) Much higher speed is possible.

(g) Steam turbines are suitable for the operation with high-pressure steam.

(h) They are well suited for large steam power plants.