THEORETICAL ANALYSIS OF THE LAST STAGE LP STEAM TURBINE …

THEORETICAL ANALYSIS OF THE

LAST STAGE LP STEAM TURBINE

BLADES REPAIR BY MEANS OF

WELDING ALICIA RICO PALACIOS

Curse 2017/2018

FINAL PROJECT PROMOTED BY DR. IN. ANNA REHMUS-FORCE The State University of Applied Sciences (PWSZ)

Final Project Alicia Rico Palacios PWSZ

1

INDEX

1. CONCEPT OF STEAM TURBINE .......................................................................................... 2

2. CLASSIFICATION................................................................................................................. 3

3. HISTORY OF THE STEAM TURBINE .................................................................................... 6

4. PARTS OF THE STEAM TURBINE ........................................................................................ 8

5. OPERATION OF THE STEAM TURBINE ............................................................................. 12

5.1. Rankine cycle ........................................................................................................... 12

5.2. Arrangement of the blades ..................................................................................... 12

5.3. Velocity triangles ..................................................................................................... 13

6. WORKING CONDITIONS................................................................................................... 15

6.1. Working conditions of last stage ............................................................................. 15

7. LOW PRESSURE STEAM TURBINE .................................................................................... 17

7.1. Blades of the low pressure turbine ......................................................................... 17

7.2. Last stage of the blades of the low pressure turbine .............................................. 18

7.3. Baumann stage ........................................................................................................ 19

8. MATERIAL OF THE BLADES .............................................................................................. 21

8.1. Stainless steels. Titanium alloys and 12%Cr ............................................................ 21

9. DAMAGE PRODUCED IN THE MATERIAL ......................................................................... 26

9.1. Erosion in the blades ............................................................................................... 27

10. REPAIR OF MATERIAL DAMAGE ...................................................................................... 30

10.1. Welding Processes ................................................................................................... 30

10.1.1. Welding Concepts ............................................................................................... 30

10.2. Laser Welding .......................................................................................................... 32

10.3. Electric arc welding ................................................................................................. 35

10.3.1. TIG Welding ......................................................................................................... 36

10.3.2. MIG/MAG Welding .............................................................................................. 40

10.4. Post Welding Processes ........................................................................................... 45

11. TURBINE MAINTENANCE ................................................................................................. 46

12. SUMMARY ....................................................................................................................... 47

13. CONCLUSIONS ................................................................................................................. 48

14. BIBLIOGRAPHY ................................................................................................................. 49


2

1. CONCEPT OF STEAM TURBINE

The steam turbine is a turbomachine, this means that it is a machine that works continuously,

whose main element is a rotor, or moving part intended to perform the rotation. In this type of

equipment the fluid transforms its amount of movement by the action of the machine.

The steam turbine is an equipment responsible for transforming the energy from a steam flow

into mechanical energy (Figure 1).

Figure 1. Steam turbine

It is a thermal machine, because it performs the work of rotation through the heat of the steam,

in which the fluid used is a compressible fluid, whose specific volume changes significantly as

the process advances. This thermal machine is external combustion, since the steam generated

by combustion is produced outside the turbine. And, it is a motor machine that transforms the

kinetic energy of the steam into energy of rotation, the steam being initially at a very high

pressure and as it transforms into a rotation movement it loses pressure.

This equipment must comply basic conditions for proper operation: a suitable working fluid, a

source that provides a high energy degree and a sink for low-grade energy.

By this way, it can try to achieve three basic principles for the steam turbine: maximum

efficiency, by means of a plant of maximum reliability at a minimum price. In addition, these

devices try to fulfill other objectives: minimum supervision and minimum startup time.


3

2. CLASSIFICATION

The steam turbine can be of different types, depending on several characteristics. The steam

turbine used is classified as: a multi-stage steam turbine, it is a reaction turbine, a condensing

turbine and an axial turbine. The following describes the different types of steam turbines

possible, to understand the previous classification:

- Depending on the number of stages, it can be a single-stage turbine or a multi-stage

turbine. In single-stage turbines the machine consists of a single device, a very robust

body and does not reach high powers. On the other hand, multistage turbines are

formed by several stages of turbines, which reach high power and can withstand very

high pressures. Its efficiency is much higher.

Figure 2. Single stage steam turbine

Figure 3. Body of a turbine formed by several stages


4

- Depending on the way to take advantage of the energy, it can be an impulse turbine or

a reaction turbine. The turbine of impulse develops the process of expansion of the

steam in the static part of the turbine, in the nozzles. However, in the turbine under

study, in the reaction turbines, steam expansion takes place both in the nozzles and in

the mobile part of the turbine, in the rotor blades.

Figure 4. Scheme of operation by impulse or reaction turbines

- Depending on the steam outlet pressure, the types of turbines are backpressure

turbines or condensation turbines. Backpressure steam turbines eject steam at a

pressure greater than atmospheric pressure. In the condensation turbines, the steam

comes out at a lower pressure than the atmospheric one and the energy use is greater

than in the other type mentioned above. In these turbines the vapor contains drops of

moisture, since it appears as condensed vapor in a small percentage. This case will be

studied later.

Figure 5. Section of a condensing turbine, in which the output section is very wild


5

Figure 6. Section of a backpressure turbine, in which the output section is smaller

- Finally, they can also be presented as axial or radial steam turbines, depending on the

path followed by the steam flow inside. The axial turbines use steam that flows in a

direction parallel to the axis of the turbine and the radial turbines carry the steam in a

plane perpendicular to the axis.

Figure 7. Axial flow turbine

Figure 8. Radial flow turbine


6

3. HISTORY OF THE STEAM TURBINE

It is not until 1884 when the steam turbine with industrial applications was invented.

A first contact with this type of machines arose in the second century BC, built by Heron of

Alexandria. Its purpose was simply to entertain. This engineer and mathematician invented a

mechanism in which the steam to rotate the system by the force of expulsion, leaving by tubes

connected to the ends of the axis of a sphere that served as an air chamber.

Figure 9. Machine invented by Herón de Alejandría

The next contact with this type of machine consists of the year 1629, when Giovanni Brance

turned a water wheel through a jet of steam. This turn was not strong enough so it had no useful

applications.

Figure 10. Machine invented by Giovanni Brance


7

In 1878 the technological development of steam turbines began, as the Swedish Gustav de Laval

patented a centrifuge skimmer that separated the milk from the butter and operated by the

mechanism of a steam turbine.

Figure 11. Centrifuge skimmer

Finally, in 1884 Charles Parsons realized the design and construction of a steam turbine of

reaction that would be used mainly in the boats that required a lot of power since they were of

great tonnage. This turbine revolutionized the world of the industry, because it managed to

reach a high speed, up to 18000 rev/min.

Figure 12. Design of an old turbine of steam

Simultaneously, Charles G. Curtis invented in the United States the turbine of impulse, which

would serve for the electricity industry. Since then, the development of steam turbine has

evolved to the present.


8

4. PARTS OF THE STEAM TURBINE

The steam turbine to be studied consists of three different turbines, depending on the force

with which the steam hits, the high (HP), medium or intermediate (IP) and low pressure (LP)

turbine. The purpose of adding different phases is to decrease the velocity that the machine has

to support, otherwise it would not be mechanically viable for high pressures and powers. The

steam expands as it moves through the different stages of the turbine.

Figure 13. The different stages of the steam turbine. High, medium and low pressure

In general, for the steam turbine the following parts can be differentiated, later we will stop in

the study of the low pressure turbine.

The fundamental parts of the turbine are the rotor, the blades, the nozzles and the stator.

- The rotor is the mobile element that consists of a solid steel shaft of the turbine.

Through which the turning movement is carried out and to which the blades are

attached.

It transmits the rotation to the alternator, in the case of producing electricity.

Figure 14. Rotor of a turbine


9

- The blades of the steam turbine are small metal structures that are placed around the

rotor and transmit its movement. These curved blades are arranged perpendicular to

the axis and in which the steam, when put in contact with these parts, rotates them.

These structures are of great importance, are responsible for producing the rotation in

the rotor to generate power. They reach very high velocities. This part of the steam

turbine will be studied later.

Figure 15. Blades coupled to the rotor

Figure 16. Blades of a turbine

- The stator is the fixed part of the turbine. It forms part of the external structure of the

turbine and protects the internal system. It is formed by a lower part, joined to the

bench and a removable upper part, to have the possibility of manipulating the rotor. A

series of blade crowns are also fixed to the stator.

Figure 17. Stator of a turbine


10

- The nozzles are part of the outer casing, consisting of tubes that are responsible for

passing the steam, distributing it correctly, decreasing the pressure and increasing the

velocity.

It is the element through which the steam passes to the first stages of the high pressure

turbine. The steam expands and are designed so that no shocks or turbulence occur.

Figure 18. Nozzle

There are other parts of the turbine, which do not perform the main functions to produce the

desired power, but are indispensable for optimal operation and serve as a help to the main parts

described above. These are elements such as:

- The sealing, responsible for covering the mechanism to prevent leaks.

- The diaphragms: elements to which the fixed parts of the turbine are sealed. These discs

are attached to the stator blade crowns.

Figure 19. Diaphragm of a steam turbine

- The valves are control and protection elements. They are responsible for supervising the

flow that circulates through the turbine. They control the velocities and work in case of

excessive working conditions.


11

Figure 20. Valve of a turbine

- The bearings prevent friction that can occur in parts such as the rotor. They support the

main parts, support the weight of the rotor and are used as guides for movement.

Figure 21. Bearing


12

5. OPERATION OF THE STEAM TURBINE

5.1. Rankine cycle

To understand the operation of the steam turbine, it must be studied as an element that is part,

as the most common application, of a cycle, in which it is an integrated part. It resembles a

Rankine cycle. In this cycle the working fluid, which is water, passes through a pump, which

performs the reverse work to the turbine, raises the fluid pressure and brings the liquid to the

boiler. The boiler heats the fluid, which is at a very high pressure and enters as liquid water and

comes out as steam, this process is carried out at constant pressure. It is then, when the steam

at high pressure expands in the turbine, thus producing mechanical movement and rotating the

shaft. The work of the turbine is equal to the difference of entrance and exit enthalpies. The

steam that leaves the turbine, at a lower pressure than the input, is directed to a condenser,

which is responsible for cooling the steam and convert it back into liquid water. Finally, the pump

brings the condensed steam back to the boiler.

Figure 22. Rankine cycle

Focusing on the steam turbine, within this cycle, steam at high pressure enters the turbine

through the nozzle. This distributes the steam to the blades, where the steam hits them at high

velocity and turns them, transmitting the rotation to the rotor.

5.2. Arrangement of the blades

The blades are distributed in pairs between the fixed blades and the mobile blades, so that the

steam passes through each pair of curved propellers and expands. These pairs of blades formed

first by a blade of the stator and then by a blade of the rotor is called step or stage. The number


13

of stages depends on the power required by the turbine. The greater the number of stages the

greater the power of the steam turbine.

Figure 23. Scheme of the mobile and fixed parts

The expansion in the blades leads to an increase in volume so that as the process progresses the

blades are larger.

The stator blades are responsible for transforming the thermal energy into kinetic energy, and

the kinetic energy is converted into mechanical movement by the rotor blades. The step section

of the stator forms a converging duct, its passage section decreasing each time, thus increasing

the kinetic energy. In the reaction turbine, the passage section of the rotor blades is also

decreasing, and its output section is smaller than the input section. Therefore, the relative

velocity is greater at the exit than at the entrance and the pressure decreases. This is achieved

by modifying the draft angle.

5.3. Velocity triangles

To represent the velocity change suffered by the steam, the velocity triangles are used (Fig 24),

in which the subscript "1" refers to the input conditions and the subscript "2" refers to the

output conditions. Each of these triangles satisfies the equation:

𝑐 = �⃗⃗� + �⃗⃗⃗�

Where “𝑐” is the vector of the absolute velocity of the fluid, “�⃗⃗�” is the circumferential velocity

of the rotor or tangential velocity and “�⃗⃗⃗�” is the relative speed of the fluid with respect to the

rotor. The angle “α” indicates the direction of the absolute inlet and outlet velocities of the fluid

and the angle “β” indicates the direction of the relative inlet and outlet velocities.

Regarding the stator, it is not important to talk about relative velocities, because the blades do

not rotate. In this area of the turbine the absolute velocity will be greater at the exit due to the

increase of the kinetic energy. However, in the rotor, the study of the velocity triangle is

important. In this triangle it is shown that the relative velocity in the output is greater than the

relative velocity in the input and it can also be observed that the absolute velocity at the output


14

is lower than at the input, this is explained because the kinetic energy of the steam becomes

mechanical energy.

In addition, the tangential velocity of the rotor is: �⃗⃗� = �⃗⃗⃗� · 𝑟, where "r" is the distance from the

axis of rotation. Since this velocity depends on the distance, the velocity will be higher at the tip

of the blade than at the root. Thus, the last stages, which reach high velocities, will have longer

lengths and will be determined to twist them.

Figure 24. Triangle of velocities at the input and output of a stage


15

6. WORKING CONDITIONS

There are three types of characteristic turbines, with different working conditions each:

- Supercritical initial steam conditions with steam reheat: 24MPa, 540ºC/540ºC. This type

of turbines is often used in fossil fuel plants. Although these plants also use turbines

with more moderate and subcritical steam pressure: 17-20 MPa. The exhaust pressure

is 4 kPa. The sum of available energy is 1800 kJ/kg.

A variant of the first mentioned turbines are the turbines with ultra-supercritical steam

pressure (USC): 30 MPa. With double stage steam reheating and steam temperatures of

up to 593-600ºC.

- Superheated subcritical steam without reheating: 13MPa, 565ºC. These turbines are

used in fossil fuel plants, in certain zones to which they are destined. The exhaust

pressure is 25kPa. The steam enthalpy drop is 1180 kJ/kg.

- Wet steam with intermediate steam separation and reheating by the same main steam:

65MPa and 5% humidity. The turbines with these working conditions work in nuclear

power plants. The exhaust pressure is 4kPa. The sum of the available energy is 1300

kJ/kg.

The circular velocity in the mean diameter of the turbine is between 200 and 400 m/s. Then the

maximum enthalpy drop in each stage is, for the high and intermediate pressure turbines 100

kJ/kg and for the low pressure turbines 200 kJ/kg. The drops of enthalpy of each stage are much

smaller than the total since each stage exploits a part of the total available, thus achieving a very

high efficiency.

6.1. Working conditions of last stage

For the working conditions of the last stage:

Assuming the upper admissible stress of 450 MPa, it is possible to achieve blades of the last

stage with a design of an available maximum exit annular area of 8.6 m2 for a frequency of 50

Hz and 6.0 m2 for a frequency of 60 Hz. The length of these blades can be 1010 mm for n=3000

rev/min and 841 mm for n=3600 rev/min.

For turbines with different operating modes different working conditions result:

- A turbine with supercritical main steam conditions and single stage steam reheating

(23.5 MPa; 540/560 ºC) that rotates at 3000 rev/min. With a pressure inside the

condenser of 3.4 kPa and a loss of exit velocity of 45 kJ/kg, the efficiency of the turbine

in this stage is 150 MW. However, for a turbine that rotates at 3600 rev/min, the

efficiency decreases to 105 MW. Therefore, for two-cylinder LP turbines with double


16

flow, the efficiency would not be higher than 600 MW for the first case and 420 MW for

the second case.

- A USC turbine with double stage steam reheating (29.4 MPa; 650/565/565 ºC) increases

the efficiency value shown in the previous case 1.1 times.

- Turbines are manufactured with titanium alloy materials whose bucket length reaches

1200 mm and an annular exit area of 11.31 m2. Its efficiency is 1200 MW for supercritical

pressures and 1000 MW for wet steam. These are TC high-speed turbines (tandem-

compound).

- It has been able to develop titanium alloy turbines of 1500 mm in length with an annular

exit area of 17.9m2 for 3000 rev/min.

- Japanese companies manufacture titanium alloy turbines 1016 mm long with an annular

exit area of 8.7 m2. For TC turbines of 1000 MW with a speed of 3000 and 3600 rev/min.

- The longest steel blades are manufactured by Turboatom, Alsthom, KWU. With lengths

of 1450-1500 mm and annular exit area of up to 20.3 m2, for rotor speeds of 1500

rev/min.

- The companies ABB, GE, Hitachi, Westinghouse have managed to develop blades with a

length of 1320-1270 mm for LSB, with an annular exit area of 16.46 m2 and with a speed

of 1800 rev/min.

Many of the turbines designed do not reach such high lengths for the blades of the last stage of

the low pressure turbine. Because the longer the blade, the design of the turbine is more

complex and the aerodynamic conditions worse. It increases the erosion that occurs in the

material because of the humidity and the vibration problems also increase.


17

7. LOW PRESSURE STEAM TURBINE

All stages of the turbine are important. The enthalpy drop is distributed by the different stages,

treating a part of this enthalpy drop each of the stages and exploiting only a part of the total

available energy, thus achieving an increase in efficiency.

The high pressure stage has great importance because it generates around 90% of the total work

that is generated in the multistage steam turbine. But the low pressure stage is also very

important and is related to the efficiency of the turbine, since the turbine's output area is the

main source of energy losses.

7.1. Blades of the low pressure turbine

The blades of the low pressure turbine, as mentioned above, have a longer length than in the

previous stages, and their length increases as the flow progresses through the different stages

(Fig 25). This design decision is explained because the density of the fluid as it advances is smaller

due to the expansion, so that the specific volume is greater and occupies more space the steam

used. This leads to increase the height of the blade according to the expansion. In addition these

stages suffer the effects of moisture from the steam.

In the last blade crowns the blades have a twisted design. Since the velocity at the tip is greater

than at the root, the velocity triangles are significantly different in the later stages (Fig 26).

Figure 25. (a) Last stages of a large high-speed steam turbine and (b) LSB with its profiles at tip, mean, and root diameters


18

Figure 26. LP last stage velocity profiles

7.2. Last stage of the blades of the low pressure turbine

Currently, it is possible to design blades of the last stage of the low pressure turbine with a length

of 940 mm, in turbines of 3000 rev/min. The design of these blades has been perfected to

achieve ever greater lengths, with a constant traction effort in practically the whole length.

Nowadays the cross section is no longer constant, and it is reduced exponentially with the

square of the radius.

The blades used have a twisted helix design, with a root with a robust low reaction section and

a tip with a high and thin reaction section (Fig 27). This is explained by the fact that there is an

increase in the pressure drop in the mobile knives, it must be compensated with a decrease in

the pressure drop in the fixed blades.

The sizing parameters of the blade explain this decision to twist the blades:

- The diameter at the tip of the blade is twice the diameter at the root. Thus, the

circumferential distance between adjacent blades is 1.5 times greater than the pitch at

the base.


19

- The peripheral velocity is 1.5 times higher at the tip than at the base and this causes the

direction of steam flow to change with respect to the moving blades.

- This change of the input velocity entails an alignment of the input angle with the

direction of the steam. The section of the moving blade is changed and the exit angle is

reduced.

Figure 27. Final blade envelopes

7.3. Baumann stage

There is a design, Baumann stage, that manages to increase the production of the turbine

without having to vary the length of the blades of the last stages. This design consists of making

partitions in the blades of the penultimate stage (Fig 28), dividing them into upper and lower

zones. By means of this form of design, the fluid in the upper part goes directly to the condenser

and the enthalpy doubles and in the lower part the fluid goes to the last stage.

It is also called "one-and-a-half", since the amount of flow in the outlet from the top of the

division is half the amount of flow from the last stage.


20

Figure 28. LP steam path with the penultimate two-tier stage (Baumann stage)

This design is complex and is not used because it has considerable disadvantages. In addition to

the manufacturing and machining difficulties, it also produces high vibrations in the machine

and losses and steam leaks in the turbine that are not admissible.


21

8. MATERIAL OF THE BLADES

The material used for the steam turbine blades is an important decision. This choice is based on

three basic factors: the temperature at which it operates, the effects of water vapor and the

stress to which they are subjected. Considering these factors, different materials will be used

according to the stage in which we are, HP, IP or LP, because the first stages operate at high

temperatures and moderate tension, the intermediate stages operate at moderate temperature

and tension; and the last stages operate at low temperature, undergo a stable and changing

tension and are affected by the action of wet steam.

The material chosen is stainless steel with a high chromium content. In the blades to be studied,

from the last stage of the low pressure turbine, materials of titanium alloys WT-3 or WT-5 and

also stainless steel 12% Cr AISI 403 are applied. These materials are chosen keeping in mind, as

it has been seen, the extreme conditions of load and corrosion, in addition to the working

temperature. Therefore, it is required that the chosen material satisfies a series of operating

conditions: high plasticity at the time of distributing the tension; low sensitivity to withstand

stress concentrations; high resistance to the temperature at which it operates; high resistance

to both corrosion and erosion; guarantee of uniform mechanical properties; appropriate

technological properties; and low cost per unit of material.

8.1. Stainless steels. Titanium alloys and 12%Cr

The components studied are made of stainless steel. This material is mainly formed by iron, to

which a carbon content is added and chromium is the element that gives it the property of

stainless, with a minimum amount of 10 to 12%.

Figure 29. Stainless steel plates


22

The fundamental characteristic of stainless steel is its resistance to corrosion, iron in the absence

of chromium would corrode. Chromium has great affinity with oxygen and react with each other,

so that a passive layer, of nanometric thickness, is created that protects the surface of the

material. This layer means that oxygen can not continue to penetrate the material.

Figure 30. Formation of the passivating layer

The use of a stainless steel with a content of 12% Cr, means that it is a martensitic stainless steel.

This material is used due to its properties, it confers resistance to corrosion, mechanical

resistance, hardness and magnetic properties, these properties are detailed in the “tables 8 and

9".

Martensitic stainless steel possesses martensite in its metallographic structure. This structure is

achieved by austenizing the steel, followed by a hardening and tempering treatment.

Another material used for the blades of the last stages are the titanium alloys. It has ideal

properties for the proper functioning of the turbine. It is a material of high resistance to

corrosion, in addition to high resistance to fatigue, so that it supports the changes of power and

high resistance to temperature. In addition, titanium is an element with a much lower density

than steel, so that the blades made with this material will be lighter.


23

Properties of the materials used:

TITANIUM ALLOY WT3:

Chemical constitution

Al=4-6.2% Cr=2-3% C≤0.1% Fe≤0.8% N2≤0.05% O2≤0.2% H2≤0.015% Remainder Ti

Table 1. Chemical constitution of titanium alloy WT3

Physical properties

Q=4460 kg/m3

αm=8.4·10-6

v=0.3

Table 2. Physical properties of titanium alloy WT3

Mechanical properties

Mechanical characteristic

20ºC 100ºC 200ºC 400ºC 500ºC

R0.2 [MPa] 835-955 745 610 490 400

Rm [Mpa] 930-1130 805-930 675-815 560-695 530-600

δ5 [%] 10-16 10 10 12 14

ψ [%] 25-40 - - - -

KM [MJ/m2] 3-6 - - - -

E·10-4 [MPa] 10.8 10.3 9.7 8.3 7.36

Z-1b [MPa] 440 - - 343 -

Table 3. Mechanical properties of titanium alloy WT3

Limit of sustained strength

R100= 590 MPa for 300ºC

550 MPa for 400ºC

340-390 MPa for 500ºC

Table 4. Limit of sustained strength of titanium alloy WT3


24

TITANIUM ALLOY WT5


Al=4-5.5%

Sn=2-3%

C≤0.1%

Fe≤0.03%

N2≤0.5%

O2≤0.02%

H2≤0.015%

Remainder Ti

Table 5. Chemical constitution of titanium alloy WT5

Physical propierties

Q=4500 kg/m3

αm=8·10-6

v=0.3

Table 6. Physical properties of titanium alloy WT5



20ºC 100ºC 200ºC 400ºC

R0.2 [MPa] 690-830 510-690 410-540 360-430

Rm [Mpa] 735-930 650-820 540-670 440-510

δ5 [%] ≥10 15 12-23 9-20

ψ [%] ≥25 - - -

KM [MJ/m2] ≥4 - - -

E·10-4 [MPa] 10.8 9.81 9.28 7.65

Z-1b [MPa] - - - 230

Table 7. Mechanical properties of titanium alloy WT5


25

STAINLESS STEEL AISI 403:


C=0.012%

Si=0.15%

Mn=0.41%

P=0.017%

S=0.015%

Cr=11.9%

Ni=0.52%

Mo=0.11%

Remainder Fe

Table 8. Chemical constitution of stainless steel AISI 403



20ºC 80ºC

R0.2 [MPa] 645-672 641-658

Rm [MPa] 759-786 736-754

δ5 [%] 20-20.5 17.6-19.0

ψ [%] 63.3-67.1 64.0-67.5

Table 9. Mechanical properties of stainless steel AISI 403

As shown in the tables (all of them were taken from the book “Dynamics of Steam Turbine Rotor

Blading”, authors: Janecki, S. et Krawczuk, M., 1998) in any of the materials chosen for blades

the main component is chromium. Chromium is the essential element for the formation of the

passivation layer. In addition, other alloys are used, which are responsible for maintaining the

formed film or reconstructing it if it had been destroyed and confer greater resistance to

corrosion.

These materials have excellent mechanical resistance and give the possibility of being welded at

high temperatures because of their resistance to creep and tension.

The material AISI 403 stainless steel is highly recommended for these critical parts, because as

seen in their properties, can withstand great stress and resist heat and erosion wear.


26

9. DAMAGE PRODUCED IN THE MATERIAL

The problem that affects the blades of the steam turbine is erosion. This harmful effect occurs

in the last stage of the low pressure steam turbine, since it is the stage of the turbine in which

moisture appears. It is the main cause of the damages and failures produced. This phenomenon

occurs due to the continuous impact of condensed vapor droplets, which cause the wastage of

the material and leads to erosion.

In these thermal machines steam flows which can damage the blades in two ways. On the one

hand the tear of the material due to the continuous impact of contaminated condensate vapor

drops. The contamination is due to the presence of solid particles of materials such as

phosphorus, silica or sulfur or oxide particles that come from other elements of the cycle, from

the boiler.

The factors that determine the degree of erosion are the material used in the blades and the

properties of the particles transported by the steam. In addition to the design parameters of the

blades that will be mentioned later.

These contaminated particles, coming from previous phases, cause loss of material and rips of

the blade (Fig 31), being able to reach the rupture (Fig 32).

Figure 31. Damage in areas of low pressure turbine


27

Figure 32. Rupture of one of the blades of the LPT

9.1. Erosion in the blades

In addition the erosion is produced by the simple impact of the drops of steam in the blades of

the last stages, this impact also causes a significant loss of power. This unwanted activity causes

the following negative consequences: efficiency decreases, so power generation is lower; entails

maintenance and repair costs; it causes security problems.

The formation of the vapor drops occurs in the low pressure stage, because the fluid expands

and the temperature of the steam in which the turbine operates is lower than the saturation

temperature. As a part of the steam that circulates in the last stages is condensed and contains

drops of liquid water, at first with a minimum diameter. In previous stages the drops present in

the vapor have an almost negligible diameter, and the humidity is not significant. However in

the last stages this humidity reaches undesired percentages.

This moisture forms a film of water on the stator blades. At the exit of the stator the film breaks

and the steam drops, now larger, travel to the rotor blades moving at high speed and generates

a centrifugal force that makes the drops hit with great intensity in the blades (Fig 33).


28

Figure 33. Diagram of the presence of drops in a stator-rotor step of the low pressure turbine

The area of the tip of the blades suffers more damage, because the drops are larger as we move

along the length of the blade and impact faster than at the root.

The operating parameters that intervene in erosion have to be studied, because they are the

way to prevent this effect. These parameters are:

- The speed of the flow of the drops of steam.

- The angle of the flow.

- The diameter of the drop.

- The percentage of humidity.

To increase the useful life of the machine studied, this erosive phenomenon must be reduced

by taking measures such as reducing the impact, modifying the trajectory of the particles hitting

the material of the blades and modifying the angle of impact of the drops with the damaged

surface.


29

Below are the images taken at GE Power, which show damage in the blades.

Figure 34. Damage produced in the steam turbine blades

Figure 35. Detailed image of a damaged blade

This damage has to be repaired as indicated in the next part of the project.


30

10. REPAIR OF MATERIAL DAMAGE

Since erosion causes significant consequences for the operation of the turbine, these damages

have to be repaired. The method chosen for the repair of the material is welding.

10.1. Welding Processes

In all the processes that are going to be explained next, the welding action takes place, which

consists of joining the piece of manufacture, guaranteeing continuity in the material.

Welding is a complex procedure that requires a study of the material to be welded, and some

stages before and after welding. The preheating and the postheating.

To repair the blades of the last stages of the low pressure turbine, we resort to different welding

methods:

- Laser Welding

- TIG Welding

- MIG/MAG Welding

These types of welding are part of the fusion welding processes, this means that the operating

temperature through which the union of the material occurs is greater than the temperature of

the liquid phase of the base metal and the filler metal (if it was necessary). As a result of this

high localized temperature, the edges of the material to be joined present a liquid or molten

phase and when cooled it is reconstructed on the metal that has not been melted, forming a

solid structure.

10.1.1. Welding Concepts

For the realization of the welding process it is necessary to have clear concepts such as

preheating, postheating and the heat affected zone, which are explained below. These concepts

influence the final result of the piece and the quality of the welding, so they are very important.

Heat Affected Zone

When the welding is done, the material has different zones, since the heat is located in the

operation zone to carry out the fusion and a thermal heterogeneity takes place in the material.

The melted zone, the fusion boundary, the heat affected zone and the unaffected base metal

are distinguished.


31

The Heat Affected Zone (HAZ) is an area in which special care must be taken. This zone is located

on both sides of the molten material. The temperature is so high that structural changes occur

in the base metal, so much attention is paid to the cooling velocities.

Preheating and postheating

To define the concepts of preheating and postheating, one must first understand the thermal

cycle of welding. This cycle, shown in Figure 36, represents the variation of the temperature at

a determined place in the weld as a function of time. Being “Tmax” the maximum temperature

reached, and obtaining from it the cooling velocity, which is the descending slope of the curve

and the heating velocity the ascending slope of the curve from the initial temperature.

Figure 36. Thermal cycle of welding

Preheating and postheating are methods that reduce the cooling velocity (Fig 37).

Figure 37. Thermal cycle of welding with and without preheating


32

Preheating consists of applying heat to the material before welding. With preheating, the heat

affected zone is wider. This method avoids the formation of a brittle material and the

appearance of hydrogen in diffusion, so as not to form cold cracks.

Post-heating consists of the heat application process once the weld has been formed. With this

method, residual stresses are relieved and more stable microstructures are achieved.

10.2. Laser Welding

Fundament of the process

Laser welding uses light particles, which are photons, which when excited by the intensity of the

current emit the energy that is expressed in the form of light. The laser ray is formed, since the

energy is concentrated in a beam. This explanation gives the name to the “laser” term: "Light

Amplification by Stimulated Emission of Radiation".

This process needs a high energy density and uses a light source. Laser beam welding uses a

high-power laser beam as a heat source and produces fusion welding, which consists of melting

and recrystallizing the material to be welded. The work area is heated and pressure is exerted

between the points.

It is a very precise method, because it uses mirrors to focus the energy of the laser beam, it

focuses on a point with a diameter of a few millimeters, which provides high energy density.

When the material reaches the melting temperature, plasma formation occurs, resulting from

the ionization of the mixture between the vaporized material and the protective gas. The basic

operation form is detailed in "Figure 38".

Figure 38. Laser welding (Source: Unpublished form material Plasma System S.A.)


33

The process does not need filler material. However, a protective gas is needed, which protects

the mirrors from splashing and protects the material from reacting with the air. The most

commonly used are helium or argon. These gases also serve to form a certain porosity and thus

let escape the oxygen bubbles that are formed in the liquid phase of the process.

Laser beam welding is used to weld stainless steels and titanium alloys, making it ideal for the

blades of the turbine under study.

Although other welding processes will be described below as an alternative to laser beam

welding (Fig 39), this is the most appropriate. The thermal input is much more precise since it

generates high energy density and the thermally affected area is much smaller, so for crack

repair it is a highly advisable technique. The main reason why this type of welding is used is that

when repairing the material it gives it a high resistance to wear and erosion.

Figure 39. Laser beam welding

Equipment used

The equipment used to repair the blades is a resonator that withstands high temperatures, uses

a wavelength of 10.6 μm, reaches a power of 2000 to 20,000 W and treats surfaces of two-

dimensional and three-dimensional pieces.


34

Figure 40. CO2 Laser, from TRUMPF

The basic working system of a laser welding equipment is based on: the laser source, a x/y

motorized cross table, the gas supply, a microscope, a mechanical arm and a base that acts as a

support.

Advantages of the method

Laser welding is the most efficient method to repair the blades of the last stage of the low

pressure steam turbine, since it has much more attractive advantages than other processes.

These advantages are the following:

- Achieves good qualities of the material. It presents fewer defects, such as deformation,

than other processes, due to high precision.

- It is an automated process.

- It does not produce noise or dirt, it is clean.

- The tool used does not wear out, since there is no direct contact with the piece to be

welded.

- There are no costs related to the filler material, because its use is not usually necessary.

- It achieves very high welding speeds. Even eight times faster than TIG welding.

- It is a highly recommended process for thin pieces.

- No subsequent processes are necessary to improve the welding.


35

As it has just been detailed, in terms of the process, it is a fast and highly accurate process. Which

does not need subsequent cleaning operations and also does not generate noise. An advantage

to highlight is its automation. In addition there is no loss of time due to changes of material of

contribution or tool. As for the welded material, the final result is a material with great qualities

and recommended for small thicknesses.

Disadvantages of the method

Laser welding has advantages that make it the most suitable welding for this repair work.

However, the disadvantages of the method must be taken into account:

- The equipment used is of high cost.

- The cords that are made by welding can not be very wide.

- It uses a lot of intensity, so it requires more control, since the high intensity could lead

to perforations in the material.

- It consumes a lot of electrical power.

- It is not applicable to materials with high reflection.

10.3. Electric arc welding

The processes that will be detailed below are electric arc welding processes. The processes by

electric arc work in the following way: the fusion takes place because an energy is generated

when putting an electrode in contact with the piece of work. The contact causes the electric arc

to jump, due to the potential difference caused when closing the circuit: both the electrode and

the piece are connected to a voltage generator. (Fig 41).

Figure 41. General scheme of electrical arc welding


36

10.3.1. TIG Welding


The welding process with tungsten electrode is called TIG welding (Tungsten Inert Gas) or also

GTAW (Gas Tungsten Arc Welding). The TIG welding strikes the electric arc between a tungsten

electrode and the piece to be welded. The tungsten electrode is not consumed and is used

through a welding gun. The fusion of the material to be welded takes place thanks to the heat

produced by the electric arc. It is not essential to use a filler material, in the case of using it, it

will be melted with the piece to be welded. This process uses a shielding gas to keep the melting

bath in the best conditions, without being contaminated. (Fig 42).

Figure 42. TIG scheme

Equipment used

The basic equipment for a TIG welding is formed by a gun that holds the tungsten electrode; a

grounding clamp attached to the piece and a wire to the current generator, which makes the

electrical circuit work; a power source, a high-frequency generator to jump the arc; and a

cylinder of inert gas, the protective gas. (Fig 43).


37

Figure 43. TIG equipment

The power source

The current generator can be direct current (DC) or alternating current (AC). The DC current is

usually used to weld stainless steels, connecting the negative pole to the electrode and

generating the highest heat in the work piece.

The welding gun

The torch or welding gun (Fig 44) is responsible for carrying the electrode. Connected to it are

the cables that feed it electrically and provide it with protective gas. These guns usually need a

method of cooling, by water or by air.

Figure 44. Welding gun


38

This gun (Fig 45) contains a nozzle that is responsible for distributing the protective gas in the

weld. It has a body for holding the torch manually and is capable of holding the electrode rigidly,

to carry out the welding correctly.

Figure 45. Parts of a torch

Tungsten electrode

The electrode is not considered a consumable, because the only thing that must be changed by

a new one is because it wears out.

The electrodes used for TIG must meet the following conditions: high melting point, low

electrical resistance, good thermal conductivity and good emission of electrons.

There are three types according to their composition: pure tungsten electrode, wolfram

electrode alloyed with thorium and tungsten electrode alloyed with zirconium. The first ones

have less cost and are usually used in alternating current and with not very high currents since

the arc is more stable. The seconds when having a thorium composition make the arc more

stable, they are used for direct current and they support very high currents. The latter have a

medium or low current intensity.

A very important parameter is the diameter of the electrode. They are usually used of 1.5, 2.5

and 3 mm.

The tip of the electrode can not be very sharp, otherwise the electric arc would be very intense.

Nor can it be worn or rounded, if so, the arch will be erratic.

Figure 46. Tungsten electrodes


39

Consumables

There are two consumables present in this process: the shielding gas and the filler material if it

is necessary.

The shielding gas

The shielding gas is responsible for protecting both the molten bath and the electrode, as well

as the contribution material from the harmful effect of the air and this gas prevents any

contamination.

The two most commonly used shielding gases for welding stainless steel and titanium are helium

and argon. Mixtures of hydrogen with helium and/or argon are also used.

Argon is an inert gas that has no interaction with other materials and its use is advisable for

sensitive materials such as stainless steel. It is used for small thicknesses because the arc is stable

and does not generate excessive amount of heat in the melting bath.

Helium is also an inert gas and provides more heat than argon. It is used for materials with high

electrical conductivity.

The filler material

In the case of using filler material, it must have properties similar to the material to be welded.

If used manually, rods are used and if it is used in an automated way, reels are used.

The filler material is placed in the area of the electric arc, lateral to the fusion bath.


This process has very suitable characteristics to be used in the blades of the turbine. It is a

appropriate method for small areas to be welded. Here are the main advantages:

- It does not produce slag or splashes.

- The process can weld a wide variety of materials.

- TIG can weld in any position.

- The welding is of great quality, smooth and regular.

- Great welding speed for small thicknesses.

- There is total control over the welding.

- The arc is stable and concentrated.


40

The TIG process is a high quality process due mainly to the great control it offers over welding.

That is why it is very convenient process taking into account the characteristics mentioned

above.


This method of welding requires conditions that make it less advantageous than other processes

such as laser welding. These drawbacks are:

- It is a complex process that requires the qualification of the welder.

- For large thicknesses it is not economical.

- It needs a continuous gas flow.

10.3.2. MIG/MAG Welding


Continuous rod welding under a gaseous atmosphere is called MIG/MAG: Metal Inert/Active

Gas, or also GMAW: Gas Metal Arc Welding.

The MIG/MAG process is a semiautomatic welding process, in which the electric arc strikes

between a continuous supply electrode and the piece to be welded. The electric arc heats the

piece and the fusion of the material takes place. This process is characterized by the use of a

shielding gas. An inert gas in the case of MIG welding and an active gas in the case of MAG

welding.


41

Figure 47. MIG/MAG Welding scheme

Figure 48. MIG/MAG Welding

Equipment used

The welding equipment is formed by the elements that are distinguished in “Figure 49”. The

power source (5) connects its positive pole to the contact tube (6) of the welding gun (4), which

contains the electrode (8). And the negative pole is connected to the work piece, so the electric

arc that makes melting for welding is possible. The contact tube mentioned serves to pass the

electrical energy to the electrode, which supplies the arc.


42

In order that the wire electrode (8) is being used continuously, a wire reel (1) is used. The reel

continuously supplies wire through drive rollers (2), which pass said wire through the hose pack

(3) to the work gun.

The shielding gas (10) protects the process, protects the arc (9) and the weld pool (11). Exiting

through a gas nozzle (7) that covers the contact tube.

Figure 49.Scheme of MIG/MAG welding equipment

Figure 50. Equipment for MIG/MAG welding


43

Consumables

Filler material

The filler material is chosen according to the material to be welded and the shielding gas used.

It must have similar properties to the base material and is covered with an outer layer of copper.

The material is supplied in reels (Fig 51).

Figure 51. Wire reel

There are two types of wires. A wire with a solid section and a wire with a section filled with flux

(Fig 52). The first ones are usually used for alloys with low carbon content and for thin materials,

since they cool quickly and do not leave scum in the cable. The seconds are used for thicker

materials and leave scum in the cable, they are formed by an external metal part and a core

filled with flux or metal powder, which provides alloys to the weld bead.

Figure 52. Cross section of the types of consumables

Shielding gas

In the atmosphere there are gases such as O2, N2 and H2 that could damage the electrode and

the weld pool. That is why a shielding gas is used that isolates the process of these polluting

gases and allows to obtain a good final quality.


44

The shielding gas chosen depends on: the type of base metal, the characteristics of the arc and

metal transfer, the welding speed and the cost and availability of the gas.

There are two types of gases that are used in this welding process. The inert gases (Metal Inert

Gas), helium and argon are the most used, these gases do not react. And the active gases, such

as H2, CO2, O2, N2, which react chemically. In addition there are commercial mixtures to improve

the final results. The mixture of inert gases is Ar + He and the mixtures with active gases are: Ar

+ CO2, Ar + CO2 + O2, Ar + O2, Ar + He + CO2 + O2.


The MIG/MAG process is another welding process that could be chosen for the repair of the

blades by welding. The two previous processes are more recommended, but this also has

considerable advantages such as:

- It does not generate scum.

- High deposition speed.

- High deposition efficiency.

- Easy to use.

- Applicable to high thickness ranges.

- Low smoke generation.

- It is economic.

- Rapidity of deposition.

- High performance.

- Possibility of automation.

It is recommended because the environment is not damaged by the slag and also taking into

account the economic factor this is the optimal process.


MIG/MAG welding has disadvantages that do not make it the first of the processes to choose

for this work. Some of these disadvantages are the following:

- Limited distance between the equipment and the workplace.


45

- Difficulty working outdoors.

- Faster cooling compared to other methods.

- Limitation in places of difficult access.

- More qualified labor than other processes.

10.4. Post Welding Processes

The last procedure related to welding is inspection. There are different inspection methods to

evaluate the quality of the welding and check that it has been done correctly.

Controls are carried out before welding, in which all the elements that are going to be part of

the process are checked. The characteristics and composition of the base material and the filler

material, its compatibility, the qualification of the operators, the availability of auxiliary means

in the laboratory and as the most important inspection the preparation of the union, making

sure of its cleaning, preparation of the edges and positioning for welding.

Regarding the welding activity, it is necessary to control parameters such as the welding velocity,

the temperature and the sequence between passes, the protective atmosphere and the

deposition and penetration of the cord.

Finally, controls are carried out after the welding, such as the cooling velocity, the deformations,

the final dimensions and to conclude the inspection processes of destructive and non-

destructive tests.

The non-destructive tests are those used in this case. Tests of visual inspection, radiographic

control, ultrasonic inspection, inspection by penetrating liquids and inspection by magnetic

particles can be carried out.


46

11. TURBINE MAINTENANCE

Lastly, the maintenance of the steam turbine must be mentioned for the correct development

of the useful life of the machine. The turbine requires daily operational maintenance. A check of

alarms and warnings, monitoring of vibrations and revolutions, as well as monitoring of

parameters such as temperature and pressure of entry and exit. Checking the correct lubrication

and pressure in the filters and bearings.

Daily inspections are carried out to monitor the occurrence of any imperfection in daily or

biweekly maintenance or in longer periods of time. In addition to a leak inspection.

If a good maintenance is carried out, a suitable life cycle of the turbine is ensured and there will

be no unexpected breakdowns.


47

12. SUMMARY

The project consists in the analysis of the steam turbines, for which a classification of them is

made, and thus the multistage reaction turbine is defined clearly, which is the object of study.

In the first place, the steam turbine is classified within all existing types and a brief review is

made of the history of the same. Then, the essential parts of the equipment and the way of

functioning for the realization of the turn, which will give rise to mechanical work, were detailed.

It is located to the steam turbine integrated into the Rankine cycle, to better understand its

purpose.

This study focuses on the low pressure stage and more in detail on the mobile blades, which

work attached to the rotor. Within this set of blades, the most important are the blades of the

last stage of the low pressure turbine, because they are the ones that can cause problems due

to their design and working conditions.

The blades of the last stage have a long length and work at very high speeds. In this area of the

turbine the fluid contains drops of condensed vapor and there is a presence of moisture. For all

these reasons, special attention must be paid to this part of the turbine and its form of

operation.

We study the material used for the blades that are case study, focusing on stainless steels such

as 12% Cr and titanium alloys. The choice of material is an important decision, because the

mentioned moisture and vapor drops cause the erosion of the material, causing damage to the

turbine. A material that increases the useful life of the turbomachine has to be chosen.

The damages have to be repaired before they are irreversible and cause the failure. For this,

welding techniques are used. Laser welding is studied as the first option, and welding with a

tungsten electrode or TIG welding and continuous rod welding under a gaseous atmosphere or

MIG/MAG welding are also studied.

Finally, the welding inspection processes used must be mentioned, and for the steam turbine to

work properly and prevent failures, good maintenance is vitally important to ensure the turbine

satisfies its efficiency objectives.


48

13. CONCLUSIONS

The steam turbine used for a power plant is a machine that produces work from a fluid. So that

the fluid, which initially is at a very high pressure and at a very high temperature, expands and

by the blades attached to the rotor rotates and produces mechanical energy.

The turbines used in these plants are large turbines, formed by stages of high medium and low

pressure, to achieve greater power and obtain a much higher efficiency. The low pressure

turbine, object of study, is important because it is closely related to the efficiency of the

machine.

The working conditions of the steam turbines vary according to the manufacturing companies

and use different inlet conditions. Turn velocities of up to 3000 rev/min and 1010 mm of length

are achieved for the blades of the last stage, reducing the length to 841 mm if the turn occurs

at 3600 rev/min. It is important to take into account the annular exit section.

The most suitable material to work under the steam conditions of the low pressure turbine is

stainless steel 12% Cr. It has a mechanical resistance and optimal properties to respond

successfully to the working conditions of the turbine.

Although this material is the most suitable, it also suffers damages that have to be repaired. In

addition to the damage caused by vibration, the steam flow hits the blades of the turbine with

great force. As the steam turbine is an equipment in which the pressure and temperature

decrease, the presence of water droplets in the steam occurs. These drops cause the erosion of

the blades of the last stages.

Therefore, it is necessary to use welding processes for its repair. Laser welding is the best option,

since even compared to TIG or MIG / MAG welding it produces very good qualities and does not

need further improvement processes. The welding speed is much higher than in the other

methods and is an automated process.

Finally, it is very important to control each part of the steam turbine to increase the useful life

of the machine. Welding inspection methods are necessary to guarantee the success of the

welding used. In addition, the machine must be checked to avoid possible damage, which if not

detected in time can cause large failures.


49

14. BIBLIOGRAPHY

[1] John, E., “Modern Power Station Practice”, Pergamon Press, 1990.

[2] Schobeiri, M., “Turbomachinery Flow Physics and Dynamic Performance”, Springer-Verlag

Berlin Heidelberg, 2005.

[3] Janecki, S., & Krawczuk, M., “Dinamics of Steam Turbine Rotor Blading”, Ossolineum, 1998.

[4] Renovetec, “Principales elementos de la turbine de vapor”, URL: www.renovetec.com

[5] Kobelco, “Pre-calentamiento y Post-calentamiento: Los Propósitos y Procedimientos”, URL:

http://www.kobelco-welding.jp

[6] Ashby, M., & Jones, D., “Engineering Materials 1”, Butterworth Heinemann, 1996.

[7] Belzunce, J., “Tecnología de Materiales. Comportamiento en Servicio de Materiales”, Escuela

Politécnica de Ingerniería de Gijón. Universidad de Oviedo, 2012.

[8] Unpublished form material Plasma System S.A.

[9] Weman, K., “Welding Processes Handbook”, CRC Press, 2003.

[10] García, M., “Apuntes de Soldadura. Conceptos Básicos”, Bellisco Editions, 2010.

[11] TRUMPF, URL: www.trumpf.com

[12] Ibarra, M., Núñez, E., & Huerta, J., “Manual Aceros Inoxidables”, Chile: INDURA S.A., 2010.

THEORETICAL ANALYSIS OF THE LAST STAGE LP STEAM TURBINE …

Documents

Transcript of THEORETICAL ANALYSIS OF THE LAST STAGE LP STEAM TURBINE …