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COMPOUNDING OF STEAM TURBINE

CHAPTER 1-INTRODUCTION

1.1GENERAL

Steam Turbine is a type of turbomachine. Turbomachine are those devices in

which energy is transferred either to or from, a continuously flowing fluid by

the dynamic action of one or more moving blade rows. In steam turbine

energy is transferred from fluid to blade rows and is decreasing along the flow

directions. It is power producing thermodynamics device.

Steam turbine converts heat energy of steam (at high pressure and

temperature) into mechanical energy. The so utilised can be used in various

filed of industry such as electricity generation, transport, in driving of pumps,

fan and compressor etc. the basic cycle on which steam turbine works is

Rankine Cycle.

The reciprocating steam engine was still inefficient, cumbersome, had a

very low power to weight ratio, and was a high maintenance piece of

machinery. The development of the steam turbine was a vast improvement in

all of these respects.

A turbine consist of one set of stationary blades or nozzles and an

adjacent set of moving blades or buckets. These stationary and rotating

elements act together to allow the steam flow to do work on the rotor. The ork

is transmitted to the load through the shaft or shafts.

pg. 1


1.2 HISTORY

Steam turbines date back to 120 B.C. when the first steam turbine was

developed by Hero of Alexandria. Subsequently number of steam turbines

came up but the practically successful steam turbine appeared at the end of

nineteenth century when Gustaf De Laval designed a high speed turbine built

on the principle of reaction turbine in 1883. Before this in 1629 G. Branca

developed the first impulse turbine.

pg. 2


Branca’s impulse turbine and Hero’s reaction turbine are shown in Fig. 1.1.

In nineteenth century some more steam turbines were developed by Sir

Charles A. Parsons and C.G. Curtis which gave a filip to the development to

the modern steam turbine. Over the period of time the modern steam turbines

evolved with capacity from few kilowatts to 350,000 kW and in speed from

1000 rpm to 40,000 rpm. Steam turbines offer the advantages over other

prime movers in terms of simplicity, reliability and low maintenance costs.

Reciprocating steam engines use pressure energy of steam while steam

turbines use dynamic action of the steam. Steam turbines require less space as

compared to diesel engine or steam engine and also the absence of

reciprocating parts & reciprocating motion in steam turbine results in lesser

vibrations and lighter foundation. In steam turbine the expanding steam does

not come into contact with lubricant and so exhaust steam leaves

uncontaminated.

pg. 3

Fig. 1.1 Hero and Branca’s turbine.


1.3 PRINCIPLE

The basic principle on which steam turbine works is Newton’s Second

law of motion. The motive power of a high velocity jet impinging on a curved

blade. The steam from boiler is expanded in a nozzle where due to fall in

pressure of steam, thermal energy of steam is converted into kinetic energy of

steam, resulting in the emission of a high velocity jet of steam which

impinges on the moving vanes or blades, mounted on a shaft; here it

undergoes a change in direction of motion which give rise to a change in

momentum and therefore, a force.

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. Steam turbines

are mostly 'axial flow' types; the steam flows over the blades in a direction

Parallel to the axis of the wheel. 'Radial flow' types are rarely used.It should

be noted that the blade obtains no motive force from the static pressure of the

steam or from any impact of the jet, because the blade is designed such that

pg. 4Fig 1.2 Working of steam turbine


the steam jet will glide on and off the blade without and tendency to strike it.

1.3 CLASSIFICATION OF STEAM TURBINE

Steam turbines may be classified into different categories based on various

attributes as given below.

1.3.1 BASED ON THE PRINCIPLE OF WORKING:

i) IMPULSE TURBINE- If the flow of steam through the nozzles and

moving blades of a turbine takes place in such a manner that “the

steam is expanded only in nozzles, and pressure at the outlet side of

blade is equal to that at the inlet side”, i.e. drop in pressure of steam

takes place only in nozzles and not in moving blades; such a turbine

is termed as impulse turbine because it works on the principle of

impulse. This is obtained by making the blade passage of constant

cross-section area. In impulse turbine, the energy transformation

takes place in nozzles while energy transfer takes place in moving

blades. Simple impulse turbine is used where small output at very

pg. 5


high speed is required or only a small pressure drop is available.

These are not suited for applications requiring conversion of large

thermal energy into work.

ii) IMPULSE-REACTION TURBINE- The expansion of steam takes

place in nozzle (fixed blades) as well as in moving blades. If the

pressure of steam at the outlet from the moving blades of a turbine is

less than that at the inlet side of blades; this pressure drop suffered

steam while

passing through the moving blades, giving rise to reaction and adds

on the propelling force which is applied through the rotor to the

turbine shaft. Such turbine is termed as impulse-and reaction both.

pg. 6


This is achieved by varying the blade passage cross-section

(converging type). Here energy transformation takes place in

nozzles (fixed blade) while both energy transfer and transformation

takes place in moving blades.

1.3.2 BASED ON THE DIRECTION OF FLOW:

Steam turbines can be classified based on the direction of flow by

which steam flows through turbine blading. Steam turbines can be:

a) AXIAL FLOW- In axial flow turbines steam flows along the axis of

turbine over blades. These axial flow turbines are well suited for large

turbo generators and very commonly used presently.

b) RADIAL FLOW-Radial flow turbine incorporates two shafts end to end

and can be of suitably small sizes. Radial flow turbines can be started

quickly and so well suited for peak load and used as stand by turbine or

peak load turbines. These are also termed as Ljungstrom turbines.

c) TANGENTIAL FLOW-In tangential flow turbines the nozzle directs

steam tangentially into buckets at the periphery of single wheel and

steam reverses back and re-enters other bucket at its’ periphery. This is

repeated several times as steam follows the helical path. Tangential

flow turbines are very robust but less efficient.

1.3.3 BASED ON THE SPEED OF TURBINE:

Steam turbines can be classified based upon the steam turbine as

low speed, normal speed and high speed turbines as given below.

a) LOW SPEED TURBINE- Low speed turbines are those steam turbines

which run at speed below 3000 rpm.

pg. 7


b) NORMAL SPEED TURBINE- Normal speed steam turbines are those

turbines which run at speed of about 3000 rpm.

c) HIGH SPEED TURBINE- High speed steam turbines are the one

which run at more than 3000 rpm.

1.3.4 BASED ON THE APPLICATION OF TURBINE:

Depending upon application the steam turbine can be classified as

below:

a) CONDENSING TURBINE-Condensing steam turbines are those in which

steam leaving turbine enters into condenser. Such type of steam

turbines permit for recirculation of condensate leaving condenser. Also

the pressure at the end of expansion can be lowered much below

atmospheric pressure as the expanded steam is rejected into condenser

where vacuum can be maintained. Condensing turbines are frequently

used in thermal power plants.

b) NON CONDENSING TURBINE- Non-condensing steam turbines are

those in which steam leaving turbine is rejected to atmosphere and not

to condenser as in case of condensing turbine.

c) BACK PRESSURE TURBINE- Back pressure turbines reject steam at a

pressure much above the atmospheric pressure and steam leaving

turbine with substantially high pressure can be used for some other

purposes such as heating or running small condensing turbines.

d) PASS OUT TURBINE- Pass out turbines are those in which certain

quantity of steam is continuously extracted for the purpose of heating

and allowing remaining steam to pass through pressure control valve

into the low pressure section of turbine. Pressure control valve and

control gear is required so as to keep the speeds of turbine and pressure

of steam constant irrespective of variations of power and heating loads

pg. 8


1.3.5 BASED ON THE PRESSURE IN STEAM TURBINE:

Steam turbines can also be classified based upon the inlet pressure of

steam turbine as follows:

a) LOW PRESSURE TURBINE- Low pressure steam turbines have pressure

of inlet steam less than 20 kg/cm2.

b) MEDIUM PRESSURE TURBINE- Medium pressure steam turbines have

steam inlet pressure between 20 kg/cm2 to 40 kg/cm2.

c) HIGH PRESSURE TURBINE- High pressure steam turbines have steam

inlet pressure lying between 40 kg/cm2 to 170 kg/cm2.

d) SUPER PRESSURE STEAM TURBINE- Turbines having inlet steam

pressure more than 170 kg/cm2 are called super pressure steam

turbines.

1.4 RANKINE CYCLE

The Rankine cycle is a steam cycle for a steam plant operating under

The best theoretical conditions for most efficient operation. This is an ideal

imaginary cycle against which all other real steam working cycles can be

compared. The theoretic cycle can be considered with reference to the figure

below. There will no losses of energy by radiation, leakage of steam, or

frictional losses in the mechanical components. The condenser cooling will

condense the steam to water with only sensible heat (saturated water). The

feed pump will add no energy to the water. The chimney gases would be at

the same pressure as the atmosphere. Within the turbine the work done would

be equal to the energy entering the turbine as steam (h1) minus the energy

leaving the turbine as steam after perfect expansion (h2) this being isentropic

(reversible adiabatic) i.e. (h1- h2).

pg. 9


The energy supplied by the steam by heat transfer from the combustion and

flue gases in the furnace to the water and steam in the boiler will be the

difference in the enthalpy of the steam leaving the boiler and the water

entering the boiler = (h1 - h3).

The various energy streams flowing in a simple steam turbine system

are as indicated in the diagram below. It is clear that the working fluid is in a

closed circuit apart from the free surface of the hot well. Every time the

working fluid flows at a uniform rate around the circuit it experiences a series

of processes making up a thermodynamic cycle. The complete plant is

enclosed in an outer boundary and the working fluid crosses inner boundaries

(control surfaces). The inner boundaries defines a flow process.

pg. 10

Fig. 2.1 Basic rankine cycle.


pg. 11


CHAPTER 2

SIMPLE IMPULSE TURBINE

This type of turbine works on the principle of impulse. It consist of a nozzles,

a rotor mounted on the shaft, one set of moving blades attached to the rotor

and a casting, etc. A set row of nozzles and moving blades constitutes a stage.

The uppermost portion of the diagram (Fig. 2.1) shows a longitudinal section

through the upper half of turbine. The middle portion shows the development

of the nozzles and blading, i.e. the actual shape of nozzle and blading, and the

bottom portion shows the variation of absolute pressure during flow of stream

through passage of nozzles and blades.

pg. 12Fig.2.1 Working of simple

impulse turbine


an example of this type of turbine is the de-Leval turbine. It has single-stage

having a nozzle fitted in the casing followed by ring of moving blades

mounted on the shaft. Variation of velocity and pressure along the axis of

turbine is also shown in the figure.

It can be seen from the figure that the complete expansion of steam

from steam chest pressure to the exhaust pressure of the condenser pressure

takes place only in one set of nozzle i.e. the pressure drop takes place only in

nozzles. It is assumed that the pressure in the recess between nozzles and

blades remain the same. The steam at the condenser pressure or exhaust

pressure enters the blades and comes out at the pressure i.e. the pressure of

steam in the blade passages remain approximately constant and equal to the

condenser pressure.

Generally, converging-diverging nozzle are used due to the relative

large ratio of expansion of steam in the nozzles, the steam leaves the nozzles

at very high velocity (supersonic) of about 1100m/s. It is assumed that the

velocity remains constant in the recess between the nozzles and the blades.

The steam at such high velocity enters the blades and comes out with a

velocity that is appreciable.

Velocity diagrams for single stage of simple impulse turbine is shown

in figure 2.1. Velocity diagram gives an account of velocity of fluid entering

and leaving the turbine.

pg. 13


Figure 2.1 gives the inlet and outlet velocity diagrams at inlet edge and outlet

edge of moving blade along with the combined inlet and outlet velocity

diagram for a stage of simple impulse turbine. The notations used for

denoting velocity angles and other parameters during calculations are

explained as under, (SI system of units is used here).

U=Linear velocity of blade.

pg. 14

Fig.2.1 Schematic diagram of an Impulse Trubine

Fig 2.1 Velocity diagram of an Impulse Turbine


V1 and V2= Inlet and outlet absolute velocity.

Vr1 and Vr2= Inlet and outlet relative velocity (Velocity relative to the

rotor blades.)

= Nozzle angle, = absolute fluid angle at outlet (It is to be

mentioned that all angles are with respect to the tangential velocity

in the direction of U.)

and = Inlet and outlet blade angles.

and = Tangential or whirl component of absolute velocity at

Inlet and outlet.

and = Axial component of velocity at inlet and outlet.

pg. 15


CHAPTER 3


3.1 WHY COMPOUNDING?

The maximum force is develops when the blades is locked while the jet

enters and leave with equal velocity. Since the blade velocity is zero, no

mechanical work is done. As the blades is allowed to speed up, the velocity of

jet from the blade reduces, which reduces the force. Due to blade velocity

work is done and maximum work is done when the blade velocity is just half

the steam velocity. Force and work done become zero when blade velocity is

equal to the steam velocity. In this case, steam velocity from the blade is near

about zero i.e. the trail of inert steam since all the kinetic energy of steam is

converted into work.

We know that for economy or maximum work, the blade velocity

should be one half of the steam velocity, blade velocity of about 500 m/s is

deemed very high. This type of turbine is generally employed where relatively

small power is required and where the rotor diameter is fairly small. The

small rotor gives a very high rotational speed, reaching 30,000 rpm. Such

high rotational speed can only be utilised to drive generators with large

reduction gearing arrangements. In this turbine, the leaving velocity of steam

is quite appreciable, resulting in an energy loss, called “carry over loss” or

“leaving velocity loss”. This leaving loss is so high that it may be as much as

11 percent of the initial kinetic energy.

pg. 16


In this turbine, the leaving velocity of steam is quite appreciable, resulting in

an energy loss, called “carry over loss” or “leaving velocity loss”. This

leaving loss is so high that it may be as much as 11 percent of the initial

in kinetic energy.

The diagram shows carry over loss or lost velocity that occurs the

simple impulse turbine. This loss very high which result in the lower

efficiency of the turbine result in the loss of the useful work. In order to

prevent this velocity loss and to reduce the maximum speed of rotor under

permissible limit compounding is employed.

3.2 COMPOUNDING OF IMPULSE TURBINE

Compounding is employed for reducing the rotational speed of the

impulse turbine to practical limits. We know that when high velocity of steam

pg. 17

Fig. 2.1 Carry over loss in impulse turbine.


is allowed to flow through one row of the moving blades, it produces a rotor

speed of about 30,000 rpm which is too high for practical use. Not only this,

the leaving velocity loss is very high. It is therefore, essential to incorporate;

such improvement in the impulse turbine as to make it more efficient and

pragmatic. This is achieved by making use of more than one set of nozzles,

blades, rotors, in series, keyed to a common shaft, so that either the steam

pressure or the jet velocity is absorbed by the turbine in stages. This also

reduces the leaving loss. This process is called compounding of steam turbine.

There are three main types of compounding turbine.

a) Pressure-compounded impulse turbine.

b) Velocity-compounded impulse turbine.

c) Pressure and velocity compounded impulse turbine.

pg. 18


CHAPTER 4

PRESSURE COMPOUNDED IMPULSE TURBINE

In this type of turbine, the compounding is done for pressure of steam

only i.e. to reduce the high rotational speed of the turbine the whole

expansion of steam is arranged in a number of steps by employing a number

of simple impulse turbine in a series on the same shaft. Each of the simple

impulse turbine consist of one set (row) of nozzles and one row of moving

blades; known as a stage of the turbine, and thus, this turbine consist os

several stages. The exhaust from each row of moving blades enters the

succeeding set of nozzles. Thus, we can say that this arrangement is nothing

but splitting up of the whole pressure drop from the steam chest pressure to

the condenser pressure into a series of smaller pressure drops across several

stages of impulse turbine, and hence, this turbine is called pressure-

compounded impulse turbine.

The pressure and velocity variation in pressure compounded impulse

turbine is shown in figure (Fig.3.1). The nozzles are fitted in the diaphragm

which is locked in the casting. This diaphragm separates one wheel chamber

from another. All rotors are mounted on the same shaft and the blades are

attached on the rotor.

The expansion of steam only takes place in the nozzles while pressure

remains constant in the moving blades because each stage is simple impulse

turbine. It can be seen from the pressure curve that the space between any

pg. 19


two consecutive diaphragm is filled with steam at constant pressure, the

pressure on either side of diaphragm is different. Since the diaphragm is a

stationary part, there must be clearance between the rotating shaft and the

diaphragm. The steam tends to leak through this clearance for which devices

like labyrinth packing, etc. are used.

pg. 20

Fig 4.1 Diagrammatic Arrangement of Pressure-compounded Impulse turbine


Since drop in pressure of steam per stage is reduced, the steam velocity

leaving the nozzles and entering the moving blades is reduced which in turn

reduces the blade velocity. Hence for economy and maximum work shaft

speed is significantly reduced to suit practical purpose. Thus, rotational speed

may be reduced to suit practical purposes. Thus rotational speed may be

reduced by increasing the number of stages according to one’s need.

The leaving velocity of the last stage is much less compared to de-

Lavel turbine and, the leaving loss is not more than 1 to 2 percent of the initial

total available energy. This turbine was invented by the late Prof. L, Rateau

and so it is called as Rateau Turbine.

pg. 21

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