boiler tubes failure
DESCRIPTION
Boiler Tubes FailureTRANSCRIPT
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BOILER TUBE FAILURESIN-SERVICE INSPECTIONS OF CONVENTIONAL POWER PLANTPractical Background Information
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Hands-On Experience and Information whilst employed by the Plant Life Integrity department:RWE Power International Owner/Operator of Conventional Power Stations
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Overheating Waterside Corrosion Fatigue
mechanical, thermo-mechanical, thermal, corrosion, creep Fireside Corrosion Oxidation Erosion Mechanical
Damage Mechanisms
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OVERHEATING
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Short term overheating
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Temperature transient reduces materials strength below the applied pressure stress Appearance
Thin edge tensile failure leading to an axial fishmouth rupture Swelling prior to thinning, evident in cracking of external / internal scale Location
Furnace wall, Pendant S/H over furnace, Radiant S/H Causes
Starvation of steam/water flow Blockages from debris Waterlogging and inadequate condensate dispersal/drainage procedures Overfiring compared to steam flow e.g. loss of HP heaters Leak upstream of failure Drum level / carry under
Short term Overheating
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Solutions Solutions are normally operational. New tubes will not prevent further failures
Drainage procedures Matching heat input with loading rate Loading rates, turbine following boiler Damage tends to be more localised than long term overheating Austenitics more tolerant than ferritic
Exceptions Austenitic tubes can give rise to thick edge short term overheating failures Accumulation of short term overheating causes damage through oxidation
and materials softening. Material replacement may offer some improvement.
Short term Overheating
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Long term Overheating
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Creep rupture due to sustained stress at elevated temperatures Appearance
Thick edge failures leading to axial rupture Thick oxide which may be crazed local to failure often with some associated fireside corrosion Relatively low ductility at failure with little measurable swelling Location
Adjacent to material or size transitions. S/H and R/H Original Top dead space header stubs and tubes
Long term Overheating
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Causes Operation beyond design life, poor steam temperature distributions,
elevated gas temperatures Increased stress due to wastage and ovality stresses Rogue material Partial blockage
Solutions Tube replacement or upgrade to remove damaged tubing Damage is more widespread than short-term overheating Replacements may be targeted by NDT oxide thickness measurements Reduction of steam temperatures and pressures Alteration to boiler design and combustion
Long term Overheating
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WATERSIDE CORROSION
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Control of boiler chemistry is fundamental to boiler availability The preservation of a thin passive oxide film on the bore of the tubes is key to
preventing corrosion Chemical species fed to the boiler concentrate as most are not carried over in
the steam Control of chemistry - pH, Conductivity, Oxygen
Blow down Chemical additions to the drum De-aeration, Physical & Chemical Chemistry problems have the potential to cause very widespread
problems throughout the furnace with large impacts on availability and maintenance
Waterside Corrosion
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Caustic Attack Caused by the concentration of NaOH Localised boiling causes concentration factor of 10,000 The caustic causes corrosion by dissolving the oxide/metal Deposits can also cause overheating failures
Thick waterside deposits reduced heat transfer causing the tube wall to overheat Oxide can be deposited in the tube with no significant tube corrosion due to the transport of corrosion products from the feed system
Waterside Corrosion ON-Load Corrosion
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Solutions Control of Boiler Chemistry Routine acid cleaning to remove deposits and prevent
concentration mechanisms Ensuring maintenance of adequate circulation
Waterside Corrosion
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FATIGUE
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Fatigue Failures
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Crack initiation and growth under cyclic loads
Nearly always low cycle fatigue rather than high cycle (
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High cycle fatigue - vibrational loads Flow induced vibration. Attemperator nozzles. Thermo- Mechanical Fatigue
Loads normally caused by constrained thermal expansion Differential expansion.
Fatigue Failures
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Pad weld repair showing renewed corrosion fatigue crack growth
Fatigue Failures Corrosion Fatigue
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Solutions Provide additional flexibility through modifications. Increasing
flexibility is normally a better solution than increasing strength of design. Take operational means to avoid thermal shocks Boiler re-circulation through economiser Control of forced cooling procedures NDT and repair at overhauls, Combinations of MPI and
Ultrasonics at targeted locations is effective at managing failures Chemical controls where appropriate for Corrosion Fatigue
Fatigue Failures
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Creep Fatigue Joint action of creep damage and fatigue damage From Creep crack growth due to a cyclic stress at
temperature enhanced fatigue Distribution tends to match temperature distribution High temperature pipe and header attachments
Fatigue Failures
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FIRESIDE CORROSION
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Appearance Thin edge split of the tube
Typically a third to half wall failure in superheater tubes Typically 1-2 mm on reheater tubes and thinner on stainless reheater tubes Thick fused deposits on ferritic tubes Pitted "orange peel" appearance or corrosion flats on Austenitic tubes Location
High temperature section of Final S/H and R/H Localised to tube attachments C+T, wrapper tubes
Fireside Corrosion
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Causes - S/H & R/H corrosion Molten sulphate attack
Formation of Na, K trisulphate. Strong evidence of a correlation with coal CI High metal temperatures
High metal temps melt sulphate. Bell shape corrosion curve, peak 650-750C Combined with accelerated oxidation High gas temperature, gas laning, tube alignment Combination with creep Materials factors
Carburisation, inadequate materials, poor HT 310 2.5 x better than 18Cr8Ni Steel ??
Fireside Corrosion
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Solutions NDT thickness testing.
EMATs on ferritic tubes Materials replacement judged on remanent life assessment Materials Upgrade - Co-extruded/Higher grade materials Reduction of metal temperatures (S/H R/H steam temps)
Fireside Corrosion
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Causes -Furnace Wall Corrosion Reducing furnace atmosphere adjacent to furnace wall
This causes sulphidation attack H2S, CO & HCl. Fuel composition There is some correlation between coal CI content and corrosion but less
than for R/H and S/H corrosion Combustion environment is the most important factor
Poor combustion, flame impingement Low furnace excess air levels, NOx abatement Worn or poorly adjusted burners PF Quality Proximity of burners to side wall, flame length and distance from rear wall
Furnace Wall Corrosion
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Mechanical Solutions NDT thickness testing
EMATs on ferritic tubes Replacement Materials Upgrade - Co-extruded Weld cladding, plasma coatings Combustion Solutions
Improvement of combustion to avoid localised reducing conditions Excess air levels, PF quality, blanket air on furnace walls, burner
maintenance Fuel specification Improvement Corrosion may be economically the best option due to savings in excess
air, NOX
Furnace Wall Corrosion
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OXIDATION
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Accelerated Oxidation Oxide growth rates normally decrease with oxide thickness Temperature transients cause cracking of the oxide and spallation and increases oxidation rate Excessive temperatures produce non protective oxides NDT preparation Oxidation and wall loss also occurs on the steam side Reheater more susceptible Stainless steel internal oxide spalling Oxide can gather in bends and cause blockage
Steam oxidation producing laminated scale on internal bore of 2Cr1Mo reheater tubing
Oxidation
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EROSION
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Dust Erosion Proportional to:
Ash wt.%, Hardness of ash Quartz content Angle of impact Gas velocity to the 3-4 power Erosion sensitive to coal diet and load Details of design that cause locally increased velocity and laning of dust Slagging and dust build up causing gas laning Solutions
Normally managed by inspection and shielding Sootblower Erosion
Mainly as above but velocity provided by s/b Largely managed by s/b maintenance and shielding
Erosion
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MECHANICAL / MANUFACTURING
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Slag falls mainly on ash slope and around the ash throat Slagging coals Operation at base load with low excess air levels Outage and maintenance damage
Removal of slag & ash bridges Sootblower lances grinding, arc strikes, burning off attachments
Mechanical - Impact
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Fretting is the wear caused by small movements between contacting metal surfaces
Rubbing contact removes protective oxides Fretting wear occurs mostly on contact of similar metals. Does not require
high contact loads. Stainless more prone than low alloy steels. Location
Vibrational contact between tubes in platen superheaters. Finger fretting on stainless element wraps.
Mechanical - Fretting
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Materials defects ERW defects, tube laps and scores, incorrect material/heat treatment Weld defects
Welding defects, pinholes, lack of fusion, porosity etc. Reheat cracking, hot cracking, Hydrogen cracking, lamella tearing Transition weld failure
Materials transition failure in a brittle manner along the weld interface Carbide migration leads to decarburised zone. Differential thermal expansion. Tri-axial constraint Stress Corrosion
Initiation and growth of cracks under stress and corrosion Austenitic Stainless steel increased due to sensitisation Ferritic steels of high hardness, bolting materials.
Other Mechanisms
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