Boilers are often referred to as steam generators, because the primary function of a boiler is to generate steam.Boiler is a heart of Fossil Fuel Plant. They seem destructive but they are not.

All boilers have something in common:

all boiler use fossil fuel all generate hot gases all use water to generate steam

INTRODUCTION:

The first boilers were not much complicated

They were open, water-filled containers heated by the combustion of fuel.

When water in the containerabsorbs enough heat, it boils and produces steam

Steam is produced but there are a number of inefficiencies caused by its design: an open container only produces steam at atmospheric pressure. an open container rapidly boils & dry the flow of steam is impossible to control

Closing the container allows steam to produce at higher pressure Constant supply of feed water is introduced thus minimizing the chances of damaging the container Flow of steam and water can be controlled by providing valve in steam and feed water line steam can now be directed to a desired location

The system still does not make efficient use of the heat producedMuch of the heat of combustion is lost to surroundings

Efficiency of a boiler has a direct bearing on the cost of power generation Power Plant boilers must be efficient Boilers with only forced draft fan are called pressurized furnace boilers.

Boilers that use both forced draft and induced draft fans are called balanced-draft boilers.

Boiler ActsIndian Boiler RegulationThe Indian Boilers Act was enacted to consolidate and amend the law relating to steam boilers. Indian Boilers Regulation (IBR) was created in exercise of the powers conferred by section 28 & 29 of the Indian Boilers Act.IBR Steam Boilers Means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is wholly, or partly under pressure when the steam is shut off.IBR Steam Pipe Means any pipe through which steam passes from a boiler to a prime mover or other user or both, if pressure at which steam passes through such pipes exceeds 3.5 kg/cm above atmospheric pressure or such pipe exceeds 254 mm in internal diameter and includes in either case any connected fitting of a steam pipe.

Definition of Boiler (as per I.B.R.)Any closed vessel exceeding 22.75 litres (5 gallons) in capacity which is used exclusively for generating steam under pressure and includes mounting or fitting attached to such vessel, which is wholly or partly under pressure when steam is shut off.INTRODUCTION:

There are two kinds of boilers: 1- Fire-tube boilers 2- Water tube boilers Water tube boilers come in two basic types: i) drum type : single drum & bi drum ii) once through boilers

1.0-TYPES OF BOILERS

Types of Furnances Dry Bottom type Wet Bottom type

Firing Systems Stroke Fired Pulverized Fuel Fired CFBC Draft Systems Natural Draft Induced Draft Forced Draft Balanced Draft

The hot gases are made to flow through its various components and come in contacts with water wall tubes, heater tubes , convection tube, economiser coils , air heater, ESP and ID fans. Boilers are also classified on the basis of circulation i.e. Natural circulation boilers Forced circulation boilers

Drum-type boilers are re-circulating boilers i.e. water that is not converted to steam continues to flow through within it

Once-through type boilers convert water to steam in one pass. Instead of drum these type boilers have moister separator.

FIRE TUBE BOILER

WATER-TUBE BOILER

GENERAL ARRANGEMENT

A coal-fired boiler produces two types of ash: (1) bottom ash and (2) fly ash Bottom Ash: Bottom ash is made up of heavy particles that fall to the bottom of the furnace in the boiler as combustion take place.Bottom ash also includes molten ash,which forms on the boilers furnace walls and runs down to the bottom of the boiler.Bottom ash poses two major problems: 1-It can build up on the boiler furnace wall, which insulates the tubes and reduces the amount of heat that reaches the boiler tubes.2-Large chunk of ash , called clinkers, can drop through the furnace and damage boilers internal components.

Fly AshFly ash is made up of light ash particles that are carried through the boiler in the gas flow.It also causes two major problems:1-Being highly abrasive in nature it causes wear in the boiler tubes.2-It can collect on and around the boiler tubes thus prevents free flow of gases through the boiler. It also reduces amount of heat that reaches boiler tubes.

Flue Gases

Steam

Three common fossil fuels are used in boilers:

(1) Natural Gas, (2) Oil and (3) CoalNatural gas is the cleanest of the three common fuels but it is in short supply for power/steam plants.It causes no maintenance problems.The major maintenance problem with oil is soot accumulation.Coal is the cheapest and most available fossil fuel, but it causes major maintenance problems.These problems are primarily a result of the ash produced when coal burns.Soot: Fine black particles, produced by incomplete combustion of coal / oil

2.0-Furnace ComponentsWATER WALLS Water-tube boilers have water walls Water walls are subject to overheating, because they are close to fire

overheating occurs if there is misalignment of burners resulting in tube failure Tube rupture also occur from flame impingement

Tangential/ Membrane tubes

MATERIAL SPECIFICATIONEco & W/wall tubes: SA 210 Gr A1 / SA 192

Bank tubes: SA 192

Superheater tubes: SA 209 T1 SA 213 T11 SA 213 T22 SA 213 TP 347H SA 213 T91

Tube ruptures and leaks are also caused by clinker impact, soot blowers-due to their improper functioning corrosion , erosion, overheating due to water side scale formation etc.

3.0-HOT GAS PATH COMPONENTS 3.1-SUPERHEATERS: Superheaters can be classified in three ways: 1) by the way they absorb heat; 2) by their tube arrangement; 3) by their location in the boiler Based on above classification, boilers have one or two or all the following: Radiant superheater Platen & Pedant superheater Horizontal Superheater or Primary and secondary superheaters

CLAMP TACK WELDED WITH SHIELD (SEE DETAIL A) PROTECTION FROM LRSB9.66 M SuperHeater TubesShielding ArrangementGas Flow

CLAMP 2 MM THK,25 MM WIDTH, SUITABLE LENGTH FOR TUBE O. D. S. STEEL HALF SEGMENT SHIELD2 MM THICKDETAIL - ASTITCH WELDShielding Arrangement

Radiant superheaters are positioned in direct sight of furnace.

Tubes in Platen superheater are widely spread to prevent bridging.

Since Pendant superheaters located in the direct path of hot gases, subject to deposits, tube failure due to erosion, and ash problem.

In pendant superheaters spacers are provided to keep the tubes at proper place.Bridging is accumulation of gas to the point that it bridges the gap between the tubes.

3.2-ECONOMIZER: Economiser heats feed water as it first enters the boiler.

Economizers have some unique maintenance problems,though the gas flow velocity and temp. are less than that of super-heater area.

Economisers particularly tube bends , become corroded, and that can lead to failure.

Ash accumulation occurs to a greater degree on the economiser tubes than on the superheaters.

Another economiser problem is unconsumed coal or oil, that sometimes gets mixed in with the accumulated ash .The combustible material catch fire and explodes.

4.0-WATERSIDE COMPONENTS STEAM DRUMS & MOISTURE SEPARATORS

The function of a steam drum is to hold large volume of water and separate steam from water.

Inside the drum are numerous components for maintaining water level in the drum, separating steam from water, and keeping steam and water free from impurities.

There are four basic connections coming off the outside steam drum are :1-Boiler water supply line 2-Chemical dosing line3-Steam/ water mixture from the water walls4-Steam outlet line

Steam drum has drum internals: turbo-separators; screen, tubes and pipes

Outside of drum is insulated. If water collects inside the insulation, corrosion starts.

Another problem with steam drum metal can crack due to heating and cooling during repeated startups and shutdowns.

Corrosive sludge accumulations may also crate problem Steam drum is also subject to erosion . It is caused by flow of water through the tube to the drum.STEAM DRUMS continued

5.0-THE OUTSIDE OF THE BOILER5.1-BOILER CASING

Outside covering of boiler is called boiler casing Casings steel plate is the outside layer Inside the casing is a layer of refractory Purpose of casing is to contain fire and hot gases inside the boiler Buckling of casing may cause maintenance problem. Sometimes buckling process may be violent and casing might rupture

Down comers are pipes, usually 6 to 8 inches dia. which run from drum to the bottom of the furnace ring header. The maintenance problem found with the downcomers is corrosion.5.2-DOWN COMERS, VENTS ,DRAINS ANDCIRCULATING PUMPS Vents and drains are other sources of maintenance They are located on drums, flash tanks, headers, economisers, superheaters etc.

6.0- AIR PRE HEATERS REQUIREMENT,TYPES, CONSTRUCTION Air pre heater is another important Boiler auxiliary which preheats combustion air for rapid and efficient combustion in the furnace Its function is to recover the waste heat from the outgoing flue gas Flue gas temp at economiser outlet is around 3800C.Every 55 0C drop in flue gas temperature improves the boiler efficiency by about 2.5% . Having air heater in the down stream of economiser, the boiler efficiency is considerably improved.

6.1- AIR PRE HEATER TYPESTypes of air pre-heaters Static recuperative type : tubular air heater Rotary regenerative type: Ljungstrom air heater In the recuperative type the flue gas is transferred to the air through the heat transfer surface normally in forms of tubes / plates In regenerative type the gas flows through a closely packed heat transfer element

CONSTRUCTION DETAILSTubular air heater consists of large no. of tubes of 40 to 65 mm dia welded / expanded to plates at the end.Tube bank is placed in a shellBaffles are generally provided between tube plates to guide the air flow across the entire surface.Gas flows inside the tubes , heating the tube surface and air flows over the tubes picking the heat.

ADVANTAGES OF TUBULAR AIRHEATER No moving parts so less maintenance No auxiliary power consumptionDISADVANTAGES Occupies more area Deposits on the tube surface reduce the heat transfer Pressure drop of flue gas across the air heater is high Cross flow Heat transfer is less efficient Severely affected due to cold end corrosion Puncture in tubes results in leaking air into flue gas stream thereby increasing load of fans Replacement of punctured tube is major task requiring more downtime on the boiler

LJUNGSTROM AIR PRE HEATERS ROTARY AIR PREHEATER3900C Flue Gas Inlet Temp3100C2900C1800C Flue Gas Outlet TempCONSTRUCTION DETAILS

The Ljungstrom airpreheater consist the following major components: Rotor Bearing Housing Connecting plates Sealing arrangements Drive units Cleaning devices safety devices

Ljungstrom air preheater rotates 2 to 3 rpm and alternately passes through the gas and air passes.The heating elements pick up the heat from the flue gas and transfers it to the air when they move through the air pass.

ADVANTAGES OF LJUNGSTROM AIRHEATER Compact and hence saves space & structural cost Economical.As the boiler size increases heat transfer area also increases Initial cost and operating cost is lower Holes in the elements due to corrosion etc. will not materially affect the performance of the heater Deposit on elements does not reduce the heat transfer Pressure drop across the elements can be kept nearly constant during its operation with on load cleaning Hot primary air for coal drying in mills is possible with tri-sector design

The Trisector design permits a single Ljungstrom regenerative air preheater to perform two functions : coal drying and combustion air heating The Trisector airpreheater is flexible in meeting operational changes and is easily adaptable to varying coal moister contents etc.

DISADVANTAGES Moving parts increases possibility of outages. Leakage of air into gas , and gas and dust into air because of imperfect sealing. To keep the elements clean both soot blowing and water washing facilities are provided.

BOILER MAINTENANCE

COMMON PRACTICES

1. Annual Overhauling

2. Running Maintenance

3. Shutdown Maintenance

4. Breakdown Maintenance

BOILER MAINTENANCEBoiler overhauling is a planned, preventive maintenance activity.Objective of this periodic long maintenance is to avoid forced outages of the unit in between two consecutive overhauls i.e. to increase MTBF.1.0-PLANNING Overhauling is a multifaceted activity. It requires proper planning & co-ordination of various agencies well before unit is taken out for the purpose. OVERHAULING

PLANNING INVOLVES: 1.1 - Identification of Areas to be attended.1.2-Identification of spares.1.3-Identification of tools and tackles.1.4-Identification of required manpower.1.1- AREAS TO BE ATTENDED Area wise detailed list of the jobs to be done, is prepared, well in advance on the basis of:previous years experience,history log books of the equipment,

For Example:

Furnace AreaConvection ZoneEconomizer CoilsDrum InternalsBurnersSoot BlowersAir Pre-heaterCoal MillsFansDucts

1.2- SPARE PARTSBefore unit is taken for overhauling, the spares required are to be arranged, based on previous records of consumption.1.3-TOOLS AND SCAFFOLDINGAll lifting tools like hook-chucks, pulleys, chain blocks, jacks, spanners and wrenches of various size, hammers, hacksaw frames with spare blades etc. are ensured and made available at site store.

consumables like shims, gaskets, asbestos ropes, welding electrodes, gas cylinders, lubricants etc. are also ensured and kept at site store.Enough scaffolding items like pipes, connecting clamps are kept ready.1.4MANPOWERSince most of the maintenance activities involve skilled jobs, it is recommended to ensure sufficient skilled manpower. It is also recommended to have exclusive gang for each equipment :

2.0-CHECKSBefore a unit is taken out for overhauling a detailed study of pre-outages and outages during operation, checks are commended.2.1-Pre-outage ChecksFor this purpose a walk down survey of the boiler, to identify the areas, which require to be attended is done.Broadly following areas are checked:Areas of gas, air, oil, steam & coal leakages. Erosion of Primary and Secondary Super- heater Tubes

Erosion of Convection Bank Tubes Leakage through Expanded JointsBurners tilting arrangements, ignitersSeal air system for any leakagesExpansion joints, indicatorsSafety valves and other valvesSoot blower system for passing from valves, or for operational defectsHangers & supports checks for failure of components, fouling etcInsulation on pressure parts, non-pressure parts.Various controls and interlocksConditions of auxiliaries.

2.2-Operating ChecksThe operating checks on unit are to be performed for operation parameters given in suppliers manuals.The operating data are noted down and compared with design data given in the manual.For example a consistent high super heater outlet temperature shows fouling of furnace wall. It requires for checking of entire furnace wall and thorough cleaning.Once again compare new operating data with the pre-outage data.

Drums Drums to be inspected for any scaling of the inside surface.Check that internals are properly fitted.Check for the orientation of the holes of chemical dosing pipers.Check turbo separators, steam separating screen.Headers to be checked and cleaned.Check for rolled tube ends.

3.0-SHUT DOWN CHECKSDuring shutdown the following areas should be thoroughly checked. 3.1-Pressure PartsFurnaceAfter thoroughly cleaning the furnace walls for slag deposits, check for alignment of water wall tubesAny bow observed should be correctedCheck for any erosion of water wall tubes particularly near to wall blowers

Check for tube thickness.Furnace bottom slope tubes to be checked for any damage due to falling of slag form the furnace.Check for the inside condition of water wall tubes for corrosion, scale formation by cutting a sample tube. If scale are noticed, this requires for improvement of water qualityFurnace roof tubes are to be checked for any bowed tubesCheck all soot blower points for any damage to sleeve.Check for all hangers and supports

CONVECTION BANK TUBESShielding ArrangementGas FlowSuperheater sideEconomiser side50.8 mm O.D. Tubes Spacing50.8 mm O.D. Tubes SpacingLRSBLRSB

HALF SEGMENT M.S. SHIELD 3.15 mm THKSTITCH WELDCLAMP 2 MM THK,25 MM WIDTH, SUITABLE LENGTH FOR TUBE O. D. SHIELDING ARRANGEMENTGas Flow

Expanding of Convection TubesF.I.D.= Final internal diameter of tube = I.D. + 2 x t x (% of wall reduction) + hole clearance 100where, I.D. = Internal diameter of tube t = Thickness of tubeFor example:Tube O.D. = 2,t = 0.200Wall reduction = 8%, Hole clearance = 0.032then calculate F.I.D.

Superheaters Check that all tubes are in alignment and gap between the panels is equalCheck for the erosion of tubes near the region of soot blower and corrosion of tubes due to flue gasCheck for swelling of tubes due to overheatingCheck for the soundness of all attachment weldsCheck for refractories below rear water wall tubes.

Economiser Conduct a survey by measuring tube thickness and diameter of tubes especially at the bends.Check for the corrosion of tubes due to low flue gas temperature in that region.Check for all attachment welds and supports.Check for the condition of baffles and restore and original condition if required.

Economiser CoilOutletHeaderInletHeader

PROTECTION COVERover Economiser bends

ECONOMISER BEND

SHIELDING ARRANGEMENT over Economiser tubes

Modifications for Maximum Plant Availability Economizer Failure Due To Gas Side Erosion Provision of spare coil assemblies. Economizer coil assemblies are replaced every year instead of on-site repair. Survey time was brought down from 50 days to 18 days.Erosion prone areas are pre-shielded over the straight length. The bends are protected by cover shields.Removed coil assemblies are repaired in workshop and kept ready for reuse in next survey.Flue gas velocity in boiler no. 1 & 2 was reduced from 17m/sec to 12m/sec by increasing the depth of economizer. More coil assemblies were added.Lower erosion.Reduced draft loss (gain of 17 mm)Gain in feed waster temperature (3oC)Reduction in tube leakageFrom 1969 to 1989 fifty six (56) leakage were detected. From 1990 to Jan. 2001 only nine leakage occurred.

Drums Drums to be inspected for any scaling of the inside surface.Check that internals are properly fitted.Check for the orientation of the holes of chemical dosing pipers.Check turbo separators, steam separating screen.Headers to be checked and cleaned.Check for rolled tube ends.

3.2-Duct SystemGas DuctsGas ducts in the Boiler are vulnerable to corrosion and erosion wear, they need a thorough checking. These ducts are: i ) Economizer to air heater. ii ) Air-heater to E.S. Precipitator iii) ESP outlet to ID Fan inlet iv) ID Fan outlet to chimneySupports and braces provided for the purpose, should also be checked.Check all the expansion joints physically for any permanent deformation due to excess loading.

Air DuctsCheck wind box compartmentCheck struts and hangersCheck the condition of measuring devices such as venturies, nozzles etc.Check for damaged insulation

Coal pipes are subjected to much abrasive wear due to erosive nature of our coal.

Bends are also subjected to severe erosion.

If the erosion is less, patch welding of bends can be carried out. For longer radius bends (R>3D) basalt lining could be provided.

All the victaulic coupling joints are to be checked.

Check all supports of pipes for load sharing while replacing portion of pipe 3.3-Coal Piping

3.4-Dampers Gates and VanesNormally gates are meant for shut off purpose and dampers for control purpose.These are to be checked for their proper functioning.

Check manually for free movement of dampers. Check that flap to flap sealing is perfect.

Check for proper linkage connectionsGates to be checked for any wearCheck gates for full opening and closingCheck gland packing of damper shaftsCheck inside surface of driving air cylinder for any corrosionCheck damaged O ringsCheck the condition of connecting pipe

3.5-Fuel Systems A-Pulverizers and Feeders Pulverizers are more susceptible to wear than other auxiliaries of Boiler.In pulverizers check for following areas for wear out:separator body liners, bottom liners, bull ring segments, grinding rolls scrapers and scraper guards, mill discharge valves/ valve assembly classifiers deflector vanes, inner cones.If amount of wear is excessive, replace, if wear is less the same could be built by hard facing electrodes.

Analyze lubricating oil used in : (i) Journals and (ii) Vertical Shaft Check the condition of bearings of (i) Journal Assembly (ii) Vertical Shaft (iii) Worm Shafts Check the pump hub, clean and fit it Check the oil cooler tubes for any choking or leakages Check the condition of worm shaft and worm gear

Check air sealing housing for any coal / ash built upCheck whether journal springs are in good conditions.Check thoroughness of tramp iron spoutVisually inspect feeders for any wear

Seal Air System Check:All seal air lines for thoroughness and cleanlinessSeal air hoses for any damageSeal air damper for proper closing and openingCheck and clean seal air line filtersService the fan bearingsDifferential pressure switches and other instrument tapping

Nozzle and Tilting Mechanism Check all the nozzles for slagging and clean the slag Check for the damages to the nozzle due to the following: (a)Warping it occurs due to excess temper(b)Erosion and(c)Welds Breaking

Refractory throat Shape Move nozzle up and down-check there is no restriction to its free movement.

Check the pattern of wear on coal nozzles.Check that all pins are OKCheck that coal pipe load does not come on to nozzleCheck that nozzles move in unison in both the directionsCheck free movements of secondary air dampers. Set for full closing.

Flame Scanners Clean the detector tube lens and check for any damage in flexible hoses.Check location of the scanner head with reference to the nozzle tip.Check whether cooling air path is through.Check the shutter for proper functioning. Check the card and control wires.Check service air fans, its bearing and motor.

4.0FUEL OIL SYSTEM - LIQUIDIGNITORS Check physical condition of ignitor hornsCheck for proper functioning of jamesburry valveCheck whether transformer is in healthy conditionClean the filters in oil and air linesClean orifice and check for flow of required quantity of oilCheck for air leakage from the wind box and stop leakage

Oil Guns Remove oil guns and check for choking and erosion of mixing plate and spray plate holes. Thoroughly clean and fit back.Check the condition of flexible hoses for any damage. Replace the damaged hoses. While fitting back, avoid kinks.Check for free operation of retract mechanism. Service all sliding parts.Check the condition of solenoid valve and replace damaged O ring. Check for electrical operations.

Check whether limit of retract mechanism act properly.Check and maintain required gap between gun tip and diffuser face.Check all hand operated and control valves of oil system for their proper functioning and attend any defect.

5.0-AIR HEATERCheck the condition of hot and cold end heating surfaces for any deposits pluggage.To remove corrosive deposits, carryout water washing of heating elements.Check for any heavy plug gage at outer baskets. This may indicate the incomplete coverage of soot blower. A check of the sweep should be made and mechanism adjusted.Check the condition of various seals such as radial, circumferential, bypass axial seals. Any erosion observed, change the seal.

Check the condition of sector plates and adjust rods.Check the condition of support bearing guide bearings and lubricating oil for any contamination. Check for proper operation of lubricant oil pumps and thermostat.Check for any ash entry to drive gear box.Service electrical motor and air motor for their operating condition.

6.0-FansCheck for erosion wear in fan casing, rotating parts, discharged guide vanes etc.Clean entire system free from ash. All the eroded portion to be restored.Check the condition of bearings and its lubricants.In order to safeguard the bearings, ground the welding machine directly to the impeller.

Check the tightness of all foundation bolts.Check for proper operation of inlet control vanes, inlet & outlet dampers. Set for proper limits of opening and closing by adjusting limit switches.Check for vane linkages of control vanes and grease.Check complete bearing lubrication system.

Check the conditions of supports and its welds with the casing and ducting.Run the fan with all interlocks and check the record of vibration. If vibration are high identify the cause and eliminate.

7.0-VALVESCheck all critical valves like low point drains, blow off, S.H. Drains, spray isolation, main steam and feed isolation, control valves.Valves in which passing was reported by operation or noted by maintenance, should be opened and checked for the condition of disk or seat. If needed, they should be lapped.Check the sealing rings, glands and change if required.

Check for free travel of valve stem & for any bend is stem.Check for erosion in valve body and in other location.For remote operated valves, check and set position of torque switches.In control valves, check the contour of disc and trim for any damage.For safety valves, check the condition of springs and valve setting.

8.0-SOOT BLOWERS AND FURNACE PROBESCheck valve seat and discCheck for free movement of lanceCheck for any bend in lance tubeCheck the gland packing and change, if requiredService the driving and rotating motor and also the gear boxClean the chain from ashSet the proper blowing pressure and limit switches for each blowers

Service the valves in pressure reducing station of soot blowerFor all wall blowers check the correct distance of the nozzle center line from furnace wall (38 mm)Check and simulate the soot blower control panel for proper functioning with all protection.Check free movement of the furnace probe and condition of thermocouple.Check and ensure working of auto retract interlock of furnace probe when gas temp exceeds preset value.

9.0INTERLOCKS, CONTROLS AND INSTRUMENTATIONFor safe and steady operation of the unit, it is essential that various controls and interlocks should function properly. A-To ensure proper functioning of boiler utmost care is required in the following: Furnace purge supervisionIgniter controlPulverizer controlSecondary air damper controlFlame scanner intelligenceBoiler trip protection

B.Check and simulate the following controls also:Feed water controlSteam temperature controlCombustion controlFurnace draft control

10.0MISCELLANEOUSPent house, lower dead spaces and arch enclosures. A- Pent House Check condition of hanger rods.Check whether all hanger rods are properly loaded. Check the rods for corrosion and thinning.Check the condition of roof casing for any crack and missed welds on roof expansion joints and joints between roofs.

Check the condition of pent house roof for insulation.Check the adequacy of braces and supports.Check attachment for integrity.The following areas should also be closely watched:The four water wall headers cornersThe under side casing adjacent to the roof tube inlet headerThe junction points on the side wall casingBuck StaysBottom Arches

Boilers Tube Failure / Repairs

Main function of tubes is to transfer the heat produced by burning fuel to water or steam.

The heat passes in through the wall of the tube and is picked up by the water or steam.

Boiler tube leaks are a common occurrence in all types of boilers TubeFurnace Flame

1.1-Overheating Overheating is the most common cause of boiler tube failures.

Tubes overheat as result of:

Deposits from boiler water Inadequate flow of water or steam through the tubesImproper flow of hot gases through the boilerRefractory failureImproper boiler operation1.0-Causes of Boilers Tube Failures

Boiler tubes are constantly exposed to heat intense enough to cause them to melt

Circulation of water and steam inside the tube keep them cool

deposits from the boiler water, that coat the inside of tube can prevent water / steam flow from cooling the tube

Scale is an example of boiler water deposits

Inadequate flow of water or steam caused by blockage can also cause a tube to overheat, resulting in tube failure

1.2-Corrosion

corrosion is deterioration of metal through chemical action

It can affect both the inside and outside of boiler tubes by thinning the walls and thus reducing the strength of the tube

Reasons of tube corrosion:

Improper treatment of water by using wrong chemicals, incorrect amount of chemicals

Oxygen in the boiler also corrodes inside boiler tubes

Rust is another form of corrosion

Sources of corrosion on the outside of the tube are: Corrosive slag and Corrosive acid

Corrosive slag is formed by contaminants in the boiler fuel Moisture that combines with the sulfur found in ash or soot produces acid. This acid attacks the outside of the tube and corrodes it

1.3-Erosion Erosion is the gradual wearing away of tube metal It can occur on the inside and /or outside of the tube On the inside of the tube erosion is caused by the flow of water or steam On the outside it is produced by the flow of gas and ash In coal fired boiler s, it is severe because of large amounts of abrasive ash produced

Mechanical stress can be caused by pressure, thermal expansion, weight of the tubes and vibration

Boiler tube are built to withstand a certain amount of mechanical stress, but excessive stress leads to mechanical failure.

Thermal expansion takes place when boiler is fired. Tubes expand when they are heated and contract when they are cooled

Stress occurs when expansion and contraction can not take place freely, 1.4-Mechanical Stress

1.5-Material Defects

Material defects are weak spots in the tube that occur at the time of tube manufacturing.

Defective tube fail during normal operation

These can be avoided by careful inspection

Tube wall lamination is the most common example of material defects in boiler tubes. This occurs during fabrication of the tube.

2.0-Types of Failure and Deformities2.1-Boiler Tube Failures There are three basic kinds of ruptures Thin-lipped rupture Thick-lipped rupture Double-ended rupture

Most boiler failures are due to tube failures

Thin-lipped rupturesThin-lipped ruptures are caused by sudden and severe over heating.There is no evidence of cracking of the metal in a thin lipped rupture.Thick-lipped ruptures Thick lipped ruptures are caused by less intense, but more prolonged heating

Thermal cracks: Thermal cracks are also called creep cracks. These are developed by prolonged mild overheating or repeated short time heating

Pinhole leaks:Pinhole leaks can result from overheating, corrosion, erosion, mechanical stress or material defectsSteam leakage from pin whole leak, if it is directed against another tube , it will erode other tube. It is called steam gouging

Running MaintenanceGenerally problems are faced in following equipment:

1. Ignitors

2. Oil Guns

3. Wall Deslaggers- Steam leakage- Mechanical jamming 4. Valves- Steam leakage from glands- Steam passing5. Bi Colour Gauge Glass

MAINTENANCE MANAGEMENT

MAINTENANCE OF BOILER WALL TUBESOpen the bottom hopper entrance door and ensure that hydraulic gate of bottom ash hopper is locked, in open position.Ensure that contractor has erected proper scaffolding and climbing arrangements in the entire furnace.Ensure that contractor has provided wire mesh at different stages as a safety measure.Ensure that Electrical Maintenance Department has provided sufficient lighting arrangement at different stages of scaffolding.

Clean each tube of all the four walls by scrappers, wire brush, sander grinder from top to bottom and ensure that no dust / slag remains on the outside surface of tubes.Number all tubes of north & south side wall east to west at different stages.Number all tubes of east and west side wall north to south at different stages. Inspect visually each tube of all four walls for dent mark, mis-alignment and bulging and record it in drawing with actual location.Measure erosion in each tubes and bends of all four walls and record in measurement book (MB).

Inspect all soot blower points for any damage to sleeve and record it in the log book.Replace / rectify all damaged shields. Mark the position of soot blower box with the help of centre punch in X and Y axis before cutting the soot blower box. Remove soot blower after disconnecting electrical connections and cut soot blower box for cutting the tubes which are connected with soot blower box.Mark the length at top and bottom from both inside and outside on the tube to be cut.

Cut the tackings of the tube with buck stay or scallop bar.Puncture the tube from inside and outside of the furnace at top and bottom positions by grinding.Cut the tube length by hacksaw blade or saber saw and take it out.Plug the cut ends of the tubes with wooden plugs.Shift the cut tube lengths to Boiler shed after marking on the tube, tube number and direction of wall from where it is cut, by white paint. Fabricate new bend as per the cut bend.

Fabricate new lengths as per the straight cut lengths and make V for butt welding by grinder or bevelling machine.Prepare V edge in both the cut ends tubes insitu.Clean and portion (externally and internally) of new length and cut ends at location.Place new length in cut position and check alignment. Get the joints welded.Ensure that the welding work is carried out by high pressure certified welders.

Ensure that the welding of joints is carried out according to the recommended welding procedures as per combustion engineering welding manual and with suitable welding electrodes.Issue clearance to radiography contractor to carryout radiography of all the new welded joints.Examine all the radiography films for any type of welding defect.Get the defective joints repaired.Ensure that no defect persists in all the repaired joints by getting it re-radiographed.Clean the dog house by vacuum cleaning through Ash Plant Maintenance Department.

Inspect the screen tubes, hanger tubes roof tubes and record it in log book.Inspect all elevation buckstay for any deformation and record it in log book.Inspect all wall tubes for misalignment or dislocation and record it in log book.Check all the welds of all the tubes with buckstay for any damage at all elevations and record it in log book.Replace the particular beam of the buck stay which is found deformed as per instruction of Maintenance I/C or HOD.

Get the alignment done of each particular tube which is found misalignment as per instruction of Maintenance I/C or HOD by using screw jack, truss, wooden block and sleeper. Get the tubes tack welded to the corresponding buckstay beam in contact to the tube after the tubes are straightened by jacking.Ensure that the tack welding of the tube to the buckstay beam is carried out by highly skilled special grade welder or certified high pressure welder. Issue clearance to Civil Maintenance Department to carryout refractory work in all inspection door/peep hole area, burner zone, rear arch, bottom hopper, roof tubes, soot blower bends and other instrument probe points.

Replace or rectify all soot blower sleeves which were found defective as per instruction of Maintenance I/C or HOD. After removing soot blowe and cutting the soot blower box.Fit all the soot blower cover boxes and fully weld them after checking that refractory material is provided inside the box by the Civil Maintenance Department.Fit soot blowers which were removed to carryout tube replacement/sleeve replacement work in respective position.

Welding Electrodes & Filler Wires

Material

Filler wire

Electrodes

SA 192

TGSM (ER70S-G)

Overcord-S

(E6013)

SA 210 Gr A1

TGSM (ER70S-G)

Overcord-S

(E6013)

SA 209 T1

TGSM (ER70S-G)

Supratherm Spl. (E7018-A1)

SA 213 T11

TGS1CM (ER80S-G)

Chromotherm-2 (E9018-B3)

SA 213 T22

TGS2CM (ER90S-G)

Chromotherm-1 (E8018-B2)

SA213 TP347H

S.S. 120S (ER347)

Superinox-1B

(E347)

SA 213 T91

TGS9cb (ER90S-B9)

CM9cb

(E9018-B3)

Calculation of Minimum Wall Thickness of TubesAs per ASME :PDt=+2f + P0.005D + CAs per IBR :t=PD2f + P+ Cwhere,t = Minimum Wall ThicknessP = Working Pressuref = Maximum Allowable StressC = ConstantD = Outside Diameter