chapter 12 steam turbine

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TURBINE AND AUXILIARIES STEAM TURBINE THEORY

CHAPTER 12

STEAM TURBINE THEORY

12.1 INTRODUCTION

The steam turbine is most versatile prime mover, which is capable of almost endless

applications. A machine, which originates mechanical motion by using some natural

force, is called prime mover. Steam turbine is a practical power source and is built in as

small size as 5 KW to as large as 1300 MW. It is relatively quiet and smooth in

operation. Its compactness is unexpelled in the high capacity region and operates on

relative speed, which permits direct connection to the alternator.

The steam turbine offers many advantages over other prime movers, both

thermodynamically and mechanically. From a thermodynamic point of view, the main

advantage of the steam turbine over, say, a reciprocating steam engine, is that in the

turbine the steam can be expanded down to a lower back-pressure, thereby making

available a greater heat drop. In addition, the internal efficiency of the turbine is high, so

it is able to convert a high proportion of this relatively large heat drop into mechanical

work.

From a mechanical point of view, the turbine is ideal, because the propelling force is

applied directly to the rotating elements of the machine and has not, as in the

reciprocating engine, to be transmitted through a system of connecting links, which are

necessary to transform a reciprocating motion into a rotary motion. Hence, since the

steam turbine possesses rotary motion only, if the manufacture is good and the machine is

correctly designed, it ought to be free from out-of-balance forces.

If the load on a turbine is kept constant, the torque developed at the coupling remains

constant. A generator at a steady load offers a constant resisting torque. Therefore, a

turbine is suitable for driving a generator, particularly as they are both high-speed

machines.

A further advantage of the turbine is that the exhaust steam is not contaminated with oil

vapour and can be condensed and fed back to the boilers without passing through filters.

It also means that there is considerable saving in lubricating oil when compared with a

reciprocating steam engine of equal power.

Yet the steam turbine is not without disadvantages. It is non-reversible. Unlike the IC

engine, it is not a complete power plant, but must be associated with a steam generator.

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Steam rates are high unless in condensing operation. The thermal (steam) power plant

therefore was a duel phase cycle, i.e. vapour and liquid. It is a closed cycle to enable the

working fluid (water) to be used again and again. The cycle used is "Rankine Cycle"

modified to include superheating of steam, regenerative feed water heating and reheating

of steam.

12.2 ENERGY CONVERSION IN STEAM TURBINE

A steam turbine basically consists of two elements or sets of elements as shown in Fig.

1.2. These are

Fig.1.2 Basic Elements of Turbine

a) NOZZLE

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The nozzle, attached to the casing of the turbine. The steam enters the nozzle at a high

pressure and a relatively low velocity. Due to nozzle action steam velocity increases at

the cost of pressure and temperature.

b) CURVED BLADES

The blades are attached to the turbine rotor. The rapidly moving particles of steam

issuing from the nozzle enter the blades. As the blades are curved (Fig. 1.3), the

direction of motion of these particles of steam is changed. This causes the change of

momentum of passing steam due to

which resultant force in the tangential to the rotor periphery is set-up. The summation of

this force acting on all the blades constitutes the driving force of the turbine.

Finally when the steam comes out of the blades, the pressure and temperature of the

steam are reduced, i.e. the drop of the enthalpy at the exhaust of the turbine due to

expansion of steam. The processes of expansion and direction changing may occur once

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nozzle exit velocity of the steam is high. For maximum blade efficiency, the blade

velocity should be slightly less than one half the steam velocity, so in this type of turbine

the blade velocity is very high. As the rotor diameter is kept fairly small, the rotational

speed is also very high, being of the order of 30,000 rpm. With speeds of this order it is

often necessary to reduce the speed of the driven machine by gear-box, thus increasing

the cost and complexity of an installation and reducing its overall efficiency. It can be

seen from Fig. 1.4 that the velocity of the steam leaving the moving blades is large which

represents the loss of Kinetic energy and is called the "Carry-over loss" or "Leaving

Loss" which may be approximately 11% of initial Kinetic energy of the steam.

An example of the simple impulse turbine is the De Laval turbine used for relatively low

power application. Rotor of simple (Single stage) impulse turbine is shown in Fig. 1.5.

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12.4.2 The Pressure-Compounded Impulse Turbine

The turbine, shown in Fig. 1.6, is basically a number of impulse turbines connected in

series on the same shaft, the exhaust steam from one stage entering the nozzles of the

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succeeding stage. In this way the whole of the pressure drop (heat drop) available, i.e.

from steam chest to condenser, is split-up into a number of smaller pressure drops.

Hence stage consists of set of nozzles and blades. As the heat drop in each stage is a

fraction of the heat drops in a simple impulse turbine working between the same limits of

pressure and temperature, the increase in Kinetic energy in each stage will be much

lower, i.e. the velocity of the steam issuing from the nozzles will be much lower.

Therefore, the blade velocities and rotational speed can be lowered. This means the

greater the number of stages, the lower the speeds.

The leaving loss in the last stage as compared to simple impulse turbine is

proportionately less, still it is appreciable.

In a pressure-compounded impulse turbine the nozzles are usually fitted into partitions,

called "diaphragms", which separate one wheel chamber from the next. The wheels are

mounted individually on the shaft and carry the blades on their periphery. As expansion

of the steam takes place wholly in the nozzles, the space between any two diaphragms is

filled with steam at a constant pressure, but the pressure on either side of any diaphragm

are different. The greatest difference occurs in the first few stages. Hence, steam will

tend to leak through the space between the bore of the diaphragm and the surface of the

shaft. Fitting of labyrinth glands usually minimizes such leakage. A.C.E. Rateau first

designed this type of turbine.

12.4.3 The Velocity-Compounded Impulse Turbine

It is similar to the simple impulse turbine in that there is only one set of nozzles. The

wheel, however, instead of being fitted with a single row of blades, is fitted with two or

more rows, between which are arranged rows of stationary guide blades. Fig. 1.7 shows

a three-row wheel. Steam enters the nozzles at the steam chest pressure and issues from

the nozzles at condenser pressure and as in the simple impulse turbine, at very high

velocity. The provision of two or more rows of moving blades, however, enables the

blade velocity for maximum efficiency to be made appreciably less than that necessary

for maximum efficiency in its simple impulse turbine. On passing through the first row

of moving blades the steam gives up only a part of its Kinetic energy and issues from this

row of blades with fairly high velocity. It then enters the first of the two rows of guide

blades and is redirected by them into the second row of moving blades. There is a slight

drop in velocity in the fixed guide blades due to friction. In passing through the second

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row of moving blades, the steam gives up another portion of its Kinetic energy to the

rotor. It is redirected in the second row of guide blades, does work on the third row of

moving blades, and finally leaves the wheel in a more or less axial direction with a

certain residual velocity. This velocity is comparatively small and therefore the leaving

loss is small, being about two percent of the initial available energy of the steam.Fig.1.7

(a) shows two-stage vel.comp.impulse turbine.

12.4.4 The Pressure-Velocity Compounded Impulse Turbine

In the same way that a number of simple impulse turbines in series on the same shaft can

be combined to form a pressure-compounded impulse turbine, so a number of simple

velocity - compounded impulse turbine as shown in Fig. 1.8. The only difference in

principle between the two types is that in the pressure-compounded type a stage consists

of a set of nozzles and a single row wheel, whereas in the pressure-velocity-compounded

type a stage consists of set of nozzles and a single row wheel, whereas in the pressure-

velocity compounded type a stage consists of a set of nozzles and a wheel with two or

more rows of blades.

As in other type of impulse turbines, the steam is expanded wholly in the nozzles and the

wheels rotate in steam at constant pressure. The total pressure drop from steam chest to

condenser being split-up into as many steps as the number of wheels on the shaft. This

type of turbine is comparatively simple in construction and is much more compact than

the multi-stage pressure-compounded impulse turbine since the pressure drop is greater

per stage and consequently fewer stages are necessary. Unfortunately its efficiency is not

high. At one time it was widely used in power stations but is now an obsolete type.

Many impulse turbines, however, incorporate a two-row velocity wheel for the first stage

in the high pressure cylinder. An American engineer, C.G. Curtis, first introduced this

turbine.

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Fig.1.6 Pressure Compounded Impulse Turbine


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Fig.1.8 Velocity &Pressure Compounded impulse Turbine

12.5 IMPLUSE-REACTION TURBINEImpulse-reaction (commonly called as "Reaction Turbine") turbines works on the

principle that the steam pressure is reduced in both fixed and moving blades unlike in

impulse turbine in which pressure was reduced only in nozzles. While the steam is

passing through the moving blades, work is still being done by the impulse effect due to

the reversal of direction of the high velocity steam, but the fixed and moving blades are

so designed that the steam expands as it passes through both thus giving, in addition, a

reaction effect due to the expansion of steam through the moving blades.

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Since in the reaction type machine a pressure drop also occurs across the moving blades

it is necessary to provide effective sealing at the blade tips. This must be done to prevent

leakage of steam past the shrouding of the wheel and consequent loss in efficiency

particularly at the high-pressure end of the machine. These turbines may be designed for

radial flow or axial flow. However, radial flow machines are absolute now a days and all

modern turbine employ axial flow designs.

The axial-flow impulse-reaction turbine consists of a number of rows of moving blades

attached to the rotor and an equal number of rows of fixed blades attached to the casing

as shown in Fig. 1.9.

The fixed blades compare to the nozzles used in the impulse turbine. Steam is admitted

over the whole circumference and in passing through the first row of fixed blades

undergoes a small drop in pressure and its velocity is increased. It then enters the first

row of moving blades and as in the impulse turbine, suffers a change in direction and

hence momentum giving an impulse on the blades. During the steam passage through the

moving blades it undergoes a further small drop in pressure resulting in increase in

velocity, which gives rise to a reaction in the direction opposite to that of the added

velocity. It is in this way that the impulse-reaction turbine differs from the pure impulse

turbine. Thus the gross propelling force in the impulse-reaction turbine, or the "reaction"

turbine, is the vector sum of the impulse and the reaction effects.

Fig. 1.9 shows how the blade heights increase as the specific volume of the steam

increases with reduction in pressure. It also shows how the pressure falls gradually as the

steam passes through the groups of blades. There is a pressure drop across each row of

blades both fixed and moving. This is of considerable practical importance, especially at

the high-pressure end of the turbine where the pressure drops are greatest, because this

difference of pressure tends to force some steam through the clearance spaces between

the moving blades and the casing, similarly between the fixed blades and rotor. These

clearances have to be carefully controlled by using axial and/or radial seals at the blade

tips; otherwise the available energy possessed by the steam, which leaked across, would

be lost. The pressure drop across the moving blades gives rise to a large axial thrust on

the rotor towards the low pressure end of the turbine, therefore special balance pistons

have to be fitted to counteract it. Fig. 1.10 shows the axial section of a turbine with

impulse/reaction stages and balance piston arrangement.

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The steam velocities in this type of turbine are moderate, the velocity for maximum blade

efficiency being roughly equal to the blade velocity. The leaving loss is normally about

the same as for the multi-stage impulse turbine.

The impulse-reaction turbine was developed by Sir Charles A. Parsons and is widely

used in power stations.

Fig.1.9 Axial Flow Reaction Turbine

12.5.1 Distinction between Impulse & Reaction Designs

The hard and fast distinction between the impulse and impulse-reaction turbine is

becoming progressively less important. The general trend of commercial development

being that the reaction turbine often to adopt a certain percentage of impulse in its design

and the impulse turbine likewise to adopt a certain percentage of reaction. At the present

time the two types are therefore characterized more by differences of constructional

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chapter 12 steam turbine

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