A REPORT

ON

BOILER TUBE COATINGS

BY

Name ID no.

Rahul Deshmukh 2009A4PS347PSarang Shamshery 2009A4PS290GAshish Chauhan 2009ABPS528P

At NTPC Badarpur

A Practice School-I Station of

BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI

2011

1

A REPORT

ON

BOILER TUBE COATINGS

BYNAME ID no. Discipline

Rahul Deshmukh 2009A4PS347P MECHANICALSarang Shamshery 2009A4PS290G MECHANICALAshish Chauhan 2009ABPS528P MANUFACTURING

Prepared in partial fulfillment of the Practice School-I

AT

NTPC Badarpur

A Practice School-I Station of

BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI

2011

2

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCEPILANI(RAJASTHAN)Practice School Division

Station: NTPC BadarpurDuration: 23-05-11 to 15-07-11 Date of Start: 23-05-11Date of Submission: 14-06-11Title of the Project: BOILER TUBE COATINGS

NAME ID no. Discipline

Rahul Deshmukh 2009A4PS347P MECHANICALSarang Shamshery 2009A4PS290G MECHANICALAshish Chauhan 2009ABPS528P MANUFACTURING

Name of and designation of the expert: Mr. Yogesh Bansal (Suptd. Boiler Mtc.)

Name of the PS faculty: Mr. Vishal Gupta

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ACKNOWLEDGEMENTS

We would like to thank Mr. Yogesh Kumar Bansal (Suptd. NTPC) for his essential inputs and guidance throughout the project. We also wish to express my gratitude to the officials and other staff members of “NATIONAL THERMAL POWER CORPORATION, DELHI (BADARPUR)” who rendered their help during the period of my project work.

We sincerely thank my PS I – Faculty Mr. Vishal Gupta for his constant

motivation.

We are very grateful to our student co-instructor Anubhav Kumar for his

support.

Last but not least we wish to avail myself of this opportunity, express a

sense of gratitude and love to my friends and my beloved parents for their

manual support, strength, and help and for everything.

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INDEX

1. ABSTRACT2. INTRODUCION3. ABOUT NTPC4. ABOUT BTPS5. BASIC STEPS OF ELECTRICITY GENERATION

i. COAL TO STEAMii. STEAM TO MECHANICAL POWERiii. MECHANICAL POWER TO ELECTRIC POWER

6. BASIC POWER PLANT CYCLE7. BOILER AND ITS DESCIPTION8. AUXILIARIES OF THE BOILER

i. FURNACEii. BOILER DRUMiii. WATERWALLSiv. REHEATERv. SUPERHEATERvi. ECONOMISERvii. AIR PREHEATER

9. PULVERIZERi. BALL MILLii. BOWL MILLiii. ADVANTAGES OF PULVERIZED COAL

10. NEED FOR COATING11. BOILER TUBE FAILURE MECHANISMS12. METHODS OF WEAR PROTECTION

i. TUBE SHIELDSii. SPRAY COATINGSiii. WELD OVERLAYSIV. INFILTRATION BRAZED TUNGSTEN CARBIDE CLADDING

13.COMPARISION14.LIST OF KNOWN VENDORS FOR WEAR RESISTANT

COATING15.CONCLUSION

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ABSTRACT

The electric power generation industry is experiencing the most complex convergence of market pressures in history. Environmental regulations are more rigorous than ever, forcing producers to make substantial capital investments in emissions conformance. At the same time, deregulation, excess capacity, and reduced market demand are driving diminishing maintenance budgets. The threat of nonconformance penalties weighs heavily against the pressure to increase profits, and the decisions between capital expenditures, potential fines, and routine equipment maintenance become a precarious balancing act.

Plant operators employ an array of methods for managing effluents and operating efficiency, including the installation of low NOx burners, over-fired air systems, and complex soot-blowing systems. These techniques, while contributing to the effective management of undesirable pollutants, significantly increase equipment wear rates, especially in waterwalls and boiler tubing.

Faced with deregulation, increasing retail competition and pressures to keep boilers online, many coal-fired power generating stations have adopted business strategies centered on increasing unit availability, reliability and increasing the operational life of critical equipment. However, boiler tube failures continue to be the number one cause of forced outages in fossil plants today. These costly forced outages are responsible for an estimated six percent overall loss of unit availability. One of the major causes for premature tube failure is excessive fireside boiler tube erosion caused by the impact, cutting action and abrasive wear of fly ash entrained flue gases undercutting the area they strike.

Power generation utilities and holding company goals are to extend times between major planned boiler outages. Systems types and configurations, the age of the plant, their specific plant operating demands and both preventative and general maintenance philosophies can dictate the accomplishment of these goals.

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INTRODUCTION

The main objective of this report is to analyze the wear and tear of boiler tubes and discuss techniques in order to prevent them.

This project analyses the reasons of wear and tear of tubes in the boiler of a thermal power plant and the extent of damage caused by them. It discusses various technologies and techniques available in the market like:

Tube Shields Spray Coatings Weld Overlays Infiltration Brazed Tungsten Carbide Cladding

Apart from the above, the project also lists the details of venders that make the techniques available in the market.

This project is concerned to analyze the method used by vendors to provide coatings for boiler, its cost and advantages & also the life of the coating.Analyzing this will help us in choosing the most suited vendor

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ABOUT NTPC

NTPC Limited is the largest thermal power generating company of India, Public Sector

Company. It was incorporated in the year 1975 to accelerate power development in the

country as a wholly owned company of the Government of India. At present,

Government of India holds 89.5% of the total equity shares of the company and the

balance 10.5% is held by FIIs, Domestic Banks, Public and others. Within a span of 31

years, NTPC has emerged as a truly national power company, with power generating

facilities in all the major regions of the country.

NTPC's core business is engineering, construction and operation of power generating

plants and providing consultancy to power utilities in India and abroad.

The total installed capacity of the company is 31134 MW (including JVs) with 15 coal

based and 7 gas based stations, located across the country. In addition under JVs, 3

stations are coal based & another station uses naphtha/LNG as fuel. By 2017, the power

generation portfolio is expected to have a diversified fuel mix with coal based capacity of

around 53000 MW, 10000 MW through gas, 9000 MW through Hydro generation, about

2000 MW from nuclear sources and around 1000 MW from Renewable Energy Sources

(RES). NTPC has adopted a multi-pronged growth strategy which includes capacity

addition through green field projects, expansion of existing stations, joint ventures,

subsidiaries and takeover of stations.

NTPC has been operating its plants at high efficiency levels. Although the company has

18.79% of the total national capacity it contributes 28.60% of total power generation due

to its focus on high efficiency. NTPC’s share at 31 Mar 2001 of the total installed

capacity of the country was 24.51% and it generated 29.68% of the power of the country

in 2008-09. Every fourth home in India is lit by NTPC. 170.88BU of electricity was

produced by its stations in the financial year 2005-2006. The Net Profit after Tax on

March 31, 2006 was INR 58,202 million. Net Profit after Tax for the quarter ended June

30, 2006 was INR 15528 million, which is 18.65% more than for the same quarter in the

previous financial year. 2005).

8

NTPC has set new benchmarks for the power industry both in the area of power plant

construction and operations. Its providing power at the cheapest average tariff in the

country. NTPC is committed to the environment, generating power at minimal

environmental cost and preserving the ecology in the vicinity of the plants. NTPC has

undertaken massive a forestation in the vicinity of its plants. Plantations have increased

forest area and reduced barren land. The massive a forestation by NTPC in and around its

Ramagundam Power station (2600 MW) have contributed reducing the temperature in the

areas by about 3°c. NTPC has also taken proactive steps for ash utilization.

In 1991, it setup Ash Utilization Division

A graphical overview:

9

Technological Initiatives

1. Introduction of steam generators (boilers) of the size of 800 MW.

2. Integrated Gasification Combined Cycle (IGCC) Technology.

3. Launch of Energy Technology Centre -A new initiative for development of

technologies with focus on fundamental R&D.

4. The company sets aside up to 0.5% of the profits for R&D.

5. Roadmap developed for adopting ‘Clean Development.

6. Mechanism’ to help get / earn ‘Certified Emission Reduction.

Environment Management

1. All stations of NTPC are ISO 14001 certified.

2. Various groups to care of environmental issues.

3. The Environment Management Group.

4. Ash Utilization Division.

5. Afforestation Group.

6. Centre for Power Efficiency & Environment Protection.

7. Group on Clean Development Mechanism.

8. NTPC is the second largest owner of trees in the country after the Forest

department.

ABOUT BTPS

Badarpur thermal power station started working in 1973 with a single 95 mw unit. There

were 2 more units (95 MW each) installed in next 2 consecutive years. Now it has total

five units with total capacity of 720 MW. Ownership of BTPS was transferred to NTPC

with effect from 01.06.2006 through GOI’s Gazette Notification .Given below are the

details of unit with the year they are installed.

10

BASIC STEPS OF ELECTRICITY GENERATION

The basic steps in the generation of electricity from coal involves following steps:

1. Coal to steam

2. Steam to mechanical power

3. Mechanical power to electrical power

The basic steps in the generation of coal to electricity are shown below:

COAL TO STEAM

Coal from the coal wagons is unloaded in the coal handling plant. This Coal is

transported up to the raw coal bunkers with the help of belt conveyors. Coal is

transported to Bowl mills by Coal Feeders. The coal is pulverized in the Bowl Mill,

where it is ground to powder form. The mill consists of a round metallic table on which

coal particles fall. This table is rotated with the help of a motor. There are three large

steel rollers, which are spaced 120 apart. When there is no coal, these rollers do not

rotate but when the coal is fed to the table it packs up between roller and the table and ths

forces the rollers to rotate. Coal is crushed by the crushing action between the rollers and

the rotating table. This crushed coal is taken away to the furnace through coal pipes with

the help of hot and cold air mixture from P.A. Fan.

P.A. Fan takes atmospheric air, a part of which is sent to Air-Preheaters for heating while

a part goes directly to the mill for temperature control. Atmospheric air from F.D. Fan is

heated in the air heaters and sent to the furnace as combustion air.

Water from the boiler feed pump passes through economizer and reaches the boiler drum.

Water from the drum passes through down comers and goes to the bottom ring header. 11

Water from the bottom ring header is divided to all the four sides of the furnace. Due to

heat and density difference, the water rises up in the water wall tubes. Water is partly

converted to steam as it rises up in the furnace. This steam and water mixture is again

taken to thee boiler drum where the steam is separated from water.

1. Cooling tower 10. Steam Control valve 19. Superheater2. Cooling water pump 11. High pressure steam turbine 20. Forced draught (draft) fan3. transmission line (3-phase) 12. Deaerator 21. Reheater4. Step-up transformer (3-phase) 13. Feedwater heater 22. Combustion air intake5. Electrical generator (3-phase) 14. Coal conveyor 23. Economiser6. Low pressure steam turbine 15. Coal hopper 24. Air preheater7. Condensate pump 16. Coal pulverizer 25. Precipitator8. Surface condenser 17. Boiler steam drum 26. Induced draught (draft) fan9. Intermediate pressure steam turbine 18. Bottom ash hopper 27. Flue gas stack

Water follows the same path while the steam is sent to superheaters for superheating. The

superheaters are located inside the furnace and the steam is superheated (540C) and

finally it goes to the turbine.

Flue gases from the furnace are extracted by induced draft fan, which maintains balance

draft in the furnace (-5 to –10 mm of wcl) with forced draft fan. These flue gases emit

their heat energy to various super heaters in the pent house and finally pass through air-

preheaters and goes to electrostatic precipitators where the ash particles are extracted.

Electrostatic Precipitator consists of metal plates, which are electrically charged. Ash

particles are attracted on to these plates, so that they do not pass through the chimney to

12

pollute the atmosphere. Regular mechanical hammer blows cause the accumulation of ash

to fall to the bottom of the precipitator where they are collected in a hopper for disposal.

STEAM TO MECHANICAL POWER

From the boiler, a steam pipe conveys steam to the turbine through a stop valve (which

can be used to shut-off the steam in case of emergency) and through control valves that

automatically regulate the supply of steam to the turbine. Stop valve and control valves

are located in a steam chest and a governor, driven from the main turbine shaft, operates

the control valves to regulate the amount of steam used. (This depends upon the speed of

the turbine and the amount of electricity required from the generator).

Steam from the control valves enters the high pressure cylinder of the turbine, where it

passes through a ring of stationary blades fixed to the cylinder wall. These act as nozzles

and direct the steam into a second ring of moving blades mounted on a disc secured to the

turbine shaft. The second ring turns the shafts as a result of the force of steam. The

stationary and moving blades together constitute a ‘stage’ of turbine and in practice many

stages are necessary, so that the cylinder contains a number of rings of stationary blades

with rings of moving blades arranged between them. The steam passes through each stage

in turn until it reaches the end of the high-pressure cylinder and in its passage some of its

heat energy is changed into mechanical energy.

The steam leaving the high pressure cylinder goes back to the boiler for reheating and

returns by a further pipe to the intermediate pressure cylinder. Here it passes through

another series of stationary and moving blades.

Finally, the steam is taken to the low-pressure cylinders, each of which enters at the

centre flowing outwards in opposite directions through the rows of turbine blades through

an arrangement called the ‘double flow’- to the extremities of the cylinder. As the steam

gives up its heat energy to drive the turbine, its temperature and pressure fall and it

expands. Because of this expansion the blades are much larger and longer towards the

low pressure ends of the turbine.

MECHANICAL POWER TO ELECTRICAL POWER

As the blades of turbine rotate, the shaft of the generator, which is coupled to tha of the

turbine, also rotates. It results in rotation of the coil of the generator, which causes

induced electricity to be produced.

13

BASIC POWER PLANT CYCLE

A simplified diagram of a thermal power plant

The thermal (steam) power plant uses a dual (vapour+ liquid) phase cycle. It is a close

cycle to enable the working fluid (water) to be used again and again. The cycle used is

Rankine Cycle modified to include superheating of steam, regenerative feed water

heating and reheating of steam.

On large turbines, it becomes economical to increase the cycle efficiency by using reheat,

which is a way of partially overcoming temperature limitations. By returning partially

expanded steam, to a reheat, the average temperature at which the heat is added, is

increased and, by expanding this reheated steam to the remaining stages of the turbine,

the exhaust wetness is considerably less than it would otherwise be conversely, if the

maximum tolerable wetness is allowed, the initial pressure of the steam can be

appreciably increased.

Bleed Steam Extraction: For regenerative system, nos. of non-regulated extractions is

taken from HP, IP turbine.

14

Regenerative heating of the boiler feed water is widely used in modern power plants; the

effect being to increase the average temperature at which heat is added to the cycle, thus

improving the cycle efficiency.

Factors Affecting Thermal Cycle Efficiency

Thermal cycle efficiency is affected by following:

1. Initial Steam Pressure.

2. Initial Steam Temperature.

3. Whether reheat is used or not, and if used reheat pressure and temperature.

4. Condenser pressure.

5. Regenerative feed water heating.

15

BOILER AND ITS DESCRIPTION

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its

walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in

diameter. Pulverized coal is air-blown into the furnace from fuel nozzles at the four

corners and it rapidly burns, forming a large fireball at the centre. The thermal radiation

of the fireball heats the water that circulates through the boiler tubes near the boiler

perimeter. The water circulation rate in the boiler is three to four times the throughput

and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and

changes into steam at 700 °F (370 °C) and 3,200 psi (22.1MPa). It is separated from the

water inside a drum at the top of the furnace.

Boiler Side of the Badarpur Thermal Power Station, New Delhi

16

The saturated steam is introduced into superheat pendant tubes that hang in the hottest

part of the combustion gases as they exit the furnace. Here the steam is superheated to

1,000 °F (540 °C) to prepare it for the turbine. The steam generating boiler has to

produce steam at the high purity, pressure and temperature required for the steam turbine

that drives the electrical generator.

The generator includes the economizer, the steam drum, the chemical dosing equipment,

and the furnace with its steam generating tubes and the superheater coils. Necessary

safety valves are located at suitable points to avoid excessive boiler pressure. The air and

flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler

furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse)

and the flue gas stack.

For units over about 210 MW capacity, redundancy of key components is provided by

installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating

dampers. On some units of about 60 MW, two boilers per unit may instead be provided.

Schematic diagram of a coal-fired power plant steam generator

17

AUXILIARIES OF THE BOILER

FURNACE

Furnace is primary part of boiler where the chemical energy of the fuel is converted to

thermal energy by combustion. Furnace is designed for efficient and complete

combustion. Major factors that assist for efficient combustion are amount of fuel inside

the furnace and turbulence, which causes rapid mixing between fuel and air. In modern

boilers, water furnaces are used.

BOILER DRUM

Drum is of fusion-welded design with welded hemispherical dished ends. It is provided

with stubs for welding all the connecting tubes, i.e. downcomers, risers, pipes, saturated

steam outlet. The function of steam drum internals is to separate the water from the steam

generated in the furnace walls and to reduce the dissolved solid contents of the steam

below the prescribed limit of 1 ppm and also take care of the sudden change of steam

demand for boiler.

The secondary stage of two opposite banks of closely spaced thin corrugated sheets,

which direct the steam and force the remaining entertained water against the corrugated

plates. Since the velocity is relatively low this water does not get picked up again but

runs down the plates and off the second stage of the two steam outlets.

From the secondary separators the steam flows upwards to the series of screen dryers,

extending in layers across the length of the drum. These screens perform the final stage of

the separation.

Once water inside the boiler or steam generator, the process of adding the latent heat of

vaporization or enthalpy is underway. The boiler transfers energy to the water by the

chemical reaction of burning some type of fuel.

The water enters the boiler through a section in the convection pass called the

economizer. From the economizer it passes to the steam drum. Once the water enters the

steam drum it goes down the down comers to the lower inlet water wall headers. From

the inlet headers the water rises through the water walls and is eventually turned into

18

steam due to the heat being generated by the burners located on the front and rear water

walls (typically). As the water is turned into steam/vapour in the water walls, the

steam/vapour once again enters the steam drum.

External View of an Industrial Boiler at BTPS, New Delhi

The steam/vapour is passed through a series of steam and water separators and then

dryers inside the steam drum. The steam separators and dryers remove the water droplets

from the steam and the cycle through the water walls is repeated. This process is known

as natural circulation.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot

blowers, water lancing and observation ports (in the furnace walls) for observation of the

furnace interior. Furnace explosions due to any accumulation of combustible gases after a

tripout are avoided by flushing out such gases from the combustion zone before igniting

the coal.

The steam drum (as well as the superheater coils and headers) have air vents and drains

needed for initial start-up. The steam drum has an internal device that removes moisture

from the wet steam entering the drum from the steam generating tubes. The dry steam

then flows into the superheater coils. Geothermal plants need no boiler since they use

naturally occurring steam sources.

Heat exchangers may be used where the geothermal steam is very corrosive or contains

excessive suspended solids. Nuclear plants also boil water to raise steam, either directly

passing the working steam through the reactor or else using an intermediate heat

exchanger.

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WATER WALLS

Water flows to the water walls from the boiler drum by natural circulation. The front and

the two side water walls constitute the main evaporation surface, absorbing the bulk of

radiant heat of the fuel burnt in the chamber. The front and rear walls are bent at the

lower ends to form a water-cooled slag hopper. The upper part of the chamber is

narrowed to achieve perfect mixing of combustion gases. The water wall tubes are

connected to headers at the top and bottom. The rear water wall tubes at the top are

grounded in four rows at a wider pitch forming g the grid tubes.

REHEATER

Reheater is used to raise the temperature of steam from which a part of energy has been

extracted in high–pressure turbine. This is another method of increasing the cycle

efficiency. Reheating requires additional equipment i.e. heating surface connecting boiler

and turbine pipe safety equipment like safety valve, non return valves, isolating valves,

high pressure feed pump, etc: Reheater is composed of two sections namely the front and

the rear pendant section, which is located above the furnace arc between water-cooled,

screen wall tubes and rear wall tubes.

Tubes of a reheater

SUPERHEATER

Whatever type of boiler is used, steam will leave the water at its surface and pass into the

steam space. Steam formed above the water surface in a shell boiler is always saturated

20

and become superheated in the boiler shell, as it is constantly. If superheated steam is

required, the saturated steam must pass through a superheater. This is simply a heat

exchanger where additional heat is added to the steam.

In water-tube boilers, the superheater may be an additional pendant suspended in the

furnace area where the hot gases will provide the degree of superheat required. In other

cases, for example in CHP schemes where the gas turbine exhaust gases are relatively

cool, a separately fired superheater may be needed to provide the additional heat.

ECONOMIZER

The function of an economizer in a steam-generating unit is to absorb heat from the flue

gases and add as a sensible heat to the feed water before the water enters the evaporation

circuit of the boiler.

Earlier economizer were introduced mainly to recover the heat available in the flue gases

that leaves the boiler and provision of this addition heating surface increases the

efficiency of steam generators. In the modern boilers used for power generation feed

water heaters were used to increase the efficiency of turbine unit and feed water

temperature.

An economizer

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Use of economizer or air heater or both is decided by the total economy that will result in

flexibility in operation, maintenance and selection of firing system and other related

equipment. Modern medium and high capacity boilers are used both as economizers and

air heaters. In low capacity, air heaters may alone be selected.

Stop valves and non-return valves may be incorporated to keep circulation in economizer

into steam drum when there is fire in the furnace but not feed flow. Tube elements

composing the unit are built up into banks and these are connected to inlet and outlet

headers.

AIR PREHEATER

Air preheater absorbs waste heat from the flue gases and transfers this heat to incoming

cold air, by means of continuously rotating heat transfer element of specially formed

metal plates. Thousands of these high efficiency elements are spaced and compactly

arranged within 12 sections. Sloped compartments of a radially divided cylindrical shell

called the rotor. The housing surrounding the rotor is provided with duct connecting both

the ends and is adequately scaled by radial and circumferential scaling.

An air preheater

Special sealing arrangements are provided in the provided in the air preheater to prevent

the leakage between the air and gas sides. Adjustable plates are also used to help the

sealing arrangements and prevent the leakage as expansion occurs. The air preheater

heating surface elements are provided with two types of cleaning devices, soot blowers to

clean normal devices and washing devices to clean the element when soot blowing alone

cannot keep the element clean.

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PULVERIZER

A pulverizer is a mechanical device for the grinding of many types of materials. For

example, they are used to pulverize coal for combustion in the steam-generating furnaces

of the fossil fuel power plants.

1. BALL AND TUBE MILLS

A ball mill is a pulverizer that consists of a horizontal cylinder, up to three diameters in

length, containing a charge of tumbling or cascading steel balls, pebbles or steel rods.

A tube mill is a revolving cylinder of up to five diameters in length used for finer

pulverization of ore, rock and other such materials; the materials mixed with water is fed

into the chamber from one end, and passes out the other end as slime.

2. BOWL MILL

It uses tires to crush coal. It is of two types; a deep bowl mill and the shallow bowl mill.

ADVANTAGES OF PULVERIZED COAL

1. Pulverized coal is used for large capacity plants.

2. It is easier to adapt to fluctuating load as there are no limitations on the

combustion capacity.

3. Coal with higher ash percentage cannot be used without pulverizing because of

the problem of large amount ash deposition after combustion.

4. Increased thermal efficiency is obtained through pulverization.

5. The use of secondary air in the combustion chamber along with the powered coal

helps in creating turbulence and therefore uniform mixing of the coal and the air

during combustion.

6. Greater surface area of coal per unit mass of coal allows faster combustion as

more coal is exposed to heat and combustion.

7. The combustion process is almost free from clinker and slag formation.

8. The boiler can be easily started from cold condition in case of emergency.

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9. Practically no ash handling problem.

10. The furnace volume required is less as the turbulence caused aids in complete

combustion of the coal with minimum travel of the particles.

Of all components boiler is of our major concern regarding this project. The inside walls of the boiler are surrounded by many pipes for the heating of water to steam.

Different types of boiler tubes

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NEED FOR COATING

Boilers operating with low NOx burners present unusually harsh environments for power plant materials. In particular, a change from normally oxidizing conditions in a standard boiler to reducing conditions in a low NOx boiler, causes usually protective oxides to give way to severe wastage by sulfidation corrosion and erosion. This change, driven by new government regulations for reduction of NOx emissions, has caused high rates of deterioration and, on occasion, early catastrophic failure of major boiler components such as waterwalls and burners. Investigators in the Energy Research Center are developing and evaluating several new coatings for use in boilers — coatings which will help prevent early failure of waterwall.

Iron aluminide weld overlays containing ternary additions and thermal spraycoatings are being investigated for corrosion protection of boiler tubes in Low NOxburners. The primary objective of the research is to identify overlay and thermal spraycompositions that provide corrosion protection of waterwall boiler tubes. In the currentphase of work, preliminary corrosion tests were conducted on a binary Fe-Al alloy inmultiple complex gases to determine which gases will be used for testing of the ternaryalloys. Preliminary solid-state corrosion tests were also conducted to simulate slag-metalinteractions seen in Low NOx furnaces. Two powder compositions were chosen fortesting of the ternary alloys. A matrix of alloys to be tested in both gaseous and solidstatecorrosion experiments was produced based on corrosion literature.

Recent clean air regulations have required electric power companies to decreaseNOx emissions, which has led to a reducing/sulfidizing environment within the boiler.Because of these new boiler conditions, protective oxide scales traditionally formed onthe low alloy Cr-Mo steel boiler tubes are now being replaced by less protective sulfidescales1. These sulfide scales are causing unacceptable waterwall wastage rates, whichhave led to costly forced outages. Therefore, to reduce the corrosion of the boiler tubes,new materials are being considered as corrosion resistant coatings.

Iron-aluminides are among the systems being considered as coatings. Iron-aluminumalloys have demonstrated excellent corrosion resistance in high-temperaturereducing environments. Iron-aluminum alloys are unsuitable for structural applicationsas they show a sharp drop in strength above 600°C and have low room temperatureductility. Fortunately, recent work has shown that they possess good weldability up to10wt% Al. Their good weldability coupled with the fact that they are less expensivethan Ni-based superalloy and stainless steel coatings, makes iron-aluminum alloysexcellent candidates for weld claddings.

Thermal spray coatings have also been considered as protection for boiler tubes inLow NOx furnaces. Thermal spray coatings can be advantageous to weld overlaysbecause they are relatively easy to apply and are not susceptible to cold cracking, unlike weld claddings. Plasma spray coatings have typically been used for corrosion anderosion protection, but they possess oxide inclusions and porosity, which can causeflaking of the coating. Recently, High-velocity Oxy-fuel (HVOF) processes have beenused to create dense, low-oxide coatings that contain little porosity. These HVOFthermal sprays have outperformed plasma spray coatings during high-temperature corrosion testing. Therefore, HVOF thermal sprays are being considered for coatings as well.

25

Boiler Tube Failure Mechanisms

Erosion

What happens?

Tube experiences metal loss from the OD of the tube. Damage will be oriented on the impact side of the tube. Ultimate failure results from rupture due to increasing strain as tube material erodes away.

Causes:

Erosion may take place by fly ash, falling slag, coal particles, soot blower, etc. Erosion of tube surfaces occurs from impingement on the external surfaces. The erosion medium can be any abrasive in the combustion gas flow stream, but most commonly is associated with impingement of fly ash or soot blowing steam. In cases where soot blower steam is the primary cause, the erosion may be accompanied by thermal fatigue.

The rate of erosion is calculated by the formula given below:

E= [K Ia R (A/C) Vn] / Tg Where,E Metal Erosion rateK constant depending up on tube materialIa Ash abrasiveness factorA/C Ash to Carbon RatioV flue gas velocity (current practice limit 15m/s)N index currently taken as 3.3Tg Flue gas absolute temp

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Water-side Corrosion

Caustic Corrosion

What Happens?

Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall.

Causes:

Caustic attack occurs when there is excessive deposition on ID tube surfaces. This leads to diminished cooling water flow in contact with the tube, which in turn causes local under-deposit boiling and concentration of boiler water chemicals. If combined with boiler water chemistry upsets of high pH, it results in a caustic condition which corrosively attacks and breaks down protective magnetite

Caustic attack at backling ring

Hydrogen Damage

What happens? Inter-granular micro-cracking. Loss of ductility or embrittlement of the tube material leading to brittle catastrophic rupture.

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Causes:

Hydrogen damage is most commonly associated with excessive deposition on ID tube surfaces, coupled with a boiler water low pH excursion. Water chemistry is upset, such as what can occur from condenser leaks, particularly with salt water cooling medium, and leads to acidic (low pH) contaminants that can be concentrated in the deposit. Under-deposit corrosion releases atomic hydrogen which migrates into the tube wall metal, reacts with carbon in the steel (decarburization) and causes intergranular separation.

Brittle failure due to hydrogen damage

Pitting

What happens?

Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feedwater inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods.

Causes:

Oxygen pitting occurs with the presence of excessive oxygen in boiler water. It can occur during operation as a result of in-leakage of air at pumps, or failure in operation of preboiler water treatment equipment. This also may occur during extended out-of-service periods, such as outages and storage, if proper procedures are not followed in lay-up. Non-drainable locations of boiler circuits, such as superheater loops, sagging horizontal superheater and reheater tubes, and supply lines, are especially susceptible. More generalized oxidation of tubes during idle periods is sometimes referred to as out-of-service corrosion. Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron oxide. When corrosive ash is present, moisture on tube surfaces from condensation or water washing can react with elements in the ash to form acids that lead to a much more aggressive attack on metal surfaces.

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Oxygen pitting on tube

Stress Corrosion Cracking

What happens?Failures from SCC are characterized by a thick wall, brittle-type crack. May be found at locations of higher external stresses, such as near attachments.

Causes:

SCC most commonly is associated with austenitic (stainless steel) superheater materials and can lead to either transgranular or intergranular crack propagation in the tube wall. It occurs where a combination of high tensile stresses and a corrosive fluid are present. The damage results from cracks that propagate from the ID. The source of corrosive fluid may be carryover into the superheater from the steam drum or from contamination during boiler acid cleaning if the superheater is not properly protected.

Lack of quality control

Acid attack:

What happens?

Corrosive attack of the internal tube metal surfaces, resulting in an irregular pitted or, in extreme cases, a “Swiss cheese” appearance of the tube ID.

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Causes:

Acid attack most commonly is associated with poor control of process during boiler chemical cleanings and/or inadequate post-cleaning passivation of residual acid.

The other causes under poor quality control may include:1) Maintenance cleaning damage2) Chemical execution damage3) Material defects4) Welding defects

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Fire-side corrosion

What happens?

External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed.

Causes:

Fireside ash corrosion is a function of the ash characteristics of the fuel and boiler design. It usually is associated with coal firing, but also can occur for certain types of oil firing. Ash characteristics are considered in the boiler design when establishing the size, geometry and materials used in the boiler. Combustion gas and metal temperatures in the convection passes are important considerations. Damage occurs when certain coal ash constituents remain in a molten state on the superheater tube surfaces. This molten ash can be highly corrosive.

High-temperature Oxidation:Similar in appearance and often confused with fireside ash corrosion, high-temperature oxidation can occur locally in areas that have the highest outside surface temperature relative to the oxidation limit of the tube material. Determining the actual root cause between the mechanisms of ash corrosion or high-temperature oxidation is best done by tube analysis and evaluation of both ID and OD scale and deposits.

Sectional photo of a tube with loss from fireside ash corrosion

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Stress rupture

Short-term Overheating:

What Happens?

Failure results in a ductile rupture of the tube metal and is normally characterized by the classic “fish mouth” opening in the tube where the fracture surface is a thin edge.

Causes:

Short-term overheat failures are most common during boiler start up. Failures result when the tube metal temperature is extremely elevated from a lack of cooling steam or water flow. A typical example is when superheater tubes have not cleared of condensation during boiler start-up, obstructing steam flow. Tube metal temperatures reach combustion gas temperatures of 1600°F or greater which lead to tube failure.

Thin-edged fish-mouth rupture

Long-term Overheating:

What happens?

The failed tube has minimal swelling and a longitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scale build-up and secondary cracking.

Causes:

Long-term overheat occurs over a period of months or years. Superheater and reheat superheater tubes commonly fail after many years of service, as a result of creep. During normal operation, alloy superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. Furnace water wall tubes also can fail from long-term overheat. In the case of water wall tubes, the tube temperature increases abnormally, most commonly from waterside problems such as

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deposits, scale or restricted flow. In the case of either superheater or water wall tubes, eventual failure is by creep rupture.

Dissimilar Metal Welds:

What happens?

Failure is preceded by little or no warning of tube degradation. Material fails at the ferritic side of the weld, along the weld fusion line. A failure tends to be catastrophic in that the entire tube will fail across the circumference of the tube section.

Causes:

DMW describes the butt weld where an autenitic (stainless steel) material joins a ferritic alloy, such as SA213T22, material. Failures at DMW locations occur on the ferritic side of the butt weld. These failures areattributed to several factors: high stresses at the austenitic to ferritic interface due to differences in expansion properties of the two materials, excessiveexternal loading stresses and thermal cycling, and creep of the ferritic material. As a consequence, failures are a function of operating temperaturesand unit design.

DMW failure where ferric Photomicrograph showing DMWmaterial is completely separated creep voids at ferric interface

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Fatigue

Mechanical Fatigue

What happens?

Damage most often results in an OD initiated crack. Tends to be localized to the area of high stress or constraint.

Causes:

Fatigue is the result of cyclical stresses in the component. Distinct from thermal fatigue effects, mechanical fatigue damage is associated with externally applied stresses. Stresses may be associated with vibration due to flue gas flow or sootblowers (high-frequency low-amplitude stresses), or they may be associated with boiler cycling (low-frequency high-amplitude stress mechanism). Fatigue failure most often occurs at areas of constraint, such as tube penetrations, welds, attachments or supports.

Mechanical fatigue failure at an attachment

Fireside corrosion Fatigue:

What happens?

Tubes develop a series of cracks that initiate on the outside diameter (OD) surface and propagate into the tube wall. Since the damage develops over longer periods, tube surfaces tend to develop appearances described as “elephant hide,” “alligator hide” or craze cracking. Most commonly seen as a series of circumferential cracks. Usually found on furnace wall tubes of coal-fired once-through boiler designs, but also has occurred on tubes in drum-type boilers.

Causes:

Damage initiation and propagation result from corrosion in combination with thermal fatigue. Tube OD surfaces experience thermal fatigue stress cycles which can occur from normal shedding of slag, sootblowing or from cyclic operation of the boiler. Thermal

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cycling, in addition to subjecting the material to cyclic stress, can initiate cracking of the less elastic external tube scales and expose the tube base material to repeated corrosion.

Craze cracking on OD surface

Waterside corrosion fatigue:

What happens?

ID initiated wide trans-granular cracks which typically occur adjacent to external attachments.

Causes:

Tube damage occurs due to the combination of thermal fatigue and corrosion. Corrosion fatigue is influenced by boiler design, water chemistry, boiler water oxygen content and boiler operation. A combination of these effects leads to the breakdown of the protective magnetite on the ID surface of the boiler tube. The loss of this protective scale exposes tube to corrosion. The locations of attachments and external weldments, such as buckstay attachments, seal plates and scallop bars, are most susceptible. The problem is most likely to progress during boiler start-up cycles.

Corrosion fatigue on tube ID adjacent to attachment

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Out of all the above mentioned causes of tube failures wear due to erosion is the major cause which is responsible for eighty percent of all the failures.

METHODS OF WEAR PROTECTION

Several methods for protecting boiler tubes from the severe wear environment in a powergeneration steam boiler have been employed, with varying degrees of success. Each method offers certain benefits, but may a dd additional risks to asset availability.

TUBE SHIELDS

Virtually every boiler maintenance team is familiar with the use of shields for the protection of boiler tubes. Shields may be as simple as a sacrificial contoured plate of carbon steel welded in place, or as complex as a “hand-cuffed” superalloy with sophisticated spray coatings. While tube shields may reduce the frequency of tube leaks and the opportunity for collateral damage caused by high-velocity steam cutting, there are multiple issues that make tube shields a poor performing, if not risky, solution.The objective of tube shields, to prevent hot gas from contacting the heat exchanger tubes of the boiler, it significantly reduces the efficiency of the unit by impeding heat transfer. This heat transfer degradation is caused not only by the increased material of the shield, but also by the shield’s tendency to entrap “dead” air between itself and the tube that it is protecting.The additional area represented by the tube shield typically constricts gas flow through and between tubes in a zone that already may have a limited flow area. Most significantly, the affixing of tube shields is very difficult and unpredictable. Because of the severe operating environment in a steam boiler furnace, attaching a tube shield (either by bolting, welding, or other method) so that it will not come free is a tenuous task. Tube shields that come loose can block gas flow, which further reduces heat transfer efficiency and can be the source of flow eddies that concentrate wear, accelerating tube failure. Tube shields that fall to the bottom of the furnace can lead to the reduced life of clinker grinders.

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TUBE SHIELDS INSTALLED IN BOILER TUBES

VARIOUS TYPES OF TUBE SHIELDS

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THERMAL SPRAY COATINGS

Thermal spraying techniques are coating processes in which melted (or heated)

materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by

electrical (plasma or arc) or chemical means (combustion flame).

Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers

to several mm, depending on the process and feedstock), over a large area at high

deposition rate as compared to other coating processes such

as electroplating, physical and chemical vapor deposition. Coating materials available for

thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed

in powder or wire form, heated to a molten or semimolten state and accelerated towards

substrates in the form of micrometer-size particles. Combustion or electrical arc

discharge is usually used as the source of energy for thermal spraying. Resulting coatings

are made by the accumulation of numerous sprayed particles. The surface may not heat

up significantly, allowing the coating of flammable substances.

A common feature of all thermal spray coatings is their lenticular or lamellar grain structure resulting from the rapid solidification of small globules, flattened from striking

a cold surface at high velocities.

Schematic diagram of thermally sprayed spherical particle impinged onto a flat substrate

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Schematic Diagram of  Thermal Spray Metal Coating

A typical microstructure of a metallic thermally sprayed coating. The lamellar structure is interspersed with oxide inclusions and porosity.

Coating quality is usually assessed by measuring its porosity, oxide content, macro and

micro-hardness, bond strength and surface roughness. Generally, the coating quality

increases with increasing particle velocities.

Protective spray coatings, including HVOF, plasma spray, laser, and others, have become very popular over the past two decades. While spray coatings have provided good wear

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resistance in some applications, their popularity has been primarily driven by the mystique of the technology, along with considerable marketing investments by OEMs and other large coating manufacturers.Though direct application of spray coatings alleviates some of the issues that plague tube shields, sprays have experienced limited success in protecting boiler tubes from fly ash erosion in severe environments.The spray coating process, which typically utilizes a hot molten carrier material to move hard particles into place, tends to produce limited hard particle density in the resulting coating layer. The higher the density of hard particles, the more difficult it is to make and keep the carrier material fluid.Spray coating is analogous to a painting process, albeit a very sophisticated one. Typically, multiple coats are applied atop one another to build up the material thickness. This process, which generally results in mechanical “sticky” bonding between particles and coating layers, can be susceptible to oxide and other contamination between layers. This limits the total available thickness, and may, in some cases, threaten the integrity of overall coating adhesion, resulting in spalling and chipping.The bond issues described above can be particularly problematic in the severe environment of a power generation furnace. Frequent thermal shock, especially in soot blowing lanes, generates the rapid heating and cooling of dissimilar materials, causing differential expansion and contraction. This differential movement creates extreme stress in the coating bond zones.Comparing properties of unsupported coatings with wrought/cast bulk equivalents:

PROPERTY COATING WROUGHT/CAST

Strength low (5-30%) high (100%)Ductility very low (1-10%) high (100%)Impact strength low highPorosity high lowHardness higher particulate/micro-hardness     higher bulk/macro-hardnessWear Resistance high lowCorrosion Resistance     low highMachining poor good

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Several variations of thermal spraying are distinguished:

Plasma spraying

Wire arc spraying

High velocity oxy-fuel coating spraying (HVOF)

PLASMA SPRAY COATINGS

In plasma spraying process, the material to be deposited (feedstock) — typically as a powder, sometimes as a liquid, suspension  or wire — is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is on the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc.

The deposits consist of a multitude of pancake-like lamellae called 'splats', formed by flattening of the liquid droplets. As the feedstock powders typically have sizes from micrometers to above 100 micrometers, the lamellae have thickness in the micrometer range and lateral dimension from several to hundreds of micrometers. Between these lamellae, there are small voids, such as pores, cracks and regions of incomplete bonding. As a result of this unique structure, the deposits can have properties significantly different

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from bulk materials. These are generally mechanical properties, such as lower strength and modulus, higher strain tolerance, and lower thermal and electrical conductivity. Also, due to the rapid solidification, metastable phases can be present in the deposits.This technique is mostly used to produce coatings on structural materials. Such coatings provide protection against high temperatures (for example thermal barrier coatings for exhaust heat management), corrosion, erosion, wear; they can also change the appearance, electrical or tribological properties of the surface, replace worn material, etc. When sprayed on substrates of various shapes and removed, free-standing parts in the form of plates, tubes, shells, etc. can be produced. It can also be used for powder processing (spheroidization, homogenization, modification of chemistry, etc.). In this case, the substrate for deposition is absent and the particles solidify during flight or in a controlled environment (e.g., water). 

PLASMA SPRAYING

WIRE ARC SPRAYING

Wire arc spray is a form of thermal spraying where two consumable metal wires are fed

independently into the spray gun. These wires are then charged and an arc is generated

between them. The heat from this arc melts the incoming wire, which is then entrained in

air jet from the gun. This entrained molten feedstock is then deposited onto a substrate.

This process is commonly used for metallic, heavy coatings.

Plasma transferred wire arc (PTWA) is another form of wire arc spray which deposits

a coating on the internal surface of a cylinder, or on the external surface of a part of any

geometry. It is predominantly known for its use in coating the cylinder bores of an

engine, enabling the use of Aluminum engine blocks without the need for heavy cast iron

sleeves. A single conductive wire is used as "feedstock" for the system. A supersonic

plasma jet melts the wire, atomizes it and propels it onto the substrate. The plasma jet is

formed by a transferred arc between a non-consumable cathode and the type of a wire.

After atomization, forced air transports the stream of molten droplets onto the bore wall.

The particles flatten when they impinge on the surface of the substrate, due to the high

kinetic energy. The particles rapidly solidify upon contact. The stacked particles make up

a high wear resistant coating. This process utilizes a single wire as the feedstock material. 42

All conductive wires up to and including 0.0625" (1.6mm) can be used as feedstock

material, including "cored" wires. It can be used to apply a coating to the wear surface of

engine or transmission components to replace a bushing or bearing. For example, using

PTWA to coat the bearing surface of a connecting rod offers a number of benefits

including reductions in weight, cost, friction potential, and stress in the connecting rod.

Arc Wire Sprayed 13Cr Steel Coating on Aluminium Substrate.Coating designed to give gripping surface to rubber.

Arc Wire Sprayed 13Cr Steel/Aluminium bronze Psuedo-alloy CoatingMade possible by feeding two different wires into the arc.

Arc Wire Sprayed Nickel Aluminium Alloy Coating

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HIGH VELOCITY OXY-FUEL PROCESS

 A mixture of gaseous or liquid fuel and oxygen is fed into a combustion chamber, where

they are ignited and combusted continuously. The resultant hot gas at a pressure close to

1 MPa emanates through a converging–diverging nozzle and travels through a straight

section. The fuels can be gases

(hydrogen, methane, propane, propylene, acetylene, natural gas, etc.) or liquids

(kerosene, etc.). The jet velocity at the exit of the barrel (>1000 m/s) exceeds the speed of

sound. A powder feed stock is injected into the gas stream, which accelerates the powder

up to 800 m/s. The stream of hot gas and powder is directed towards the surface to be

coated. The powder partially melts in the stream, and deposits upon the substrate. The

resulting coating has low porosity and high bond strength.

HVOF coatings may be as thick as 12 mm (1/2"). It is typically used to

deposit wear and corrosion resistant coatings on materials, such as ceramic and metallic

layers. Common powders include WC-Co, chromium carbide, MCrAlY, and alumina.

The process has been most successful for depositing cermet materials (WC–Co, etc.) and

other corrosion-resistant alloys (stainless steels, nickel-based alloys,

aluminium, hydroxyapatite for medical implants, etc.).

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Thermal Spray Coating Photomicrographs HVOF Spray Chromium Carbide/Nickel Chromium Coating

HVOF sprayed Chromium Carbide/Nickel Chromium Coating on Cast Iron Substrate

HVOF sprayed Chromium Carbide/Nickel Chromium Coating

WELD OVERLAYS

Weld overlay materials are available in a variety of compositions, including chrome carbide, vanadium carbide, tungsten carbide, inconnels, stainless steel, and other exotic materials. All weld overlays generally utilize the same approach to inhibiting wear, with varying compositions selected depending upon environmental variables. In an environment where corrosion is a significant factor, a stainless steel overlay might provide adequate protection. Whereas, a tungsten carbide weld overlay may perform better when corrosion is nominal and the primary wear mode is erosion.While their use in boiler tubes has been limited over the past several years, some weld overlay compositions have shown promise in protecting tubes against corrosive attack.

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This is particularly true of inconnel weld overlays, which are becoming commonly used as protection against waterwall corrosive attack. While these, and other weld overlays, are often used for corrosive applications, their resistance to erosion is minimal when compared against more advanced erosion resistant materials.Due to the relatively “low tech” nature of welding, many of the constraints related to spray coatings are even more relevant to weld overlays. Weld overlay procedures do not typically achieve the same temperatures or particle movement velocities as those attained with spray coating processes. This significantly limits the amount of hard particles that can be deposited for a given amount of carrier material. Typical hard particle volumetric densities for weld overlay materials rarely exceed 28% by volume, the remainder consisting of relatively soft carrier material. While there are many corrosion resistant carrier materials that can be used in the weld overlay process, the nature of such materials (which makes them useful as a carrier in the weld process) precludes them from withstanding high velocity particle erosion. Erosion resistance is directly proportional to the density of the material’s hard particles (see Figures 3 and 4).High localized heating in the weld overlay process typically causes absorption of substrate carbon, known as “carbon dilution”, into the weld overlay material. This dilution of carbon causes a reduction in the material’s ability to withstand high velocity erosive attack. This extreme localized heating, combined with the difficulty in controlling cooling rates, also results in check cracking of the weld overlay material. This check cracking may propagate into the base tube material, leading to premature tube failure. Check cracking at the weld surface offers a path for high velocity erosive material to penetrate into the protective layer; a phenomenon known as “channeling”.

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INFILTRATION BRAZED TUNGSTEN CARBIDE CLADDING

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 Infiltration brazing means to fill by capillary action with molten fillermetal, a porous coating or structure that has a melting point higher that the filler metal.While there are many means for applying the carbide and braze in preparation forinfiltration braze coating, the principal method involves a non-wovenpreformed cloth. The particles used in this process are sized and mixed to provide astable, dense coating. Figure 1 illustrates the process steps from powder mixing tobrazing. Figure 2 shows the resulting microstructure of the cladding and the interface.The constraints on this process are straight-forward. Infiltration brazing is performedin a vacuum furnace. Large parts as measured by volume, weight or area, are oftensegmented prior to cladding to avoid limitations of furnace size.

When Tungsten (W) and Carbon (C) unite they become one of the hardest of all carbides.  Tungsten Carbide is an ideal material when confronting severe abrasion applications.  Tungsten Carbide Hardfacing will reduce eductor wear and increase the eductor's service life.

Tungsten Carbide is the choice to provide the best protection available for extreme wear and impact. It and Alpha silicon carbide offer the best wear performance and corrosion resistance of any available materials.  The impact resistant and corrosion resistant properties of tungsten carbide platelets and forms can be improved by selecting from the different grades of the material or by selecting from the various application technologies.   Used alone or in combination with other wear resistant materials, tungsten carbide whether in the form of cladding, cast in steel or in the form of a weld overlay simply lasts where others fail. It can offer wear life in excess of 5 times that of alumina ceramic and has ability for continuous operation a temperatures up to 900 degrees F (480 C). It can also be machined to very tight tolerances and to fine surface finishes as needed. It is excellent product for use on fan blades, High Efficiency, Fan Wheels pulverizer and roller mill liners, target plates in high wear areas, Sand Mixers, Sand Mullers and Sand Plows and in pipes and elbows where extreme abrasion exists.

Tungsten Carbide Cladding – is applied in the form of solid carbide platelets either AeroTech Bonded or Brazed onto the desired protection area. Brazing offers the highest performing tungsten carbide cladded wear parts with metallurgical bond strengths in excess of 22,000 psi.

   

Tungsten Carbide Shapes - like ceramics tungsten carbide may also be formed into monolithic shapes for applications such as valve seats, seal rings, nozzles, bushings and other mechanical components. 

 

Carbistel –parts formed from casting abrasion resistant steel around tungsten carbide granules forming a carbide steel matrix.  Ideal for the highest impact applications where gouging and crushing wear is present. 

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 Infiltration brazing means to fill by capillary action with molten fillermetal, a porous coating or structure that has a melting point higher that the filler metal.While there are many means for applying the carbide and braze in preparation forinfiltration braze coating, the principal method involves a non-wovenpreformed cloth. The particles used in this process are sized and mixed to provide astable, dense coating. Figure 1 illustrates the process steps from powder mixing tobrazing. Figure 2 shows the resulting microstructure of the cladding and the interface.The constraints on this process are straight-forward. Infiltration brazing is performedin a vacuum furnace. Large parts as measured by volume, weight or area, are oftensegmented prior to cladding to avoid limitations of furnace size.

When Tungsten (W) and Carbon (C) unite they become one of the hardest of all carbides.  Tungsten Carbide is an ideal material when confronting severe abrasion applications.  Tungsten Carbide Hardfacing will reduce eductor wear and increase the eductor's service life.

 

TufCote – Is made of cast tungsten carbide granules in a self fluxing nickel matrix that is both abrasion and corrosion resistant.  TufCote can be applied in various grade and granule sizes to meet your application needs.  It is ideal for shredding, crushing and pulverizing applications where impact and fine particle wear are both present.

Infiltration brazed tungsten carbide cladding overcomes the constraints of tube shields, spray coatings, and weld overlay materials, as described below.• Infiltration brazing does not require the movement of hard particles. Therefore, hardparticle densities of more than 70%, by volume, can be achieved.• Infiltration brazing creates a metallurgical bond between the hard particles, the carriermaterial, and the boiler tube to be protected, thus keeping the particles in place.• Brazing precludes the introduction of contaminants and the development of oxides in the protective layer, ensuring consistent cladding integrity.• Infiltration brazing does not allow for significant carbon dilution into the protective layer, ensuring uniform wear resistance from top to bottom.• Infiltration brazing allows for a deposition thickness as high as 120 mils, providingextreme life extension when required.• The metallurgical bond and high toughness of the carrier material enables infiltrationbrazed cladding to withstand extreme thermal shock and impact.

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• Infiltration brazed tungsten carbide has a heat transfer coefficient similar in magnitude to common tube steels (~30 W/ (m-ºK)), and the metallurgical bond ensures that there is no significant heat transfer impediment at the bond surface.• Controlled application and cooling during infiltration brazing ensures that protection isvirtually crack-free.

Based on more than twenty years of research, Conforma Clad has developed three standard tungsten carbide cladding formulas (WC200, WC210, WC219) that provide superior wear protection for a wide range of operating conditions and wear modes (including abrasion, erosion, corrosion and impact, individually or in a combination). Engineers evaluate each substrate and operating environment to determine the best cladding formula. In addition to the standard tungsten carbide cladding formulas, offer custom formulations to fit specific customer requirements.

Cladding Specifications

Standard cladding thickness ranges from 0.020 to 0.125 inches.

Most cladding is applied by hand.

In cases where there is not sufficient hand access, mechanical aids may be employed. Inside diameters (ID’s) as small as 1/4" and 6” long have been clad with the aid of an application rod.

Chamfers and radii as small as 1/8” can be clad.

Sharp edges (where cladding is required on both sides) will be clad with two types of cloth. The side experiencing greater wear will be clad with the more wear resistant cladding. The other side will be clad with a more ductile material. This combination helps relieve excess stresses that can cause fracturing during the cool-down portion of the cycle.

Standard Tungsten Carbide Cladding Formulas

Cladding Composition (Weight Percentage)

 WC 200 WC 210 WC 219

Tungsten Carbide* 62% 55% 48%

Nickel 30% 34% 39%

Chromium 6% 7% 8%

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Other 2% 4% 5%

Total carbide loading from other carbide formation

68% 66% 62%

*Tungsten Carbide (WC) includes cobalt bonded WC.

Cladding Properties

                                             WC 200 WC 210 WC 219

Density (lb/in3) 0.44 0.42 0.40

Thermal Conductivity (BTU in/h•ft2•°F)

230 200 170

Metallurgical Bond Strength (psi) >70,000 >70,000 >70,000

Porosity <3% <3% <3%

Rockwell Hardness (HRC)** 64-70 60-66 56-62

**Cladding is a composite of tungsten carbide particles dispersed in a nickel-based alloy matrix. The extremely hard carbide particles, with a Vickers Diamond Pyramid Hardness of about 2000 DPH50g[1865 DPH50g is equivalent to 80 Rockwell C Hardness (HRC)], are surrounded by a two-phase matrix (300-800 DPH50g, equivalent to 30-64 HRC). Because of the heterogeneous structure of the cladding, direct Rockwell hardness measurements are an average of the hard particles and matrix, and are not representative of the individual components of the composite.

 

Characteristics of Finished Conforma Clad Surfaces

Percentage of tungsten  carbide                

48%-62%

DistributionUniform distribution of large & small carbide particles throughout the composite

No interconnected porositySince pores at such low levels are not interconnected, claddings are impermeable to high pressure gases and corrosive fluids

Rockwell C Hardness 64 to 70

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Shear strength at bond lines 70,000 PSI

Transverse rupture strength strength

80,000-100,000 PSI

The result is a hard protective layer that is extremely wear resistant and very durable. Cladding combines the hardness of tungsten carbide and the corrosion resistance of nickel chrome boron, resulting in protection that outperforms the competition. Bond strengths are estimated to be in excess of 70,000 PSI, making material highly resistant to chipping, cracking and flaking.

Field applications verify that 1/16" of brazed tungsten carbide cladding provides the same erosion protection as 3/4" of chrome carbide weld overlay or 3" of carbon steel.

It develop custom cloth formulations for industry specific applications, and even for individual customers. It clad customers’ components in certain situations, but also can fabricate and provide finished parts that meet customer specifications.

BOILER TUBE SOLUTIONS:THEY LAST FOR MORE THAN 10 TIMES IN COMPARISON TO OTHER WEAR SOLUTIONSSOME MORE PROPERTIES:

Conforma Clad Technology Conforma Clad can help reduce the risk of boiler tube leaks by protecting your replacement boiler tube segments from severe wear. Cladding withstands the extremes of thermal shock, erosion, abrasion, corrosion and impact — and because application process creates a true metallurgical bond, it is not subject to chipping and spalling

Erosion Protection in Sootblower Lanes• Eliminates need for tube shields • Composite/Profiled Cladding • Proprietary process ensures consistent thickness

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and density

Extended Predictable Life in Fluidized Beds• Linear wear offers predictable life extrapolation • Minimal dilution ratio,true metallurgical bond• Hot erosion 10X Inconel 622, 11X 312SS weld overlay

Random 22 ft. Lengths Available• Meets the requirements of the ASME Boiler and Pressure Vessel Code (S Stamp)• Inventory programs available• Pendants, U-bends, complex shapes clad after fabrication

 Diameter Range Length Range

ID Clad3/4" - 12" 1/2" - 4'

OD Clad3/4" - 12" 1/2" - 24'

 

SubstratesCladding can be applied to most carbon steels, stainless steels and alloy steels.

Temperature

Continuous operation at temperatures up to 1900° F (1038° C) with nominal performance impact. Able to withstand transients in excess of 2000° F.

Compatibility Compatible with chemicals commonly found in coal and fly ash, including hydrochloric acid, hydrogen fluoride and sulfuric acid.

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Comparison:- The various researches are going on the world on boiler tube coating to characterize and compare them and suggest which one is better.

So various equipments and criteria are used for their comparison:The ERC team uses a wide range of laboratory facilities to fully characterize and compare.

Processing facilities include an automated welding laboratory for developing weld overlay coatings and an electrode position laboratory for applying electrodeposited intermetallic coatings. The thermal spray coatings evaluated by the group are prepared by National Laboratories and commercial thermal spray vendor. A state-of-the-art high temperature corrosion facility is used to evaluate materials in both sulfidation and oxidation corrosion environments. The corrosion laboratory facility includes a thermo gravimetric (TG) corrosion test unit for quantitative kinetic studies and furnaces dedicated to sulfidation, oxidation and mixed environments. Of special value to the project is a unique Environmental Scanning Electron Microscope which makes it possible to directly observe high temperature corrosion and oxidation processes in real time under actual environmental conditions. The erosion resistance of coatings is measured in a special test facility specifically built for this application. This facility is capable of evaluating erosion velocities between 20 and 90 m/s (impact velocities are measured by a laser velocimeter),temperatures up to 500°C, a range of impact angles from 20° to 90°, and a variety of erodents from hard alumina particles to simulated fly ash. Coating microstructures are characterized both before and after exposure in a boiler in the University’s microscopy laboratories. These include light optical microscopy with quantitative image analysis for measurement of coating constituents, a variety of scanning electron microscopes for higher magnification of planar and cross-sectional analysis and several analytical electron microscopes for precise measurement of the chemical constituents in the coatings.

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Comparison on the basis of properties :-

Properties Conforma clad

Thermal spray

Weldoverlays

WearTiles

Ar plates

Bond strength

Very high Very low High Low N/A

Complex geometries

Yes No Difficult Difficult Very low

Abrasion

resistance

Very high Modetare High Very high Very low

Erosion resistance

Very high Low to moderate

Low Low Very low

Corrosion resistance

High Low Low Low Low

Impact resistance

Moderate Low Moderate Very low Low

Oxide Level

Low High Low Low Low

TemperatureResistance

High Moderate Low Very low High

Resist multiple modes of

wear

Yes No Yes No No

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Case study:

NTPC BADARPUR:-

1 PROBLEM: - EROSION IN ECONOMISER TUBE OF UNIT 1 ECO

There was four failures in eco 1 of unit 1 in year 2006 all were due to erosion. On analysis with the abovementioned formula for erosion rate, it was found that

all factor are constant then why erosion at only few places

ANALYSIS:-

Factor influencing equation greatly is V to the power n. And on critical analysis it was found that erosion is localized and somehow

velocity is increasing beyond 15 m/s at some particular places. Cause for this was found misalignment Causes for misalignment are: Bad shape and design of eco clamps permitting coils misalignment and hence

increased velocities at some locations resulting increase rate of erosion and hence failures.

Method of wear protection:-

TUBE SHIELD – Objective of tube shield is to prevent hot gas from contacting heat exchanger. This leads to:

- Reduced efficiency. - Constricts gas flow through and between tubes in a zone. - Tube shields that become loose block gas path and can be source of flow eddies.

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Alignment:-

Assessment of the damage reveals that poor alignment is the main cause for localized erosion which in turn leads to high maintenance cost and unreliable equipment. Misalignment may be due to:

Poor maintenance. Unfriendly supporting system.

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Modification of eco 1 supporting system:-

OLDER SYSTEM Made of 6 mm flats. Highly flexible makes coil alignment poor. Tube fixing is a problem (Maintenance Unfriendly). Gap between tubes and coils not maintained.

Modified support:-

Made of 20 mm thick plates. Maintenance friendly. It itself cares the positioning of tubes and the distance between coils and/or tubes. Tube placement is not a problem. Itself make coil alignment perfect. Highly robust and sturdy Coil bending can be completely avoided while maintenance.

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Result:-

There is not even a single leakage due to erosion in unit 1 eco 1 after replacement of clamps (90% replacement done).

On inspection of 8 coils in next o/h, even a single meter of tube replacement was not required.

Savings:-

Savings due to reduction in outage of unit. Saving due to reduction in material consumption for overhauling. Reduction in overhauling time. Desired quality can be achieved and reduction in maintenance induced defects.

Outage costRs.13.68 lakhs per leakage (approx.)Assuming 24 hours outageAnd for 4 leakages13.68 X4 = 54.72 lakhs (approx.)

Maintenance costAnnual O/H 16 lakhs Capital O/H 68.35 lakhs In this overhauling we have used no material hence saving of 16 lakhs

TOTAL16+54 = 70 Lakhs in one year.Note: saving estimate is indicative only for quantifying the gain from modification. Although detailed study required for payback period and other gain in long period.

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LIST OF KNOWN VENDORS FOR WEAR RESISTANT COATING

1. M/s. Metallizing Equipment Co,Pvt.Limited.

2. M/s. Anode Plasma spray Limited

3. M/s. S V X Powder M surface Engg. Pvt. Limited.

4. M/s. Industrial Processors & Metallizors Pvt. Limited.

5. M/s Wear Resist Technologies Pvt. Limited.

6. M/s. LARSEN & TOUBRO LIMITED

CONCLUSION

Power generation plant owners and maintenance teams are expected to use innovative methods to increase the availability and productivity of their steam boilers and other large capital assets. Infiltration brazed tungsten carbide, weld overlays, thermal sprays etc are proven technology available to provide substantial protection against the most common causes of aggressive equipment wear. Infiltration brazed tungsten carbide, while not as well known as spray coatings and other older technologies, has proven its ability to simultaneously withstand extremes of thermal shock, erosion, abrasion, corrosion, and impact. Through the use of this ultra-high performance protection material, plant operators can significantly reduce the risk of boiler tube leaks, greatly increasing levels of unit availability and overall capital asset productivity.

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