Boiler tube failure analysis
is more complicated than the water main pipe since temperature is involved. In the Figure we can see that the major four
causes of boiler tube failure is wear, corrosion, temperature and stress.
Wear
Wear, which lead to locally metal loss mechanically seldom comes from the improper installation of boiler which can be avoid by following installation procedure and regularly inspection. Erosion is the major reason that caused the decreasing of OD/ID which has less mechanical strength and rupture eventually. The four major locations that erosion occurs are at the fire side, the preboiler, the afterboiler, and the water side and steam side.
Fire-side mechanisms cause most erosion-related failures and can be further subdivided as erosion related to soot blowing, steam cutting, fly- ash attack, coal-particle impingement, and falling slag. Fluidized-bed and other special-purpose boilers sometimes suffer severe attack. Boilers burning wood chips and bagasse are often eroded by entrainment of tramp contaminants such as sand and other foreign material in furnace gases. Incinerators suffer similar fire-side problems. Because fire-side mechanisms cause most erosion-related failures, each mechanism will be dis cussed in detail.
Erosive metal loss on water-side surfaces is comparatively rare. Cases do occur, however. Internal-surface discontinuities or solid foreign objects lodged within tubes can disturb flow, increase turbulence, and cause wastage.
Preboiler attack is confined primarily to feedwater systems. Turbine erosion is common in afterboiler regions. Burner nozzles, blowdown piping, condensate return lines, and many other boiler components are also eroded.
Corrosion
Corrosion is one of the major causes the boiler tube failure which will lead to the metal loss on the tube surface. Leakage and rupture of tube due to increasing stress and strain are two common failure mode of corrosion attack. Sometime the build-up of corrosion product (debris) will cause clogging of tube. The corrosive media can come from environment, coal ash, water even from service and maintenance. Different corrosion mechanism can be characterized by visual exam generally.
Caustic corrosion attack --- Localized wall loss on the ID surface of the tube, resulting in increased stress and strain in the tube wall
Oxygen Pitting --- 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.
Acid Attack --- Corrosive attack of the internal tube metal surface, resulting in an irregular pitted or, in extreme case, a ‘’swiss cheese’’ appearance of the tube ID
Superheater Fireside Ash Corrosion --- External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed.
Waterwall Fireside Corrosion --- External tube metal loss( wastage) leading to thinning and increasing tube strain.
Temperature
In condition that boiler tube metal exposed under temperature exceed design limit, rupture will happen. Rupture part shows a longitudinal open on the tube wall, usually described as ‘’fish mouth’’.
Short-term Overheat --- 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.
Long-term Overheat(Creep) --- 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.
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.
Graphitization --- Long-term operation at relatively high metal temperatures can result in damage in carbon steels of higher carbon content, or carbon-molybdenum steel, and result in a unique degradation of the material in a manner referred to as graphitization.
Stress
Both static load and cyclic load will cause the failure of boiler tube. Static load usually shows in the case called “stress corrosion crack” which is the failure happens in the combination of tensile stress and corrosive media. The tensile stress can come from external force or from improper heat treatment on boiler tube. Cyclic load comes from vibration will cause fatigue failure. Internal pressure is also a physical cause leading to the rupture or leakage when tube wall is thinned by other failure mechanism.
Stress Corrosion Cracking --- 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.
Mechanical Fatigue --- Damage most often results in an OD initiated crack. Tends to
be localized to the area of high stress or constraint.
Corrosion Fatigue --- ID initiated, wide transgranular cracks which typically occur adjacent to external attachments.
Wear
Wear, which lead to locally metal loss mechanically seldom comes from the improper installation of boiler which can be avoid by following installation procedure and regularly inspection. Erosion is the major reason that caused the decreasing of OD/ID which has less mechanical strength and rupture eventually. The four major locations that erosion occurs are at the fire side, the preboiler, the afterboiler, and the water side and steam side.
Fire-side mechanisms cause most erosion-related failures and can be further subdivided as erosion related to soot blowing, steam cutting, fly- ash attack, coal-particle impingement, and falling slag. Fluidized-bed and other special-purpose boilers sometimes suffer severe attack. Boilers burning wood chips and bagasse are often eroded by entrainment of tramp contaminants such as sand and other foreign material in furnace gases. Incinerators suffer similar fire-side problems. Because fire-side mechanisms cause most erosion-related failures, each mechanism will be dis cussed in detail.
Erosive metal loss on water-side surfaces is comparatively rare. Cases do occur, however. Internal-surface discontinuities or solid foreign objects lodged within tubes can disturb flow, increase turbulence, and cause wastage.
Preboiler attack is confined primarily to feedwater systems. Turbine erosion is common in afterboiler regions. Burner nozzles, blowdown piping, condensate return lines, and many other boiler components are also eroded.
Corrosion
Corrosion is one of the major causes the boiler tube failure which will lead to the metal loss on the tube surface. Leakage and rupture of tube due to increasing stress and strain are two common failure mode of corrosion attack. Sometime the build-up of corrosion product (debris) will cause clogging of tube. The corrosive media can come from environment, coal ash, water even from service and maintenance. Different corrosion mechanism can be characterized by visual exam generally.
Caustic corrosion attack --- Localized wall loss on the ID surface of the tube, resulting in increased stress and strain in the tube wall
Oxygen Pitting --- 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.
Acid Attack --- Corrosive attack of the internal tube metal surface, resulting in an irregular pitted or, in extreme case, a ‘’swiss cheese’’ appearance of the tube ID
Superheater Fireside Ash Corrosion --- External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed.
Waterwall Fireside Corrosion --- External tube metal loss( wastage) leading to thinning and increasing tube strain.
Temperature
In condition that boiler tube metal exposed under temperature exceed design limit, rupture will happen. Rupture part shows a longitudinal open on the tube wall, usually described as ‘’fish mouth’’.
Short-term Overheat --- 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.
Long-term Overheat(Creep) --- 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.
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.
Graphitization --- Long-term operation at relatively high metal temperatures can result in damage in carbon steels of higher carbon content, or carbon-molybdenum steel, and result in a unique degradation of the material in a manner referred to as graphitization.
Stress
Both static load and cyclic load will cause the failure of boiler tube. Static load usually shows in the case called “stress corrosion crack” which is the failure happens in the combination of tensile stress and corrosive media. The tensile stress can come from external force or from improper heat treatment on boiler tube. Cyclic load comes from vibration will cause fatigue failure. Internal pressure is also a physical cause leading to the rupture or leakage when tube wall is thinned by other failure mechanism.
Stress Corrosion Cracking --- 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.
Mechanical Fatigue --- Damage most often results in an OD initiated crack. Tends to
be localized to the area of high stress or constraint.
Corrosion Fatigue --- ID initiated, wide transgranular cracks which typically occur adjacent to external attachments.