Exp 7

Introduction
Turbine:
Turbines are defined as the hydraulic machines which convert hydraulic energy into mechanical
energy. This mechanical energy is used in running an electric generator which is directly coupled
to the shaft of the turbine. Thus, the mechanical energy is converted into electrical energy. The
electric power which is obtained from the hydraulic energy (energy of water) is known as
Hydroelectric power. At present the generation of hydroelectric power is the cheapest as compared
by the power generated by other sources such as oil, coal etc.

Steam Turbine:

Steam turbines are mechanical devices that convert the thermal energy of pressurized steam into
mechanical work, which is then used to generate electricity or perform other types of mechanical
tasks. They are widely used in power plants, industrial processes, and marine propulsion systems.
A machine that collects thermal energy from pressurised steam and uses that energy to perform
mechanical work on a rotating output shaft is a ‘Steam turbine’.

The steam is produced by heating water using a heat source such as coal, gas, solar, or nuclear
energy. As the steam flows past the spinning blades of the turbine, it cools and expands, converting
its potential energy into kinetic energy in the blades. This makes steam turbines ideal for driving
electrical generators, as they generate rotary motion that can be converted into electrical energy
through a magnetic field.

Parts of Steam turbine:

Rotor and Blades:

Rotor:

It consists of a wheel mounted to a shaft. The rotor is positioned on the bearing so that it can rotate
smoothly and effectively. The rotor's blades are installed along its full circle. The rotor in a typical
turbine is of two types, Drum-type and Disk-type.

Blades:
On the rotor's whole circumference, a great number of blades are erected. The efficiency of the
turbine depends on more than anything else on the design of the turbine blades. The impulse blades
must be designed to convert the kinetic energy of the steam into mechanical energy. The blades
are strong enough to withstand the following factors:

• High temperatures and stresses due to the pulsating steam load.

• Stress due to centrifugal force.
• Erosion and corrosion resistance.

Here, there are only two types of blades, Fixed-type and Moving-type.

Impulse and Reaction Turbine:

In Impulse Steam Turbine, there are some fixed nozzles and moving blades. As steam enters, it
passes through nozzles into turbine. The relative velocity of steam at the outlet of the moving
blades is same as the inlet to the blades. During Expansion, high-pressure drop in nozzles causes
increase in velocity of steam. This high-velocity jet of steam strikes the blade with constant
pressure. Now blades are starting to move in the same direction of the steam flow. Due to change
in momentum, turbine's shaft is starting to rotate.

In a reaction turbine, nozzles will move on bearing in the opposite direction of the steam flow and
the pressure is not constant in this turbine. That's why; a reaction force is always applied on the
nozzles and tubes. In this turbine steam produces both impulsive and reactive force. So, the
resultant force produces to the rotor is the vector sum of impulsive and reactive force and the
reaction force is an unbalanced condition. Generally, this turbine is not used for commercial
purpose. Due to this reactive force, it is called reaction turbine. An example of this turbine is
Parson's Turbine.

Other Parts:

Nozzle:

In a steam turbine or steam turbine generator, a steam nozzle is a channel or duct with a cross-
sectional area that varies gradually and is used to transform the heat energy of steam into kinetic
energy.

Casing:
It is an enclosure that houses the blades, and rotor assembly, and holds the nozzle in place. By
using some sort of packing or oil seal to allow the shaft to run freely and stop leaks, the casing is
kept apart from the rotor. There are two casings, an inner and an exterior casing.

Turbine Seals:

Seals are used to reduce the leakage of steam between the rotary and stationary parts of the

steam turbine. Depend upon the location of seal, the seals are classified as two types, they are:

• Shaft Seal
• Blade Seal

Types of Steam Turbine:

Based on the action of steam:

• Impulse Turbine
• Reaction Turbine

Based on the steam flow direction:

• Axial flow turbine

• Radial flow turbine

Types of Compounding:

There are two types of compounding:

• Pressure compounding (The Rateau Turbine)

• Velocity compounding (The Curtis Turbine)

Velocity compounding of Impulse Turbine:

In the simple impulse stage, the optimum condition of blade speed is hardly practicable, and with
the speeds used only a small amount of the kinetic energy of the steam can be utilized. The velocity
compounded stage is used to employ lower blade speeds and a higher utilization of the kinetic
energy of the steam. In this type all the expansion takes place in a single set of nozzles, and the
steam then passes through a series of blades attached to a single wheel or rotor. Since the blades
move in the same direction it is necessary to change the direction of the steam between one set of
moving blades and the next. For this purpose, a stationary ring of blades is fitted between each
pair of moving blades. The steam velocity is high but the blade velocity is less than that for the
single row turbine. The kinetic energy of the jet is thus utilized. in the multiple stages. The inlet
velocity to the fixed blades is the absolute exit velocity from the first row of moving blades. The
absolute inlet velocity to the second row of moving blades is the exit velocity from the fixed blades.

Disadvantages of velocity compounding:

• In a single row wheel, steam temperature is high, so cast iron cylinder cannot be use due
to phenomenon of enlargement; cast steel cylinder is use which is costlier than cast iron.
• It has a high steam use and low efficiency.

Pressure Compounding of Impulse Turbine:

The pressure drop available to the turbine is used in a series of small increments; each increment
being associated with one stage of the turbine. The nozzles are carried in diaphragms which
separate each stage from the next.

The steam pressure in the space between each pair of diaphragms is constant, but there is a pressure
drop across each diaphragm as required by the nozzles. Precaution must be taken to prevent
leakage of the steam from one section to the next at the shaft and outer casing.

The steam speeds, and hence the blade speeds, are low if the number of stages is high. In figure
the variations in pressure and velocity through the turbine are shown, the final pressure being that
of the condenser, and the final velocity that required for the steam to leave the turbine.

In figure only one set of wheels is shown, but these may be followed by another set with a larger
mean radius. Each of the stages can be analyzed by the method used previously for the single stage.
A turbine with a series of simple impulse stages is called a Rateau turbine.

Disadvantages of pressure compounding:

• Due to high velocity, it has a high frictional loss.

• The consumption of steam is very high. To resist the high velocity the blades require
specific designing and fabrication.
Pressure-Velocity Compounded Impulse Turbine:

Pressure-velocity compounding is a combination of the above two types of compounding. In fact,

a series of velocity-compounded impulse stages is called a pressure-velocity compounded turbine.
Each stage consists of rings of fixed and moving blades. Each set of rings of moving blades is
separated by a single ring of fixed nozzles. In each stage, there is one ring of fixed nozzles and 3-
4 rings of moving blades. Each stage acts as a velocity compounded impulse turbine.

The steam coming from the steam generator is passed to the first ring of fixed nozzles, which gets
partially expanded. The pressure partially decreases, and the velocity rises correspondingly. It then
passes over the 3-4 rings of moving blades (with fixed blades between them), where nearly all of
its velocity is absorbed. From the last ring of the stage, it exhausts into the next nozzle ring and is
again partially expanded.

This has the advantage of allowing a bigger pressure to drop in each stage and, consequently, fewer
stages are necessary, resulting in a shorter turbine for a given pressure drop. We may see that the
pressure is constant during each stage; the turbine is, therefore, an impulse turbine. The method of
pressure-velocity compounding is used in the Curtis turbine.

Applications

Pressure-velocity compounded impulse steam turbines are commonly used in power generation
applications, particularly in steam power plants. They are employed in various industrial processes
for power generation, such as in refineries, chemical plants, and pulp and paper mills. They can
find applications in marine propulsion systems, where they drive ship propellers using steam
generated onboard.