Steam Turbine Principle

Prof. Dr. Ping He

1
Introduction

2
Steam (Thermal) Power Plant

3
Steam (Thermal) Power Plant

4
Steam Power Plant is an energy converter

5
 Steam (Thermal) Power Plant
p Fuel:Coal or Oil
p Main parts:Boiler, Turbine,Generator,Condenser,Pump

6
The Simple Ideal Rankine Cycle

7
 The Simple Ideal Rankine Cycle
Feedwater pump(1-2):Promotion the pressure of
feedwater. : Isentropic compression

Boiler(2-3):Heat the feedwater to superheated steam

and reheat steam. Isobaric heat supply

Steam Turbine(3-4):Steam expansion in steam

turbine, transfer the steam heat energy to mechanical
enery of rotor. Isentropic expansion

Condenser(4-1): Exhaust steam flow into condenser to

be condensed to saturated water. Isobaric heat
simple flow diagram and T-S rejection
chart of Rankine cycle
8
Simple Rankine Cycle Diagram

9
Reheat Cycle
 What is Steam Turbine
A steam turbine is a
machine that extracts
thermal energy from
pressurized steam and uses
it to do mechanical work on
a rotating output shaft.
 Physical Principles: Steam Turbines
l High Pressure Steam
expands through a
governor valve and a
nozzle.
l Experiences an increase in
velocity and momentum.
l Pushes the impeller to
drive the turbine.
12
Physical Principles: Steam Turbines
Turbines perform the energy conversion in two steps:

l Step 1: Thermal energy of the steam to kinetic energy of

the steam

l Step 2: kinetic energy of the steam to mechanical energy

of the rotor

13
 Principle & Types of Steam Turbine
Types:
Ø Impulse Turbine
Ø Reaction Turbine

Principle: when steam is allowed to expand through a

narrow orifice, it assumes kinetic energy at the expense
of its enthalpy (heat energy). This kinetic energy of steam
is changed to mechanical (rotational) energy through the
impact (impulse) or reaction of steam against the blades.

14
Construction of steam turbines

15
Cross Section of Steam Turbine
Construction of steam turbines

17
Classification of Steam Turbines
 Classification of Steam Turbines
a) way of energy c) number of stages
conversion - single stage
- impulse turbines - multi-stage
- reaction turbines

b) flow direction
- axial
- radial
 Classification of Steam Turbines
d) rotational speed
- Fixed speed
- Variable speed
- Low Speed (≤ 3000 rpm)
- High Speed(≥ 3000 rpm)
e) inlet steam pressure
- Low-Pressure Steam Turbine
- Medium Pressure Steam Turbine
- High-Pressure Steam Turbine
- Very High-Pressure Steam Turbine
- Supercritical Pressure Steam
turbine
 Classification of Steam Turbines
f) way of energy utilisation
- condensing
- extraction
- back-pressure
 Steam Turbine Configurations
l Back-pressure: where the exhaust steam is used as heat
supply to a process or DH system.
l Condensing: where all of the steam is exhausted to a
condenser and no steam passes to other processes.
l Extraction – Condensing: where part of the steam flow
is extracted from the main flow and is used as heat supply
to a process or DH system, while the remaining steam is
exhausted to a condenser.
Typical steam conditions:
40 bar a 40 bar a 40 bar a
400 oC 400 oC 400 oC

2-5 bar a 2-5 bar a

~0.1
to heating ~0.5 bar a to heating
bar a
system system

Extraction-
Back-Pressure Condensing Condensing
Classification of Steam Turbines

One section Two sections

Classification of Steam Turbines

Reheat Turbine
 Classification of Steam Turbines

g) application
- power station
- industrial
- transpor
 History
A Rotation Turbomachine
Sir Charles Algernon Parsons (13 June 1854 – 11 February 1931)

an Anglo-Irish engineer，inventor of the steam turbine

Trinity College, Dublin

St. John's College, Cambridge

Heaton, Newcastle
in 1884 , Steam turbine
Franklin Medal (1920)
Faraday Medal (1923)
Copley Medal (1928)
Parsons' first 1MW turbogenerator in
1899
First Steam Turbine made in China（6MW）
 Usage
Thermal Power Plant Nuclear Power
Plant
Nuclear Steam Turbine
Overview Of USC Technology Development
(USC --Ultra Super Critical)
1.1 Parameters of SC/USC
The critical point parameters of water ：
Pc=22.115MPa Tc=374.15 ℃
（P↗ -- Saturation temperature ↗ -- latent
heat of vaporization ↘ -- Density difference ↘ ）

SC: Main steam parameter＞ critical point parameter

Typical ：24.2MPa，538 ℃-566 ℃（Efficiency41-42%）
USC： Parameters to further improve ， Typical ：
Japan—P＞24.2MPa or T＞580℃
Europe—P＞28-30MPa
China—P＞25MPa，T＞580℃ （Efficiency42-46%）
Efficient USC：
China—P＞27MPa，T＞620℃ （Efficiency46%or more）
l Advantages of turbines

l Large power achieved by relatively small size

l High efficiency

l Simple Design

l High revolution
 Type:

N 300—16.7／538／538
Main(/reheat) steam
temperature

Main steam pressure

Power rating（MW)

With condenser
There are two basic types of steam turbine blading
systems:
1. Impulse: where the pressure drop occurs in a
convergent/divergent nozzle located in a
stationary diaphragm.
2. Reaction: where the pressure drop occurs equally
in the rotating blades and in the stationary blades,
which are held in stator blade-carriers.
Although these two types lead to very different
turbine designs, in modern machines there is often a
mixture of both types.
Energy Conversion in Steam Turbine

• Blade

• Cascade
 Energy Conversion in Steam Turbine

• Stage
Stage is a basic unit for
conversing heat energy into
mechanical energy. It consists
of fixed blades and moving
blades。
stator 汽缸
stator blades 静叶片
damping wire 拉筋
rotor blades 动叶片
rotor 转子
 Stage work process
thermodynamic energy
(nozzle) kinetic energy
Pressure drop，velocity increase

Pressure drop，velocity
increase 0

1 2

blade ( reaction )

Steam flow changes direction 0

mechanical ( impulse)
energy
1 2

Characteristic section or calculation

section:
nozzle: 0-0;

After the nozzle (before blade): 1-1;
after blade: 2-2.


Flow in steam blades and nozzles

Force Analysis of moving blade
⑴ The high speed steam from the nozzle hits on the moving blade of the turbine,
and the steam exerts an impulse force on the moving blade Fi.

⑵ The steam expands and accelerates inside the moving blade channel, and
when it leaves the moving blade channel, the moving blade is given a force
in the opposite direction of the steam flow, called the reaction force Fr.

⑶ In general, when steam flows in the blade

channel, on the one hand, impulse force Fi to
the moving blade, on the other hand, On the
other hand, when the moving blade passage
continues to expand, a reaction force Fr is
exerted on the moving blade cascade. The Fz

direction of the two forces is not consistent

with the direction of the wheel circumference.
The resultant force F of the two forces acts
on the moving blade cascade, and its
component Fu on the side of the wheel
circumference makes the moving blade
cascade rotate
 pressure [bar]
等压线

specific volume [m3/kg]

等容线

temperature [°C]
enthalpy [kJ/kg]

等温线
saturation curve
饱和曲线

relative humidity [%]

相对湿度

entropy [kJ/kg·K]
 Thermodynamic process of
steam in the stage
• Ideal enthalpy dropht*
• Real enthalpy  h
• Heat drop and enthalpy
drop in fixed blades and
moving blades
• Reaction Ω
The ratio of isentropic
enthalpy drops in the moving
blade and in the stage
 hb
 
 ht*
 Impulse and reaction turbine
• Impulse stage
Enthalpy drop of the stage is
only in the moving blades or
most part in the moving blades
• Reaction stage
50％ enthalpy drop in the fixed
blades and another half in the
moving blades
Note: Most stages in real
steam turbine are not pure
impulse stages or reaction
stages, The reaction varies with
the height of the blades.
Velocity at outlet of the nozzle
• Energy conservation： c02
h0   h1t 
c12t
2 2
2 *
• Velocity： c1t  2( h0  h1t )  c  2h
0 n

• Nozzle coefficient：
c1

c1t
 Velocity Triangle
  
inlet velocity triangle c1  u  w 1
  
outlet velocity triangle c2  u  w2
d m n
u  60
c is absolute velocities at the inlet and outlet respectively
w is relative velocities at the inlet and outlet respectively
u is blade velocities

1  2*  2*
 1 
c1  c2
 w1 52  
u u w2
 Velocity Triangle
• Velocities at inlet and out
let of moving blades
（vector）
• Flow direction matches up
the inlet angle of moving
blades, and has minimum
leaving loss c22
 hc 2 
• Leaving loss： 2
• Utilization coefficient of
leaving energy：μ＝0～1
 Wheel Power
• Effective Power of steam flow acting on the
blades in whole wheel at a unit time.
Pu  Fu u  Gu(c1 cos 1  c2 cos  2 )
2 2 2 2
• Or Pu  [(c  c )  ( w  w )]
1 2 2 1
 Wheel efficiency and optimum
ratio of velocity
• The ratio of wheel power Pu and ideal energy in this stage
E0 :
*
Pu 1  hu  h t  (  hn   hb   hc 2 )
u   
E0 E0 E0
• Ideal energy E0：Energy able to be used in this stage
c02
E0  0  ht  1 hc 2
2
• The view of losses：
u  1   n  b  (1  1 ) c 2
56
57
Fine and coarse water droplets in
wet steam
Unique waterproof corrosion structure
 stage efficiency

hi ht  ( hn  hb  hc 2   h) hu   h 

*
h
i     u 
E0 E0 E0 E0
1
 [ ht*  ( hn  hb  hc 2  hl  h  h f  he  h  hx )]
E0
c 02 c 22
E 0   ht  1 hc 2   ht   0
*
 1
2 2
stage efficiency is the final measure of the degree of perfection within the stage energy
transition. h*  (h  h  h )
 hu t n b c2
u  
E0 E0
Note: The wheel efficiency is greater than the relative internal efficiency
Stage power
Dhi Dhi
Pi 
3600
Pi  Pi  Dhi
3 .6
D-----Stage intake kg/h t/h kg/s
hi -----The effective enthalpy drop kJ/kg
Working process of multi-stage steam turbine

The basic characteristics of multi-stage steam turbine

operation process

l Multi-stage steam turbine is impulsive and reactionary.Multi-stage steam turbine usually adopts nozzle adjustment
(control steam intake), called adjustment stage, and the other stages are called pressure stage.Small and medium-sized
steam turbines, which usually use the double-column stage as the adjustment stage, and the high-power steam turbines
mostly use the single-column stage as the adjustment stage.

l When the steam expands into the steam turbine at all stages for work, the pressure and temperature decrease step by
step, and the specific volum is constantly increasing.Therefore, the through flow part size increases step by step,
especially in the low pressure part, and the average diameter increases quickly.That is, the blade height is getting longer
and longer.
l Due to the strength of the material, the blades can not be too long, so large steam turbines use multi- exhaust
ports.For example, domestic 200MW steam turbine is designed as three exhaust ports and two exhaust ports; domestic
300MW steam turbine adopts two exhaust ports . 61
Working process of multi-stage steam turbine:
The steam expansion is the same as in the stage.The work process is repeated,
but the parameters are varied.

62
 Medium pressure Low
steam inlet Pressure combined steam inlet pressure
loss pressure loss part inlet,
p0 pr' pr'' steam
p '0 pressure
loss
ht1
'
pr ps p ' Steam
ht1 s exhaust
ht 2 pressure
loss
ht1 ht' 2
pc pc'
h Reheat pipe
and, ht''2
reheater
pressure loss ht' 3
s ht 3
ht 2 htc ht 2
63
 Evaluation index of steam turbine and its devices
The production process of thermal power plant takes a series of energy conversion before finally
transforming the chemical energy of mineral fuel into electric energy.During these transformations, various
efficiencies are used to describe the perfection of the whole energy conversion process.
n steam turbine efficiency The ratio of the effective enthalpy reduction (useful
work) to the ideal enthalpy reduction
H
nPower in steam turbine D 0  H t i i  i
pi   G 0  H t i H t
3 .6
n mechanical efficiency The ratio of the output power Pe and Pi of the internal power P i
Overcome the friction resistance of the bearing in the steam turbine, and drive the main oil
pump and speed governor. P
m  e

Pi
ngenerator efficiency The ratio of generator power output Pel to steam
turbine shaft end power Pe
 g  Pe l / Pe
¨ Generator loss is mainly mechanical loss (mechanical friction and fan power consumption) and
electrical loss (excitation power consumption, iron loss, copper loss). 64
 Evaluation index of steam turbine and its device
Relative electric efficiency of steam turbine generator unit

The part converted into electric energy in the 1kg vapor ideal ratio enthalpy reduction.

p el  G 0  H t el  e l   i m  g
n of steam turbine generator unit

The ratio of the part converted into electric energy in the 1kg steam ideal ratio enthalpy
drop to the heat added to 1kg steam throughout the thermodynamic cycle.

 H t el H t
 a , el  '
  t i m  g t 
h0  hc h 0  h c'

65
 Evaluation index of steam turbine and its devices
nsteam consumption rate
n The amount of steam consumed by the unit for generating 1KW · h power
n The steam consumption rate does not fully indicate the advantages and disadvantages of
the unit economy.The steam consumption rate of the return pumping units is greater than
the non-heat pumping units, but the circulation efficiency of the former is higher than the
latter. d=1000D0/Pel=3600/(Δhtηel)

nFor the steam turbine set with the same power, although the same power, the steam
consumption is different due to the different initial and final parameters of the
steam.
nThen steam consumption rate d should not be used to compare the economy of different
types of units, but another index reflecting the unit economy is the thermal
consumption rate q.
nheat consumption rate
n The heat consumed by the unit at 1KW · h

n For no intermediate reheat unit,

n q  d ( h 0  h c )  3 6 0 0 ( h 0  h c ) / (  H t e l )  3 6 0 0 /  a , e l
For the intermediate reheat unit,
66
Dr
q  d [ ( h 0  h c )  D0 ( h r  h r ) ]
Axial thrust of multi-stage steam turbines
When the steam expands through the flow of the steam turbine, the
force on the blade is composed of circular and axial force.Among
them, the circular dividing force pushes the impeller to work, while
the axial dividing force produces an axial thrust on the rotor.
In general, the axial thrust acting on
an impulse stage is composed of 3
parts:

1、Axial force acting on the

moving blade F z1

2、Axial force acting on the impeller surface Fz 2

3、Axial force acting on the convex shoulders of the spindle 67

Fz 3
 Balance method of axial thrust
In multistage steam turbines, the overall axial thrust is large.In particular, reactionary steam
turbines, with total axial thrust of 200 to 300 T and impulse turbines, with total axial thrust of up to
.Such a large axial thrust is unbearable to the thrust bearing.Therefore, attempts must be
made to reduce the total axial thrust to meet the energy bearing capacity of the thrust bearing.That is,
the total axial thrust of the steam turbine should be balanced
1.Set the balance piston

2.The rotor is designed in a rotary drum

form
Common axial thrust
balance methods 3.Open the balance hole on the impeller

4.Cylinder symmetric layout method

68
5.Cylinder symmetric layout method
 1. Balance piston method
A tooth shaft seal is mounted on the balance
px p1
piston, and when steam flows from the high
pressure side to the piston, the pressure drops
from p1 to px.The equilibrium piston, under the
pressure difference, produces a left-facing
Fz
force.This force is exactly opposite to the Fz
direction and acts in equilibrium.
2. The rotor is designed as a rotating
drum form: suitable for the reactionary
steam turbine
Reaction steam turbine, at all levels of the reactionary degree is greater

The pressure difference on both sides of the moving blade is very large

The rotor is designed to turn the drum form to reduce the axial thrust generated on each stage of impeller

69
 3. balance hole on the impeller

Suitable for impulse steam turbines

balance hole
p2
balance hole on the impeller reduces the pressure pd
difference on both sides of the impeller, thus
reducing the axial force acting on the
impeller.Usually open 5 to 7 balance holes on the
impeller

70
 4.Cycylinder symmetric arrangement
the most effective way to balance axial thrust for large multi-cylinder turbines

Multi-cylinder reverse (two cylinder symmetrical arrangement) arrangement, so

that the steam flow for reverse flow in different cylinders, its axial force in the
opposite direction, to achieve the purpose of balance.The following diagram shows
a schematic diagram of the multi-cylinder reverse layout.Domestic 125MW,
200MW and 300MW steam turbines all adopt the multi-cylinder reverse
arrangement method to balance the axial force.
5.Use of thrust bearings In order to
ensure the stability of the steam turbine generator
rotor position when the operating conditions of the
steam turbine change, and to achieve the purpose of
stable operation of the unit.
After the above measures are taken to balance most
of the axial thrust, the thrust bearing is used to high intermed low
assume the rest of the axial thrust.。 pressure iate pressure
The axial thrust borne by the thrust bearing is: cylinder pressure cylinder
cylinder
Fb  Fz  F
71
Steam Turbine Foundation of Frame