CFBC BOILER

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FAILURE MECHANISM OF CFBC BOILER

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CFBC BOILER:

CFBC BOILER Prepared By: Jaivin Ghelani

INTRODUCTION TO CFBC BOILER:

INTRODUCTION TO CFBC BOILER CFBC boiler work on principle of w hen an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”. In circulating fluidized bed combustion boiler, generally two types of air primary as well as secondary are feeded in the boiler. A primary air is introduced through bottom of the bed. The bottom of the bed supported by water cooled membrane walls with specially designed air nozzles which distributes the air uniformly. The fuel and limestones are fed into the lower bed. Carbon particles in the fuel are exposed to the combustion air. The balance of combustion air is introduced at the top of the lower, dense bed. This staged combustion limits the formation of nitrogen oxides.

Introduction:

Introduction The bed fluidizing air velocity is greater than the terminal velocity of most of the particles in bed and thus fluidizing air elutriates the particles through combustion chamber to u-beam separator at furnace exist. The captured solids including any unburned carbon and unutilized carbon oxide, are re-injected directly back into combustion chamber without passing through an external recirculation. This internal solids circulation provides longer residence time for fuel and limestone, resulting in proper combustion and improved sulphur capture.

Failure mechanisms:

Failure mechanisms Primary Failure Mechanisms – A mechanism is defined as the process by which something comes into being. As listed below: Stress rapture Water-side corrosion Fire side corrosion Erosion

Failure mechhanism:

Failure mechhanism Stress rapture: Short tern over heating High temperature creep Dissimilar metal weld

Failure meechanism:

Failure meechanism Short term over heating : Symptoms: 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: • Blockage of tube internally • Loss of boiler coolant circulation or low water level • Loss of coolant due to an upstream tube failure • Over firing or uneven firing of boiler fuel burners

Failure mechanism:

Failure mechanism High temperature creep: Symptoms: 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: • Partial blockage by debris, scale, or deposits • Exposure to radiant heat • Before the change to a higher grade material • Just above the final outlet header • Exposure to high gas temperature due to blockage of gas passages or laning • Have incorrect grade of steel material • Have higher stresses due to welded attachments

Failure mechanism:

Failure mechanism Dissimilar metal weld cracking: Symptoms: 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: Failures at DMW locations occur on the ferritic side of the butt weld. These failures are attributed to several factors: high stresses at the austenitic to ferritic interface due to differences in expansion properties of the two materials excessive external loading stresses and thermal cycling, creep of the ferritic material. As a consequence, failures are a function of operating temperatures and unit design. DMW failure where ferritic material hascompletely separated,leaving the DMW Photomicrographvshowing DMW creep voids at ferritic interface s

Failure mechanism:

Failure mechanism Water-Side Corrosion Caustic Corrosion Hydrogen Damage Pitting (Localized Corrosion) Stress Corrosion Cracking

Failure mechanism:

Failure mechanism Caustic corrosion: Symptoms: Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall. Causes: • Selective deposition of feedwater system or preboiler corrosion products at locations of high heat flux • Concentration of sodium hydroxide from boiler water chemicals or from upsets in the water chemistry

Failure mechanism:

Failure mechanism Pitting (Localized corrosion) Symptoms: Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feed water inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods. Causes: • Exposure of the tube to water with high acidic or oxygen concentrations • Existence of close-fitting surfaces and deposits where differences in oxygen concentration can be produced

Failure mechanism:

Failure mechanism Hydrogen damage Symptoms: Intergranular micro-cracking. Loss of ductility or embrittlement of the tube material leading to brittle catastrophic rupture. 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.

Failure mechanism:

Failure mechanism Stress corrosion cracking Symptoms : 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.

Failure mechanism:

Failure mechanism Erosion Erosion due to fly Ash Erosion due to soot blower Erosion due to air ingress

Failure mechanism:

Failure mechanism Fly ash: Factors influencing fly ash erosion in coal fired boilers are The velocity of flue gas: For low ash coals, the weight loss in pressure parts due to erosion is proportional to flue gas velocity to the power of 1.99. However for high ash Gondwana coals the erosion rate is velocity to the power of 3 to 5. The power depends upon the percentage of ash in coal, the percentage of silica in coal ash, the percentage of quartz in this silica, the percentage of alpha quartz in this quartz, and the structure of alpha quartz. The temperature of flue gas: Higher temperature softens the minerals in the ash as well as reduces the strength properties of the material of pressure parts; due to this ash erosion is not predominant in high temperature zones like furnaces, final superheaters , exit reheaters , etc. The ash erosion mainly starts in the conventional two-pass boilers from the area where gas temperature is around 700 – 750 deg.C . The low temperature superheater (LTSH) and economizer are the areas where ash erosion is severe in a conventional two-pass boiler. The temperature of flue gas entry to LTSH can be around 650 to 700 degree C and leaving, the economizer can be around 350 – 300 degree C. The minerals, which mainly constitute the ash in flue gas at these temperatures, become hard and attain its full abrasiveness

Failure mechanism:

Failure mechanism Erosion due to soot blower While second pass of the boiler is a major zone prone to erosion due to fly ash in flue gases, the furnace water wall tubes are mainly prone to erosion due to soot blower steam. Causes: Condensation in the soot blowing system Improper setting of soot blower pressure Poorly angled blowers Condition of soot blower nozzle Remaining of soot blower inside furnace accidentally with steam supply to the blower continuing While the first four causes result in gradual reduction in tube thickness of the water tubes around the soot blower, the fifth reason can result in immediate tube failure.

Failure mechanism:

Failure mechanism Erosion due to air ingress: Any ingress of air in the boiler from atmosphere is highly detrimental to the boiler operation as well as availability. The air ingress affects the boiler performance as well as can result in erosion of boiler tubes resulting in boiler tube failure. The source of air is through incomplete welded fins of the boiler tubes.

Failure mechanism:

Failure mechanism Fire side corrosion: Water wall high temperature corrosion Super heater fireside ash corrosion low temperature corrosion

PowerPoint Presentation:

Waterwall Fireside Corrosion Symptoms : External tube metal loss (wastage) leading to thinning and increasing tube strain. Causes: Corrosion occurs on external surfaces of waterwall tubes when the combustion process produces a reducing atmosphere (sub stoichiometric ). This is common in the lower furnace of process recovery boilers in the pulp and paper industry. For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing. Boilers having maladjusted burners or operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates.

Failure mechanism:

Failure mechanism Super heater fireside ash corrosion Symptoms: 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 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.

Failure mechanism:

Failure mechanism Low temperature corrosion:

Methodology :

Methodology Boiler tube failure analysis. Find out main causes of tube failure. Suggested solution and check feasibility Optimum solution and implementation

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