boiler failure mechanism.pdf

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Long-term fa ilures by cr ee p dam age can occu r w ith little or no detectablechan ges in the tube wall thick ness . M icrostructural ex am in ation is ane ffe ctiv e means of c onfi rm in g long-term overheating. The plate lets of ironcarbid e in the pearlite stru cture of ca rbon s te els w ill thermally decompose tosp he ro id ized iro n carbide as show n in Figures 5 through 7.

C on tin ued decomposit ion w ill result in total degrad ation to graphite plusf er rit e, o r g raph it ization as shown in Figure 8.

Fig.4 Internal and Extcmul Oxidrzatron.

Fig.3 Long Term Ovthe


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Where:P the Larsen-Miller parameterT Absolute metal temperature, degrees Rankine, OF 460)t Time for rupture, hours

Fig. 9 - Creep VOids

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: . ~ ~ : ~ - : ~ - . -- - : .- :-of: \.

. . < - , : : . , : . ; \ : , Fig 8 Grap hlill ut>on

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fig.7 SphelOidil.


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Fig. 11 t{ydogcn Damage MiClOstruc;turc.Fig. 10 Hydrogen Dalll


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Oxygen ttackThe pres ence of diss olved oxygen in boiler w at er causes th e cath ode of anycorrosion ce ll to depolarize , thereby sustaining the corr osion process.Form at io n of re ddish brown hematite (FeZO )) or ru st deposi ts or tuberclesaccompanied by hem ispher ical pitting is th e most famil iar fo rm of oxygenattack as shown in Figures 14 and 15.D iss olved oxygen is also a si gnifi ca nt component in ammonia co rr osion ofcopper alloys, stress-corr osio n cracking of austenit ic sta in less steels andchelant co rr osion.

Fig 3 C llJ 5t> C Gougin gig 2 Caustic Gouging

austic GougingSodium hydroxide (NaOH) is used ex tensiv ely in boiler w ater trea tm ent tomain tain the optimum hydroxyl ion concentrat ion ran ge to form a pro tectivemag netite film on stee l surfaces and to help form nonadherent sludges whenhardness enters the boile r w ater. However, excessive so dium hydroxide candes troy the protective film and co rr ode the base meta l as shown in eq uations2 and 3. N aO H can concentrate durin g departure from nuclea te boil ing(DNB), fi lm boil ing or st eam blanketing conditions. Concent rat ion alsooccu rs when normal boiler water evaporates beneath deposits lea ving behindthe ca ustic at the met al surface . The effect of tube metal gougin g beneathdeposits is shown in Fig ure 12.

C aust ic gouging can also occur due to e va po ratio n along a wat erlinew ithout sig nificant ac cumulation of deposit as shown in Figure 13.

In these cas es, so lubi l ized sodium ferroite is removed fr om th e base tubemetal, but then hydrolyzes and precipitates el se where in th e boiler asmag netite when th e co nce ntrat ed wate r is diluted by normal boiler wate r,

hemically Induced Failures


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Flow ssisted orrosionFlow ass isted corrosion FAC) is the local ized thinning of a co mponentrelat ed to the diss olu tion of th e protective oxide fi lm and the underlyin g basemet al. The mech anisms involved in FAC are very complex due to the ef fectof m an y variables , which influence its occurrence , In utili ty oper ations FACis ty pica lly seen in single or two phase water flow in g in ca rbon steel pipin g.The m ore com mon locations where FAC is dete cted ar e:

Fig 17 Thertt1,11FatiguQ , ~icrosliuclurc,

Fig. 16 Thermal Filligue.

Thermal FatigueThermal fatigue dam age typica lly exhibits numerous cracks and cr azin g asshown in Figure 16.

This dam age results from cyclic and exce ss ive therm al fluctuat ionsaccompanied by m echanical co nst ra int. Exces sive temperat ure gradients canadd to intern al str ain to initiate or enhan ce the crac king proce ss . Onceinitiat ed, fres hly exposed metal w ithin th e cracks can undergo oxid ation,creating a wedge effe ct at th e crack tip, sin ce the oxide occ upies a greatervolum e than th e base metal. See Figure 17,Thermal f at igue ca n occ ur in waterwalls or other areas subject ed to ONB orrapid ly f luc tuat ing flows. Low-am plitude vibrat ions of entire superh eate rs canlea d to thermal fa tigue.

Fig. 15 OX fg~n Pilling.Fig. 14 OX~g;;nCorroslon .

:~.. ~


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The great es t potentia l fo r normalize d wear occurs at aro und 300F 149C ,w ith very litt le poten tial for FAC below IS0F 6 6C and ab ove SOOF2 60C . This is il lustrat ed in Figure 18.

As w ith general co rrosion, pH has a significa nt effec t on the norm alize dwear rate of carbon stee l. There is alm ost a forty 40 fold reduct ion in w earrate betwee n a pH of 8.6 and a pH of 9.4 , as shown in Figure 19.

Dissolved oxygen D O concentration has a direc t im pact of the potentialfo r flow ass isted corrosion. Above a DO conce ntrat ion of 30 mg/L ppb , th enorm alize d wea r rate on carbon steel is at a m inimum. Below 30 mg/L p pbDO, th e normaliz ed wear rate on carbon steel in creases exponentially, asdepicted in Figure 20.

Fig. 19 Effect 01pl1 on FAC.

4fJ r rQ 3-~ 20


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Wear due toSocondaryRow atMedium R .eNumbers

Woar atlow R .eNumbers

Geometry, or general layout of thepiping systems has an effect on thelocation of FAC, regardless of theReynolds Number Re). Consequently,changes in flow rate may notsignificantly reduce FAC. Figure 23shows areas of wear at differentReynolds Numbers.Flow assisted corrosion is most often

Fig. 22 Effect of Velocity on FAC.

Mu. Tftn. er ActillaUonControllod Region Mech.nl(alRlfmQ lConUolRegion Roglong: ?~ie x :. ~ I _ _ _Z 1/1-Quu01 {~G :

Fluid Voloclty Increulng

t only stands to reason that the mass velocity will have a significantinfluence on flow-assisted corrosion. Below 10 ftlsec 3.05 M/sec thenormalized wear rate is minimal, increasing 2.8 times at 100 ft/sec 30.5M/sec). Figure 21 and Figure 22 illustrate the effect of velocity on normalizedwear rate.

Fig. 21 Effect of Ve locrtv on FAC.

2.82,~ \I ~ 2.42 20 :t f.8 2 u1.4g -:1.21.0 :

3 4 60 70 60 ~O 1)0Velocity ft/sec

Fig. 20 - Effect of O,ssolvQdOxygen on FAC.

31

\~ 25 \~ 20 \i 15 \1 ~~ 5 r-, r--

I 2 ) 31 I ~I 1 IX 10{O~on ConcMtr~tJon hl9/l


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back to 2 00 3 c on ten ts

s paper was presented as part a/the 6Jst Annual International WaterConference/ , which took place in Pittsburgh, PA (USA), October 22-26,2002.

bout the uthorsThe authors can be rea ched at Ashland S pecialty Chemic al CompanyOne Drew Plaz aBoonton, NJ 07005

I . F . C ja ro fa lo, Fundamentals of C reep and StressR upture in M eta ls, Macrnillam, New Y ork, 1965.2, F .R . L arso n and J. M iller, Tim e-TemperatureRela tio nship fo r Ruptu re and Cree p Stresses,Trans ASME, Ju ly 1952, pp. 765-775. Drew Principles of Industr ial Water Trea tment, E leventh Editio n,Boonton, NJ: D rew Industr ial Divisi on, Ashland Chemical Co., 1994 p 275.4D.N . French , Meta llurgical Failure s in Fossil Fired Boilers, New York ,NY: John W iley Sons, 1983 p 189.5. D rew P rincip les of Ind ustri al W ate r Trea tmen t, p 233.6. Ibid., P 234.7 Ib id ., p . 215.8 D .N . F re nc h, p 25.9 Hea t Rec overy Steam Genera tor C orro sio nC au ses an d C on trol, W eatoug, CTTetra E ng inee ring Group, Inc. , 2000).

References

Fig. 23 . V\leiH S,te ill Oiffcf( J\t Reynold's NUl11ben;.

Wear atHgh ReNumbersound in system s w ith all ferr ousmetallurgy. The in co rporatio n of aslittle as 0.1 chrom ium into lowcarbon steels h as b een sh own to havebenefi ts in red ucing FAe.


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1 Peakmetal operating temperature above eutectoid can be estimated by:A the amount of bainite/martensite mixed with ferrite in microstructure at the failureoriginBthe amount of austenite mixed with ferrite in microstructure at the failure originCthe amount of austenite mixed with ferrite in microstructure remote from thefailure originD the amount of marensite mixed with austenite in microstructure at the failureorigin

2 What can cause short term overheating?A High StacktemperatureB low water levelC High steam pressure settingD Defective steam king valve

3 Short term overheating usually exhibits:A Thin lip longitudinal ruptureB Thick lip longitudinal ruptureC TransversecracksD Severepitting4 Which of the following can result in long term overheating:A Undesired channeling of fireside gasesB Persistent hardness in boiler waterC Excessivelyhigh boiler water pHD Chronically high total alkalinityLongterm overheating exhibits:A Thin lip longitudinal ruptureB Thick lip longitudinal ruptureC TransversecracksD Severepitting

6 Creep damage can be detected with:A Changesin tube wall thicknessB Micro structural examination of iron carbidesC Severebulging of tube wall

Boiler Failure Mechanisms by Douglas DeWittwDick Stephan Mcintyre andJoseph Hofilena

Analyst Fall 2003QUIZ


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D. Transverse cracking of tube wall.7. High Temperature stress rupture and oxidation resistance can be increased by:

A. Alloying additives of Nickel and TinB. Alloying additives of Phosphorous and SiliconeC. Alloying additives of Molybdenum and ChromiumD. Alloying additives of Aluminum and Boron.

Hydrogen damage is done thru releasing atomic hydrogen to diffuse into steel to form:A. AcidsB. Iron hydrideC. Methane and ironD. Positive charge resistance

9 Hydrogen damage is characterized with:A. Thin lipped ruptureB. TransversecracksC. Thick lipped brittle ruptureD. Severebulging.

lO.Caustic gouging destroys protective film by:A. Dissolving carbonate scale.B. Dissolving silicate scale.C. Dissolving iron and magnetite to form sodium ferroate.D. Depositing metal hydroxides.

Boiler Failure Mechanisms by Douglas DeWittDick Stephan Mcintyre andJoseph Hofilena

Analyst Fall 2003QUIZ

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