Steam Turbine

Principle of operation of steam turbine:

An ideal steam turbine is considered to undergo an isentropic process, or constant entropy process, in which the entropy of the inlet steam is equal to the entropy of the outlet steam in the turbine. No steam turbine is purely isentropic, however, with distinctive isentropic efficiencies of 20–90% on the basis of the application of the turbine. The inner side of a turbine is made up of several sets of blades/buckets. A set of stationary blades is fixed to the casing and a set of rotating blades is linked to the shaft. These sets intermesh with certain minimum clearance, with the size and configuration of sets changing to efficiently exploit the expansion of steam at each stage.

Types of steam turbines

Generally, the turbines are classified into two types,

1. Impulse Turbine
2. Reaction Turbine
   

Fig 1. Impulse and Reaction turbine

Impulse Turbines

In this type of turbine, the steam jets are directed at the turbine’s bucket shaped rotor blades so that the pressure exerted by the jets causes the rotor to rotate and there is a reduction in the velocity of the steam as it expenses its kinetic energy in rotating the blades. As a result blades change the direction of flow of the steam with its pressure remaining constant as it passes through the rotor blades meanwhile the cross section of the chamber between the blades is constant. So impulse turbines are also called as constant pressure turbines. The next series of the fixed blades reverses the direction of the steam and then it passes to the second row of moving blades.

Reaction Turbines

The rotor blades of the reaction turbine are shaped like airfoils, arranged in such a way that the cross-section of the chambers between the fixed blades lessens from the inlet side towards the outlet side of the blades. The chambers in between the rotor blades importantly form nozzles so that as the steam advances through the chambers its velocity rises but at the same time its pressure decreases, just as in the nozzles made up of the fixed blades. Thus, the pressure reduces in both blades; fixed and moving. As the steam arises in a jet from between the rotor blades, it forms a reactive force on the blades which then creates the turning moment on the turbine rotor, just like in Hero’s steam engine. [Newton’s Third Law – For every action there is an equal and opposite reaction]

Compounding of impulse turbine:

Compounding of impulse turbine is done to reduce the rotational speed of the impulse turbine to practical limits. (A rotor speed of 35,000 rpm is possible, but is very high for the practical uses.). Compounding is done by using more than one set of nozzles, blades, rotors, in a series, fixed to a common shaft; so that either the steam pressure or the jet velocity is utilized by the turbine in stages.

There are three main types of compounding done on impulse turbines. They are:

1. Pressure compounding,
2. velocity compounding and
3. Pressure and velocity compounding impulse turbines.

1. Velocity Compounding:

Fig 2. Velocity Compounding

Pi = Inlet Pressure, Pe = Exit Pressure, Vi = Inlet Velocity, and Ve = Exit Velocity

The idea of the velocity-compounded impulse turbine was first introduced by C.G. Curtis for solving the problems of a single-stage impulse turbine for the use with high pressure and temperature steam. So it is also called “The Curtis Stage Turbine”. The Curtis stage turbine is formed of 1 stage of nozzles as the single-stage turbine, followed by 2 rows of moving blades. These two rows of moving blades are separated by one row of fixed blades attached to the turbine stator, which redirects the steam exiting the first row of moving blades towards the second row of moving blades. A Curtis stage impulse turbine is shown in Fig. below with the schematic pressure and absolute steam-velocity changes through the stage. In the Curtis stage, the overall enthalpy drop. So, pressure drop occurs in the nozzles so that the pressure does not vary in all three rows of blades.

2. Pressure Compounding:

Fig 3. Pressure Compounding

Pressure compounding involves dividing up of the whole pressure drop from the steam chest pressure to the condenser pressure into a series of smaller pressure drops across the several stages of impulse turbine. The nozzles are installed into a diaphragm locked in the casing. This diaphragm separates the wheel chamber from each other. All rotors are mounted on the same shaft and the blades are fixed on the rotor.

3. Pressure-Velocity Compounding:

It is a combination of pressure compounding and velocity compounding.

Fig 4. Pressure-Velocity Compounding

Two-row velocity compounded turbine is found to be more efficient than the three-row type. In a two-step pressure velocity compounded turbine, at first pressure drop arises in the first set of nozzles, the resulting addition in the kinetic energy is absorbed successively in two rows of moving blades before the second pressure drop takes place in the second set of nozzles. As the K.E gained in each step is used up completely before the next pressure drop, the turbine is both pressure as well as velocity compounded. The kinetic energy added due to the second pressure drop in the second set of nozzles is taken up successively in the two rows of moving blades. This type of steam turbine is comparatively simple in construction and is very much compact than that of the pressure compounded turbine.

Fig 5(a),(b). Velocity triangle of impulse turbine

The fixed blades guide the outlet steam from the previous stage in such a way so as to smoothen the entry at the next stage is ensured. K, the blade velocity coefficient may be different in each row of blades.

Reaction Turbine:

A reaction turbine consists of rows of fixed and rows of moving blades. The fixed blades function as nozzles. The moving blades get motion as a result of the impulse of steam received and also due to expansion and acceleration of the steam relative to them i.e. they also act as nozzles. The drop in enthalpy per stage of one row fixed and one-row moving blades are shared among them, often equally. Thus, a blade having 50 % degree of reaction is the one in which half of the total enthalpy of the stage drops in the fixed blades and the half in the moving blades. But the pressure drops will not be equal. They are larger for the fixed blades and larger for the high-pressure than the low-pressure stages. The moving blades of a reaction turbine can be easily distinguished from those of an impulse turbine because they are not symmetrical and, also because they act partly as nozzles, have a shape similar to that of the fixed blades, but curved in the opposite direction. The schematic pressure line in the figure given below shows that pressure drops continuously through all rows of blades, fixed and moving. The absolute steam velocity varies within each stage as shown and keeps repeating from stage to stage. The second figure shows the velocity diagram for the reaction stage.

Fig 6. Compounded Reaction Turbine

Fig 7. Velocity triangle for Reaction Turbine

A very widely used design has half a degree of reaction or 50% reaction and this is known as Parson’s Turbine. This consists of a symmetrical stator and rotor blades. The velocity triangles are symmetrical and we have,

Things to rememer

• Generally the turbines are classified into two types,
  1. Impulse Turbine
  2. Reaction Turbine
• In impulse steam turbine, the steam jets are directed at the turbine’s bucket shaped rotor blades so that the pressure exerted by the jets causes the rotor to rotate and there is reduction in the velocity of the steam as it expenses its kinetic energy in rotating the blades.
• The rotor blades of the reaction turbine are shaped like airfoils, arranged in such a way that the cross section of the chambers between the fixed blades lessens from the inlet side towards the outlet side of the blades.
• There are three main types of compounding done on impulse turbines. They are:
  1. Pressure compounding,
  2. velocity compounding and
  3. Pressure and velocity compounding impulse turbines.
• A reaction turbine consists of rows of fixed and rows of moving blades. The fixed blades functions as nozzles. The moving blades get motion as a result of the impulse of steam received and also due to expansion and acceleration of the steam relative to them.
• A very widely used design has half degree of reaction or 50% reaction and this is known as Parson’s Turbine. This consists of symmetrical stator and rotor blades.