CONTENTS Sl.No Lecture Notes Page No. Dynamic Action of ...

CONTENTS

Sl.No Lecture Notes Page No.

1Dynamic Action of Fluid and Concept of Velocity

Triangles1

2 Water Turbines 24

3 Gas and Steam Turbines 38

4 Industrial Pumps and its Applications 79

5 Computational Fluid Dynamics 87

6 Dimensional Analysis 106

7 Pnematics 119

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Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Dynamic Action of Fluid on Vanes and Concept of Velocity Triangle

Prepared by: Prof. A. MURUGAPPANProfessor and HeadDepartment of Civil Engineering, Annamalai University, Annamalainagar – 608 002

1

DYNAMIC ACTION OF FLUID ON VANESDynamic force

Consider a stream of fluid entering a machine such as a hydraulic turbine or steam

turbine, or a pump or a fan. The stream of fluid has a more or less defined direction. For a

force to be exerted by the fluid on the machine, the stream of fluid must undergo a

change in its velocity either in its magnitude or direction or both. When the fluid stream

enters the machine, the machine exerts a force on the fluid bringing about a change in the

velocity of fluid either in its magnitude or in its direction. According to Newton’s third

law of motion for every action there is an equal and opposite reaction. Hence, the fluid

stream exerts an equal and opposite force upon the machine that causes the change in

velocity of fluid stream. This force exerted by the virtue of fluid in motion is called the

dynamic force of fluid. The dynamic force of fluid always involves a change in its

velocity and thus a change in its momentum. Hence, the force exerted by the machine on

the fluid is the action and the force, in turn, exerted by the fluid on the machine is the

reaction.

The major problem in hydraulic machinery is to determine the power developed by a

particular machine or the power consumed in a particular machine. For instance, a

hydraulic turbine develops power while a pump consumes power in order to run. The

power consumed or developed by a machine can be determined from the dynamic force

or forces exerted by the flowing fluid on the boundaries of the flow passage and which

are due to the change of momentum. These are determined by applying Newton’s second

law of motion.

Fundamental principle of dynamics

The fundamental principle of dynamics is Newton’s Second Law of Motion. It states that

“the rate of change of momentum (linear momentum) is proportional to the applied force

and takes place in the direction of the force”. More precisely, this statement may be

written as “the resultant external force Fx acting on the particle of mass m along any

arbitrarily chosen direction x is equal to the time rate of change of linear momentum of

the particle in the same direction, i.e., x – direction”.





2

Linear momentum of a body is the product of its mass and its velocity. Let m be the mass

of the fluid moving with a velocity v. Let the fluid mass undergoes a change in velocity

dv in time dt.

Hence, change in linear momentum of fluid mass = m.dv

Time rate of change of linear momentum of fluid mass = m.dt

dv

According to Newton’s Second Law of Motion,

Dynamic force applied in x – direction= time rate of change of linear momentum of fluid mass in x – direction

i.e.,dt

dvmF x

x . …… (1)

where the suffix x, denotes the components in x – direction.

Equation (1) is known as linear momentum equation. Equation (1) can also be written as:

xx dvmdtF .. …… (2)

The term on the LHS of equation (2) represents the product of the dynamic force Fx and

the time increment dt during which it acts. This is known as the impulse of applied

dynamic force. The term on the RHS of equation (2) represents the product of mass of

fluid and the change in velocity dvx undergone by the fluid mass in x-direction in time

increment dt. This term represents the change in linear momentum of fluid mass.

Note: As velocity is a vector quantity, any change in magnitude or direction or both will

change the velocity, and hence momentum.

Equation (2) is known as the Impulse-Momentum Equation. It states that the impulse of

the dynamic force is equal to the resulting change in linear momentum of body.

Newton’s Second Law of Motion is generally applied to a system. A system is a definite

mass of fluid (or material) and all other matter around it is known as its surroundings.

The boundaries of the system will form a closed surface and this surface may change





3

with time so that it contains the same mass during which the change takes place. When

Newton’s Second Law of Motion is applied to a system, equation (1) may be written as:

dt

dvmF x

x . …… (3)

where m is the constant mass of the system. xF represents the algebraic sum (or

resultant) of all body forces such as gravity as well as surface forces acting on fluid

mass m in any arbitrary direction x. vx is the velocity of the centre of mass of the system

in x-direction. dvx is the change in vx in time dt.

Concept of Control Volume and One-dimensional form of momentum equation

Control volume is a specific region in space, its size and shape being entirely arbitrary.

However, the shape and size of a control volume are made to coincide with solid

boundaries.

Considering a control volume and a fluid entering the control volume with uniform

velocity1x

v in the arbitrary x – direction and leaving the control volume after time t with

a uniform velocity2xv in the x – direction, equation (3) can be written as

12 xxx vv

t

mF …… (4)

Sincet

mrepresents the mass of fluid flowing per unit time, we have,

fluid)offlowof(rate xfluid)ofdensity(mass Qt

m

where = mass density of fluid (kg m-3)Q = rate of flow of fluid or discharge (m3s-1)

t

mhas units of kg s-1.

Hence, equation (4) can be written as

12 xxx vvQF …… (5)





4

Equation (5) represents the one-dimensional form of steady flow momentum equation.

This equation is important in the study of hydraulic machines as it enables the

determination of forces developed by the flow of fluid in the machine.

Dynamic force exerted by a fluid jet on a stationary flat plate held normal todirection of jet

Let a fluid jet issued from a nozzle strikes a flat plate with a velocity v. Let the fluid jet

be oriented in x – direction (horizontal direction). The flat plate is held stationary, vertical

and normal (perpendicular) to the direction of the jet. Let the flow rate of fluid issued

from the nozzle and impinging on the plate be Q. Let be the specific weight of fluid and

be the mass density of fluid. Then weight of fluid flowing per second is Q.

Mass of fluid issued from nozzle and striking the plate per second, QQgt

m

Velocity of fluid issued from the nozzle = v

Velocity with which the fluid jet strikes the plate in x – direction = v

The fluid jet after striking the plate gets divided into two equal halves, with each half

deflected by 90 from the original direction of the fluid jet (at the instant of striking the

plate). One half of the jet moves in the vertical upward direction and the other half of the

x x

Fluid jet moving withvelocity v

Nozzle

v

Stationary flat plate heldnormal to fluid jet

v

Figure 1 Fluid jet impinging on a stationary flat plate held normal to jet





5

jet moves in the vertical downward direction as shown in Figure 1. Assuming the plate to

be frictionless, the jet leaving the plate will move with the same magnitude v of the jet

incident on the plate. As the direction of the velocity of fluid jet is changed after

impingement on the plate, it is said that the velocity of fluid jet has changed causing a

linear momentum.

Final velocity of fluid jet in x - direction after striking the plate

= component of velocity of fluid jet leaving the plate in x – direction= v cos 90 = v x 0 = 0 ms-1

Hence change in velocity of fluid jet in x – direction

= final velocity of fluid jet in x – direction (after impingement on plate) – initial velocityof fluid jet in x - direction (before impingement)

= 0 – v = - v ms-1

Dynamic force exerted by plate on fluid

= rate of change of momentum of fluid= (mass of fluid striking the plate per second) x

(change of velocity of fluid normal to the plate)Applying Equation (5)

12 xxx vvQF = vQ 0 Qv

- QvFx …… (6)

The negative sign on RHS of above equation represents that the velocity of fluid jet is

decreasing, while the negative sign on the LHS of the equation represents that the force

exerted by the plate on the fluid jet is acting in the negative direction of x – axis.

Equation (6) gives the force exerted by the plate on the fluid jet. Here, the plate is

responsible for effecting a change in the velocity of fluid jet. Therefore, the force is

exerted by the plate on the fluid jet (this is the action). Since the final velocity of fluid jet

is less than the initial velocity, the force exerted by the plate on the fluid jet is a retarding

force, thus it acts in a direction opposite to the direction of flow of fluid.

As per Newton’s Third Law of motion, for every action there is an equal and opposite

reaction. Now, the reaction, in this case, is the force exerted by the fluid jet on the plate.





6

The magnitude of the force exerted by the fluid jet on the plate is equal to Qv . The

direction of force exerted by fluid jet on the plate is diametrically opposite to that exerted

by the plate on the fluid jet. As the direction of the force exerted by the plate on the fluid

jet is in the negative x - direction, the direction of the force exerted by the fluid jet on the

plate must be in x - direction. Hence, the dynamic force exerted by the fluid jet on the

plate is given by

QvFx …… (7)

Dynamic force exerted by a fluid jet on a stationary flat plate held inclined todirection of jet

Let a fluid jet of diameter d and cross-sectional area a issued from a nozzle and moving

with a velocity v impinges on a stationary flat plate held inclined to the direction of jet.

The jet is oriented in the horizontal direction (x – direction). The plate makes an angle

with the horizontal. After impingement on the plate, the jet gets divided into two equal

parts, with one part moving upward at an angle to the horizontal and the other part

moving downward at the same angle to the horizontal. Assuming the plate to be

frictionless, the jet leaves the plate with a velocity whose magnitude is equal to v which is

the same as that of the velocity of the jet incident on the plate.

x

Nozzle

Fluid jet moving withvelocity v Stationary flat plate held

inclined to fluid jet

v

v

xv





7

The velocity of jet issued form nozzle and incident on the inclined plate can be resolved

into two mutually perpendicular components, one component normal (perpendicular) to

the plate and other component along the plate (parallel to the plate). Let the velocity

component defined normal to the plate be denoted by vn and the component defined along

the plate be defined by the symbol vt.

Here, sin)90cos( vvvn

and cosvvt

As the plate surface is frictionless, the jet leaving the plate has the same magnitude v as

that of the jet incident on the plate. The jet leaving the plate is tangential to the plane

surface of the plate. The component of the velocity of jet leaving the plate in a direction

normal to the plate is .sm00 x90cos -1 vv Hence, the change in velocity of jet in

a direction normal to the plate is given by

Change in velocity of jet in a direction normal to the plate

= final velocity of jet normal to the plate – initial velocity of jet normal to the plate

= sinsin0 vv

Dynamic force exerted by plate on fluid in a direction normal to the plate, Fn

= rate of change of momentum of fluid in a direction normal to the plate

= (mass of fluid striking the plate per second) x

(change of velocity of fluid normal to the plate)

90

v

90 -

vn = v sin vt = v cos





8

Replacing the direction x (horizontal) by n (normal) in Equation (5)

12 nnn vvQF = sin0 vQ sinQv

- sinQvFn …… (8)

The negative sign on RHS of above equation represents that the velocity of fluid jet is

decreasing in the direction normal to the jet, while the negative sign on the LHS of the

equation represents that the force exerted by the plate on the fluid jet in a direction

normal to the fluid jet is acting in the negative direction of x – axis.

The dynamic force exerted by the fluid jet on the plate in a direction normal to the plate is

oriented in a direction diametrically opposite to that exerted by the plate on the fluid jet.

It is given by

sin)sin( QvQvFF nn …… (9)

The component of Fn in x – direction (along the direction of jet issued from nozzle) is

given by

2sinsin)sin(sin)90cos( QvQvFFF nnx …… (10)

Dynamic Force exerted by Fluid Jet on a Moving Flat Plate

The plate is held normal to the direction of fluid jet. The fluid jet moving with velocity v

impinges on the plate and after the jet strikes the plate, the plate moves with a velocity u

in the direction same as the direction of the fluid jet incident on the plate. Hence, the

velocity of jet leaving the plate becomes (v - u). as the plate moves away progressively

from the jet with velocity u, the quantity of fluid striking the plate per second is given by

multiplying the cross-sectional area of fluid jet and the velocity of jet relative to the plate,

w.

uvaawQ

where, w = velocity of fluid jet relative to the motion of the plate

v = absolute velocity of fluid jet





9

Force exerted by the fluid on the plate (in x – direction)

= (mass of fluid striking the plate per second) x

(change of velocity of fluid jet (in x – direction))

uvQuvQuvuvQFx 090cos

Here, uvaQ

Hence, 2uvauvuvaFx

Here, under the impact of the jet on the plate, the plate starts moving away from the jet at

a velocity u. that is, the distance between the plate and the nozzle from which the jet is

issued increases constantly at u m s-1. Therefore, a single moving plate is not a practical

case. However, if a series of plates (refer Figure below) were so arranged that each plate

appeared successively before the fluid jet in the same position and always moving with a

velocity u in the direction of the jet, the entire flow issued from the nozzle will be utilized

in making impact on all the plates. Thus, the mass of fluid striking the plates per second

would be avQ .

Hence, uF

(mass of water striking the plates per second) x (change in velocity of fluid jet)

= uvavuvavuvuvQ 090cos

Work done by the fluid jet on the plates = uuvQuFu .

x x

Fluid jet moving withvelocity v

Nozzle

Moving flat plate heldnormal to fluid jet

Figure. Fluid jet impinging on a moving flat plate held normal to jet

u

v - u

v - u

v





10

This gives the output of the system under consideration

Kinetic energy of the fluid jet = 2

2

1mv

Where m = mass of fluid issued from the nozzle per second = Q

Hence, kinetic energy of the fluid jet (input to the system under consideration)

= 2

2

1vQ

Efficiency of the system,jetfluid theofenergykinetic

plateson thejetfluidby thedonework

input

output

=

2

2

1Qv

uuvQ

=

2

2

v

uuv

For maximum efficiency, max , 0

du

d

v

Figure. Fluid jet impinging on aseries of moving flat plates

u





11

i.e.,

02

2

v

uuv

du

d

022

uuvdu

d

v

02 uvudu

d

02 uv

2

vu

Substituting the value of u in the expression for , the value of max can be obtained.

5.02

12222

222

2

2

22max

v

v

v

vv

v

vvv

(or) 50%

Dynamic force exerted by fluid jet on a stationary curved plate

The fluid jet moving with a velocity v1 impinges on a curved plate such that it makes an

angle 1 with the x – axis at the point of impingement (inlet of plate). After impingement

1

2v2

v1

xx

xx

Inlet

Outlet

Figure. Fluid jet impinging on a stationary curved plate withacute discharge angle 2





12

at inlet, the jet glides over the surface of the plate and leaves the plate tangentially at its

outlet. The jet leaving the plate makes an angle 2 with the x – axis. The angle 2 also

indicates the angle of curvature of the plate at its outlet as the jet leaves the plate

tangentially to the curvature of the plate. Let v2 be the velocity of fluid jet leaving the

plate. If the surface of the plate along which the jet glides past the plate is frictionless, the

magnitude v2 of the velocity of fluid jet leaving the plate will be the same as the

magnitude v1 of the velocity of the jet incident on the plate at its inlet.

Velocity of fluid jet at inlet in x - direction = 11 cosv

Velocity of fluid jet at outlet in x - direction = 22 cosv

Therefore, force exerted by fluid jet on plate in x - direction is given by

xF (mass of fluid striking the plate per second) x

(change of velocity of fluid jet in x – direction)

= 2211 coscos vvQ

where

inlet)itsatplateon theincidentjetfluidof(velocity xjet)fluidofareasectional-cross(

secondperplate thestrikingfluidofquantity

Q

= av1

Hence, 22111 coscos vvavFx

If the curvature of plate at its outlet is such that the discharge angle, 2, is more than 90

(as shown in Figure below), then cos 2 is negative, hence, 22 cosv is negative. Hence,

in order to get more force, the curvature of the plate should be such that the discharge

angle 2 must be more than 90.





13

Dynamic force exerted by a fluid jet on a single moving curved plate

Let a fluid jet of velocity v1 issued from a nozzle impinges at the inlet ‘1’ of a curved

plate which moves as a result of impingement of fluid jet at its inlet. The velocity with

which the fluid jet is issued from the nozzle and impinges on the plate at tits inlet is

termed as the absolute velocity of jet at inlet. The absolute velocity of fluid jet makes an

angle 1 with the X-direction.

1

2

v2

v1

Inlet

Outlet

x x

x x

Figure. Fluid jet impinging on a stationary curved plate withobtuse discharge angle 2





14

Once the jet is incident on the plate at inlet, let the plate move in X-direction with a

velocity u. Let the velocity of plate at inlet be referred by u1. It should be noted that as the

entire plate moves in a unique direction (X-direction) with the same velocity, u, we have,

u1 = u. Once the jet strikes the plate at its inlet, the velocity of jet remains no more equal

to v1, but it becomes the velocity of jet relative to the motion of the plate at inlet (that is,

relative velocity of fluid jet) be referred by the symbol w1. The direction of relative

velocity of jet at inlet is tangential to the curvature of the plate at inlet. The tangent to the

curvature of the plate at its inlet may or may not make the same angle as that of the fluid

jet with the X-direction. In Figure drawn above, the angle which the tangent to the

curvature of the plate makes with the X-direction is different from that which the fluid jet

at inlet makes with the X-direction. Let the angle which the relative velocity of jet at inlet,

w1, makes with the reversed direction of motion of plate at inlet, that is, - u2, be 1. That

is, the angle of curvature of the plate at inlet (tangent to the curvature of the plate at inlet)

makes an angle 1 with the reversed direction of motion of plate at inlet.

X

XX1

1w1

v1

u11

1 - Inlet

2 - Outlet

u = u1 = u2

22

w2 v2

u2

X2





15

Here, the magnitudes of v1 and u1 (= u) are known. The magnitude of w1 can be

determined by subtracting u1 from v1 vectorially by applying the Law of Parallelogram of

Velocities.

Vecorially speaking, we have, by Law of Parallelogram of Velocities,

111 uvw

Construction of Parallelogram of Velocities at Inlet:

Draw 1vAC to a suitable scale such that 1v makes an angle 1 with the X-direction.

At A, draw 1uAB in X –direction to the same scale as that of v1. Join B and C so that

1wBC . Measure BC which gives the magnitude of 1w to the same scale as those of

11 and uv .

Then, the jet glides past the surface of the plate and leaves the plate at its outlet with a

relative velocity equal to w2. The magnitude of the relative velocity of jet at outlet, w2,

may remain equal to the relative velocity of jet at inlet, w1, provided the surface of the

plate is perfectly smooth, that is, when there is no energy loss due to friction of the

surface of the plate. Let the plate at outlet be inclined at an angle 2. This angle 2 is

measured such that it is the angle between relative velocity of jet at outlet, w2, and

reversed direction of motion of plate at outlet, i.e., - u2.

v1

u1

w1

A

X

B

1XX

1

C

1 - Inlet





16

Construction of Parallelogram of Velocities at Outlet:

Draw 2uAB to a suitable scale in X – direction. At B set out angle 2 representing the

direction of 2w . Draw 2wBC along the set direction. Join A and C so that 2vAC .

Measure AC which gives the magnitude of 2v . Vectorially speaking, we have,

222 wuv

The magnitude and direction of 2v can be determined by applying the Law of

Parallelogram of Forces. The angle which the absolute velocity of jet at outlet, 2v , makes

with X – direction is referred by 2. It is the angle between 2v and 2u .

Force exerted by the fluid jet on the plate in X – direction or in the direction of motion of

plate is determined by applying the Linear-Momentum Equation:

direction-Xinjetfluidofyin velocitchange xunit timeperplate thestrikingfluidofmassxF

= 2211 coscos x vvQ

where uvauvaQ 111 , since the plate progressively moves away from the jet

and hence, the velocity of jet falling on the plate is reduced by u, or the velocity with

which the jet falls on the plate is (v1 – u). The quantity of fluid jet issued from the nozzle

(i.e., av1). For 2 <2

, cos 2 > 0, while for 2 >

2

, cos 2 < 0, hence, when cos 2 < 0,

the second term (v2 cos 2) within the parentheses will become negative. Hence, the

quantity 2211 coscos vv will be higher for the same Q, v1, 1 and v2. Hence, in

22

w2 v2

u2X

2

2

w2 v2

u2X

2

2





17

order to get more magnitude for force F, the curvature of the plate at outlet should be

such that 2 is obtuse.

Velocity Diagrams for Turbine Blades

At Inlet:

u1 = circumferential or peripheral velocity of vanes at inlet

w1 = velocity of jet relative to motion of vane at inlet

v1 = absolute velocity (i.e., relative to earth) of jet at inlet

1 = angle between v1 and u1, i.e., angle of jet with the direction of motion of vane atinlet

1 = angle between w1 and – u1, i.e., angle of vane tip at inlet. This angle is measuredbetween w1 and u1 reversed

The absolute velocity of jet, v1, can be resolved into two mutually perpendicularcomponents.

1

1

w1v1

u1Inlet

1uvu1

1 1

v1

w1

1mv

AB

C

D

=

Figure. Typical velocity triangle for flow over turbine blade at Inlet





18

(i) Tangential component,1uv , equal to v1 cos 1. This component is parallel to

the direction of motion of vane, i.e., u1, and hence is responsible for doing

work. Therefore, this velocity component is referred to as Velocity of Whirl at

inlet.

(ii) Radial component, ,vm1equal to v1 sin 1. This component is perpendicular to

the direction of motion of vane and hence they do not do any work on the

blades (runner). This component causes the water to flow through the turbine

blade and therefore, is called the Velocity of Flow at inlet.

Drawing Velocity Triangle at Inlet:

Step 1: Draw 1uAB , in the horizontal direction (x – direction) to a suitable scale.

Step 2: At A, set out angle 1 downward, to mark the direction of 1v . It should be noted

that the angle between 1v and 1u is 1

Step 3: Along the direction set out at A as outlined in Step 2, set out 1v to the same scale

so that 1vAC

Step 4: Join B and C so that 1wBC

Step 5: Measure ABC = 1, the vane angle at inlet

The relationship between the velocity vectors, 11 u,v and 1w is

111 wuv

111 uvw

The absolute velocity of water jet at inlet, v1, is resolved into two mutually perpendicular

velocity components namely, the Velocity of whirl at inlet,1uv , which is the tangential

component, and the Velocity of flow at inlet, ,vm1which is the normal or radial





19

component. It should be noted that the direction of1uv is along the direction of u1. The

direction of1mv is perpendicular (normal) to u1

The arrow heads mark the directions of the respective velocity vectors.

At outlet:

u2 = circumferential or peripheral velocity of vanes at outlet

w2 = velocity of jet relative to motion of vane at outlet

v2 = absolute velocity (i.e., relative to earth) of jet at outlet

2 = angle between v2 and u2, i.e., angle of jet with the direction of motion of vane at

2

u2

v2

w2

2

Outlet

=

2

u2

v2w2

2

2mv

2uv

A B

C

D

Figure. Typical velocity triangle for flow over turbine blade at Outlet





20

outlet

2 = angle between w2 and – u2, i.e., angle of vane tip at outlet. This angle is measured

between w2 and u2 reversed

The absolute velocity of jet, v2, can be resolved into two mutually perpendicular

components.

(i) Tangential component,2uv , equal to v2 cos 2. This component is parallel to the

direction of motion of vane at outlet, i.e., u2, and hence is responsible for doing work.

Therefore, this velocity component is referred to as Velocity of Whirl at outlet.

(ii) Radial component, ,vm2equal to v2 sin 2. This component is perpendicular to the

direction of motion of vane and hence they do not do any work on the blades

(runner). This component causes the water to flow through the turbine blade and

therefore, is called the Velocity of Flow at inlet.

Drawing Velocity Triangle at Outlet:

Step 1: Draw 2uAB , in the horizontal direction (x – direction) to a suitable scale.

Step 2: At B, set out angle 2 downward, to mark the direction of 2w . It should be noted

that the angle between 2w and 2u reversed is 2

Step 3: Along the direction set out at B as outlined in Step 2, set out 2w to the same scale

so that 2wBC

Step 4: Join A and C so that 2vAC

Step 5: Measure BAC = 2, the angle which the absolute velocity of jet at outlet, v2,

makes with the circumferential or peripheral velocity of runner at outlet, u2.

The relationship between the velocity vectors, 22 u,v and 2w is





21

222 wuv

The absolute velocity of water jet at outlet, v2, is resolved into two mutually

perpendicular velocity components namely, the Velocity of whirl at outlet,2uv , which is

the tangential component, and the Velocity of flow at outlet, ,vm2which is the normal or

radial component. It should be noted that the direction of2uv is along the direction of u2.

The direction of2mv is perpendicular (normal) to u2.

Note: The velocity of whirl at outlet,2uv , may be positive or negative depending upon

whether the angle 2 is acute or obtuse. When 2 is acute, cos 2 is positive, and hence,

222cosvvu , is positive. When 2 is obtuse, cos 2 is negative, and hence,

222cosvvu , is negative.

Fluid jet on moving curved surface of a Pelton turbine blade

Figure: Plan view of a double hemispherical bucket of a Pelton runner

1

2

2

NozzleAbsolute path

Relative path

1 – Inlet (splitter)

2 – Outlet





22

Each bucket consists of two approximately hemispherical cups separated by a sharp edge(called ‘splitter’) at the centre. The water jet impinges at the centre of the bucket (splitter)and is divided by the splitter into two equal halves, each half moving sideways inopposite direction. Thus the incident jet at inlet is deflected backward when leaving thebucket at outlet. The theoretical angle of deflection of jet is 180. But due to practicalreasons, the actual angle of deflection of jet is kept less than 180, say, about 165.

The direction of absolute velocity of jet at inlet is tangential to the curvature of blade atinlet. Further, the direction of absolute velocity of jet at inlet is along the direction of

peripheral velocity of runner at inlet, i.e., 1v and 1u are in the same direction. Hence,

01 . Hence, the direction of relative velocity of jet at inlet is also along the same

direction as those of 1v and 1u . Therefore, 1 = 180. There is no formation of velocity

triangle at inlet, as all the three velocity vectors, 1v , 1u and 1w are along the samedirection. Further, we have,

111 uvw

As the outlet of the runner blade is located at the same radial distance (in the same plane)

from the axis of runner, as that of the inlet, the peripheral velocity of blade at outlet, 2u

peripheral velocity of blade at inlet, 1u . The shape of the outlet velocity triangle is shownbelow.

u1

u2

v1

v2

w1

w2

22

1 = 01 = 180





23

The direction of absolute velocity of jet at outlet is against the direction of motion of

runner, i.e., 2v is against 2u . Further, as the angle between 2v and 2u , i.e., 2, is obtuse

(more than 90), cos 2 is negative. Hence,2uv = v2 cos 2, is negative. What is the

impact of this condition in development of the dynamic force by the fluid jet on the runnervanes?

u2

v2

w1

w2

2

2

v2

w2

u2

2mv

2uv

2

2



Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Water Turbines

Prepared by: Prof. N. Ashok KumarAssistant ProfessorDepartment of Civil Engineering, Annamalai University, Annamalainagar – 608 002

24

WATER TURBINESIntroduction:

Water turbines were developed in the 19th century and were widely used for industrial

power prior to electrical grids. Now they are mostly used for electric power generation.

Water turbines are mostly found in dams to generate electric power from water kinetic

energy. Water wheels have been used for hundreds of years for industrial power. Their

main shortcoming is size, which limits the flow rate and head that can be harnessed. The

migration from water wheels to modern turbines took about one hundred years.

Development occurred during the Industrial revolution, using scientific principles and

methods. They also made extensive use of new materials and manufacturing methods

developed at the time.

Turbine is a device in which a mechanical energy is transferred from the flowing liquid

through the machine to its operating member. The inlet energy of the liquid is greater

than the outlet energy of the liquid are referred as Water turbines. It is well known from

Newton’s law that to change of momentum of Fluid, a force is required. Similarly, when

momentum of fluid is changed, a force is generated. This principle is made use in

hydraulic turbine.

It converts energy in the form of falling water into rotating shaft power. The geometry of

turbines is such that the fluid exerts a torque on the rotor in the direction of its rotation.

The shaft power generated is available to derive generators or other devices.

Classification of turbines:According to action of water on moving blades:The two basic types of hydraulic turbines are impulse and reaction turbines.

In impulse turbine the entire pressure energy of water is converted into kinetic energy,

also it converts the kinetic energy of a jet of water to mechanical energy.

The static pressure of water at the entrance to the runner is equal to the static pressure at

exit and the rotation of wheel is caused purely due to tangential force created by the

impact of the jet, and hence it is called as impulse turbine.

Reaction turbines convert potential in pressurized water to mechanical energy. The static

pressure at inlet to the runner is higher than the static pressure at the exit, and there is a

gradual conversion of static pressure into kinetic energy while water is flowing through

the runner. In this type of turbine, the rotation of the runner is partly due to impulse





25

action and partly due to change in pressure over the runner blades, hence the turbine is

called reaction turbine.

According to available head:

Turbines can be classified as high head, medium head or low head machines. The

turbines which are capable of working under high heads (i.e greater than 400m) are called

high head turbines. When the gross head lies between 50m to 400m, the turbines are

medium head turbines. Turbines capable of working under heads varying from 50m to

2.5m are called low head turbines.

Type High head Medium head Low head

Impulse turbines pelton

turgo

cross-flow

multi-jet pelton

turgo

cross-flow

Reaction turbines francis propeller

kaplan

According to direction of flow of water:

The turbines are classified into Tangential flow, Radial flow, Axial and mixed flowturbines.

Tangential flow turbines: In this type of turbines, the water strikes the runner in the

direction tangent to the wheel. Example: pelton wheel

Radial flow turbines: In this type of turbines, the water strikes in the radial direction. It is

further classified as inward radial flow turbine and outward radial flow turbine.

Inward radial flow turbine: The flow is inward from periphery to the centre. Example: old

Francis turbine.

Outward radial flow turbine: The flow is outward from centre to periphery. Example:

Fourneyron turbine.

Axial flow turbine: When flow of water is in the direction parallel to the axis of the shaft.

Example: Kaplan and propeller turbine.

Mixed flow turbine: The water enters the runner in the radial direction and leaves in the

axial direction. Example: Modern Francis turbine.





26

Selection of a type of turbine:The selection of the best turbine for any particular hydro site depends on the site

characteristics, the dominant ones being the head and flow available. Selection also

depends on the desired running speed of the generator or other device loading the

turbine. Other considerations such as whether the turbine is expected to produce power

under part-flow conditions also play an important role in the selection. All turbines have

a power-speed characteristic. They will tend to run most efficiently at a particular speed,

head and flow combination.

In large scale hydro installation Pelton turbines are normally only considered for heads

above 150 m, but for micro-hydro applications Pelton turbines can be used effectively at

heads down to about 20 m. Pelton turbines are not used at lower heads because their

rotational speeds become very slow and the runner required is very large and heavy. If

runner size and low speed do not pose a problem for a particular installation, then a

Pelton turbine can be used efficiently with fairly low heads. If a higher running speed

and smaller runner are required then there are two further options:

• Increasing the number of jets.

Having two or more jets enables a smaller runner to be used for a given flow and

increases the rotational speed. The required power can still be attained and the

part-flow efficiency is especially good because the wheel can be run on a reduced

number of jets with each jet in use still receiving the optimum flow.

• Twin runners.

Two runners can be placed on the same shaft either side by side or on opposite sides of

the generator. This configuration is unusual and would only be used if number of jets

per runner had already been maximized, but it allows the use of smaller diameter and

hence faster rotating runners.

Impulse Turbine:

A Pelton turbine consists of a set of specially shaped buckets mounted on a periphery of a

circular disc. It is turned by jets of water which are discharged from one or more nozzles

and strike the buckets. The buckets are split into two halves so that the central area does

not act as a dead spot incapable of deflecting water away from the oncoming jet. The





27

cutaway on the lower lip allows the following bucket to move further before cutting off

the jet propelling the bucket ahead of it and also permits a smoother entrance of the

bucket into the jet. The Pelton bucket is designed to deflect the jet through 165 degrees

(not 180 degrees) which is the maximum angle possible without the return jet interfering

with the following bucket for the oncoming jet.

Impulse turbines are generally more suitable for micro-hydro applications compared with

reaction turbines because they have the following advantages:

• greater tolerance of sand and other particles in the water,

• better access to working parts,

• no pressure seals around the shaft,

• easier to fabricate and maintain,

• better part-flow efficiency.

Runner of a Pelton wheel. Source: http://www.hydrowest.com/runners1.htm

Components of Pelton Turbine:

The main components of pelton wheel are

1. Nozzle and flow regulating arrangements,

2. Runner with buckets,

3. Casing,

4. Breaking jet.

Working Principle of Pelton Turbine

High speed water jets emerging from the nozzles (obtained by expanding high pressure

water to the atmospheric pressure in the nozzle) strike a series of spoon-shaped buckets





28

mounted around the edge of the pelton wheel. High pressure water can be obtained from

any water body situated at some height or streams of water flowing down the hills.

Components of Pelton Turbine. Source: Web image

As water flows into the bucket, the direction of the water velocity changes to follow the

contour of the bucket. These jets flow along the inner curve of the bucket and leave it in

the direction opposite to that of incoming jet. When the water-jet contacts the bucket, the

water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and

flows out the other side of the bucket at low velocity.The change in momentum (direction

as well as speed) of water jet produces an impulse on the blades of the wheel of Pelton

Turbine. This "impulse" does work on the turbine and generates the torque and rotation in

the shaft of Pelton Turbine.To obtain the optimum output from the Pelton Turbine the

impulse received by the blades should be maximum. For that, change in momentum of

the water jet should be maximum possible. This is obtained when the water jet is

deflected in the direction opposite to which it strikes the buckets and with the same speed

relative to the buckets. For maximum power and efficiency, the turbine system is

designed such that the water-jet velocity is twice the velocity of the bucket. A very small

percentage of the water's original kinetic energy will still remain in the water. However,

this allows the bucket to be emptied at the same rate at which it is filled, thus allowing

the water flow to continue uninterrupted. Often two buckets are mounted side-by-side,





29

thus splitting the water jet in half (see photo). The high speed water jets emerging form

the nozzles strike the buckets at splitters, placed at the middle of the buckets, from where

jets are divided into two equal streams. This balances the side-load forces on the wheel,

and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine

wheel. Because water and most liquids are nearly incompressible, almost all of the

available energy is extracted in the first stage of the hydraulic turbine. Therefore, Pelton

wheels have only one turbine stage, unlike gas turbines that operate with compressible

fluid.

Bucket shape. Source: http://europa.eu.int/en/comm/dg17/hydro/layman2.pdf

Velocity triangle for pelton Wheel:





30

Velocity triangles for the jet striking the bucket . Source VTU Learning

From the impulse momentum equation

Force= (Mass/second)× change in velocity in x direction

Applications of Pelton Wheel:

Pelton wheels are the preferred turbine for hydro-power, when the available water source

has relatively high hydraulic head at low flow rates. Pelton wheels are made in all sizes.

There exist multi-ton Pelton wheels mounted on vertical oil pad bearings in hydroelectric

plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels are only

a few inches across, and can be used to tap power from mountain streams having flows of

a few gallons per minute. Some of these systems utilize household plumbing fixtures for

water delivery. These small units are recommended for use with thirty meters or more of

head, in order to generate significant power levels. Depending on water flow and design,

Pelton wheels operate best with heads from 15 meters to 1,800 meters, although there is

no theoretical limit.

Thus, more power can be extracted from a water source with high-pressure and low-flow

than from a source with low-pressure and high-flow, even though the two flows





31

theoretically contain the same power. Also a comparable amount of pipe material is

required for each of the two sources, one requiring a long thin pipe, and the other a short

wide pipe.

Reaction Turbines:

The reaction turbines considered here are the Francis turbine and the propeller turbine. A

special case of the propeller turbine is the Kaplan. In all these cases, specific speed is

high, i.e. reaction turbines rotate faster than impulse turbines given the same head and

flow conditions. This has the very important consequences in that a reaction turbine can

often be coupled directly to an alternator without requiring a speed-increasing drive

system. Some manufacturers make combined turbine-generator sets of this sort.

Significant cost savings are made in eliminating the drive and the maintenance of the

hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while

the propeller is more suitable for low heads.

On the whole, reaction turbines require more sophisticated fabrication than impulse

turbines because they involve the use of larger and more intricated profile blades together

with carefully profiled casings.

Francis turbines can either be volute-cased or open-flume machines. The spiral casing is

tapered to distribute water uniformly around the entire perimeter of the runner and the

guide vanes feed the water into the runner at the correct angle. The runner blades are

profiled in a complex manner and direct the water so that it exits axially from the centre

of the runner. In doing so, the water imparts most of its pressure energy to the runner

before leaving the turbine via a draft tube.

The Francis turbine is generally fitted with adjustable guide vanes. These regulates the

water flow as it enters the runner and are usually coupled to a governing system which

equals flow to turbine loading in the same way as a spear valve or deflector plate in a

Pelton turbine. When the flow is reduced the efficiency of the turbine falls away.

Francis Turbine.(Radial flow turbine)Construction and Working: Figure shows schematic diagram of a Francis turbine.





32

The main parts are:

• Penstock: It is a large size conduit which conveys water from the upstream to the

dam/reservoir to the turbine runner.

• Spiral Casing: It constitutes a closed passage whose cross-sectional area

gradually decreases along the flow direction; area is maximum at inlet and nearly

zero at exit.

• Guide Vanes: These vanes direct the water on to the runner at an angle

appropriate to the design, the motion of them is given by means of hand wheel or by a

governor.

• Governing Mechanism: It changes the position of the guide blades/vanes to

affect a variation in water flow rate, when the load conditions on the turbine change.

• Runner and Runner Blades: The driving force on the runner is both due to

impulse and reaction effect. The number of runner blades usually varies between 16

to 24.

• Draft Tube: It is gradually expanding tube which discharges water, passing

through the runner to the tail race.

http://www.ululu.in/first-year/elements-of-mechanical-engineering/img/francis-

turbine.jpg





33

Source:VTU LearningWorking: Francis turbine has a purely radiate flow runner. Water under pressure, enters

the runner from the guide vanes towards the center in radial direction and discharges out





34

of the runner axially. Francis turbine operates under medium heads. Water is brought

down to the turbine through a penstock and directed to a number of stationary orifices

fixed all around the circumference of the runner. These stationary orifices are called as

guide vanes.

The head acting on the turbine is transformed into kinetic energy and pressure head. Due

to the difference of pressure between guide vanes and the runner (called reaction

pressure), the motion of runner occurs. That is why a Francis turbine is also known as

reactionturbine.

The pressure at inlet is more than that at outlet. In Francis turbine runner is always full of

water. The moment of runner is affected by the change of both the potential and kinetic

energies of water. After doing the work the water is discharged to the tail race through a

closed tube called draft tube.

Draft Tubes:

In radial flow turbines, as the water flows from higher pressure to lower pressure, it

cannot come out of the turbine and hence a divergent tube is connected to the end of the

turbine. This divergent tube, one end of which is connected to the outlet of the turbine

and the other is immersed well below the tail race.

Functions of Draft tube:

1. It is to increase the pressure from inlet to outlet of the draft tube as it flows through it

and hence increase it more than atmospheric pressure.

2. To safely discharge the water that has worked on the turbine to the tail race.

Velocity Triangle for a radial flow Turbines:





35

Source VTU Learning

Component of Velocity at inlet ×

Component of Velocity at outlet ×

Now angular momentum per second at inlet and outlet is the product of momentum inlet

and outlet with respect to their radial distance R1 and R2.

Torque=

Work done per second= Torque× ω

= ×ω

Kaplan Turbine (axial flow turbine)





36

Kaplan turbine is a low head reaction turbine, in which water flows axially. It was

developed by German Engineer Kaplan in 1916.

In this type of turbine, the water flows parallel to the axis of rotation. The shaft of the

turbine may be either vertical or horizontal. The lower end of the shaft is made of larger

to form the boss or the hub. When the vanes are composite with the boss the turbine is

called as propeller turbine. When the vanes are adjustable the turbine is called as Kaplan

turbine.

All the parts of the Kaplan turbine (viz, spiral casing, guide wheel and guide blades) are

similar to that of the Francis turbine, except the runner blades, runner and draft tube. The

runner and runner blades of the Kaplan turbine resemble with the propeller of the ship.

Hence, Kaplan turbine is also called as Propeller Turbine. The blades of a Kaplan turbine,

three to eight in number are pivoted around the central hub or boss, thus permitting

adjustment of their orientation changes in load and head.

Source: VTU Learning





37

Governor of Turbines:

Source: VTU Learning

A Governor is a mechanism to regulate the speed of the turbine. The turbine is coupled to

shaft of the generator, which is generating power. The power generated should have

uniform rating of current and frequency which in turn depends on the speed of the shaft

of the turbine. The above figure shows the oil pressure governor of the turbine.

Specific speed of the turbine: Ns

The specific speed of a turbine is the speed at which the turbine will run when developing

unit power under a unit head.

Consider P as the power developed by the turbine.

Where Ns= specific speed of turbine, H= Head, N= speed of the turbine, and

P= power developed by the turbine.

****************************************



Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Gas and Steam Turbines

Prepared by: Prof. C. SivarajanAssociate ProfessorDepartment of Mech. Engineering, Annamalai University, Annamalainagar – 608 002

38

GAS AND STEAM TURBINESINTRODUCTIONTurbines and compressors are used in electric power generation, aircraft propulsion and a

wide variety of medium and heavy industries. Small and heavy duty fans and blowers

cover a wide range of industrial applications. Though the steam turbine was perfected

much earlier than the gas turbine engine, the last two decades have seen almost parallel

development in aeroengines and steam turbine power plant. As a result today on the one

hand we have the jumbo-jet and their high thrust engines in the aeronautical field, while

on the other there are giant steam turbine plants operating in the “super thermal powerstations”. These developments suggest that the 2000MW steam turbine plant will beoperating in many countries by the turn of century. Along with this the “super jumbo-jet”air liners will also be flying between the major cities of the world.

ROTODYNAMIC MACHINES

A Rotodynamic machine is one in which a fluid flows freely through an impeller of

rotor, the transfer of energy between the fluid and the rotor is continuous and the

change of angular momentum of the fluid causes , or is the result of, a torque on the

rotor. When energy is transferred from the fluid to the rotor the machine is known as

turbine, when the energy is transferred to the fluid from the rotor the machine is known

as fan, pump or compressor.





39

COMPRESSIBLE FLOW MACHINES

The pressure, temperature and density changes occurring in fluids passing through steam

and gas turbines, and compressors are appreciable. A finite change in the temperature of

the working fluid is a typical characteristic of this class of machines which distinguishes

them from other turbo machine . These classes of machines with predominantly

compressible flows are refereed to as compressible flow or thermal turbomachnies. They

are characterized by higher temperature and peripheral speeds of rotor. Therefore their

design and operation are influenced by compressible flows, high temperature and speed

problems.

INCOMPRESSIBLE FLOW MACHINES

Hydraulic pumps and turbines are examples of turbo machines working with a liquid. The

fluid or water is incompressible giving a constant volume flow rate for a given mass flow

rate in steady operation. Water and air are considered here as typical working fluid in

turbomachines handling liquids and gases. The density of water is about 800 times that of

atmospheric air. Therefore the force required to accelerate a given quantity of water is

much larger compared to that required for air. This factor largely accounts for much

lower fluid and rotor velocity in hydro -turbomachines.

Turbomachines dealing with gases over a small pressure difference also behave as

incompressible flow machines. This is because of negligible changes in the temperature

and density of the fluid across the machine. Fans, low pressure blowers, airscrews and

windmills are examples of such machines.

Thus a majority of incompressible flow machines work near ambient conditions and are

comparatively low speed and low temperature machines. This makes their running and

maintenance much easier compared to thermal turbomachines

AXIAL STAGES

In an axial flow turbo machine or its stage shown in Figure 1.1 the radial component of

the fluid velocity is negligible. The change in radius between the entry and exit of the

stage is small. The through flow in such machines mainly occurs in the axial direction,

hence the term “axial stage”.

An axial machine can be easily connected with other components. For example in a gas

turbine plant this configuration offers mechanically and aerodynamically a convenient





40

connection between the compressor, combustion chamber and turbine. For the same

reason, axial stages are widely employed in multi stage turbo machines. Such a stage is

ideally suited for high flow rates. The area of cross section available to the flow in an

axial stage is

1.1Suitable values of the hub and tip diameters can be chosen to provide required area. For

aircraft propulsion, the axial flow configuration of compressors and turbines has special

advantages of low frontal area resulting in a lower aircraft drag.

The turning of the fluid in axial stages is not too severe and the length of the blade

passages is short. This leads to lower aerodynamic losses and higher stage efficiencies.

On account of the individual blade root fixtures, the root of an axial stage has limited

mechanical strength. This restricts the maximum permissible peripheral speed of the

rotor.

Figure 1.1 An axial flow stage

RADIAL STAGES

In the radial stage of a turbo machine the through flow of the fluid occurs mainly in the

radial direction, i.e. perpendicular to the axis of rotation. Therefore the change of radius

between the entry and exit of the stage is finite. This causes a finite change in the energy

level of the fluid due to the centrifugal energy.

A radial turbo machine may be inward flow type or outward flow type. Since the purpose

of compressors, blowers, fans and pumps is to increase the energy level of the fluid, they





41

are of the outward flow radial type as shown in figure1. 2. Radial gas turbines are mostly

of the inward

Figure 1.2 A centrifugal compressor stage

Figure 1.3 An inward-flow radial turbine stage

Figure 1.4 An outward flow radial turbine ( Ljungstrom turbine)





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flow type as shown in figure 1.3; the fluid transfers its centrifugal energy to the rotor in

flowing from a larger to a smaller radius. The Ljungstrom stem turbine shown in

figure1.4 is a double rotation outward flow radial turbine. The outward flow

configuration is chosen here to accommodate the large volume flow rate of the rapidly

expanding steam.

dhddA hti

)(4

22

1.2

111 bdA

1.3

222 bdA

1.4

For given impeller and shaft diameters and change of radius from entry to exit the area at

the entry to the stage is restricted by equation1. 2. At this station a compressible fluid has

the largest volume requiring a correspondingly large area. Conversely, the same is true

for an inward flow gas turbine figure 3. On account of this, radial flow stages do not offer

the best geometrical configuration for high flow rates. In radial flow stages the flow

invariably turns through 90o traversing a much longer blade passage compared to that in

the axial types. This leads to comparatively higher losses and lower efficiencies. In a

multi stage radial machine the flow is required to change its direction drastically number

of times in long interconnecting flow passages. This is obviously an undesirable feature

both mechanically and aerodynamically. Therefore a majority of radial machines are

single stage machines; very few multistage radial machines employ more than three

stages.

Since the power developed is proportional to the mass flow rate, and the number of stages

that can be employed is much smaller compared to axial machine s, radial flow machines

are not suited for large power requirement.

Radial stages employ ‘one piece’ rotors in which the blades are an integral part of the

main body. This makes a radial rotor mechanically stronger than an axial type in which

the blades are separately fixed. Therefore radial machines can employ higher peripheral





43

speeds.On account of higher peripheral speeds and additional change in the energy level

of the fluid caused due to centrifugal energy, much higher values of the pressure ratio per

stage are obtained in the radial stage compared to the axial type.

Radial flow compressors and turbines for large power and thrust requirements have a

larger overall diameter of the aero-engine, leading to an unacceptably large frontal area.

Therefore radial machines are unsuitable for the propulsion of large aircrafts.

STEAM TURBINEIntroduction to Steam TurbineA steam turbine is a mechanical device that extracts thermal energy from

pressurized steam, and converts it into rotary motion. Its modern manifestation was

invented by Sir Charles Parsons in 1884.

Definitions of Steam turbine

Turbine in which steam strikes blades and makes them turn

A system of angled and shaped blades arranged on a rotor through which steam is

passed to generate rotational energy. Today, normally used in power stations

A device for converting energy of high-pressure steam (produced in a boiler) into

mechanical power which can then be used to generate electricity.

Equipment unit flown through by steam, used to convert the energy of the steam

into rotational energy.

Principle of Operation

In reciprocating steam engine, the pressure of energy of steam is used to overcome

external resistance and dynamic action of the steam is negligibly small. Steam engine

may be return by using the full pressure without any expansion or drop of pressure in the

cylinder.

The steam energy is converted mechanical work by expansion through the turbine. The

expansion takes place through a series of fixed blades (nozzles) and moving blades each

row of fixed blades and moving blades is called a stage. The moving blades rotate on the

central turbine rotor and the fixed blades are concentrically arranged within the circular

turbine casing which is substantially designed to withstand the steam pressure.





44

Classification of Steam Turbines

The first steam turbine, at its time indeed did spark off the industrial revolution through

out the west. However, the turbine at that time was still an inefficient piece of heavy

weighing high maintenance machine. The power to weight ratio of the first reciprocating

steam turbine was extremely low, and this led to a great focus improving the design,

efficiency and usability of the basic steam turbine, the result of which are the power

horses that currently produce more than 80% of today’s electricity at power plants!

Steam Turbines are Classified as

Steam Turbines can be classified on the basis of a number of factors. Some of the

important methods of steam turbine classification are enunciated below:

On the basis of Stage Design:

Steam turbines use different stages to achieve their ultimate power conversion

goal. Depending on the stages used by a particular turbine, it is classified as Impulse

Turbine, or Reaction type.

On the Basis of the Arrangement of its Main Shaft:

Depending on the shaft arrangement of the steam turbine, they may be classified

as Single housing (casing), tandem compound (two or more housings, with shafts

that are coupled in line with each other) and Cross compound turbines (the shafts

here are not in line).

On the Basis of Supply of Steam and Steam Exhaust Condition:

They may be classified as Condensing, Non Condensing, Controlled or Automatic

extraction type, Reheat (the steam is bypassed at an intermediate level, reheated

and sent again) and Mixed pressure steam turbines (they have more than one source

of steam at different pressures).

On the basis of Direction of Steam Flow:

They may be axial, radial or tangential flow steam turbines.

On the Basis of Steam Supply:

Superheated steam turbine or saturated steam turbine.

According to method of governing

Throttle governing turbine, Nozzle governing turbine and By pass governing

turbine.





45

Basic types of turbine

The two most basic and fundamental types of steam turbines are the impulse turbine and

the impulse reaction turbine.

The Impulse Turbine

The impulse turbine consists of a set of stationary blades followed by a set of rotor

blades which rotate to produce the rotary power. The high pressure steam flows through

the fixed blades, which are nothing but nozzles, and undergo a decrease in pressure

energy, which is converted to kinetic energy to give the steam high velocity levels. This

high velocity steam strikes the moving blades or rotor and causes them to rotate. The

fixed blades do not completely convert all the pressure energy of the steam to kinetic

energy, hence there is some residual pressure energy associated with the steam on exit.

Therefore the efficiency of this turbine is very limited as compared to the next turbine we

are going to review- the reaction turbine or impulse reaction turbine.

Fig.2.4.1 Diagram of an Impulse Turbine Fig.2.4.1a. An Impulse TurbineStage

Working of Impulse Turbine

The impulse turbine involves striking of the blades by a stream or a jet of high pressure

steam, which causes the blades of the turbine to rotate. The direction of the jet is

perpendicular to the axis of the blade. It was realized that the impulse turbine is not very

efficient and requires high pressures, which is also quite difficult to maintain. The





46

impulse turbine has nozzles ,that are fixed, to convert the steam to high pressure steam

before letting it strike the blades.

Impulse turbine mechanism

Impulse turbine mechanism deals with the Impulse force action-reaction.

It works on Newton third law which state that," Every action has equal and

opposite reaction".

As the water falls on the blade of the rotor it generates the impact force on the

blade surface, The blade tends to give the same reaction to the fluid, but the rotor

is attached to the rotating assembly, it absorb the force impact and give the

reaction in the direction of the fluid flow. Thus the turbine rotates.

The rotational speed of the turbine depends on the fluid velocity. More the fluid velocity,

greater is the rotational speed, and greater the speed have the power generation.

The Reaction Turbine

The reaction turbine is a turbine that makes use of both the impulse and the reaction of

the steam to produce the rotary effect on the rotors. The moving blades or the rotors here

are also nozzle shaped (They are aerodynamically designed for this) and hence there is a

drop in pressure while moving through the rotor as well. Therefore in this turbine the

pressure drops occur not only in the fixed blades, but a further pressure drop occurs in the

rotor stage as well. This is the reason why this turbine is more efficient as the exit

pressure of the steam is lesser, and the conversion is more. The velocity drop between the

fixed blades and moving blades is almost zero, and the main velocity drop occurs only in

the rotor stage.





47

Fig.2.4.2 Diagram of a Reaction Turbine Fig.2.4.2a. A Reaction Turbine

Stage

WORKING OF REACTION TURBINE

In the reaction turbine, the rotor blades themselves are arranged to form convergent

nozzles. This type of turbine makes use of the reaction force produced as the steam

accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by

the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference

of the rotor. The steam then changes direction and increases its speed relative to the speed

of the blades. A pressure drop occurs across both the stator and the rotor, with steam

accelerating through the stator and decelerating through the rotor, with no net change in

steam velocity across the stage but with a decrease in both pressure and temperature,

reflecting the work performed in the driving of the rotor.

This type of turbine makes use of the reaction force produced as the steam accelerates

through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed

vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the

rotor. The steam then changes direction and increases its speed relative to the speed of the

blades. A pressure drop occurs across both the stator and the rotor, with steam

accelerating through the stator and decelerating through the rotor, with no net change in

steam velocity across the stage but with a decrease in both pressure and temperature,

reflecting the work performed in the driving of the rotor.

Applications of Steam Turbine

The Steam turbines of today are mostly used in the power production field. Steam

turbines are used to efficiently produce electricity from solar, coal and nuclear power

plants owing to the harmlessness of its working fluid, water/steam, and its wide

availability. Modern steam turbines have come a long way in increasing efficiency in

performance and more and more efforts are being made to try and reach the ideal steam

turbine conditions, though this is physically impossible! Almost every power plant in the

world, other than hydro electric power plants, that use turbines that run on water (the

Francis, Pelton turbines also have the influence of steam turbines) , use steam turbines for





48

power conversion. With all the scientific advancement in power generation being

attributed to them, steam turbines really have changed the way the world moves!

Steam turbines are devices which convert the energy stored in steam into rotational

mechanical energy. These machines are widely used for the generation of electricity

Utility Steam Turbine Applications:

Applications for utility Steam Turbines are applied for control of straight condensing,

reheat and non-reheat steam turbines up to 300MW. These upgrades may include

integrated generator control for generator protection and excitation/ AVR upgrades,

utilizing the latest commonly available industry-standard digital equipment.

Industrial application of steam turbine:

Applications of Industrial Steam Turbines cover all straight condensing, non-condensing,

and automatic extraction steam turbines. Specific design features are incorporated to

address control issues often unique to process plants including paper mills, oil refineries,

chemical plants, and other industrial applications, generator and mechanical drive.

Some of the world’s largest turbines manufacturing companies that are seeing the

rewards of research and steam turbine advances are coming together to develop highly

efficient turbines. The collaboration of Mitsubishi Heavy Machinery and General Electric

Energy (GE Energy) for the conceptualization and design of a highly efficient “next-

generation” steam turbine for its inception in combined cycle gas turbine power plants

recently has further proved that there is still a lot to be achieved in steam turbine related

research and development, and that the scope for improvement can be much higher.

Compounding:

When expansion of steam takes place from the high initial pressure to the exhaust

pressure in only one stage, the velocity of the steam will be very high and this will set up

excessive blade speeds, far above the normal useful speeds. Further, “the lost velocity or

the leaving loss", namely the kinetic energy of the steam leaving the turbine will also be

high. Therefore, in order to restrict the rotational speed to the turbine and also to

minimize the leaving loss, the exhaust steam from the first ring of moving blades is

diverted to a second ring of moving blades with the help of a ring of stationary or fixed

blades. There may be two or more rings of moving blades keyed to a common shaft and

in between two rings of moving blades there will be a ring of fixed blades usually





49

anchored to the turbine casing. This way of reducing rotor speed is known as

“COMPOUNDING ".

Methods of Compounding:

1. Pressure Compounding

2. Velocity Compounding

Pressure Compounding (The Rateau 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 physical arrangement is

shown in the figure 2.6.1. The nozzles are carried in diaphragms which separate each

stage from 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 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 3) the variation of pressure & 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 fig. 3 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 analysed by the method used previously for the single stage. A turbine with a

series of simple impulse stage is called a pressure compounding.





50

Figure 2.6.1 Pressure –compounded impulse turbineshowing pressure and velocity variations

Velocity compounding (the Curtis turbine):

From, previous considerations it is seen that in the simple impulse stage the optimum

condition of blade speed is hardly practical, and with the speeds actually used only a

small amount of the kinetic energy of the steam can be utilized. The velocity

compounded stage, called the Curtis stage after its designer, 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 blade moves in the same

direction it is necessary to change the direction of the steam between one set of

moving blades & the next. For this purpose a stationary ring of blades is fitted between

each pair of moving blades. A two row wheel version of this turbine is shown in fig

2.6.2.





51

Figure 2.6.2 Two-row velocity-compounded impulse turbineshowing pressure and velocity variations

Axial Turbine stage velocity Triangles -Impulse steam turbine:

The steam supplied to a single wheel impulse turbine expands completely in the nozzles

and leaves with a high absolute velocity. This is the absolute inlet velocity to the blades

as shown in figure 2.7.1(a). The steam is delivered to the wheel at an angle α.i. The

absolute velocity Cai can be considered as the resultant of the blade velocity Cb, and the

velocity of the steam relative to the blade at inlet, Cri. The two points of particular interest

are those at the inlet and exit of the blades. The velocities of these points are as shown in

figure 2.7.1 (b) as Cri a Cre respectively, and the directions are defined by the angles βi &

βe as shown. The velocity triangle for the inlet conditions is drawn in Figure 2.7.2 (a)

from the information of figure 2.7.1. The steam leaves the blade with a velocity, Cre,

relative to the blade, and at the blade exit angle of βe. the absolute velocity at exit Cae is

determined from the velocity triangle of Figure 2.7.2(b) and its direction is α .e. Since both

triangles have the common side OA = Cb, the triangles can be combined to give a single

diagram shown in figure 2.7.2(c).





52

Driving force on wheel = m ∆Cw

Power output = m Cb ∆Cw

Figure 2.7.1 Absolute and relative velocities for a simple impulse turbine blade

Figure 2.7.2 Inlet (a) and Outlet (b) blade velocity diagrams for an impulse turbineand a composite diagram (c)

Figure 2.7.3Absolute Velocities at inlet and exit and the forces produced

Axial Turbine stage velocity Triangles - Reaction steam turbine:





53

The reaction turbine applies the principle of both the pure impulse and the pure

reaction turbine. Each stage of the reaction turbine consists of a fixed row of blades over

the whole of the circumferential annulus, and an equal number of blades on a wheel.

Admission of fluid in the reaction turbine takes place over the complete annulus, and so

there is full admission. The fixed blade channels are of nozzle shape and there is a

comparatively small drop in pressure accompanied by an increase in velocity. The fluid

then passes over the moving blades and as in the pure impulse turbine, a force is exerted

on the blades by the fluid. There is a further drop in pressure as the fluid passes through

the moving blades, since the moving blade channels are also of nozzle shape, and

therefore there is an increase in the fluid velocity relative to the blades. This is illustrated

in the velocity diagram of figure 2.8.1(a). With a simple impulse type the value of Cre

would be given by AD, but in the reaction turbine this velocity is increased to AC by

further expansion of the fluid in the blade channels. The net change in velocity of the

fluid is given by BC and the resultant force on the blades by m(CB) and shown in figure

2.8.1(b). This force can be resolved into the tangential and axial thrust, m(CE) and m(EB)

as shown in figure2.8.1(b).





54

Figure 2.8.1 Blade velocity (a) and thrust diagram (b) for an axial-flow reaction

turbine

Ranking Cycle

A power cycle continuously converts heat (energy released by the burning of fuel) into

work (shaft work), in which a working fluid repeatedly performs a succession of

processes. In the vapour power cycle, the working fluid, which is water, undergoes a

change of phase. Figure 2.9.1 gives the schematic of a simple steam power plant working

on the vapour power cycle. Heat is transferred to water in the boiler from an external

source (furnace, where fuel is continuously burnt) to raise steam, the high pressure, high

temperature steam leaving the boiler burnt) to raise steam, the high pressure, high

temperature steam leaving the boiler expands in the turbine to produce shaft work, the

steam leaving the turbine condenses into water in the condenser (where cooling water

circulates), rejecting heat, and then the water is pumped back to the boiler.





55

For each process in the vapour power cycle, it is possible to assume a hypothetical or

ideal process which represents the basic intended operation and involves no extraneous

effects. For the steam boiler, this would be a reversible constant pressure heating process

of water to form steam, for the turbine the ideal process would be a reversible adiabatic

expansion of steam, for the condenser it would be a reversible constant pressure heat

rejection as the steam condenses till it becomes saturated liquid, and for heat pump, the

ideal process would be the reversible adiabatic compression of this liquid ending at the

initial pressure. When all these four processes are idea, the cycle is an ideal cycle, called

a Ranking cycle. This is a reversible cycle. Figure 2.9.1 shows the flow diagram of the

Rankine cycle, and in Figure 2.9.2, the cycle has been plotted on the p-v, T-s, and h-s

planes. The numbers on the plots correspond to the numbers on the flow diagram. For

any given pressure, the steam approaching the turbine may be dry saturated (state 1)

given pressure, the steam approaching the turbine may be dry saturated (state 1) wet

(state 1’), or superheated (state l1), but the fluid approaching the pump is, in each case,

saturated liquid (state 3). Steam expands reversibly and adiabatically in the turbine from

state 1 to state 2 (or 1 to 2, or 1 to 2), the steam leaving the turbine condenses to water

in the condenser reversibly at constant pressure from state 2 (or 2, or 2) to state 3, the

water at state 3 is then pumped to the boiler at state 4 reversibly and adiabatically, and the

water at state 3 is then pumped to the boiler at state 4 reversibly and adiabatically, and the

water is heated in the boiler to form steam reversibly at constant pressure from state 4 to

state 1 (or 1 or 1).

Figure 2.9.1 A simple steam plant





56

For purposes of analysis the Rankine cycle is assumed to be carried out in a steady flow

operation. Applying the steady flow energy equation to each of the processes on the basis

of unit mass of fluid, and neglecting changes in kinetic and potential energy, the work

and heat quantities can be evaluated in terms of the properties of the fluid.

Figure 2.9.2 Rankine cycle on p-v, T-s and h-s diagrams

41

3421

11

)()(

hh

hhhh

Q

WW

Q

W pTnet

The work ratio is defined as the ratio of net work output to positive work output.

Work ratioT

PT

T

net

W

WW

W

W

Usually, the pump work is quite small compared to the turbine work and is sometimes

neglected. Then h4 = h3, and the cycle efficiency approximately becomes

41

21

hh

hh





57

The efficiency of the Rankine cycle is presented graphically in the T-s plot in Fig.2.9.3.

Thus Q1 is proportional to area 1564, Q2 is proportional to area 2563, and Wnet (=Q1-Q2)

is proportional to area 1 2 3 4 enclosed by the cycle.

Figure 2.9.3 Q1, Wnet and Q2 are proportional to areas

The capacity of a steam plant is often expressed in terms of steam rate, which is defined

as the rate of steam flow (kg/h) required producing unit shaft output (1 kW). Therefore

kW

skJ

kJ

kg

WWareSteam

PT 1

/1.

1

kWs

kJ

WWkWs

kg

WW PTPT

36001

The cycle efficiency is sometimes expressed alternatively as heat rate which is the rate

output (Q1) required to produce unit work output (1kW)

Heat ratekWs

kJ

WW cyclePT 36003600

From the equation 2

1

,dpWrev it is obvious that the reversible steady-flow work is

closely associated with the specific volume, of fluid flowing through the device. The

larger the specific volume, the larger the reversible work produced or consumed by the

steady-flow device. Therefore, every effort should be made to keep the specific volume

of a fluid as small as possible during a compression process to minimize the work input

and as large as possible, during an expansion process to maximize the work output.

In steam or gas power plants , the pressure rise in the pump or compressor is equal to the

pressure drop in the turbine if we neglect the pressure losses in various other components.





58

In steam power plants, the pump handles liquid, which has a very small specific volume,

and the turbine handles vapour, whose specific volume is many times larger. Therefore,

the work output of the turbine is much larger than the work input to the pump. This is one

of the reasons for the overwhelming popularity of steam power plants in electric power

generation.

If we were to compress the steam exiting the turbine back to the turbine inlet pressure

before cooling it first in the condenser in order to “save” the heat rejected, we would have

to supply all the work produced by the turbine back to the compressor. In reality, the

required work input would be still greater than the work output of the turbine because of

the irreversibilities present in both processes.

LOSSES AND EFFICIENCY

Energy Flow diagram for the impulse stage of an axial turbine

GAS TURBINE

A simple gas turbine unit consists of three components, viz., a compressor, a heat

addition device and a turbine. These three components can be arranged either in an open

or a closed form. Accordingly, a gas turbine cycle can be classified into two categories:

i) Open-cycle arrangement, ii) Closed-cycle arrangement





59

Of the two, open-cycle arrangements are much more common. In this arrangement fresh

atmospheric air is drawn into the system continuously and energy is added by combustion

of fuel in the working fluid itself. The products of combustion are expanded through the

turbine and exhausted into the atmosphere. In the closed-cycle, the same working fluid,

be it air or some other gas, is repeatedly circulated through the system. It may be noted

that in this type of plant whether the working fluid. is air or some other gas, fuel cannot

be burnt directly in the working fluid and the necessary energy must be added in a heater

or gas boiler.

OPEN-CYCLE ARRANGEMENTS

If a gas turbine is to be operated at a fixed speed and fixed load condition such as peak-

load power generation, a single shaft arrangement as shown in Fig. 3.1.1 may be suitable.

Flexibility of operation, i.e., the rapidity with which the machine can accommodate itself

to changes of load and speed, and efficiency at part load are in this case considered

unimportant. A heat exchanger can be added as shown in Fig. 3.1.2 to improve the

thermal efficiency, although for a given size of the plant, power output may be reduced

by 10% due to pressure losses in the heat exchanger.

Figure 3.1.1 A simple gas turbine

Figure 3.1.2 A simple gas turbine with a heat exchanger

The modified form, as shown in Fig. 3.1.3, is more suitable for fuels whose products of

combustion contain constituents which may corrode or erode the turbine blades. It is

much less efficient than the simple cycle power plant because the heat exchanger,





60

inevitably less than perfect, in transferring the energy input because of the effectiveness

of the heat exchanger. Such a cycle may be considered only if inferior fuels are to be

used. When flexibility of operation is of paramount importance, such as in road, rail and

marine applications, a mechanically independent power turbine is used.

In this twin-shaft arrangement (Fig. 3.1.4), the compressor and high- pressure turbine

combination acts as a gas generator for the low pressure turbine. Fuel flow to the

combustion chamber is controlled to achieve van- action of power. It should be noted that

this will cause a decrease in cycle pressure ratio and maximum temperature. At off-

design conditions the power output reduces with the result that the thermal efficiency

deteriorates considerably at part loads.

Alternative arrangements to overcome the above disadvantages are the series flow and

parallel flow gas turbines (Figs. 3.1.5 and 3.1.6). In these arrangements power output is

controlled by the adjustment of fuel supply to the combustion chamber in the power

turbine line.

Fig. 3.1.3 A simple gas turbine with heat exchanger – an alternative arrangement





61

Fig. 3.1.4 A simple twin shaft arrangement

Fig. 3.1.5 Series flow twin shaft arrangement

Fig. 3.1.6 Parallel flow twin shaft arrangement

The performance of a gas turbine may be improved substantially by reducing the work of

compression and/or increasing the work of expansion. For any given pressure ratio, the

power required for compression per kg of working fluid is directly proportional to the

inlet temperature. If, therefore, the compression process is carried out in two or more

stages with intervolving, the work of compression will be reduced. This arrangement is

shown in (Fig. 3.1.7).

Similarly, the turbine output can be increased by dividing the expansion into two or more

stages and reheating the gas to the maximum permissible temperature between the stages

(Fig. 3.1.8). By employing a heat exchanger, the cost of additional fuel can be minimized.

Complex cycles offer good part load performance and high flexibility but it is to be noted

that the inherent simplicity and compactness of the power plant are lost.





62

To obtain higher thermal efficiencies without a heat exchanger, a high pressure ratio is

essential. Axial compressors are normally preferred, particularly for large units, as its

efficiency is appreciably higher than that of the centrifugal compressor. Unfortunately,

axial compressors are more prone to instability when operating at off-design conditions.

The unstable operation, manifested by violent aerodynamic vibration, is likely when a gas

turbine is started up or operated at low power. The problem is particularly severe if an

attempt is made to obtain a pressure ratio of more than 8:1 in one compressor.

Fig. 3.1.7 Series flow with intercooling

Fig. 3.1.8 Series flow with reheating

One way of overcoming this difficulty is to divide the compressor into two or more

sections. This mechanical separation permits each section to run at different rotational

speed. When the compressors are mechanically independent, each will have its own





63

turbine. The LP compressor is driven by the LP turbine and the HP compressor by the

FTP turbine. This arrangement is called straight compounding. Power is normally taken

either from the low pressure turbine shaft or from an additional free power turbine. This

configuration is generally referred to as a twin-spool engine (Fig. 3.1.9). This

arrangement is widely used both for shaft power units and for the turbojet aircraft

engines, employing pressure ratios in the range of 10:1 to 20:1.

Fig. 3.1.9 Straight compounded twin spool arrangement

An arrangement, known as cross compounding wherein the LP compressor is driven by

the HP turbine and the HP compressor by the LP turbine, is claimed to give better

efficiency at part load. Unfortunately, the effect on stability of operation is the opposite

of straight compounding, i.e., it makes the problem worse instead of better.

THE CLOSED-CYCLE

In all the arrangements discussed so far, atmospheric pressure and temperature have been

considered as the datum, and the exhaust gases were discharged at atmospheric pressure.

The average exhaust temperature will be around 700 K for an average maximum

temperature of about 1000 K. It follows that 1 kg of gas will occupy a volume according

to the law PV = mRT which is about of 2.34 m. At low pressure-ratios the constant-

pressure cycle requires large mass flow rate, thus the total volume flow rate for any given

power output will be quite large. In order to accommodate such large flow, a large rotor

diameter and long blades are required. This results in excessive stress at the root of the





64

blades due to large centrifugal force. Hence, there is an upper limit on the size and power

output of a turbine or compressor.

It will be shown in the next chapter that the efficiency expression of various constant-

pressure cycles is a function of pressure-ratio and the gas temperatures: i.e., the efficiency

depends not upon the magnitude of the pressures, but only upon their ratio. However, the

volume occupied by a gas depends upon the magnitude of the pressure. When the volume

to be circulated becomes large, the pressure level can be raised so that the volume per kg

of fluid, i.e., the specific volume, is reduced to the level desired. However, the same

pressure ratio (i.e., ratio of maximum to minimum pressures) should be maintained as

before for attaining the same efficiency. It is true that the maximum pressure will

increase, but only in proportion to the change of absolute pressures on the system.

Thus, when the system is closed from the atmosphere, the same fluid will circulate again

and again. It follows that if the fuel is burnt directly in the circulating air, the oxygen will

soon deplete and combustion will fail. It is, thus necessary to supply a certain amount of

fresh air to the closed circuit. The normal overall air-fuel ratio of a gas turbine is between

60:1 to 100:1. It follows that only a small fraction of burnt oxygen is to be supplied each

time through the combustion chamber in order to have continuous operation. Thus,

majority of the exhaust gas leaving the turbine would return to the compressor entrance;,

i.e., the inlet temperature to the compressor would be greatly increased. It may be noted

that the work of compression per kg can be shown to be

11

1

C

TCW p

c

which is seen to depend upon the inlet temperature, T1 and the pressure ratio, . Thus, in a

closed-cycle, the net output would be much reduced unless a gas cooler is added between

the turbine exhaust and the compressor inlet to cool the gas which is being recalculated,

down to approximately normal inlet temperature. The detail of such a system with

internal firing is shown in Fig. 3.2.1.





65

Fig. 3.2.1 A closed cycle arrangement with air as working medium

The compressor draws gas from turbine exhaust at point (1) through a regenerator and

gas cooler under pressure p1, which is greater than atmospheric pressure. After

compression, the gas at pressure p2 is delivered to combustion chamber through

regenerator. In the regenerator the corn- pressed gas is heated to higher temperature by

the main turbine exhaust. In the combustion chamber, fuel is added to obtain the desired

maximum gas temperature. From combustion chamber the gases pass on to main turbine,

where they expand from pressure P2 to pressure P1 . The turbine provides net work to the

load after meeting the compression work of the cycle. A small portion of the gases

leaving the combustion chamber is by-passed to auxiliary turbine, where expansion takes

place from p2 to patm.

The auxiliary turbine develops enough work to run a compressor, which draws air from

atmosphere and delivers it at pressure p2 of the combustion chamber. The amount of fresh

air supplied by this compressor must be equal to the amount of gases by-passed after

combustion to atmosphere via auxiliary turbine and also to the amount of air which is

sufficient to burn completely the fuel that is added to maintain the maximum temperature

of the cycle and thereby the maximum output of the main turbine.

Figure 4.11 illustrates another method on the closed-cycle concept, where instead of

supplying the fuel directly to, and burning it with, the air inside the system, the fuel could

be burned in a separate combustion chamber built like a regenerator. The heat of

combustion is transferred through the containing walls of the furnace to the air flow in





66

the turbines as shown in Fig. 3.2.2.. Thus, the air or gas contained in the closed system

can be used over and over again without the necessity for make-up air. These results in

that the products of combustion do not come in contact with the moving parts and no

deposit will accumulate on the turbine blades. In such type of closed circuit every kind of

solid fuels like coal, can also be burned in the furnace. Further the working medium other

than air having the desired properties can be used. The details of working of such a

system are shown in Fig. 3.2.2. Advantages and disadvantages of a closed-cycle system

are enumerated in the following sections.

Fig. 3.2.2 Another closed cycle arrangement with working medium other than air

Advantages(i) Use of high pressure (and hence gas density) level throughout the cycle would result

in a reduced size of the plant for a give output. (ii)Wide range of load variation is

possible by varying the pressure levels without altering the maximum cycle temperature.

Hence, there will be almost no variation of overall efficiency. (iii)Erosion of the turbine

blades due to the products of combustion is eliminated. (iv)Filtration of working medium

is not required except charging for the first time. (v) High density of the working medium

improves the effectiveness of the heat exchanger. (vi) Gases other than air having more

desirable thermal properties, such as helium etc., with = 1.66 can be used to increase

the power output and thermal efficiency. (vii) Cheaper fuels can be used.

Disadvantages





67

(i)The heating system is quite bulky. (ii)It is quite difficult to make the system

absolutely leak proof. (iii) Large capacity cooler is necessary. (iv) Useful only for

stationary power plants.

BASIC REQUIREMENTS OF THE WORKING MEDIUM

In a closed-cycle arrangement, the operating medium can be other than air and it must

satisfy the following requirements.

(i) Availability as well as cheapness’ of the working medium.

(ii) The circulating working medium must be stable, non-explosive and non-

corrosive.

(iii) It should be non-toxic and non-inflammable.

(iv) It should have high specific heat value Cp, and high specific heat ratio,

(v) It must have a higher thermal conductivity, k.

Applications of Gas Turbine

Gas turbines can he classified into aircraft and industrial gas turbines, the second term

meaning all those gas turbine power plants which are not included in the first category.

The aircraft gas turbines differ from the industrial gas turbines in three main aspects.

(i) The life of the industrial gas turbine is expected to be of the order of 120,000

hours without major overhaul as against 600-1200 hours for aircraft gas

turbines.

(ii) Size and the weight of an aircraft power plant is very crucial compared to

industrial units. .

(iii) The aircraft power plant can make use of kinetic energy of the gases leaving the

exhaust whereas it is wasted in other types and consequently, this energy loss

must be kept as minimum as possible.

These differences in the requirements have considerable effect on design, although

fundamental theory is same for both the categories. Industrial gas turbines are rugged in

construction, with many auxiliary types of equipment. They often employ a single, large

cylindrical combustion chamber. They are also designed for multifuel capability.

Apart from the aircraft market, the widest application of gas turbines has been in pump

sets for oil and gas transmission pipe lines and generation of electricity. So far gas

turbines have made no inroads into the world of merchant shipping but it is extensively





68

used in naval operations. A major disadvantage of the gas turbine in naval use is its poor

part load performance and higher specific fuel consumption. To overcome this problem,

combined power plants consisting of gas turbines in conjunction with steam turbines,

diesel engines and other gas turbines have been considered.

To date, little impact has been made in the field of rail transport. Experimental trains

have been operating in some countries. The high speed passenger train with gas turbine

power is an attractive concept for the future. Maybe in the near future, a long haul truck

market will provide a major application for the gas turbine: Major automobile industries

are active in developing engines in the range of 200 — 300 kW. These vehicular engines

employ low pressure ratio, centrifugal compressor, free power turbine and a rotary heat

exchanger. Concern with exhaust pollution will be a critical factor in favour of gas

turbine. The major problem is still with high part-load fuel consumption.

Another concept of potentially great importance is the so-called Total Energy Plant,

where exhaust heat is used to provide building heating in winter and refrigeration/air

conditioning in the summer. Other uses for energy in a gas turbine’s exhaust are found in

process industries. The gas turbine can also be used as a compact air compressor suitable

for supplying large quantities of air at moderate pressures.

The principle of jet propulsion is obtained from the application of New- ton’s laws of

motion. We know that when a fluid is to be accelerated, a force is required to produce

this acceleration in the fluid. At the same time, there is an equal and opposite reaction

force of the fluid on the engine which is known as the thrust. Hence, it may be stated that

the working of jet propulsion is based on the reaction principle. Thus all devices that

move through fluids must follow this basic principle.

In principle, any fluid can be used to achieve the jet propulsion. Thus water, steam or

combustion gases can be used to propel a body in a fluid. But there are limitations in the

choice of the fluid when the bodies are to be propelled in the atmosphere. Experience

shows that only two types of fluids are particularly suitable for jet propulsion.

i. A heated and compressed atmospheric air — admixed with the products of

combustion produced by burning fuel in that air can be used for jet propulsion.

The thermochemical energy of the fuel is utilized for increasing the temperature





69

of the air to the desired value. The jet of this character is called a thermal jet and

the jet propulsion engine using atmospheric air is called air breathing engines.

ii. Another class of jet-propulsion engines use a jet of gas produced by the

chemical reactions of fuel and oxidizer. Each of them is carried with the system

itself The fuel-oxidant mixture is called the propellant. No atmospheric air is used

for the formation of the jet. But the oxidant in the ropel1ant is used for generating

the thermal jet. A jet produced in this way is known as rocket jet and the

equipment wherein the chemical reaction takes place is called a rocket motor. The

complete unit including the propellant is, called a rocket engine.

From the above discussion it is clear that jet-propulsion engines may be classified

broadly into two groups. .

(i) air breathing engines and (ii) rocket engines

Air breathing engines can be further classified as follows:

(i) Reciprocating or propeller engines (ii) Gas turbine engines.

GAS TURBINE ENGINES

World War II was the turning point for the development of gas turbine technology. All

modern aircrafts are fitted with gas turbines. Gas turbine engines can be classified into.

(i) ramjet engines, (ii)pulse jet engines, (iii) turbojet engines, (iv)turboprop engines, and

(v)turbofan engines.

Taken in the above order they provide propulsive jets of increasing mass flow and

decreasing jet velocity. Therefore, in that order, it will be seen that ramjet can be used for

highest cruising speed whereas the turboprop engine will be useful for the lower cruising

speed at low altitudes. In practice, the choice of the power plant will depend on the

required cruising speed, desired range of the aircraft and maximum rate of climb.

The details of various gas turbine engines mentioned above are discussed under two

categories: (i) pilotless operation, and (ii) piloted operation. The ramjet and pulse jet

engines come under the category of pilotless operation whereas the turboprop and

turbojet engines are used for piloted operation,

GAS TURBINES FOR SURFACE VEHICLES

The problems and design features of gas turbines employed by surface vehicles are

considerably different from those of aircraft gas turbines.





70

Automobiles

Attempts were made by a number of automobile manufacturing companies in various

countries to perfect gas turbine engines for cars. An exhaust heat exchanger was used to

improve the fuel economy. However, inspite of a high degree of technological success,

the gas turbine car engine at present cannot commercially compete with the well-

established piston engine. Some degree of success was achieved in the field of heavy

vehicles with engines of over 200 kW. Many designs employed the combination of an

axial turbine and a low pressure centrifugal compressor along with a rotary heat

exchanger.

The gas turbine automobile engine is mechanically sound and pollutes the atmosphere at

a lesser rate. However it suffers from its inherent high speed, and poor part-load

performance.

Railway locomotives

Long distance passenger trains have employed gas turbine locomotives in many

countries. Gas turbine locomotives (with electrical transmission) can be introduced in

sectors where electric traction is uneconomical.

Exhaust superchargers

Small gas turbines are also used in automobiles in another way. All large truck and

railway diesel locomotive engines are supercharged. They employ exhaust gas driven

turbines (axial or inward flow radial) to drive the centrifugal air compressors (super

chargers).

Hovercrafts

Commercial and naval services are now employing an ever increasing number of air

cushion crafts. They have certain advantages over marine vehicles. Gas turbine provides

all the power-for lift and propulsion in such crafts. In a typical 100 t U.S. design, the air

cushion is generated and maintained by eight lift fans. Three marine gas turbines provide

propulsion through the propellers located astern.

The world’s first commercial hovertrain developed in France employs air screw

propulsion and has a speed of 300 km/h. The train has two sets of six air cushions.

Hydrofoils





71

Hydrofoils employ the lift on an aerofoil to lift the craft above the water surface. This

enables them to move at comparatively higher speeds (120 km/h). The application of gas

turbines for powering hydrofoils is on a relatively small scale.

3.6 GAS TURBINES FOR ELECTRIC POWER GENERATION

Gas turbines are used for electric power generation in a number of ways. Some of its

main advantages are ability to start quickly, lower cooling water requirement, and high

temperature and low pressure of the working medium.

Aeroengines

The life of a terrestrial gas turbine between overhauls is 20,000-30,000 h compared to

3000-7000 h for aero engines. Full advantage of the high level of design of aero engines

is taken in designing gas turbine power plants for electric power generation.

Turboprop engines with some modifications of the combustion chamber and bearings can

be used to drive electric generators through reduction gears. A number of derated turbojet

engines (gas generators) can supply the working gas to a separate power turbine. A large

plant uses eight jet engines feeding four power turbines giving a total output of 120-140

MW. Another 100 MW single-stage gas turbine plant employees ten jet engines around

the periphery of a single turbine rotor for supplying gas to its various sectors of nozzles.

Aeroengines in power stations can be brought to full-load operation from cold in a few

minutes. This makes them ideal for peak load operation.

Topping plant

The temperature of the exhaust gases in a gas turbine is high. Therefore the use of gas

turbine plants in electric power stations without any heat recuperating apparatus makes

them uneconomical. Figure 3.6.1 shows a gas turbine as a topping plant. The gas

turbine forms the high temperature loop, whereas the steam plant forms the low

temperature loop. The connecting link between the two loops (or cycles) is the steam

boiler working on the exhaust heat of the gas turbine. The outgoing exhaust gases also

heat the feed water of the steam cycle. The gas turbine, as shown here, woks wholly as a

gas generator for the steam plant, whereas the steam turbine drives the generator. In

another arrangement the gas turbine can also drive a generator, thus contributing to the

output of the combined gas – steam plant.





72

Fig. 3.6.1 A topping gas turbine plant (combined gas – steam plant)

Fig. 3.6.2 Gas turbine plant in the total energy system

Total energy plant

Satisfying the demands of heating, cooling and electrical energy from a single source is

the total energy concept. The type of fuel used in a total energy system may be liquid or

gaseous and is chosen on economic considerations, variability, cost and transportation.

Figure 3.6.2 shows a gas turbine plant in the-total energy system. The steam boiler

utilizes the energy in the high temperature turbine exhaust gases. Steam can be used

directly for space heating. For cooling purposes, steam is utilized in producing chilled

water in an absorption chiller. The overall efficiency of the total energy plant is between

60 and 75%.

Nuclear plant

Figure 3.6.3 shows a closed circuit nuclear gas turbine plant. Helium gas is used both as a

coolant in the reactor and the working fluid in the closed circuit gas turbine plant. Helium

after compression is first heated in the heat exchanger and then in the reactor. The high

pressure and temperature (p 25-50 bar, T=1000-1200K) gas drives the helium turbine.

The turbine drives both the compressor and the load (electric generator).





73

Fig. 3.6.3 A closed circuit nuclear gas turbine plant

3.7 GAS TURBINES IN PETRO-CHEMICAL INDUSTRIES

Gas turbines have special applications in a variety of industries. Some advantages of the

gas turbines in these applications are:

1. A variety of fuels can be used in gas turbine plants. Some process gases (which

are otherwise lost) can also be used. 2. The energy its exhaust gas can be used in

various processes. 3.They can he used conveniently for industrial utilities, such as

corn- pressed air, hot gases, steam, hot water, mechanical and electrical power. 4.

It is easy to install, cheap, compact and competitive (cost wise); it has ability to

combine with other equipment. 5. Ease in speed regulation in industrial drives.

Figure 3.7.1 shows a gas turbine supplying preheated combustion air to boilers. The

cooling of air after the supercharger reduces the compressor size and its work.

Fig. 3.7.1 Gas turbine supplying preheated combustion air for steam boilers

Figure 3.7.2 shows a gasifier supplying hot gases for an industrial process. Additional

fuel is burnt in a combustion chamber placed after the turbine depending on the heat

requirement in the process. The hot gases, after the process can be further used in steam

boilers. The starting turbine runs of compressed air.





74

Fig. 3.7.2 Gasifier supplying hot gases for an industrial process

Figure 3.7.3 shows the application of the gas turbine and compressor in the manufacture

of nitric acid. The gas turbine works on the waste heat of the process it drives the axial

and centrifugal stages of the compressor. Oxygen is removed from the high pressure air

before injecting steam. A steam turbine is employed for starting the plant.

Fig. 3.7.3 Pressurized process used in the manufacture of nitric acid

3.8 GAS TURBINES IN CRYOGENICS

Engineering and scientific aspects of considerably low temperature (- 157°C) form the

subject matter of cryogenics. Low temperatures can be obtained by:

1. Isenthalpic Joule-Thomson expansion and 2.Isentropic expansion.

Isentropic expansion was first obtained by reciprocating expanders which had problems

at very low temperatures. Rotating machines on account of high speeds (up to 6x105 rpm)

are most suitable for this purpose. High speed turbo-expanders employing 8-16 mm

diameter inward flow radial turbines give very low temperatures. Helium and hydrogen

turbo-expanders do not have problems of high Mach number because of the high speed of

sound.

Figure 3.8.1 shows the La-Fleur helium gas turbine cryogenic refrigeration system.

Various processes occurring in this plait are shown in the T-s plane in Fig. 3.8.2.





75

Helium is first compressed (AB) to a high pressure in a common compressor at the exit of

the compressor helium s divided between the power and the refrigeration cycles.

In the closed circuit power cycle helium is passed through a regenerator (BC) before

heating it by burning fuel. The heater or combustion chamber (CD) raises its temperature

for doing work in the turbine (DE). The turbine power is used to drive the compressor.

The exhaust from the turbine is sent back to the compressor inlet through the hot side

(EF) of the regenerator and a precooler. Thus the output of the closed circuit helium gas

turbine plant is the high pressure helium available for the refrigeration cycle. This is first

cooled in the after cooler (BM) and

Fig. 3.8.1 The La-Fleur helium gas turbine refrigeration system

Fig. 3.8.2 T-s diagram of the La-Fleur refrigeration system

the regenerator (MP) before expanding it in the high-speed inward flow radial cryogenic

turbine (PQ). The expansion of helium in the turbine reduces it to a very low temperature.

The power output of the cryogenic turbine can also be utilized in driving the helium

compressor. Low temperature helium after having been used for low temperature

refrigeration (QN) goes back to the compressor through the cold side of the regenerator.

MISCELLANEOUS APPLICATIONS OF GAS TURBINES





76

Only some major applications of gas turbines have been described in this chapter the aim

of this is to highlight the significance of the gas turbine as a single element in the given

total system.

By virtue of the extreme simplicity and light-weight design, small gas turbines (some of

them working on hot air) have found wide applications in many fields from surgery to

aerospace. Small air turbines are used to operate the drills used by orthopedic and dental

surgeons. The low temperature air after expansion in these turbines is used for cooling

the drilled region. Small gas turbines working on high energy fluids at low flow rates are

employed in the auxiliary power units in under-water and aerospace vehicles. Gas turbine

drives for turbo-pumps are used in rockets and missiles.

Other applications of gas turbines are in steel making, oil and gas pumping, marine

propulsion and helicopter rotor drives.

COMP4RISON OF GAS TURBINES WITH RECIPROCATING ENGINES

Gas turbine is also an internal combustion engine. Its competitor in early stages was the

reciprocating internal combustion engines. Let us compare them.

Advantages of Gas Turbines over Reciprocating Engines

(i) Mechanical efficiency Mechanical efficiency of the gas turbine is considerably

higher than that of the best reciprocating engine. For simple gas turbine design

mechanical efficiency of 90% to 95% has been claimed while for reciprocating

engine it is from 85 to 90% under full load conditions. It is due to more

frictional losses in reciprocating engines.

(ii) Balancing Due to absence of any reciprocating mass in gas turbine engine,

balancing can be near perfect. Torsional vibrations are absent because gas

turbine is a steady flow machine.

(iii)Cost In case of larger output gas turbine units of 2500 kW; it can be built at an

appreciably lower cost and in a shorter time than the corresponding multi

cylinder petrol or diesel engines.

(iv)Weight The fuel consumption per kW hour of best available aircraft gas turbine is

almost twice that of the normal petrol engine. However, it has much lighter

weight per kW so that the total weight of turbine plus fuel does not compare

unfavourably with reciprocating type of engine and its fuel. To give quantitative





77

example, the specific weight of (a) steam turbine is about 53 kg/kW, (b) diesel

engine is about 115 kg/kW and (c) gas turbine is about 20 kg/kW.

(v) External shape and size The basic cylindrical shape of turbine and compressor

unit renders the gas turbine more convenient to start, especially in aircraft and

locomotives.

(vi)Fuel The turbine can be designed to operate with cheaper and more readily

available fuels such as benzene, powdered coal, and heavy graded

hydrocarbons. Promising results have been obtained using furnace oil and also

pulverized coal as fuel.

(vii) Lubrication Compared with reciprocating engines the lubrication of gas

turbines is comparatively simpler. The requirement is chiefly to lubricate the

main bearing, compressor shaft and bearings of auxiliaries.

(viii) Maintenance The fact that the gas turbine consists of essentially a single

turbine and compressor unit with a common or coupled shaft running in a

relatively smaller number of main bearings, only minimum maintenance is

necessary as compared to the reciprocating internal combustion engines.

(ix)Low operating pressures The gas turbine generally operates at relatively low

pressures so that the parts exposed to these pressures can be made light although

the effects of thermal expansion and contraction must be taken into account.

The maximum combustion pressure is much lower than that in reciprocating

engines so that the pressure joints and piping do not pose any difficulty.

(x) Silent operation Since the exhaust from a gas turbine occurs under practically

constant-pressure conditions unlike the pulsating nature of reciprocating engine

exhaust, the turbine and compressor, if dynamically balanced, can run very

smoothly. The usual vibrational noises as in the case of reciprocating engine are

almost absent.

(xi)Smokeless exhaust With the present tendency to use relatively large surplus air

for combustion in order to reduce temperature of gases, the exhaust from the

turbine is almost smokeless and generally free from pungent odour associated

with optimum and rich fuel mixture which is characteristic of reciprocating

engines.





78

(xii) High operational speed Turbine can be made lighter than the reciprocating

engine of similar output. It can be run at much higher speed than reciprocating

engines. The output of any engine varies directly as the product of the driving

shaft torque and its rpm. Therefore, for a given output and higher speed the

torque will be lower. It may be noted that the torque characteristics of the gas

turbine is much better than that of reciprocating engine, since the former gives a

high initial torque and its variation with speed is comparatively less.

Advantages of Reciprocating Engines over Gas Turbines

1. Efficiency The overall efficiency of the turbine is much less than the

reciprocating engine since 70% of the output of the turbine is to be fed to the

compressor and other accessories and auxiliary parts.

2. Temperature limitation The maximum temperature in gas turbine cannot exceed

1500 K because of the material consideration of the blade while in reciprocating

engines with complete combustion of the fuel the maximum temperature can be

raised to 2000 K. This high temperature is permitted since the piston and cylinder

head are subjected to this high temperature only for a fraction of a second.

3. Cooling We can achieve very good results by efficient cooling in reciprocating

engine by which the heat of the cylinder walls is taken away, which enables to

keep the wall temperature only around 500 K but in gas turbine, cooling is

complicated, and, therefore, much higher temperature cannot be allowed to reach.

4. Starting difficulties It is more difficult to start a gas turbine than a reciprocating

engine as it requires compressed air or some suitable starter mechanism which are

complicated.

***



Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Industrial Pumps and its Applications

Prepared by: Prof. K. MuthuAssistant ProfessorDepartment of Chemical Engineering, Annamalai University, Annamalainagar.

79

INDUSTRIAL PUMPS AND ITS APPLICATIONS

Pumps are used to move fluids (liquids or gases) and slurries from one place to another

by mechanical action. Normally, liquids are moved by gravity through pipes and channels

from elevated tanks. Sometimes, a storage vessel pressurized by an external source of

compressed gas, the liquids are moved somewhere. But by far the most common devices

for the purpose are pumps. The pump is installed in a pipeline to provide the energy

needed to draw liquid from a reservoir and discharge a constant volumetric flow rate at

the exit of the pipeline. Severe erosion and cavity problems may reduce the pump

capacity. So always it is essential to maintain the suction pressure higher than the vapour

pressure of the liquid.

Pumps increase the mechanical energy of the liquid, velocity of the liquid and

pressure of the liquid. Pumps can be classified by their method of displacement

as positive displacement pumps, impulse pumps, velocity pumps, gravity pumps, steam

pumps and valve less and Centrifugal pumps. Positive displacement units apply pressure

directly to the liquid by a reciprocating piston, or by rotating members which form

chambers alternately filled by and emptied of the liquid. Centrifugal pumps generate high

rotational velocities and kinetic energy of the liquid to pressure energy. In pumps, the

liquid density does not change and remains constant.

Many different industries employ different pumps for varied uses. For example,

cryogenics use centrifugal pumps in extreme cold applications; dairy farmers use

centrifugal pumps to keep their product at the proper temperatures, hot and cold; electric

utility companies use centrifugal pumps, or turbines, to produce energy; food service,

construction, distillery, and automotive companies are a few more examples of industries

that employ centrifugal pumps for their many applications. Positive displacement pumps

are used to transport wide range of liquids, slurry and foams to be transported without

product degradation. For example, Bakery – dough, fats, fruit filling, icing, oil, yeast;

Beverages – beer, fruit concentrate, fruit juices, mash; Candy – chocolate, cocoa butter,

corn syrup, gelatin, sugar; Canned Foods – baby food, jams, jellies, mayonnaise, potato

salad, pudding, relish, stews; Cosmetics – creams, emulsions, jellies, lotions, shampoo,





80

toothpaste; Dairy – butter, cream, curds, ice cream, margarine, milk, soft cheese, yogurt

etc.

Various types of pumps used in the chemical industry are centrifugal pumps,

reciprocating pumps and helical rotor pumps. Centrifugal pumps operate by applying a

centrifugal force to fluids, many times with the assistance of impellers. These pumps are

typically used in moderate to high flow applications with low-pressure head, and are very

common in chemical process industries. There are three types of centrifugal pumps—

radial, mixed, and axial flow pumps. In the radial pumps, pressure is developed

completely through a centrifugal force, while in axial pumps pressure is developed by lift

generated by the impeller. Mixed flow pumps develop flow through a centrifugal force

and the impeller.

Reciprocating pumps compress liquid in small chambers via pistons or diaphragms.

These pumps are typically used in low-flow and high-head applications. Piston pumps

may have single or multiple stages and are generally not suitable for transferring toxic or

explosive material. Diaphragm pumps are more commonly used for toxic or explosive

materials. Helical rotor pumps use a rotor within a helical cavity to develop pressure.

All pumps use basic forces of nature to move a liquid. As the moving pump part

(impeller, vane, piston diaphragm, etc.) begins to move, air is pushed out of the way. The

movement of air creates a partial vacuum (low pressure) which can be filled up by more

air, or in the case of water pumps.

Mechanical pumps serve in a wide range of applications such as pumping water from

wells, aquarium filtering, pond filtering and aeration. In the car industry for water-

cooling and fuel injection, In the energy industry for pumping oil and natural gas or for

operating cooling towers. In the medical industry, pumps are used for biochemical

processes in developing and manufacturing medicine, and as artificial replacements for

body parts, in particular the artificial heart and penile prosthesis.





81

The diagram below provides an overview of pump classification by type





82





83

Rotary positive displacement pumps

The reciprocating pumps, rotary pumps contain no check valves, close tolerances

between the moving and stationary parts minimize leakage from the discharge space back

to the suction space they also limit the operating speed. Rotary pumps operate best on

clean, moderately viscous fluids, such as light lubricating oil.

Gear pumps: - A simple type of rotary pump where the liquid is pushed between two

gears. The gears rotate with close clearance inside the casing.

This is the simplest of rotary positive displacement pumps. It consists of two meshed

gears that rotate in a closely fitted casing. The tooth spaces trap fluid and force it around

the outer periphery. The fluid does not travel back on the meshed part, because the teeth

mesh closely in the centre. Gear pumps see wide use in car engine oil pumps and in

various hydraulic power packs.

Screw pumps - the shape of the internals of this pump is usually two screws turning

against each other to pump the liquid





84

Rotary vane pumps

A cylindrical rotor encased in a similarly shaped housing. As the rotor orbits, the vanes

trap fluid between the rotor and the casing, drawing the fluid through the pump

Positive displacement pumps

Positive displacement pumps have an expanding cavity on the suction side and a

decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the

suction side expands and the liquid flows out of the discharge as the cavity collapses. The

volume is constant given each cycle of operation.

Reciprocating pumps are classified as follows:

Plunger pumps

A reciprocating plunger pushes the fluid through one or two open valves, closed by

suction on the way back.

Diaphragm pumps

Similar to plunger pumps, where the plunger pressurizes hydraulic oil which is used to

flex a diaphragm in the pumping cylinder. Diaphragm valves are used to pump hazardous

and toxic fluids.

Piston pumps displacement pumps

Usually simple device for pumping small amount of liquid or gel manually. The common

hand soap dispenser is such a pump.

Air Operating Diaphragm Pump

An air operated double diaphragm pump has two diaphragms. These diaphragms are

connected by a shaft in the center section. The diaphragms are working as separation wall

between the air and the liquid. The air valve is located in the center section of the

diaphragm pump. The air valve directs the compressed air to the back of diaphragm

number one. This way, diaphragm number one moves away from the center section. This

diaphragm causes a press stroke moving liquid out of the pump. At the same time





85

diaphragm number two is performing a suction stroke. The air behind diaphragm number

two is being pushed out to the atmosphere. Atmospheric pressure pushes the liquid to the

suction side. The suction ball valve is being pushed away off its seat. This allows the

fluid to flow along the ball valve into the liquid chamber.

When the pressurized diaphragm number one has reached the end of its stroke, the

movement of the air is switched from diaphragm number one to the back of diaphragm

number two by the air valve. The compressed air pushes diaphragm number two away

from the center block. Doing so, diaphragm number one is pulled toward the center

block. In pump chamber number two the discharge ball valve is pushed off its seat. In

pump chamber number one the opposite occurs. Upon completion of the stroke the air

valve leads the air again to the back of diaphragm number one and restarts the cycle as

described above.

Vacuum Pump

A vacuum pump converts the mechanical input energy of a rotating shaft into pneumatic

energy by evacuating the air contained within a system. The internal pressure level thus

becomes lower than that of the outside atmosphere. The amount of energy produced

depends on the volume evacuated and the pressure difference produced. Pumps typically

operate to serve various chemical process support equipments such as chillers, cooling

towers, material transfer, etc., pumping is considered an individual process separate from

the processes of the aforementioned equipment.

Advantages of Centrifugal Pump

As there is no drive seal so there is no leakage in pump, It can pump hazardous liquids,

There are very less frictional losses, There in almost no noise, Pump has almost have 100

efficiency.

Advantages of Reciprocating pump

Reciprocating pumps will deliver fluid at high pressure. They are 'Self-priming' - No

need to fill the cylinders before starting.





86

Disadvantages of Piston Pumps

Reciprocating pumps give a pulsating flow. The suction stroke is difficult when pumping

viscous liquids. The cost of producing piston pumps is high. This is due to the very

accurate sizes of the cylinders and pistons. Also, the gearing needed to convert the

rotation of the drive motor into a reciprocating action involves extra equipment and cost.

The close fitting moving parts cause maintenance problems, especially when the pump is

handling fluids containing suspended solids, as the particles can get into the small

clearances and cause severe wear. The piston pump therefore, should not be used for

slurries.

Disadvantages of Centrifugal Pumps

Most centrifugal pumps are not self-priming. In other words, the pump casing must be

filled with liquid before the pump is started, or the pump will not be able to function.

References

http://en.wikipedia.org/wiki/Centrifugal_pump

http://www.renewables-info.com/drawbacks_and_benefits/geothermal_heat_pumps_%25E2%2580%2593

www.italvacuum.it/, www.ksb.com/_Pumps





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Prepared by: Dr. S. Vaidyanathan Professor Department of Manufacturing Engineering, Annamalai University, Annamalainagar.

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Topic: Computational Fluid Dynamics



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Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Dimensional Analysis

Prepared by: Dr. B. KumaravelAssistant Professor,Dept. of Civil Engineering, Annamalai University, Annamalainagar

106

DIMENSIONAL ANALYSISINTRODUCTION

Dimensional Analysis is a mathematical technique that makes use of the dimensions as a

tool to the solution of several engineering problems. Each physical phenomenon can be

expressed by an equation composed of physical quantities (or variables). These physical

quantities may be dimensional or non-dimensional quantities. Through dimensional

analysis, the physical quantities or variables can be arranged in a systematic fashion and

the physical quantities can be combined to form non-dimensional parameters.

Uses of dimensional analysis in the study of fluid mechanics:

1. Testing the dimensional homogeneity of any equation in fluid mechanics

2. Deriving equations expressed in terms of non-dimensional parameters to show the

relative significance of each parameter

3. Planning model tests and presenting experimental results in a systematic manner

using non-dimensional parameters; this enables analysis of even complex fluid

flow phenomenon.

DIMENSIONS

Engineers and scientists use various physical quantities to describe a physical

phenomenon. These physical quantities can be described by a set of quantities which are

in a sense independent of each other. These quantities are called fundamental quantities

or primary quantities.

The primary quantities are mass, length, time, and temperature denoted by M, L, T and

respectively.

All other physical quantities such as area, volume, acceleration, force, energy, power, etc.

are termed as derived quantities or secondary quantities. These quantities are called

secondary quantities because they can be expressed in terms of physical quantities.

The expression for a derived quantity in terms of the primary quantities is called the

dimension of the physical quantity. For instance, let us derive the dimension of the

derived quantity namely, force.

As per Newton’s second law of motion, the dynamic force is the product of mass and

acceleration. Acceleration, too, is a derived quantity which is the rate of change of





107

velocity. Velocity is yet another derived quantity which represents the rate of change of

displacement. The dimensions of velocity are: LT-1. Hence, the dimensions of

acceleration are: LT-2; so, the dimensions of force are: MLT-2.

Some engineers prefer to use force instead of mass as fundamental quantity because force

is easy to measure. In such a case, the physical phenomenon is represented by variables

expressed in F-L-T system instead of M-L-T system. The advantage with the dimensional

form of any quantity is that it is independent of the system of units and enables us to

convert from one system of units to the other system of units.

DIMENSIONAL HOMOGENEITY

The Fourier’s principle of dimensional homogeneity states that an equation which

expresses a physical phenomenon must be algebraically correct and dimensionally

homogeneous.

An equation is said to dimensionally homogeneous, if the dimensions of the terms on the

left hand side of the equation are same as the dimensions of the terms on the right hand

side of the equation.

Illustration of dimensional homogeneity

Consider the expression for discharge in a rectangular weir,

Q = (2/3)Cd(2g)1/2 LH3/2

Let us list the SI units and dimensions of the various quantities in the above expression

Quantity SI unitsDimensions

(M-L-T system)

Discharge, Q m3/s L3T-1

Coefficint of discharge, Cd No units Dimensionless

(Acceleration due togravity)1/2, g1/2

(m/s2)1/2 (LT-2)1/2 = L1/2T-1

Length of the notch, L M L

(Head over the sill ofnotch)3/2, H3/2

(m)3/2 L3/2





108

The dimensions of the left hand side of the equation are: L3T-1. the dimensions of the

right hand side of the equation are: (L1/2T-1).L.L3/2 = L1/2+1+3/2. T-1 = L3T-1 Thus we find

that the dimensions of both the LHS and RHS of the equation are the same. Hence, the

equation is dimensionally homogeneous.

The unique characteristic of a dimensionally homogeneous equation is that it is

independent of the system of units chosen for measurement, i.e., if an equation is

dimensionally homogeneous, it can be used without any modification with either system

of units.

METHODS OF DIMENSIONAL ANALYSIS

There are two methods of dimensional analyses as follows:

(A) Rayleigh Method

(B) Buckingham - Method

(A) Rayleigh Method

This method was proposed by Lord Rayleigh in the year 1989 to determine the

effect of temperature on viscosity of a gas. Let X be a variable which is a function of

different variables namely, X1, X2, ……, Xn. This can be written in the general form as

nXXXfX ,......,, 21 ……

(1)

In the above equation, X is the dependent variable and X1, X2, ……, Xn are the

independent variables. In the Rayleigh method, the functional relationship of the

variables X1, X2, ……, Xn is expressed in the form of an exponential equation which must

be dimensionally homogeneous. Hence, equation (1) can be expressed as

nn

ba XXXCX ......21 ……

(2)

where C is a dimensionless constant; C can be determined either from the physical

characteristics of the problem or from experimental measurements. a, b, ……, n are the

exponents of X1, X2, ……, Xn respectively which can be evaluated on the basis that the

equation is dimensionally homogeneous. By grouping together the variables with like

powers, the dimensionless parameters are formed. The Rayleigh method is illustrated in

the following example.





109

Illustration

Let us consider the problem of flow of liquid through a circular orifice

discharging freely into the atmosphere under a constant head. Let Q be the discharge

passing through the orifice of diameter d, under a constant head H. Let be the mass

density and let the dynamic viscosity of the liquid discharged through the orifice. Now,

the discharge Q through the orifice can be assumed to be dependent on the variables

namely, diameter d of the orifice, constant head H, mass density of liquid, dynamic

viscosity of liquid and the acceleration due to gravity g since the flow is freely into the

atmosphere. Hence, the general functional relationship for the dependent variable Q can

be written as

),,,,( gHdfQ ……

(3)

Equation (3) can be expressed by Rayleigh method in the exponential form as

edcba gHdCQ ……

(4)

where C is a dimensionless constant

The following Table shows the SI units and the dimensions of the various quantities

considered in this illustration.

Quantity with symbol SI units Dimensions (in MLT system)

Discharge, Q m3s-1 M0L3T-1

Dynamic viscosity, kg(mass)m-1s-1 ML-1T-1

Mass density, kg(mass)m-3 ML-3T0

Diameter, d m M0LT0

Head, H m M0LT0

Gravitational constant, g ms-2 M0LT-2

Dimensionless constant, C - M0L0T0





110

Substituting the dimensions for each variable in equation (4)

M0L3T-1 =(M0L0T0) (ML-1T-1)a (ML-3T0)b (M0LT0)c (M0LT0)d (M0LT-2)e

For dimensional homogeneity of the above equation, the exponents of each of the

dimensions M, L and T on both sides of the equation must be identical. Thus

for M: 0 = a + b

(5a)

for L: 3 = - a – 3b + c + d + e

(5b)

for T: -1 = - a – 2e

(5c)

Now, there are 5 unknowns namely a, b, c, d and e; but there are only 3 equations; hence,

three of the unknowns must be expressed in terms of the other two.

From equation (5a), b = - a ……

(6a)

From equation (5c),22

1 ae ……

(6b)

From equation (5b),22

1)(33

adcaa

21

23

3 dca

da

c 2

325

……

(6c)

Substituting the values of b, c and e from equations (6a), (6c) and (6b) in (4), we have,

22

1

2

3

2

5 a

dd

a

aa gHdCQ

=

ddaa

aa dHgdgdC 22

3

2

1

2

5





111

=

da

d

H

gdgddC

2/12/32

1

2

12

=

da

d

H

gdg

ddC

2/12/32

1

2/12 1

=

2/1

2/12/32

12 1

dd

H

gdgdC

da

=

2/1

2/1

2/12/32

12 H

d

H

gdgdC

da

=

2/1

2/12/32

12/12

da

d

H

gdgHdC

=

2/1

2/12/32 2

424

da

d

H

gdgHd

C

=

d

H

gdfgHa ,2

2/12/31

This expression may be written in the usual form as

gHaCQ d 2 ……

(7)

where Cd is the coefficient of discharge of the orifice

dC

d

H

gdf ,

2/12/31 ……

(8)

In the above expression, both the terms

d

H

gd,

2/12/3

are dimensionless and Cd is

also a dimensionless factor.

Buckingham - Method





112

Statement of Buckingham’s - Theorem: If a phenomenon is described by n

dimensional variables, and if these n dimensional variables can be completely described

by m fundamental quantities or dimensions (such as mass, length, time, etc.), and are

related by a dimensionally homogeneous equation, then the relationship among the n

quantities (or variables) can always be expressed by (n – m) dimensionless and

independent terms.

Let Y be a variable which depends on the independent variables X1, X2, X3, ……,

Xn. Then, the functional equation can be written as

Y = f(X1, X2, X3,……, Xn) ……

(9)

Equation (9) can be transformed to another functional relationship as

f1(Y, X1, X2, X3,……, Xn) = C ……

(10)

where C is a dimensionless constant. This is as if Y = f(X) = X2 + C; hence, Y – X2 = f1(X,

Y) = C. In accordance with the Buckingham’s - theorem, a non-dimensional equation

can be obtained as

f2(1, 2, 3, ……, n-m) = C1 ……

(11)

How are these - terms formed?

Each dimensionless - term is formed by combining m variables out of the total n

variables with one of the remaining (n – m) variables. These m variables in each of the -

terms are the same. As these m variables appear repeatedly in each of the - terms, these

variables are called repeating variables.

How are these repeating variables chosen?

These repeating variables are chosen from among the n variables such that they

involve all the m fundamental quantities or dimensions and they themselves do not form

any dimensionless number. Thus the different π - terms may be established as below.

132111111 ...... m

mm

cba XXXXX |





113

232122222 ...... m

mm

cba XXXXX | ……

(12)

………………………………….. |

nmm

cbamn XXXXX mnmnmnmn ......321 |

In equation (12), each individual equation is dimensionless and the exponents a, b, c, d,

……, m, etc., are determined by considering the dimensional homogeneity for each

equation so that each - term is dimensionless.

The final general equation for the phenomenon may be obtained by expressing one

- term as a function of other - terms. That is,

mnf ,......,, 43211 |

mnf ,......,, 43122 |

……………………………… | (13)

13211 ,......,, mnmn f |

Illustration of Buckingham’s - method

Let us consider the same problem of flow through a small orifice as considered

under the Rayleigh’s method.

Step 1. The discharge of an orifice depends upon the diameter d of orifice, constant

supply head H, acceleration due to gravity g, dynamic viscosity of liquid and mass

density of liquid. The functional equation for discharge Q can be written as

),,,,( gHdfQ ……

(14)

Equation (14) can be expressed in its most general form as

CgHdQf ),,,,,(1 ……

(15)

The total number of variables (including both the dependent variable Q and all the

independent variables) n = 6. All these variables can be expressed by the three

fundamental dimensions of either the M-L-T or F-L-T system. Hence, the number of





114

fundamental quantities m = 3. Therefore, the number of dimensionless - terms to be

formed are (n – m) = (6 – 3) = 3, so that

13212 ),,( Cf ……

(16)

Step 2. Selection of Repeating Variables.

In order to form these - terms, we have to choose m = 3 repeating variables. The

criteria for choosing these m repeating variables is that these variables among themselves

contain all the three fundamental dimensions and they themselves do not form any

dimensionless parameter. Thus let us choose the dynamic viscosity with dimensions

ML-1T-1, constant supply head H with dimension L and acceleration due to gravity g with

dimensions LT-2 as the repeating variables.

Step 3. Formulation of the different - terms.

QgH cba 1111

2222

cba gH ……

(17)

dgH cba 3333

Step 4. Determination of the - terms

Let us express the 1 – term in the dimensional form using the M-L-T system.

132110001

111 TLLTLTMLTLMcba

Equating the exponents of M, L and T, we obtain

for M: 0 = a1 ……(18a)

for L: 0 = - a1 + b1 + c1 + 3 ……(18b)

for T: 0 = - a1 – 2c1 – 1 ……(18c)

From (18a), a1 = 0; from (18c), c1 = - ½; from (18b), b1 = - 5/2

Hence,2/12/5

2/12/501 gH

QQgH

Now, Let us express the 1 – term in the dimensional form using the M-L-T system.





115

32110002

222 MLLTLTMLTLMcba


for M: 0 = a2 + 1 ……(19a)

for L: 0 = - a2 + b2 + c2 - 3 ……(19b)

for T: 0 = - a2 – 2c2 ……(19c)

From (19a), a2 = - 1; from (19c), c2 = ½; from (19b), b2 = 3/2

Hence, 2 2/12/31 gH =

2/32/1 Hg

Now, Let us express the 3 – term in the dimensional form using the M-L-T system.

LLTLTMLTLMcba 333 211000

3


for M: 0 = a3 ……(20a)

for L: 0 = - a3+ b3+ c3 + 1 ……(20b)

for T: 0 = - a3 – 2c3 ……(20c)

From (20a), a3 = 0; from (20c), c3 = 0; from (20b), b3 = - 1

Hence, 3 dgH 010 =H

d

Step 5.

As per equation (16), we have,

13212 ),,( Cf

H

dHg

gH

Qf ,,

2/32/1

2/12/52

= C1

or

2/12/5 gH

Q=

H

dHgfC ,

2/32/1

32

----------*****************----------

Modeling and Similitude

Modeling and Similitude is to develop the procedures for designing models so that the

model and prototype will behave in a similar fashion.





116

Model vs. Prototype

A model is a representation of a physical system that may be used to predict the behavior

of the system in some desired respect. Mathematical or computer models may also

conform to this definition, our interest will be in physical model.The physical system for

which the prediction are to be made.

Models that resemble the prototype but are generally of a different size, may involve

different fluid, and often operate under different conditions. Usually a model is smaller

than the prototype. Occasionally, if the prototype is very small, it may be advantageous to

have a model that is larger than the prototype so that it can be more easily studied. For

example, large models have been used to study the motion of red blood cells.

With the successful development of a valid model, it is possible to predict the behavior of

the prototype under a certain set of conditions. There is an inherent danger in the use of

models in that the predictions can be made that are in error and the error not detected

until the prototype is found not to perform as predicted. It is imperative that the model be

properly designed and tested and that the results be interpreted correctly.

Similarity of Model and Prototype

Hydraulic models may be either true or distorted models. True models reproduce features

of the prototype but at a scale - that is they are geometrically similar.

Geometric similarity

Geometric similarity exists between model and prototype if the ratio of all corresponding

dimensions in the model and prototype are equal.

Lprototype

el

Lp

Lm

L

Lmod

whereL is the scale factor for length.

For area

L

prototype

el

pL

mL

A

A 22

2mod

All corresponding angles are the same.

Kinematic similarity





117

Kinematic similarity is the similarity of time as well as geometry. It exists between model

and prototype

i). If the paths of moving particles are geometrically similar

ii). If the rations of the velocities of particles are similar

Some useful ratios are:

Velocity vLmLp

TmLm

Vp

Vm

T

L

/

/

Acceleration aT

L

pp

mm

p

m

LL

TL

a

a

22

2

/

/

Discharge QT

L

PP

mm

p

m

TL

TL

Q

Q

3

3

3

/

/

This has the consequence that streamline patterns are the same.

Dynamic similarity

Dynamic similarity exists between geometrically and kinematically similar systems if the

ratios of all forces in the model and prototype are the same.

Force ratio 22

2

223

3

ULT

LL

T

L

P

M

MP

MM

p

m

L

L

aM

aM

F

F

This occurs when the controlling dimensionless group on the right hand side of the

defining equation is the same for model and prototype.

Validation of Models Design

The purpose of model design is to predict the effects of certain proposed changes in a

given prototype, and in this instance some actual prototype data may be available. The

model can be designed, constructed, and tested, and the model prediction can be

compared with these data. If the agreement is satisfactory, then the model can be changed

in the desired manner, and the corresponding effect on the prototype can be predicted

with increased confidence.

Distorted Models

In many model studies, to achieve dynamic similarity requires duplication of

several dimensionless groups. In some cases, complete dynamic similarity between

model and prototype may not be attainable. If one or more of the similarity requirements





118

are not met, for example, if π2 = π2m , then it follows that the prediction π1= π1m

equation is not true; that is, π1≠ π1m. Models for which one or more of the similarity

requirements are not satisfied are called distorted models.

Example

Determine the drag force on a surface ship, complete dynamic similarity requires thatboth Reynolds and Froude numbers be duplicated between model and prototype.

21

21

)()( P

PP

m

mm

gl

VFr

gl

VFr Froude numbers

P

PPP

m

mmm U

lV

U

lV ReRe Reynolds numbers

To match Froude numbers between model and prototype2

1

P

m

P

m

l

l

V

V

To match Reynolds numbers between model and prototype

2

1

P

m

P

m

P

m

P

m

P

m

l

l

U

U

l

l

V

V

U

U 2

3

P

m

P

m

l

l

l

l

If lm/ lp equals 1/100 (a typical length scale for ship model tests) , then υm/υp must be1/1000. Thus, the kinematic viscosity ratio required to duplicate Reynolds numberscannot be attained.Reference1. http://www.freestudy.co.uk/fluid%20mechanics/t6203.pdf2. http://www.efm.leeds.ac.uk/CIVE/CIVE1400/PDF/Notes/section5.pdf3. www.iust.ac.ir/files/mech/mazidi_9920c/fluid_i/Lecture9.ppt4. http://www.docstoc.com/docs/168425252/5. http://www.fkm.utm.my/~syahruls/3-teaching/1-fluid-I/2note/9%20buckingham%

20note % 201.pdf6. http://www.daniel-huilier.fr/Enseignement/Notes_Cours/AnalyseDimensionnelle/

Taiwan Shieh fluid07.pdf

----------***************---------



Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Pneumatics

Prepared by: Prof. P. NarendersinghAssociate ProfessorDepartment of Manufacturing Engineering, Annamalai University, Annamalainagar.

119

PNEUMATICS – An Overview

Pneumatics is the study of air and gases and the relationship between volume, pressure

and temperature of the air or gases. Initially used for carrying out simplest mechanical

tasks but is playing an important role in the development in the development of

pneumatic technology for automation.

Compressed air used for:

The use of sensors to determine the status of processes

Information processing

Switching of actuators by means of final control elements

Carrying out work

The pneumatic cylinder has a significant role as a linear drive unit due to its,

Relatively low cost

Ease of installation

Simple and robust

Ready availability in various sizes and lengths

Pneumatic components can perform the following types of motion:

Linear

Swivel

Rotary

Some industrial applications of pneumatics:

General methods of

material handling:

General applications: Machining and

working operations:

Clamping

Shifting

Positioning

Orienting

Packaging

Feeding

Metering

Door control

Transfer of materials

Turning or inverting parts

Sorting of parts

Drilling

Turning

Milling

Sawing

Forming

Finishing

Quality control





120

Stocking of components

Stamping or embossing of

components

Advantages and distinguishing characteristics

Availability - Air is available in unlimited quantity

Transport - Can be easily transported in pipelines even over long distances

Storage - Compressor need not be in continuous operation. Even reservoir is

transportable

Temperature - Compressed air is relatively insensitive to temperature fluctuations

Explosion proof- Compressed air offers minimum risk of explosion

Cleanliness- Unlubricated exhaust air is clean

Components- Components are relatively inexpensive

Speed- Compressed air is a very fast working medium

Adjustable- Speeds& forces of pneumatic components are infinitely variable

Overload safe- Pneumatic tools and operating components can be loaded to the

point of stopping

Disadvantages

Preparation - Compressed air requires good preparation. Dirt and condensate should

not be present

Compressible- Not always possible to achieve uniform and constant piston speeds

with compressed air

Force requirement- Compressed air is economical only up to a certain force

requirement

Noise level- Exhaust air is loud

Costs- Compressed air is a relatively expensive of conveying power

A comparison with other forms of energy is essential

Factors to be considered if pneumatics is to be used as a control or working medium:

Work output requirements





121

Preferred control methods

Resources & expertise available to support project

Systems currently installed which are to be integrated with the new project

Choice of working media:

Electrics

Hydraulics

Pneumatics

A combination of the above

Selection criteria for the working section:

Force, Stroke, Type of motion, Speed, Size

Service life, Safety, Reliability

Sensitivity, Controllability

Energy Costs

Handling, Storage

Choice of control media:

Mechanical

Electrical / Electronic

Pneumatic (normal pressure or high pressure)

Hydraulic

Selection criteria for the control section:

Reliability of components

Sensitivity to environmental influences

Ease of maintenance and repair

Switching time of components

Signal speed

Space requirements

Service life

Training requirements of operators and maintenance personnel

Modification of the control system





122

A pneumatic system can be broken down into a number of levels, representing hardware

and signal flow, as shown in Fig. 1.

Figure 1 Pneumatic system structure and signal flow

The primary levels in a pneumatic system are:

Energy supply

Input elements (Sensors)

Processing elements (Processors)

Actuating devices (Actuators)

ENERGY SUPPLY

Pneumatic Gases

Qualities

The ideal fluid medium for a pneumatic system is a readily available gas that is

nonpoisonous (nontoxic), chemically stable, free from any acids that cause corrosion of

system components, and nonflammable. It also will not support combustion of other

elements.

Gases that have these desired qualities may not have the required lubricating power.

Therefore, lubrication of the components of some pneumatic systems must be arranged

by other means. For example, some air compressors are provided with a lubricating

system, some components are lubricated upon installation or, in some cases, lubrication is





123

introduced into the air supply line. Two gases meeting these qualities and most

commonly used in pneumatic systems are compressed air and nitrogen.

Compressed Air

Compressed air has most of the desired properties and characteristics of a gas for

pneumatic systems. The unlimited supply of air and the ease of compression make

compressed air the most widely used fluid for pneumatic systems. Compressed air is a

mixture of all gases contained in the atmosphere. It is nonpoisonous and nonflammable

but does contain oxygen, which supports combustion.

One of the most undesirable qualities of compressed air as a fluid medium for pneumatic

systems is moisture content. The atmosphere contains varying amounts of moisture in

vapor form. Changes in the temperature of compressed air will cause condensation of

moisture in the pneumatic system. This condensed moisture can be very harmful to the

system, as it increases corrosion, dilutes lubricants, and may freeze in lines and

components during cold weather. Although moisture and solid particles must be removed

from the air, it does not require the extensive distillation or separation process required in

the production of other gases.

The supply of compressed air at the required volume and pressure is provided by an air

compressor.

Compressed air systems are categorized by their operating pressures as follows: high-

pressure (HP) air, medium-pressure (MP) air, and low-pressure (LP) air.

Nitrogen

For all practical purposes, nitrogen is considered to be an inert gas. It is nonflammable,

does not form explosive mixtures with air or oxygen, and does not cause rust or decay.

Due to these qualities, its use is preferred over compressed air in many pneumatic

systems, especially aircraft and missile systems, and wherever an inert gas blanket is

required.

Contamination Control

As in hydraulic systems, fluid contamination is also a leading cause of malfunctions in

pneumatic systems. In addition to the solid particles of foreign matter which find a way to





124

enter the system, there is also the problem of moisture. Most systems are equipped with

one or more devices to remove this contamination. These include filters, water separators,

air dehydrators, and chemical driers. In addition, most systems contain drain valves at

critical low points in the system. These valves are opened and closed periodically (Either

automatically or manually) to allow the escaping gas to purge a large percentage of the

contaminants, both solids and moisture, from the system.

The air service unit consists of the following:

Compressed air filter

Compresses air regulator

Compressed air lubricator

INPUT ELEMENTS (SENSORS)

Valves

Primary function of the valves is to alter, generate or cancel signals for the purpose of

sensing, processing and controlling. Additionally, the valve is used as a power valve for

the supply of working air to the environment.

They can be divided into a number of groups according to their function in relation to,

Signal type

Actuation method

Construction

Therefore the following categories are relevant

Directional Control Valves (DCV)

Signal elements

Processing elements

Power elements

As a signal element the DCV is operated by roller lever to detect the piston rod position

of a cylinder. The signal element can be small in size and create a small air pulse. A

signal pulse created will be at full operating pressure but have a small flow rate.

As a processing element the DCV redirects, generates or cancels signals depending on the

signal inputs received.





125

It can be supplemented with additional elements such as the AND function and the OR

function valves to create desired control conditions.

As a power element the DCV must deliver the required quantity of air to match the

actuator requirements and hence there is a need for larger volume rates and therefore

larger sizes.

DCV can be of,

The poppet type used for small flow rates and for generation of input and process

signals, Or The slide type used for larger flow rates and hence lends itself to the power

and actuator control valve.

Non-return valves and its derivatives:

The non-return valve allows a signal to flow through the device in one direction and in

the other blocks the flow.

There many variations in construction and size derived from the basic non return valve.

Other derived valves utilize features of the non-return valve by the incorporation of non-

return elements.

Flow control valves

A flow control valve restricts or throttles the air in a particular direction to reduce the

flow rate of the air and hence control the signal flow.

If flow control valve is wide open then the flow should be almost the same as if restrictor

not fitted can be fitted with a non-return valve then flow is uni-directional. Flow control

valve as close as possible to working element

Pressure control valves

Pressure regulating valves controls the pressure in a control circuit and keeps the

pressure constant irrespective of any pressure fluctuations in the system.

Pressure limiting valves are utilized on the up-stream side of the compressor to ensure the

receiver pressure is limited, for safety, and that the supply pressure to the system is set to

the correct pressure.





126

Pressure sequence valve senses the pressure of any external line and compares the

pressure of the line against a pre-set adjustable value, creating a signal when the pre-set

limit is reached.

Combinational valves

The combined functions of various elements can produce a new function. The new

component can be constructed by the combination of individual elements or

manufactured in a combined configuration to reduce size and complexity.

Valves are described by:

No. of ports or openings (ways)⇒2 way, 3 way, 4 way, etc.

No. of positions⇒2 positions, 3 positions, etc.

Method of actuation of the valve⇒Manual, air-pilot, solenoid, etc.

Methods of return actuation⇒ Spring-return, air-return, etc.

Special features of operation⇒Manual overrides, etc.

PROCESSING ELEMENTS (PROCESSORS)

To support the DCV at the processing level, there are various elements which condition

the signal for the task, viz.

Two pressure valve (AND function)

Shuttle valve (OR function)

They have logic based role and are fitted at the junction of three lines. They have three

connections, 2 in and 1 out.

Modular processing unit consisting of DCV functions and logic elements to perform a

combined processing task have been designed to reduce cost, size and complexity of

system.





127

ACTUATING DEVICES (ACTUATORS)

Actuator group includes various types of linear and rotary actuators of varying size and

construction. The actuators are complemented by the final control element, which

transfers the required quantity of air to drive the actuator.

Normally this valve is fitted close to actuator to minimize losses and is connected directly

to the air supply.

Linear actuators

Rotary Actuators

ELECTRO-PNUEMATICS

Electro pneumatics is now commonly used in many areas of industrial low cost

automation. They are also used extensively in production, assembly, pharmaceutical,

chemical and packaging systems. There is a significant change in controls systems. In

recent developments, relays have increasingly been replaced by the programmable logic

controllers in order to meet the growing demand for more flexible automation.

Electro-pneumatic control consists of electrical control systems operating pneumatic

power systems. In this solenoid valves are used as interface between the electrical and

pneumatic systems. Devices like limit switches and proximity sensors are used as

feedback elements.

Electro-pneumatic control integrates pneumatic and electrical technologies, is more

widely used for large applications. In Electro-pneumatics, the signal medium is the

electrical signal either AC or DC source is used. Working medium is compressed air.

Operating voltages from around 12 V to 220 Volts are often used. The final control valve

is activated by solenoid actuation

The resetting of the valve is either by spring [single Solenoid]or using another solenoid

[Double solenoid Valve]. More often the valve actuation/reset is achieved by pilot-

assisted solenoid actuation to reduce the size and cost of the valve

Control of electro-pneumatic system is carried out either using combination of

relays and contactors or with the help of Programmable Logic Controllers [PLC]. A





128

Relay is often is used to convert signal input from sensors and switches to number of

output signals [either normally closed or normally open].Signal processing can be easily

achieved using relay and contactor combinations

A PLC can be conveniently used to obtain the out puts as per the required logic, time

delay and sequential operation. Finally the output signals are supplied to the solenoids

activating the final control valves which control the movement of various cylinders. The

greatest advantage of electro pneumatics is the integration of various types of proximity

sensors [electrical] and PLC for very effective control. As the signal speed with electrical

signal, can be much higher, cycle time can be reduced and signal can be conveyed over

long distances.

In Electro pneumatic controls, mainly three important steps are involved:

Signal input devices -Signal generation such as switches and contactor, Various

types of contact and proximity sensors

Signal Processing - Use of combination of Contactors of Relay or using

Programmable Logic Controllers

Signal Outputs -Outputs obtained after processing are used for activation of

solenoids, indicators or audible alarms

ELECTRICAL DEVICES

Seven basic electrical devices commonly used in the control of fluid power systems are:

1. Manually actuated push button switches

2. Limit switches

3. Pressure switches

4. Solenoids

5. Relays

6. Timers

7. Temperature switches

Other devices used in electro pneumatics are:

1. Proximity sensors

2. Electric counters





129

Programmable Logic Controllers [PLC]

A PLC can be defined as the digitally operating electronic apparatus which uses a

programmable memory for the internal storage of instructions for implementing specific

functions such as logic, sequencing, timing, counting, and arithmetic to control, through

digital or analog input/output modules, various types of machines or processes.

In essence, the programmable logic controller consists of computer hardware, which is

programmed to simulate the operation of the individual logic and sequence elements that

might be contained in a bank of relays, timers, counters, and other hard-wired

components.

The PLC was introduced around 1969 largely as a result of specifications written by the

General Motors Corporation. The automotive industry had traditionally been a large

buyer and user of electro-mechanical relays to control transfer lines, mechanized

production lines, and other automated systems.

There are significant advantages in using a programmable logic controller rather than

conventional relays, timers, counters, and other hardware elements. These advantages

include:

Programming the PLC is easier than wiring the relay control panel.

The PLC can be reprogrammed. Conventional controls must be rewired and are

often scrapped instead.

PLCs take less floor space then relay control panels.

Maintenance is easier, and reliability is greater.

The PLC can be connected to the plant computer systems more easily than relays

can.

Components of the PLC:

A schematic diagram of a programmable logic controller is presented in fig. 2. The basic

components of the OPLC are the following:

Input module

Output module

Processor





130

Memory

Power supply

Programming device

Figure 2 Schematic sketch of the Programmable Logic Controller

The components are housed in a suitable cabinet designed for the industrial environment.

The input module and output module are the connections to the industrial process that is

to be controlled. The inputs to the controller are signals from limit switches, pushbuttons,

sensors, and other ON-OFF devices. In addition, as we will describe later, larger PLCs

are capable of accepting signals from analog devices of the type modeled. The outputs

from the controller are ON-OFF signals to operate motors, valves, and other devices

required to actuate the process.

The processor is the central processing unit (CPU) of the programmable controller. It

executes the various logic and sequencing functions by operating on the PLC inputs to

determine the appropriate output signals. The processor is microprocessor very similar in

its construction to those used in personal computers and other data-processing equipment.

Tied to the CPU is the PLC memory, which contains the program of logic, sequencing,

and other input/output operations. The memory for a programmable logic controller is

specified in the same way as of storage capacity for a computer.





131

The PLC is programmed by means of a programming device. The programming device

(sometimes referred to as a programmer) is usually detachable from the PLC cabinet so

that it can be shared between different controllers. Different PLC manufactures provide

different devices, ranging from simple teach pendant-type devices, similar to those used

in robotics, to special PLC programming keyboards and CRT displays.



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