A Report on Steam Turbine

A
TECHNICAL SEMINAR REPORT
ON
“WORKING OF STEAM TURBINES”
Submitted in partial fulfilment of requirements for the degree of
BACHELOR OF TECHNOLOGY
IN
MECHANICAL ENGINEERING
Submitted By: Submitted To:
Divya Pratap Singh Chauhan _____________________
B.TECH-ME (FINAL YEAR) _____________________
ROLL NO.: 110106086
DEPARTMENT OF MECHANICAL ENGINEERING
SCHOOL OF ENGINEERING & TECHNOLOGY
SHARDA UNIVERSITY, GREATER NOIDA
SESSION 2014-15

APPROVAL SHEET
Technical seminar report, entitled: “Working of Steam Turbines” is
approved for award of 4 credits.
Coordinator
__________________
Date

ACKNOWLEDGEMENT
I express my sincere gratitude to MR. NAVEEN KUMAR, Dept. of Mechanical and Automotive
Engineering, Sharda University, Greater Noida, for his stimulating guidance, continuous
encouragement and supervision throughout the course of present work.
I also wish to extend my thanks to Manish Jha and other colleagues for attending my seminars
and for their insightful comments and constructive suggestions to improve the quality of this
research work.
I am extremely thankful to Prof. BHARAT B GUPTA Dept. of Mechanical and Automotive
Engineering, Sharda University, Greater Noida, for providing me infrastructural facilities to work
in, without which this work would not have been possible.
Signature of Student

INDEX
1. History 1
2. Introduction 3
3. Working 3
4. Advantages of Steam turbines 5
5. Disadvantages of Steam turbines 6
6. Types of Steam Turbines 6
6.1. Blade & Stage Design 6
6.2. Impulse Turbines 7
6.3. Reaction Turbines 7
7. Compounding in Steam Turbines 8
7.1. Velocity Compounding 9
7.2. Pressure Compounding of Impulse turbines 11
7.3. Pressure-Velocity Compounded Impulse turbines 13
7.4. Pressure compounding of Reaction turbines 14
8. Steam Supply & Exhaust Conditions 14
9. Casing & Shafts Arrangements 15
10. Two Flow Rotors 16
11. Principle of operation & Design 16
11.1. Turbine Efficiency 17
11.2. Operation & Maintenance 18
11.3. Speed Regulation & Governing System 18
12. Thermodynamics of Steam Turbines 19
13. Isentropic Efficiency of Steam Turbines 20
14. Applications of Steam Turbines 21
14.1. Direct Drive 21
14.2. Marine Propulsion 21
14.3. Turbo-Electric Drive 23
15. References 24

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STEAM TURBINES
1. History
The first device that may be classified as a reaction steam turbine was little more than a toy,
the classic Aeolipile, described in the 1st century by Greek mathematician Hero of
Alexandria in Roman Egypt. In 1551, Taqi al-Din in Ottoman Egypt described a steam
turbine with the practical application of rotating a spit. Steam turbines were also described by
the Italian Giovanni Branca (1629) and John Wilkins in England (1648). The devices
described by Taqi al-Din and Wilkins are today known as steam jacks.
A 250 kW industrial steam turbine
from 1910 (right) directly linked to a
generator (left)
The modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model
was connected to a dynamo that generated 7.5 kW (10 hp) of electricity. The invention of
Parsons' steam turbine made cheap and plentiful electricity possible and revolutionized
marine transport and naval warfare. Parsons' design was a reaction type. His patent was
licensed and the turbine scaled-up shortly after by an American, George Westinghouse. The
Parsons turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing
his invention adopted for all major world power stations, and the size of generators had
increased from his first 7.5 kW set up to units of 50,000 kW capacity. Within Parson's
lifetime, the generating capacity of a unit was scaled up by about 10,000 times, and the total
output from turbo-generators constructed by his firm C. A. Parsons and Company and by
their licensees, for land purposes alone, had exceeded thirty million horse-power.

A number of other variations of turbines have been developed that work effectively with
steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed
before running it against a turbine blade. De Laval's impulse turbine is simpler, less
expensive and does not need to be pressure-proof. It can operate with any pressure of steam,
but is considerably less efficient. fr:Auguste Rateau developed a pressure compounded
impulse turbine using the de Laval principle as early as 1900, obtained a US patent in 1903,
and applied the turbine to a French torpedo boat in 1904.
He taught at the École des mines de Saint-Étienne for a decade until 1897, and later founded
a successful company that was incorporated into the Alstom firm after his death. One of the
founders of the modern theory of steam and gas turbines was Aurel Stodola, a Slovak
physicist and engineer and professor at the Swiss Polytechnical Institute (now ETH) in
Zurich. His work Die Dampfturbinen und ihre Aussichten als Wärmekraftmaschinen
(English: The Steam Turbine and its prospective use as a Mechanical Engine) was published
in Berlin in 1903. A further book Dampf und Gas-Turbinen (English: Steam and Gas
Turbines) was published in 1922.
The Brown-Curtis turbine, an impulse type, which had been originally developed and
patented by the U.S. company International Curtis Marine Turbine Company, was developed
in the 1900s in conjunction with John Brown & Company. It was used in John Brawn-engined
merchant ships and warships, including liners and Royal Navy warships.
2 | P a g e
Fig:Diagram of an AEG marine steam
turbine circa 1905

2. Introduction:
A turbine is a device that converts chemical energy into mechanical energy, specifically
when a rotor of multiple blades or vanes is driven by the movement of a fluid or gas. In the
case of a steam turbine, the pressure and flow of newly condensed steam rapidly turns the
rotor. This movement is possible because the water to steam conversion results in a
rapidly expanding gas.
As the turbine’s rotor turns, the rotating shaft can work to accomplish numerous applications,
often electricity generation.
3. Working:
In a steam turbine, the steam’s energy is extracted through the turbine and the steam
leaves the turbine at a lower energy state. High pressure and temperature fluid at the inlet of
the turbine exit as lower pressure and temperature fluid. The difference is energy
converted by the turbine to mechanical rotational energy, less any aerodynamic and
mechanical inefficiencies incurred in the process. Since the fluid is at a lower pressure at the
exit of the turbine than at the inlet, it is common to say the fluid has been “expanded” across
the turbine. Because of the expanding flow, higher volumetric flow occurs at the turbine exit
(at least for compressible fluids) leading to the need for larger turbine exit areas than at the
inlet.
The generic symbol for a turbine used in a flow diagram is shown in Figure below. The
symbol diverges with a larger area at the exit than at the inlet. This is how one can tell a
3 | P a g e
Fig.1 Sectional View of a Steam
Turbine

turbine symbol from a compressor symbol. In Figure, the graphic is colored to indicate the
general trend of temperature drop through a turbine. In a turbine with a high inlet
pressure, the turbine blades convert this pressure energy into velocity or kinetic energy,
which causes the blades to rotate.
Many green cycles use a turbine in this fashion, although the inlet conditions may not be the
same as for a conventional high pressure and temperature steam turbine. Bottoming
cycles, for instance, extract fluid energy that is at a lower pressure and temperature than a
turbine in a conventional power plant. A bottoming cycle might be used to extract
energy from the exhaust gases of a large diesel engine, but the fluid in a bottoming cycle
still has sufficient energy to be extracted across a turbine, with the energy converted into
rotational energy
Turbines also extract energy in fluid flow where the pressure is not high but where the fluid
has sufficient fluid kinetic energy. The classic example is a wind turbine, which converts the
wind’s kinetic energy to rotational energy. This type of kinetic energy conversion is
common in green energy cycles for applications ranging from larger wind turbines to
smaller hydrokinetic turbines currently being designed for and demonstrated in river and
tidal applications. Turbines can be designed to work well in a variety of fluids, including
gases and liquids, where they are used not only to drive generators, but also to drive
compressors or pumps.
An additional use for turbines in industrial applications that may also be applicable in some
green energy systems is to cool a fluid. As previously mentioned, when a turbine
extracts energy from a fluid, the fluid temperature is reduced. Some industries, such
as the gas processing industry, use turbines as sources of refrigeration, dropping the
temperature of the gas going through the turbine. In other words, the primary purpose of the
turbine is to reduce the temperature of the working fluid as opposed to providing power.
4 | P a g e
Fig: Flow Diagram of Multistage
Steam Turbine

Generally speaking, the higher the pressure ratio across a turbine, the greater the
expansion and the greater the temperature drop.
Even where turbines are used to cool fluids, the turbines still produce power and must be
connected to a power absorbing device that is part of an overall system.
An additional use for turbines in industrial applications that may also be applicable in some
green energy systems is to cool a fluid. As previously mentioned, when a turbine
extracts energy from a fluid, the fluid temperature is reduced. Some industries, such
as the gas processing industry, use turbines as sources of refrigeration, dropping the
temperature of the gas going through the turbine. In other words, the primary purpose of the
turbine is to reduce the temperature of the working fluid as opposed to providing power.
Generally speaking, the higher the pressure ratio across a turbine, the greater the
expansion and the greater the temperature drop.
Also note that turbines in high inlet-pressure applications are sometimes called
expanders. The terms “turbine” and “expander” can be used interchangeably for most
applications, but expander is not used when referring to kinetic energy applications, as the
fluid does not go through significant expansion.
4. Advantages of Steam Turbines:
 Ability to use high Pressure & high Temperature.
 High Efficiency.
 High Rotational Speed.
 High capacity/weight ratio.
 Smooth nearly vibration free operation.
 No internal lubrication required.
 Oil free Exhaust System.
5 | P a g e
Fig: 3 Main Stages of Multistage
Steam Turbines

5.Disadvantages of steam Turbines:
For slow speed application reduction gears are required. The steam turbine cannot be made
reversible. The efficiency of small simple steam turbines is poor.
6.Types of Steam Turbines:
Steam turbines are made in a variety of sizes ranging from small <0.75 kW (<1 hp) units
(rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to
1 500 000 kW (1.5 GW; 2 000 000 hp) turbines used to generate electricity. There are several
classifications for modern steam turbines.
6.1.Blade & Stage Design
Turbine blades are of two basic types, blades and nozzles. Blades move entirely due to the
impact of steam on them and their profiles do not converge. This results in a steam velocity
drop and essentially no pressure drop as steam moves through the blades. A turbine
composed of blades alternating with fixed nozzles is called an impulse turbine, Curtis turbine,
Rateau turbine, or Brown-Curtis turbine. Nozzles appear similar to blades, but their profiles
converge near the exit. This results in a steam pressure drop and velocity increase as steam
moves through the nozzles. Nozzles move due to both the impact of steam on them and the
reaction due to the high-velocity steam at the exit. A turbine composed of moving nozzles
alternating with fixed nozzles is called a reaction turbine or Parsons turbine.
Except for low-power applications, turbine blades are arranged in multiple stages in series,
called compounding, which greatly improves efficiency at low speeds. A reaction stage is a
row of fixed nozzles followed by a row of moving nozzles. Multiple reaction stages divide
6 | P a g e

the pressure drop between the steam inlet and exhaust into numerous small drops, resulting in
a pressure-compounded turbine. Impulse stages may be either pressure-compounded,
velocity-compounded, or pressure-velocity compounded. A pressure-compounded impulse
stage is a row of fixed nozzles followed by a row of moving blades, with multiple stages for
compounding. This is also known as a Rateau turbine, after its inventor. A velocity-compounded
7 | P a g e
impulse stage (invented by Curtis and also called a "Curtis wheel") is a row of
fixed nozzles followed by two or more rows of moving blades alternating with rows of fixed
blades. This divides the velocity drop across the stage into several smaller drops. A series of
velocity-compounded impulse stages is called a pressure-velocity compounded turbine.
6.2.Impulse turbines
In an impulse turbine, a fast-moving fluid is fired through a narrow nozzle at the turbine
blades to make them spin around. The blades of an impulse turbine are usually bucket-shaped
so they catch the fluid and direct it off at an angle or sometimes even back the way it came
(because that gives the most efficient transfer of energy from the fluid to the turbine). In an
impulse turbine, the fluid is forced to hit the turbine at high speed. Imagine trying to make a
wheel like this turn around by kicking soccer balls into its paddles.
You'd need the balls to hit hard and bounce back well to get the wheel spinning—and those
constant energy impulses are the key to how it works. Water turbines are often based around
an impulse turbine (though some do work using reaction turbines).
6.3.Reaction turbines
In a reaction turbine, the blades sit in a much larger volume of fluid and turn around as the
fluid flows past them. A reaction turbine doesn't change the direction of the fluid flow as
drastically as an impulse turbine: it simply spins as the fluid pushes through and past its
blades. Wind turbines are perhaps the most familiar examples of reaction turbines.

If an impulse turbine is a bit like kicking soccer balls, a reaction turbine is more like
swimming—in reverse. Let me explain! Think of how you do freestyle (front crawl) by
hauling your arms through the water, starting with each hand as far in front as you can reach
and ending with a "follow through" that throws your arm well behind you. What you're trying
to achieve is to keep your hand and forearm pushing against the water for as long as possible,
so you transfer as much energy as you can in each stroke. A reaction turbine is using the
same idea in reverse: imagine fast-flowing water moving past you so it makes your arms and
legs move and supplies energy to your body! With a reaction turbine, you want the water to
touch the blades smoothly, for as long as it can, so it gives up as much energy as possible.
The water isn't hitting the blades and bouncing off, as it does in an impulse turbine: instead,
the blades are moving more smoothly, "going with the flow."
7. Compounding in Steam Turbines:
Compounding of steam turbines is the method in which energy from the steam is extracted
in a number of stages rather than a single stage in a turbine. A compounded steam turbine has
multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft
or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the
turbine in number of stages.
The steam produced in the boiler has very high enthalpy. In all turbines the blade velocity is
directly proportional to the velocity of the steam passing over the blade. Now, if the entire
energy of the steam is extracted in one stage, i.e. if the steam is expanded from the boiler
pressure to the condenser pressure in a single stage, then its velocity will be very high. Hence
the velocity of the rotor (to which the blades are keyed) can reach to about 30,000 rpm, which
is pretty high for practical uses because of very high vibration. Moreover at such high speeds
the centrifugal forces are immense, which can damage the structure. Hence, compounding is
8 | P a g e

needed. The high velocity which is used for impulse turbine just strikes on single ring of rotor
that cause wastage of steam ranges 10% to 12%. To overcome the wastage of steam
compounding of steam turbine is used.
Types of Compounding
In an Impulse steam turbine compounding can be achieved in the following three ways: -
1. Velocity compounding
2. Pressure compounding
3. Pressure-Velocity Compounding
In a Reaction turbine compounding can be achieved only by Pressure compounding.
7.1.Velocity compounding of Impulse Turbine:
The velocity compounded Impulse turbine was first proposed by C G Curtis to solve the
problem of single stage Impulse turbine for use of high pressure and temperature steam.
The rings of moving blades are separated by rings of fixed blades. The moving blades are
keyed to the turbine shaft and the fixed blades are fixed to the casing. The high pressure
steam coming from the boiler is expanded in the nozzle first. The Nozzle converts the
pressure energy of the steam into kinetic energy. It is interesting to note that the total
enthalpy drop and hence the pressure drop occurs in the nozzle. Hence, the pressure thereafter
remains constant.
This high velocity steam is directed on to the first set (ring) of moving blades. As the steam
flows over the blades, due the shape of the blades, it imparts some of its momentum to the
blades and losses some velocity. Only a part of the high kinetic energy is absorbed by these
blades. The remainder is exhausted on to the next ring of fixed blade. The function of the
fixed blades is to redirect the steam leaving from the first ring moving blades to the second
ring of moving blades. There is no change in the velocity of the steam as it passes through the
fixed blades. The steam then enters the next ring of moving blades; this process is repeated
until practically all the energy of the steam has been absorbed.
9 | P a g e

A schematic diagram of the Curtis stage impulse turbine, with two rings of moving blades
one ring of fixed blades is shown in above diagram. The Diagram also shows the changes in
the pressure and the absolute steam velocity as it passes through the stages.
where,
Pi = pressure of steam at inlet
Vi = velocity of steam at inlet
Po = pressure of steam at outlet
Vo = velocity of steam at outlet
In the above figure there are two rings of moving blades separated by a single of ring of fixed
blades. As discussed earlier the entire pressure drop occurs in the nozzle, and there are no
subsequent pressure losses in any of the following stages. Velocity drop occurs in the moving
blades and not in fixed blades.
Optimum Velocity
It is the velocity of the blades at which maximum power output can be achieved. Hence, the
optimum blade velocity for this case is,
10 | P a g e
Fig: Schematic Diagram of Curtis
Stage Impulse Turbine

where n is the number of stages.
It is interesting to note that this value of optimum velocity is 1/n times that of the single stage
turbine. This means that maximum power can be produced at much lower blade velocities.
However, the work produced in each stage is not the same. The ratio of work produced in a 2
stage turbine is 3:1 as one move from higher to lower pressure. This ratio is 5:3:1 in three
stage turbine and changes to 7:5:3:1 in a four stage turbine.
Disadvantages of Velocity Compounding
 Due to the high steam velocity there are high friction losses
 Work produced in the low-pressure stages is much less.
 The designing and fabrication of blades which can withstand such high velocities is
difficult.
7.2. Pressure compounding of Impulse Turbine
The pressure compounded Impulse turbine is also called as Rateau turbine, after its inventor.
This is used to solve the problem of high blade velocity in the single-stage impulse turbine.
It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing
and the blades are keyed to the turbine shaft.
In this type of compounding the steam is expanded in a number of stages, instead of just one
(nozzle) in the velocity compounding. It is done by the fixed blades which act as nozzles. The
steam expands equally in all rows of fixed blade. The steam coming from the boiler is fed to
the first set of fixed blades i.e. the nozzle ring. The steam is partially expanded in the nozzle
ring. Hence, there is a partial decrease in pressure of the incoming steam. This leads to an
increase in the velocity of the steam. Therefore the pressure decreases and velocity increases
partially in the nozzle.
This is then passed over the set of moving blades. As the steam flows over the moving blades
nearly all its velocity is absorbed. However, the pressure remains constant during this
process. After this it is passed into the nozzle ring and is again partially expanded. Then it is
fed into the next set of moving blades, and this process is repeated until the condenser
pressure is reached.
It is a three stage pressure compounded impulse turbine. Each stage consists of one ring of
fixed blades, which act as nozzles, and one ring of moving blades. As shown in the figure
pressure drop takes place in the nozzles and is distributed in many stages.
11 | P a g e

An important point to note here is that the inlet steam velocities to each stage of moving
blades are essentially equal. It is because the velocity corresponds to the lowering of the
pressure. Since, in a pressure compounded steam turbine only a part of the steam is expanded
in each nozzle, the steam velocity is lower than of the previous case. It can be explained
mathematically from the following formula i.e.
.
where,
V1 = absolute exit velocity of fluid
h1 = enthalpy of fluid at exit
V2 = absolute entry velocity of fluid
h2 = enthalpy of fluid at entry
12 | P a g e
Fig: Schematic Diagram of Pressure
compounded Impulse Turbine

One can see from the formula that only a fraction of the enthalpy is converted into velocity in
the fixed blades. Hence, velocity is very less as compared to the previous case.
Disadvantages of Pressure Compounding
 The disadvantage is that since there is pressure drop in the nozzles, it has to be made
air-tight.
 They are bigger and bulkier in size.
7.3. Pressure-Velocity compounded Impulse Turbine:
It is a combination of the above two types of compounding. The total pressure drop of the
steam is divided into a number of stages. 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 blades. In
each stage there is one ring of fixed blades and 3-4 rings of moving blades. Each stage acts as
a velocity compounded impulse turbine.
The fixed blades act as nozzles. The steam coming from the boiler is passed to the first ring
of fixed blades, where it gets partially expanded. The pressure partially decreases and the
velocity rises correspondingly. The velocity is absorbed by the following rings of moving
blades until it reaches the next ring of fixed blades and the whole process is repeated once
again.
13 | P a g e
Fig: Schematic Diagram of
Pressure-Velocity compounded
Impulse Turbine

7.4. Pressure compounding of Reaction Turbine:
As explained earlier a reaction turbine is one which there is pressure and velocity loss in the
moving blades. The moving blades have a converging steam nozzle. Hence when the steam
passes over the fixed blades, it expands with decrease in steam pressure and increase in
kinetic energy.
This type of turbine has a number of rings of moving blades attached to the rotor and an equal
number of fixed blades attached to the casing. In this type of turbine the pressure drops take
place in a number of stages.
The steam passes over a series of alternate fixed and moving blades. The fixed blades act as
nozzles i.e. they change the direction of the steam and also expand it. Then steam is passed
on the moving blades, which further expand the steam and also absorb its velocity.
8. Steam Supply and Exhaust Conditions:
These types include condensing, non-condensing, reheat, extraction and induction.
Condensing turbines are most commonly found in electrical power plants. These turbines
exhaust steam from a boiler in a partially condensed state, typically of a quality near 90%, at
a pressure well below atmospheric to a condenser.
14 | P a g e
Fig: Schematic Diagram of
Pressure compounded Reaction
Turbine

Non-condensing or back pressure turbines are most widely used for process steam
applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the
process steam pressure. These are commonly found at refineries, district heating units, pulp
and paper plants, and desalination facilities where large amounts of low pressure process
steam are needed.
Reheat turbines are also used almost exclusively in electrical power plants. In a reheat
turbine, steam flow exits from a high pressure section of the turbine and is returned to the
boiler where additional superheat is added. The steam then goes back into an intermediate
pressure section of the turbine and continues its expansion. Using reheat in a cycle increases
the work output from the turbine and also the expansion reaches conclusion before the steam
condenses, there by minimizing the erosion of the blades in last rows. In most of the cases,
maximum number of reheats employed in a cycle is 2 as the cost of super-heating the steam
negates the increase in the work output from turbine.
Extracting type turbines are common in all applications. In an extracting type turbine,
steam is released from various stages of the turbine, and used for industrial process needs or
sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be
controlled with a valve, or left uncontrolled.
Induction turbines introduce low pressure steam at an intermediate stage to produce
additional power.
9. Casing or Shaft Arrangements:
These arrangements include single casing, tandem compound and cross compound turbines.
Single casing units are the most basic style where a single casing and shaft are coupled to a
generator. Tandem compound are used where two or more casings are directly coupled
together to drive a single generator. A cross compound turbine arrangement features two or
more shafts not in line driving two or more generators that often operate at different speeds.
A cross compound turbine is typically used for many large applications.
15 | P a g e
Fig: A Typical LP Steam Turbine
Outer Casing

10. Two-flow rotors:
The moving steam imparts both a tangential and axial thrust on the turbine shaft, but the axial
thrust in a simple turbine is unopposed. To maintain the correct rotor position and balancing,
this force must be counteracted by an opposing force. Thrust bearings can be used for the
shaft bearings, the rotor can use dummy pistons, it can be double flow- the steam enters in
the middle of the shaft and exits at both ends, or a combination of any of these. In a double
flow rotor, the blades in each half face opposite ways, so that the axial forces negate each
other but the tangential forces act together. This design of rotor is also called two-flow,
double-axial-flow, or double-exhaust. This arrangement is common in low-pressure casings
of a compound turbine.
11. Principle of Operation & Design:
An ideal steam turbine is considered to be an isentropic process, or constant entropy process,
in which the entropy of the steam entering the turbine is equal to the entropy of the steam
leaving the turbine. No steam turbine is truly isentropic, however, with typical isentropic
efficiencies ranging from 20–90% based on the application of the turbine. The interior of a
turbine comprises several sets of blades or buckets. One set of stationary blades is connected
to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with
certain minimum clearances, with the size and configuration of sets varying to efficiently
exploit the expansion of steam at each stage.
16 | P a g e
Fig: A two-flow turbine rotor. The steam
enters in the middle of the shaft, and exits
at each end, balancing the axial force.

11.1.Turbine efficiency
To maximize turbine efficiency the steam is expanded, doing work, in a number of stages.
These stages are characterized by how the energy is extracted from them and are known as
either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and
impulse designs: each stage behaves as either one or the other, but the overall turbine uses
both. Typically, higher pressure sections are reaction type and lower pressure stages are
impulse type.
11.2. Operation and maintenance:
Because of the high pressures used in the steam circuits and the materials used, steam
turbines and their casings have high thermal inertia. When warming up a steam turbine for
use, the main steam stop valves (after the boiler) have a bypass line to allow superheated
steam to slowly bypass the valve and proceed to heat up the lines in the system along with the
steam turbine. Also, a turning gear is engaged when there is no steam to slowly rotate the
turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by
the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the
turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then
to the ahead blades slowly rotating the turbine at 10–15 RPM (0.17–0.25 Hz) to slowly warm
the turbine. The warm up procedure for large steam turbines may exceed ten hours.[15]
During normal operation, rotor imbalance can lead to vibration, which, because of the high
rotation velocities, could lead to a blade breaking away from the rotor and through the casing.
To reduce this risk, considerable efforts are spent to balance the turbine. Also, turbines are
run with high quality steam: either superheated (dry) steam, or saturated steam with a high
17 | P a g e
Fig: Maintenance & Balancing
operations Carried out by Engineers

dryness fraction. This prevents the rapid impingement and erosion of the blades which occurs
when condensed water is blasted onto the blades (moisture carry over). Also, liquid water
entering the blades may damage the thrust bearings for the turbine shaft. To prevent this,
along with controls and baffles in the boilers to ensure high quality steam, condensate drains
are installed in the steam piping leading to the turbine.
11.3. Speed Regulation in Steam Turbines
The control of a turbine with a governor is essential, as turbines need to be run up slowly to
prevent damage and some applications (such as the generation of alternating current
electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can
lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the
turbine to close. If this fails then the turbine may continue accelerating until it breaks apart,
often catastrophically. Turbines are expensive to make, requiring precision manufacture and
special quality materials.
During normal operation in synchronization with the electricity network, power plants are
governed with a five percent droop speed control. This means the full load speed is 100% and
the no-load speed is 105%. This is required for the stable operation of the network without
hunting and drop-outs of power plants. Normally the changes in speed are minor.
Adjustments in power output are made by slowly raising the droop curve by increasing the
spring pressure on a centrifugal governor. Generally this is a basic system requirement for all
power plants because the older and newer plants have to be compatible in response to the
instantaneous changes in frequency without depending on outside communication.
18 | P a g e
Fig: Diagram of a steam turbine
governor system

12. Thermodynamics of Steam Turbines:
The steam turbine operates on basic principles of thermodynamics using the part 3-4 of the
Rankine cycle shown in the adjoining diagram. Superheated vapor (or dry saturated vapor,
depending on application) enters the turbine, after it having exited the boiler, at high
temperature and high pressure. The high heat/pressure steam is converted into kinetic energy
using a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type
turbine). Once the steam has exited the nozzle it is moving at high velocity and is sent to the
blades of the turbine. A force is created on the blades due to the pressure of the vapor on the
blades causing them to move. A generator or other such device can be placed on the shaft,
and the energy that was in the vapor can now be stored and used. The gas exits the turbine as
a saturated vapor (or liquid-vapor mix depending on application) at a lower temperature and
pressure than it entered with and is sent to the condenser to be cooled.[18] If we look at the
first law we can find an equation comparing the rate at which work is developed per unit
mass. Assuming there is no heat transfer to the surrounding environment and that the change
in kinetic and potential energy is negligible when compared to the change in specific enthalpy
we come up with the following equation
where
 Ẇis the rate at which work is developed per unit time
 ṁ is the rate of mass flow through the turbine
19 | P a g e
Fig: T-s diagram of a superheated
Rankine cycle

13. Isentropic efficiency:
To measure how well a turbine is performing we can look at its isentropic efficiency. This
compares the actual performance of the turbine with the performance that would be achieved
by an ideal, isentropic, turbine. When calculating this efficiency, heat lost to the surroundings
is assumed to be zero. The starting pressure and temperature is the same for both the actual
and the ideal turbines, but at turbine exit the energy content specific enthalpy for the actual
turbine is greater than that for the ideal turbine because of irreversibility in the actual turbine.
The specific enthalpy is evaluated at the same pressure for the actual and ideal turbines in
order to give a good comparison between the two.
The isentropic efficiency is found by dividing the actual work by the ideal work.
where
 h3 is the specific enthalpy at state three
 h4 is the specific enthalpy at state four for the actual turbine
 h4s is the specific enthalpy at state four for the isentropic turbine
20 | P a g e
Fig: H-S Diagram showing efficiencies
of various stages of steam turbines

14. Applications of Steam Turbines:
14.1. Direct drive
Electrical power stations use large steam turbines driving electric generators to produce most
(about 80%) of the world's electricity. The advent of large steam turbines made central-station
electricity generation practical, since reciprocating steam engines of large rating
became very bulky, and operated at slow speeds. Most central stations are fossil fuel power
plants and nuclear power plants; some installations use geothermal steam, or use concentrated
solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large
centrifugal pumps, such as feed water pumps at a thermal power plant.
The turbines used for electric power generation are most often directly coupled to their
generators. As the generators must rotate at constant synchronous speeds according to the
frequency of the electric power system, the most common speeds are 3,000 RPM for 50 Hz
systems, and 3,600 RPM for 60 Hz systems. Since nuclear reactors have lower temperature
limits than fossil-fired plants, with lower steam quality, the turbine generator sets may be
arranged to operate at half these speeds, but with four-pole generators, to reduce erosion of
turbine blades.
14.2. Marine propulsion
In steam-powered ships, compelling advantages of steam turbines over reciprocating engines
are smaller size, lower maintenance, lighter weight, and lower vibration. A steam turbine is
only efficient when operating in the thousands of RPM, while the most effective propeller
designs are for speeds less than 300 RPM; consequently, precise (thus expensive) reduction
21 | P a g e
Fig: 500MW Steam Powered
Turbo- genertor

gears are usually required, although numerous early ships through World War I, such as
Turbinia , had direct drive from the steam turbines to the propeller shafts. Another
alternative is turbo-electric transmission, in which an electrical generator run by the high-speed
turbine is used to run one or more slow-speed electric motors connected to the
propeller shafts; precision gear cutting may be a production bottleneck during wartime.
Turbo-electric drive was most used in large US warships designed during World War I and in
some fast liners, and was used in some troop transports and mass-production destroyer
escorts in World War II. The purchase cost of turbines is offset by much lower fuel and
maintenance requirements and the small size of a turbine when compared to a reciprocating
engine having an equivalent power. However, from the 1950s diesel engines were capable of
greater reliability and higher efficiencies: propulsion steam turbine cycle efficiencies have yet
to break 50%, yet diesel engines today routinely exceed 50%, especially in marine
applications.[21][22][23] Diesel power plants also have lower operating costs since fewer
operators are required. Thus, conventional steam power is used in very few new ships. An
exception is LNG carriers which often find it more efficient to use boil-off gas with a steam
turbine than to re- liquify it.
Nuclear-powered ships and submarines use a nuclear reactor to create steam for turbines.
Nuclear power is often chosen where diesel power would be impractical (as in submarine
applications) or the logistics of refuelling pose significant problems (for example,
icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard class
submarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for
the vessel's entire service life. Nuclear propulsion has only been applied to a very few
commercial vessels due to the expense of maintenance and the regulatory controls required
on nuclear systems and fuel cycles.
22 | P a g e
Fig: The Turbinia , 1894, the first
steam turbine-powered ship

14.3. Turbo Electric Drive System
Turbo-electric drive was introduced on the USS New Mexico (BB-40), launched in 1917.
Over the next eight years the US Navy launched five additional turbo-electric-powered
battleships and two aircraft carriers (initially ordered as Lexington-class battle cruisers). Ten
more turbo-electric capital ships were planned, but cancelled due to the limits imposed by the
Washington Naval Treaty. Although New Mexico was refitted with geared turbines in a
1931-33 refit, the remaining turbo-electric ships retained the system throughout their careers.
This system used two large steam turbine generators to drive an electric motor on each of
four shafts. The system was less costly initially than reduction gears and made the ships more
maneuverable in port, with the shafts able to reverse rapidly and deliver more reverse power
than with most geared systems. Some ocean liners were also built with turbo-electric drive, as
were some troop transports and mass-production destroyer escorts in World War II. However,
when the US designed the "treaty cruisers", beginning with the USS Pensacola (CA-24)
launched in 1927, geared turbines were used for all fast steam-powered ships thereafter.
23 | P a g e
Fig: The 50 Let Pobedy nuclear
icebreaker with nuclear-turbo-electric
propulsion

24 | P a g e
REFERENCE
 Encyclopædia Britannica (1931-02-11). "Sir Charles Algernon Parsons (British
engineer) - Britannica Online Encyclopedia". Britannica.com. Retrieved 2010-09-12.
 Wiser, Wendell H. (2000). Energy resources: occurrence, production, conversion,
use. Birkhäuser. p. 190. ISBN 978-0-387-98744-6.
 Turbine. Encyclopædia Britannica Online
 A new look at Heron's 'steam engine'" (1992-06-25). Archive for History of Exact
Sciences 44 (2): 107-124.
 O'Connor, J. J.; E. F. Robertson (1999). Heron of Alexandria. MacTutor

A Report on Steam Turbine

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