CHAPTER 6

HYDRAULIC MACHINERY

2INTRODUCTION

What is turbine?

Turbine is a hydraulic machine that utilises the energy of fluids to move other types of machineries.

An example of turbine usage can be seen in a hydroelectric power plant.

Turbines are generally divided into Impulse and Reaction turbines.

3INTRODUCTION

Impulse turbines

This turbine derive its energy

from a jet of water exiting out of

a nozzle and shooting at the

blades of the turbine. The most

common type is Pelton Wheel

Turbine and its suitable for

medium head and low

discharge.

4INTRODUCTION

Reaction turbines

A reaction turbine derives its power from the equal

and opposite reactive power of fluid passing between

its blades and classified in 3 types of flows which are

radial, axial and mixed flow. Two popular types are

the Francis turbine and the propeller turbine.

5INTRODUCTION

Francis turbines are effective on a very wide range of

heads (medium head) and are very much used in spite

of their relatively high cost. Usually work in radial flow

but also can in mixed flow.

A propeller (Kaplan) turbine is an axial flow machine

with its runner confined in a closed conduit. A propeller

turbine is often set on a vertical axis, and can also be

set on a horizontal axis or a slightly inclined axis. A

propeller turbine is suitable for operation with low head

and large amount of discharge.

6What is pump?

Pump is a hydraulic machine which supply energy

to fluid in certain operation.

An example of pump usage can be seen in such as

in water distribution system.

Pumps are generally divided into positive

displacement and rotodynamic pumps.

INTRODUCTION

7Rotodynamic pumps consist of a rotating device known as an impeller. The fluid that needs to be pumped enters a casing near the shaft of the impeller. Vanesattached to the spinning impeller increases the velocity of the pumped fluid and moves the fluid out through an outlet.

The most common and popular pump under the rotodynamic pump category is the centrifugal pumpand the propeller pumps.

INTRODUCTION

8Centrifugal pumps produce radial flow and

mixed flow according to the fluid path. Thus,

centrifugal pumps are also referred to as

radial and mixed flow pumps.

Meanwhile, propeller pumps also consist of

an impeller, which produces axial flow for the

fluid.

INTRODUCTION

9POWER AND EFFICIENCY OF PUMP

Figure below shows the process of pump in a operation.

The mechanical energy through the shaft and impeller is

converted to fluid energy. The difference between the total

energy heads at the intake and discharge flanges of the

pump is denoted as net head, H developed in the pump.

The intake end (inlet flow), of a pump is commonly known

as a suction end while the discharge end (outlet flow) of a

pump is called a delivery end.

10

POWER AND EFFICIENCY OF PUMP

The equation below shows the relationship between a

suction head and a discharge head.

g

Vz

P

g

Vz

PHHH ss

sdd

dsd

22'

22

Where:

b = width

Vf= V sin = flow velocity

11

POWER AND EFFICIENCY OF PUMP

The power absorbed by the water from the actions of an

impeller is given as shown on the right.

Power at suction end also known as power in while

power at discharge end is called power out.

Where:

Vu = V cos = swirl velocity

= rate of shaft rotation in radians per second

Ps = Pi Pd = Po

Note :

12

POWER AND EFFICIENCY OF PUMP

power Brakeshafttheintopowerfluidthetodeliveredpower

0

Hmv

s

dO

P

P

The overall efficiency of a pump is given as,

However, the sum of hydraulic, volumetric and

mechanical efficiency also yields the overall efficiency

for a pump. Thus, the overall efficiency of a pump

can also be written shown below.

13

EXAMPLE 10.1

A centrifugal pump is needed to supply 23m3/s ofwater for a city. This operation will utilise a net head Hof 20 m with a specific speed N of 450 rpm. Given thatthe inflow power Ps is 5000 kW, calculate

a) Outflow power, Pdb) The overall efficiency, oAssume that the density of water is 1000 kg/m3 at 5oC.

14

EXAMPLE 10.1

Outflow Power, Po

= 1000 X 9.81 X 23 X 20

= 4512.6 kW

'QHPo

100% x i

oo

P

P

o

100%5000

6.4512x

Overall Efficiency

= 90.3%

15

CHARACTERISTICS OF PUMP CURVE

The efficiency of a pump varies considerably

depending upon the conditions under which it must

operate. It is important to have information regarding

the performance of various pumps when selecting a

pump for a given situation. Though some centrifugal

pumps are driven by variable speed motors, the usual

mode of operation of pump is at constant speed.

16

CHARACTERISTICS OF PUMP CURVE

The characteristic curve of

pump and other performance

curves for a typical mixed-

flow centrifugal pump are

shown in figure on the right.

Curves such as shown in the

figure are usually determined

by pump manufacturers

through laboratory testing.

17

CHARACTERISTICS OF PUMP CURVE

Relationship between power input, P, efficiency, and head, H starts when intake pipe valve closed,

impeller will spin the water until pressure at pump

output point increase to maximum head (shut-off

head).

Then when the valve open, water will flow through

the pipe and the head of pump will decrease.

With addition of flow rate, the pump efficiency will

increase until reach maximum and then decrease to

end of operation.

18

CHARACTERISTICS OF PUMP CURVE

Intersection between head and power corresponds to

the point of optimum efficiency is the best point to

use pump or BEP (best efficiency point).

This particular pump has a normal capacity or rated

capacity of 10 500 gpm when developing a normal

head of 60 ft at an opening speed of 1450 rpm.

19

CAVITATION

An important factor in the satisfactory operation of apump is the avoidance of cavitation, both for the goodefficiency and for the prevention of impeller damage.

As liquid passes through the impeller of a pump, thereis a change in pressure. If the absolute pressure ofthe liquid drops the vapour pressure, cavitation willoccur.

The region of vaporization hinders the flow andplaces a limit on the capacity of the pump.

20

As the fluid moves further into a region of higher

pressure, the bubbles collapse and the implosion of

the bubbles may cause pitting of the impeller.

Cavitation is most likely to occur near the point of

discharge (periphery) of radial flow and mixed flow

impellers, where velocities are highest.

It may also occur on the suction side of the impeller,

where the pressures are the lowest.

CAVITATION

21

PARALLEL PUMP

However, the head, h (pressure head) is same in

both pumps and will be the net head of combined

discharge.

If two similar pumps A and B are connected in

parallel, the combined discharge will be the sum of

individual discharges QA and QB.

Qtotal = QA + QB

htotal = hA = hB

Qtotal = QA + QB

htotal = hA = hB

22

SERIES PUMP

If two similar pumps 1 and 2 are connected in

series, the discharge will not change and the head

will added up.

Qtotal = QA = QB

htotal = hA + hB

23

SIMILITUDE

Similitude is also used in the design and analysis

of turbines and pump.

Similarity laws help us interpret the results of model studies. The relation between model and prototype is classified into three:

Geometry Similarity, Kinematics Similarity and Dynamic Similarity

24

SIMILITUDEGEOMETRY SIMILARITY - The prototype and model

have identical shapes but differ only in size.

KINEMATIC SIMILARITY - ratio of velocities at all

corresponding points in flow are the same and

involve length and time.

DYNAMIC SIMILARITY-Two systems have

dynamic similarity if, in addition to dynamic

similarity, corresponding forces are in the

same ratio in both.

25

SCALE RATIO

MODEL (m)

- Similar with object/structure required in certain

scale ratio.

- tested in laboratory and similar in real

phenomenon.

- not necessary its smaller than prototype

26

SCALE RATIO

PROTOTAIP (p) - object/actual structure- tested in actual phenomenon, example:

structure in open channel, ship etc

27

ADVANTAGES USING SIMILARITY

1. Performances of object can be predicted.2. Economy and easy to build, where design

of model can be done many times until reach a certain values.

3. Nonfunctional structure also can be measured such as dam.

28

SIMILARITY (PUMP)

In similarity relations, the basic repeating variables are

rotative speed (N) and pump diameter (D). Therefore,

the similitude laws for head (H), discharge (Q), and

power (P) can be expressed as below.

2222

mp

p

mm

m

ND

H

ND

H

3535

ppp

p

mmm

m

ND

P

ND

P

33

pp

p

mm

m

DN

Q

DN

Q

29

SIMILARITY (PUMP)

From the given laws of similitude, we conclude that any

two homologous pumps would have the same specific

speed (Ns).

Therefore, the relationship between a prototype pump

and its geometric model satisfy the following equation.

sp/p

pp

/m

mmsm N

H

QN

H

QNN

4343

30

Two homologous pumps A and B use an operation

at the speed of 600 rpm. Pump A has an impeller

with a 50 cm diameter and discharges 0.4 m3/s of

water under a net head of 50 m. Determine the

size of pump B and its net head if it is to discharge

0.3m3/s.

EXAMPLE 10.2

31

EXAMPLE 10.2

32

SIMILARITY (TURBINE)

The characteristic relationships between a turbine

model and its prototype can be expressed in terms of

variables as shown below.

p

pp

m

mm

H

DN

H

DN

33

pp

p

mm

m

DN

Q

DN

Q

3535

pp

p

mm

m

ND

P

ND

P

33

SIMILARITY (TURBINE)

Thus, we conclude that two homologous turbines have

the same specific speed Ns. Therefore

sp/p

pp

/m

mmsm N

H

PN

H

PNN

4545

34

A 1:5 model of turbine develops 2 kW of power at

400 rpm under head of 3.0 m. What is the specific

speed? Assuming the overall efficiency of 0.85 for

both the model and prototype, calculate the

rotational speed, power and discharge of the

prototype when run under a head of 20 m?

EXAMPLE 10.3

35

EXAMPLE 10.3

3.1433

24004/54/5

m

mm

SH

PNN

rpmN

N

p

p

6.206

20

)5(

3

)1)(400(

Specific Speed

Speed

p

pp

m

mm

H

DN

H

DN

36

EXAMPLE 10.4

kWP

Pk

p

p

2.861

)6.206()5()400()1(

23535

Power

Discharge

mmom HQP

3535

pp

p

mm

m

ND

P

ND

P

smQ

Qxx

m

m

/0799.0

)3)()(85.0)(100081.9(10002

3

37

EXAMPLE 10.4

33

pp

p

mm

m

DN

Q

DN

Q

Discharge

smQ

Qx

Q

p

p

p

/159.5

2582510998.1

)5)(6.206()1)(400(

0799.0

3

4

33

38

UNIT QUANTITIES

(a)Unit Discharge

Unit discharge Qu is defined as the flow rate of a geometrically similar turbine which is run under a head of 1 m

SIMILARITY (TURBINE)

H

QQu

1

1

H

Q

2

2

H

Q

If comparing between 2 turbine, the equation will be as below.

39

UNIT QUANTITIES

(b)Unit Speed

Unit speed Nu is defined as the speed of a

geometrically similar turbine which is run under a

head of 1 m

SIMILARITY (TURBINE)

If comparing between 2 turbine, the equation will be as

below.

H

NNu

1

1

H

N

2

2

H

N

40

UNIT QUANTITIES

(c)Unit Power

Unit power Pu is defined as the power of a geometrically similar turbine which is run under a head of 1 m

SIMILARITY (TURBINE)

If comparing between 2 turbine, the equation will be as

below.

2/3H

PPu

41

A Francis turbine produces 6750 kW at 300 rpm

under a net head of 45 m with an overall efficiency

of 85%. Determine the revolution per-minute (rpm),

discharge and brake power of the same turbine

under a net head of 60 m under homologous

conditions.

EXAMPLE 10.5

42

EXAMPLE 10.5

43

EXAMPLE 10.5