5 FOCUSING NDE SPECIFICALLY FOR KNOWN OR SUSPECTED MECHANISMS OF DEGRADATION
5.1 Service-Induced Damage Mechanisms
Tables 5-1 through 5-35 provide a concise guide to each of these service-induced damage mechanisms. These tables should be used when one or more of the damage mechanisms are known to be or suspected of being active. The format for each damage mechanism includes these items:
• A brief summary of the mechanism
• A description of the damage and a picture showing the damage’s appearance
• The locations where it occurs
• How to find and quantify the damage
• Optional NDE techniques
• Acceptable levels of damage
• Repair options
• Corrective actions
• Any related damage mechanisms that may also be active
• Other comments
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-1
Corrosion Fatigue
Damage Mechanism: Corrosion fatigue Summary of Mechanism:
Damage and cracking resulting from the combined action of cyclic stresses and a corrosive environment. Crack initiation involves the repeated mechanical disruption and healing of the protective oxide scale on water-touched surfaces, resulting in aligned corrosion pits, then crack-like pits, and finally, multiple, transgranular, oxide-filled cracks oriented normal to the predominant tensile stress field. Cracking is influenced by a complex interaction of the rate and magnitude of loading (strain), fluid pH and dissolved oxygen levels, and alloy chemistry.
Appearance of Damage:
• Outside surface: Since corrosion fatigue initiates at the inside surface, damage may not be visually apparent; damage that has penetrated the wall thickness may first appear as a pinhole leak (most common).
• Inside surface: Multiple, parallel cracks, often associated with corrosion pits or surface discontinuities, can be either axial or circumferential, but cracks are oriented normal to the predominant tensile stress.
Where Damage Occurs:
• The primary occurrence is on the water side in waterwall and economizer tubing, usually located adjacent to attachments or restraints. Corrosion fatigue also occurs in tube/pipes outside the gas path, such as riser and downcomer tubes to and from the drum.
• Typical locations include windbox casing attachments, buckstay attachments, and scallop bar attachments. In economizer tubing, failures have been reported in bends or the heat-affected zone of welds.
How to Find Damage:
• Internal: visual inspection (video probe, borescope)
• External: magnetic particle or penetrant testing
• Digital, film, or phosphor plate radiography How to Quantify the Severity of the Damage:
• Ultrasonic flaw sizing
• Radiographic density measurements
• Tube sampling Optional NDE:
• UT (phased array) What Is Acceptable:
• No cracks. If cracks are detected, a fitness-for-service analysis should be performed to justify continued operation.
Repair Options:
• Replace the affected area. Temporary pad weld repairs are not recommended.
Corrective Actions:
• See Chapter 19 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
• Thermal-mechanical fatigue Other Comments:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-2
Fly Ash Erosion
Damage Mechanism: Fly ash erosion Summary of Mechanism:
An increase in erosion rates occurs with higher local velocities, high ash loads containing abrasive particles such as quartz (particularly alpha quartz), and low angles of incident attack. Fly ash erosion accelerates tube wastage by direct mechanical material removal. Removal of fire-side oxide also increases the fire-side oxidation rate. The rate and extent of erosive processes are affected by particle velocity, angle of impact, particle composition and shape, and erosive resistance of the tube surface including compositional and temperature variations. Fly ash erosion damage is usually very localized, and as erosion becomes more severe, tubes begin to thin, flattened areas develop, and eventually internal pressure leads to tube rupture.
Appearance of Damage:
• Outside surface: Fly ash erosion causes flat spots, ovality, and formation of edges on straight tube sections. There may also be some surface hardening caused by cold working of the surface by particle impingement. Fresh rust appears on tubes only a few hours after boiler washing.
• Inside surface: The damage is external in origin and yields no detectable signs internally until failure has occurred.
Where Damage Occurs:
• Fly ash erosion will be a concern where nonuniform, high gas flows develop locally anywhere in the boiler, particularly on the waterwall in the back pass, economizer, primary SH, and inlet sections of RH tubes.
How to Find Damage:
• Rusted tube locations within a few hours of a boiler wash indicating the removal of protective surface oxides
• Ultrasonic testing (UT) to detect wall thinning How to Quantify the Severity of the Damage:
• Pulsed eddy current (PEC) testing
• Low-frequency electromagnetic technique (LFET)
• Tube sampling Optional NDE:
• Radiographic testing What Is Acceptable:
• The minimum tube wall thickness is greater than the specified minimum wall thickness. For more severe tube loss, a fitness-for-service analysis should be performed to justify continued operation.
Repair Options:
• Change problematic geometries; repair, replace, and align damaged components. Temporary pad weld, spray coating, or shielding may be used.
Corrective Actions:
• See Chapter 21 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
Other Comments:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-3
Hydrogen Damage
Damage Mechanism: Hydrogen damage Summary of Mechanism:
Hydrogen damage is caused by the reaction of iron carbides in the boiler tube steel with hydrogen produced as a result of increased corrosion reactions, particularly those taking place in low pH water with dirty tubes in high heat transfer areas. The combination produces methane (CH4) at the grain boundaries of the tube steel. As the relatively large, insoluble methane gas molecules accumulate, they force open microfissures in the metal. As a result, local decarburization results in a loss of material strength. With time, the depth of fissuring increases and eventually leads to through-wall failure.
Appearance of Damage:
• Outside surface: Since this is a water-side damage mechanism, no external surface damage will be visible until failure when there will be a thick-edged, brittle, final fracture.
• Inside surface: Gouged areas and/or thick deposits, along with localized internal deposition, wall loss, pitting, or gouging, may or may not be present beneath the deposits.
Where Damage Occurs:
• Locations where the water/fluid flow adjacent to the tube wall is disrupted
• Locations with existing internal deposits or with a high heat flux/transfer
• Locations where boiling first initiates, locations with thermal-hydraulic flow disruptions or where there is localized overheating of the tubes (fire-side conditions)
How to Find Damage:
• Internal visual (video probe, borescope)
• Digital radiography (RT)
• Thermography (thermal line scan)
How to Quantify the Severity of the Damage:
• Ultrasonic testing (UT)
• Tube sampling Optional NDE:
• Macro etching of a tube section with a 50% heated solution of hydrochloric acid What Is Acceptable:
• The predicted minimum wall thickness at the end of the next service period (time to the next re-examination) is greater than the minimum Code-allowable wall thickness
Repair Options:
• Replace tubing when the wall thickness is less than the specified minimum.
• Damage should not be repaired locally by the use of pad welding or canoe/window welds.
Corrective Actions:
• See Chapter 22 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
Other Comments:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-4
Acid Phosphate Corrosion
Damage Mechanism: Acid phosphate corrosion Summary of Mechanism:
Corrosion that results from the breakdown of the protective water-side oxide layer, due to the buildup of internal deposits and the local concentration of phosphate salts, leads to under-deposit corrosion. Damage normally begins with disruption of the fluid flow adjacent to the tube wall and/or the formation of deposits from feedwater corrosion products. A subsequent reaction concentrates acid phosphates in the deposit and eventually leads to corrosion and failure.
Appearance of Damage:
• Outside surface: Since this is a water-side damage mechanism, no outer surface damage is visible unless damage has progressed to cause a pinhole leak.
• Inside surface: Gouges, partially or completely filled with layered deposits, may be present on the inside surface, often with a distinct transition between mild and severe corrosion.
Where Damage Occurs:
• Locations where the water/fluid flow adjacent to the tube wall is disrupted
• Locations with existing internal deposits, or with a high heat flux/transfer
• Locations where boiling first initiates, thermal-hydraulic flow disruptions, or where there is Localized overheating of the tubes (fire-side conditions)
How To Find Damage:
• Internal visual (video probe, borescope).
• Continuous thickness measurement techniques for the tube crown are especially effective.
How To Quantify the Severity of the Damage:
• Ultrasonic thickness measurements (internal rotary probe or from the outside surface)
• Tube sampling
• Pulsed eddy current (PEC) testing Optional NDE:
• Thermography
• Electromagnetic acoustic transducer (EMAT) technology
• Low-frequency electromagnetic technique (LFET) What Is Acceptable:
• The predicted minimum wall thickness at the end of the next service period (time to the next re-examination) is greater than the minimum Code-allowable wall thickness.
Repair Options:
• Repair/replace tubing when the wall thickness is less than the specified minimum.
Corrective Actions:
• See Chapter 23 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-5
Caustic Gouging
Damage Mechanism: Caustic gouging Summary of Mechanism:
Corrosion that results from the breakdown of the protective water-side oxide layer, due to the buildup of internal deposits and the local concentration of caustic to high pH levels, leads to under-deposit corrosion. As with the other under-deposit corrosion mechanisms, caustic gouging damage normally begins with disruption of the fluid flow (water/steam) adjacent to the tube wall and/or the formation of deposits in which a concentration of caustic leads eventually to failure.
Appearance of Damage:
• Outside surface: Since this is a water-side damage mechanism, no outer surface damage is visible unless damage has progressed to cause a pinhole leak.
• Inside surface: The surface may exhibit internal hemispherical or elliptical depressions, partially or completely filled with thick layered deposits.
Corrosion deposits that contain distinctive crystals of sodium ferroate and/or sodium ferroite in the layered deposits may also be present.
Where Damage Occurs:
• Locations where the water/fluid flow adjacent to the tube wall is disrupted
• Locations with existing internal deposits or with a high heat flux/transfer
• Locations where boiling first initiates, locations with thermal-hydraulic flow disruptions or where there is localized overheating of the tubes (fire-side conditions)
How To Find Damage:
• Internal visual (video probe, borescope)
• Radiography
How To Quantify the Severity of the Damage:
• Ultrasonic thickness measurements (internal rotary probe or from the outer surface)
• Electromagnetic acoustic transducer (EMAT) technology
• Tube sampling Optional NDE:
• Thermography
• Pulsed eddy current (PEC) testing
• Internal visual (video probe, borescope) after chemical cleaning
• Low-frequency electromagnetic technique (LFET) What Is Acceptable:
• The predicted minimum wall thickness at the end of the next service period (time to the next re-examination) is greater than the minimum Code-allowable wall thickness.
Repair Options:
• Replace tubing when the wall thickness is less than the specified minimum.
Corrective Actions:
• See Chapter 24 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
Other Comments:
• Elevated caustic levels over a long period increase the potential for carryover into steam, causing damage to
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-6
Waterwall Fire-Side Corrosion
Damage Mechanism: Waterwall fire-side corrosion Summary of Mechanism:
Gaseous species released by the combustion of coal contain a number of potential corrodents. The condensable vapor species will deposit on contact with surfaces that are at temperatures lower than their respective condensation temperatures. This, in turn, results in the deposit of fly ash and the accumulation of alkali and sulfur species, which eventually leads to wall thinning via corrosion wall wastage.
Appearance of Damage:
• Outside surface: Damage is usually found with hard, fired inner-layer deposits on tubes with loosely bonded ash on the outer layers. Unburned carbon, iron oxides, and iron sulfides are found in scale overlaid by sintered deposits. These are indicative of poor combustion or local reducing conditions.
• Inside surface: The damage is external in origin and will yield no detectable signs internally until failure has occurred.
Where Damage Occurs:
• The maximum attack is generally found at the crown of the tube facing the flame and usually encompasses about 120° of the tube circumferentially. If a tube burst occurs, it is often in this location.
How To Find Damage:
• Visual examination (looking for signs of corrosion, wear, grooving, roughness of the tube surface, fresh oxidation ,or rust
• Magnetic particle examination to identify tight cracks How To Quantify the Severity of the Damage:
• Ultrasonic thickness measurements
• Tube sampling Optional NDE:
• Thermography
• Pulsed eddy current (PEC) testing
• Low frequency electromagnetic technique (LFET) What Is Acceptable:
• The predicted minimum wall thickness at the end of the next service period (time to the next re-examination) is greater than the minimum Code-allowable wall thickness.
Repair Options:
• Replace tubing when the wall thickness is less than the specified minimum.
• Install weld overlays (if the minimum thickness has not been reached).
Corrective Actions:
• See Chapter 25 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
• Thermal fatigue Other Comments:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-7
Thermal Fatigue in Waterwalls
Damage Mechanism: Thermal fatigue in waterwalls Summary of Mechanism:
Thermal fatigue is the leading boiler tube failure mechanism in the waterwalls of supercritical boilers. The underlying thermal fatigue mechanism relating to frequent thermal spikes may be exacerbated by corrosion on the fire side.
Thermal fatigue involves multiple cyclic strains that exceed the fracture strain of the fire-side oxide and lead to cracking of the protective oxide. Corrosive ash species may accelerate the cracking, but they are not the main drivers. Tubes containing thermal fatigue cracks may be found in conjunction with significant fire-side wastage, up to 50% of the tube wall thickness in some units.
Appearance of Damage:
• Outer surface: In cross-section, the cracks are sharp, vee-shaped, or finger-like and are primarily transgranular in nature. Circumferential cracks on a given tube are often uniformly spaced with a density that is typically ~ 20–40 cracks per tube inch (1–1.5 cracks per tube mm).
• Inside surface: Since cracking most often initiates on the outside surface, only cracks that have penetrated the wall thickness will be visible on the inside surface.
Where Damage Occurs:
• Failures are located primarily on the fire side of waterwall tubing and to some extent in the membranes between tubes.
• The most susceptible locations will be those where there is slag buildup and shedding, wall blower quenching, high heat fluxes, or flame impingement.
How To Find Damage:
• Visual examination
• Liquid penetrant testing or magnetic particle testing How To Quantify the Severity of the Damage:
• Radiography (RT)
• AC potential drop Optional NDE:
What Is Acceptable:
• No cracks. If cracks are detected, a fitness-for-service analysis should be performed to justify continued operation.
Repair Options:
• Leave cracks in place (if justified).
• Perform local excavation.
• Excavation and weld repair.
• Perform a complete replacement of the tube.
Corrective Actions:
• See Chapter 26 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
• Overheating
• Fire-side wastage Other Comments:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-8
Thermal Fatigue in Economizer Header Tubes Damage Mechanism: Thermal fatigue in economizer header tubes Summary of Mechanism:
The generic mechanism is induced primarily by cyclic or transient thermal loading. The damage has the
characteristics of thermal fatigue, and the cracking has a morphology that is manifested by multiple cracks that are perpendicular to the principal direction of stress, with one crack usually becoming dominant and causing wall penetration. Evidence of enhancement of the damage by corrosion, such as the presence of thick oxides associated with the thermal fatigue cracks, has also been observed.
Appearance of Damage:
• Outer surface: Since cracking most often initiates on the inside surface, only cracks that have penetrated the wall thickness will be visible on the outside surface.
• Inside surface: The damaged stub tube will manifest numerous longitudinal cracks. The cracks may be distributed completely around the bore of the tube, or they may appear only in the higher stress locations.
Where Damage Occurs:
• Cracks can begin to form at any location along the header where the cyclic thermal stress is sufficiently high.
• The first appearance of thermally induced damage is often a pin-hole leak in the toe of the weld at the header-to- stub-tube attachment weld of the economizer inlet.
• The worst damage is usually found in tubes closest to the feedwater inlet.
• Stub tube failures have generally occurred at the toe of the fillet weld on the tube side.
How To Find Damage:
• Visual inspection of the borehole and across the inside ligament
• Liquid penetrant testing and/or magnetic particle testing (stub tubes)
• Ultrasonic testing (shear wave or phased array) (ID cracks) How To Quantify the Severity of the Damage:
• Ultrasonic testing (shear wave or phased array) (ID cracks) Optional NDE:
• Eddy current testing What Is Acceptable:
• No cracks. If cracks are detected, a fitness-for-service analysis should be performed to justify continued operation.
Repair Options:
• Leave cracks in place (if justified).
• Perform a complete replacement of the header.
• Redesign the header.
Corrective Actions:
• See Chapter 27 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
Focusing NDE Specifically for Known or Suspected Mechanisms of Degradation Table 5-9
Thermal-Mechanical Fatigue and Vibration-Induced Fatigue Damage Mechanism: Thermal-mechanical fatigue and vibration-induced fatigue Summary of Mechanism:
Fatigue is the phenomenon of damage accumulation caused by cyclic or fluctuating stresses. It is manifested as the initiation and stable propagation of a crack. Final failure ensues when a critical crack size is reached, and failure occurs by fracture or overload. Fatigue is dependent upon the frequency and magnitude of the stress cycles and is generally independent of stress duration. Stress cycling may be induced mechanically or thermally.
Appearance of Damage:
• Outer surface: The cracking tends to be straight and transgranular, generally resulting in thick-edged failures. The appearance of beach marks or ratchet marks is typical.
• Inside surface: Since cracking most often initiates on the outside surface, only cracks that have penetrated the wall thickness will be visible on the inside surface.
Where Damage Occurs:
• Locations associated with attachments, particularly solid attachments or jammed sliding attachments, or with bends in tubing. Examples are the end of the membrane of waterwall tubing—either in the lower slope region near the ash hopper or at the top of the rear wall at the entrance to the rear gas passage—or tie bars, K bars, or beams.
How To Find Damage:
• Once a surface-connected crack has initiated, liquid penetrant, magnetic particle, eddy current, ultrasonic, and radiographic testing may be used to detect fatigue damage.
How To Quantify the Severity of the Damage:
• Ultrasonic testing
• Radiography (crack sizing)
• AC potential drop (crack sizing) Optional NDE:
What Is Acceptable:
• No cracks. If cracks are detected, a fitness-for-service analysis should be performed to justify continued operation.
Repair Options:
• Leave cracks in place (if justified).
• Grind out and weld the crack.
• Replace if necessary due to the geometry of the damaged area, for example, U-bends.
Corrective Actions:
• See Chapter 28 of the Boiler and HRSG Tube Failures Manual.
Other Potentially Active Damage Mechanisms:
• Creep fatigue
• Corrosion fatigue Other Comments: