BOILER TUBE FAILURESIN-SERVICE INSPECTIONS OF CONVENTIONAL POWER PLANTPractical Background Information

Hands-On Experience and Information whilst employed by the Plant Life Integrity department:

RWE Power International – Owner/Operator of Conventional Power Stations

Overheating

Waterside Corrosion

Fatigue

mechanical, thermo-mechanical, thermal, corrosion, creep

Fireside Corrosion

Oxidation

Erosion

Mechanical

Damage Mechanisms

OVERHEATING

Short term overheating

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

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 Austenitic’s 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

Long term Overheating

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

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

WATERSIDE CORROSION

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

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

Solutions Control of Boiler Chemistry Routine acid cleaning to remove deposits and prevent

concentration mechanisms Ensuring maintenance of adequate circulation

Waterside Corrosion

FATIGUE

Fatigue Failures

Crack initiation and growth under cyclic loads

Nearly always low cycle fatigue rather than high cycle (<104) and loading normally of yield level and above.

Ductility exhausted in 100-1000’s of stress cycles. Failures are not initiation controlled. In welded components initiating

defects are normally present. Cracking propagates at stress concentrations such as notches and changes of section.

Locations of stress concentration defects. Weld toes, Attachments, Bends, Lower furnace walls, Economiser, Header

stub welds. Pressure stresses are not normally a significant contribution to the loading

mechanism. Understanding the causes of fatigue cracking depends on understanding

the loading mechanism.

Fatigue Failures

High cycle fatigue - vibrational loads Flow induced vibration. Attemperator nozzles.

Thermo- Mechanical Fatigue Loads normally caused by constrained thermal

expansion Differential expansion.

Fatigue Failures

Pad weld repair showing renewed corrosion fatigue crack growth

Fatigue Failures Corrosion Fatigue

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

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

FIRESIDE CORROSION

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

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 650-750°C – 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

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

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

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

OXIDATION

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 2¼Cr1Mo reheater tubing

Oxidation

EROSION

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

MECHANICAL / MANUFACTURING

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

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

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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