Boiler Pressure Component Failure Mechanisms

7/24/2019 Boiler Pressure Component Failure Mechanisms

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Typical Failure mechanisms

Boiler Pressure Component FailureMechanisms

The boiler tubes are under high pressure and/or high-temperatureconditions.They are subject to potential degradation by a variety of mechanicaland thermal stresses and environmental attack on both the uid andreside.Mechanical components can fail due to creep, fatigue, erosion, andcorrosionCreepreep is a time-dependent deformation that takes place at elevatedtemperature under mechanical stresses.such failure results in overheating or overstressing the tube material

beyond its capabilities for either a short-term or a long-term period.


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Creep failures-Overheating9/4/2013 Steag O&M Company

Short-term Overheat !ailure results in a ductile rupture of the

tube metal, "t is characteri#ed by $sh mouth% opening in the tube

&here the fracture surface is a thin edge. Causes: Short-term

overheat failures are most common during boiler start up.

!ailures result &hen the tube metal temperature is e'tremely

elevated from a lack of cooling steam or &ater o& Long-term

OverheatTube metal often has heavy e'ternal scale build-up and

secondary cracking. (uper heater and reheat super heater tubes

commonly fail after many years of service, as a result of creep.

)uring normal operation, alloy super heater tubes &ill e'perience

increasing temperature and strain over the life of the tube until the

creep life is e'pended. !urnace &ater &all tubes also can fail from

long-term overheat.

Fatigue!atigue is a phenomenon of damage accumulation caused by cyclicor uctuating stresses, &hich are caused by mechanical loads, o&-induced vibration.components are subjected to cyclic temperature and o&uctuations restrictthermal expansion.Thermal fatigue is classied in t&o categories,corrosion fatigue and thermal fatigue.Corrosion fatigue, *the uctuations in circulation of &ater in theboiler tubeThermal fatigue+ freuent starts and stops

Typically occurs at areas such as header ligaments, &elded

attachments , tube stub &elds, circumferential e'ternal surface

cracking of &ater &all tubes in supercritical units, and fabrication

notches

Erosion and corrosion9/4/2013 Steag O&M CompanyErosion is metal removal caused by particles striking the metalssurface. arious mechanisms, such as y ash erosion, soot blo&ingerosion, falling slag erosion, and coal particle erosion can causeerosion on the boiler tubes. !ly ash erosion, is a signicant boilertube failure concern, occurs in the regions &ith high local ue gas


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velocities, &ith high ash loading, and &ith abrasive particles such asuart#Corrosion: )eterioration and loss of material due to chemicalattack.There are t&o basic categories in boiler tubes+

nternal corrosion+ hydrogen damage, acid phosphate corrosion,caustic gouging, and pitting

E!ternal corrosion+ &ater &all reside corrosion, super heater

0(12/re heater 0312 reside corrosion, and ash de& point corrosion.

nternal Corrosion-"ydrogen #amage"ydrogen damage is most commonly associated &ith e'cessivedeposition on ") tube surfaces, coupled &ith a boiler &ater lo& p1e'cursion.4ater chemistry is upset, &hich can occur from condenser leaks,particularly &ith salt &ater cooling medium, and leads to acidic 0lo&p12 contaminants that can be concentrated in the deposit.

5nder-deposit releases atomic hydrogen &hich migrates into the

tube &all metal, reacts &ith carbon in the steel 0decarburi#ation2

and causes inter granular separation.

$ater%all Fireside CorrosionCauses& Corrosion occurs on e!ternal surfaces of %ater%alltu'es %hen thecombustion process produces a reducing atmosphere0substoichiometric2.This is common in the lo&er furnace of process recovery boilers inthe pulp and paper industry.!or conventional fossil fuel boilers, corrosion in the burner #oneusually is associated &ith coal ring.

"mproper burners or operating &ith staged air #ones to control

combustion are more susceptible to larger local regions possessing

a reducing atmosphere, resulting in increased corrosion rates.

#issimilar Metal $eld (#M$)Failure

#issimilar Metal $eld (#M$) Failure Material fails at the ferritic

side of the &eld, along the &eld fusion line. 6 failure tends to be

catastrophic in that the entire tube &ill fail across the circumference

of the tube section. Causes& #M$ descri'es the 'utt %eld

%here an autenitic (stainless steel) material joins a ferriticalloy, such as (6789 T77, material. These failures are attributed to


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several factors+ high stresses at the austenitic to ferritic interface

due to di*erences in e!pansion properties of the t%o

materials, e'cessive e'ternal loading stresses and thermal cycling,

and creep of the ferritic material.

$aterside Corrosion Fatigue

Causes& Tu'e damage occurs due to the com'ination of

thermal fatigue and corrosion. orrosion fatigue is inuenced by

boiler design, &ater chemistry, boiler &ater o'ygen content and

boiler operation. :eads to the breakdo&n of the protective

magnetite on the ") surface of the boiler tube. The loss of this

protective scale e'poses tube to corrosion. The locations of

attachments and e'ternal &eldments, such as buckstay

attachments, seal plates and scallop bars, are most susceptible. The

problem is most likely to progress during boiler start-up cycles.

Critical considerations during

design of Boiler

Material selection deciencies in the design

Material and fabrication a&s

4elding a&s

The primary consideration in material choice is a function of

e'pected tube temperature of operation. Economi+ers and

%ater%all sections are usually constructed &ith a mild or

medium car'on steel, Lo% alloy ferritic steels are used for most

superheater and reheater sections, &ith austenitic stainless

steels specied for the highest-temperature circuits or corrosion

performance. ;ach manufacturer species a ma'imum operating

temperature for each material, based on laboratory o'idation

e'periments. The 6(M; code is based on the mid-&all tube

temperature.


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Factors in,uencing Boiler Tu'e

failures


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Metal temperature limits

Material and Fa'rication

Fla%s

Material ,a%s include defects introduced into the tube during its

manufacture, fabrication, storage, and/or installation (uch defects

might include+ !orging laps "nclusions or laminations in the metal

:ack of fusion of the &elded seams )eep tool marks or scoresfrom tube piercing, e'trusion, or rolling operations

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Boiler Pressure Component Failure Mechanisms