Boiler Waterside Deposition

8/18/2019 Boiler Waterside Deposition

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Boiler waterside deposition

Power (15-Sep-04)

Boiler waterside deposition has long been recognized as a cause ofboiler tube failures. widel! used wa! to identif! the a"ount andche"ical co"position of deposition in a suspected area of thefurnace is to e#tract a tube section and "easure its deposit weightdensit! ($%$). &his test "ethod is less than ideal' for two reasons.

t is destructi e' re*uiring the re"o al and replace"ent of a tubethat "a! be in a critical area of the furnace. nd operators rightfull!*uestion the alidit! of considering a single tube speci"en indicati eof the entire furnace+s deposition buildup. ,ow "uch infor"ation'the! as ' can !ou get fro" a single tube when deposition "ight beoccurring on its neighbors at a er! different rate

&o answer this *uestion' /alco o. (/aper ille' ll.) last !earconducted an e#tensi e set of $%$ anal!ses on the lower furnace ofan industrial heat-reco er! boiler. ather than pic one or a fewtubes for laborator! anal!sis' an atte"pt was "ade to deter"inetube $%$ alues across an e#tended area of the furnace and at"ultiple ele ations on the sa"e tubes. &he goal of the stud! was togain "ore insight into the deposition process.

/o deposit' no return

2ild steel is the "aterial "ost co""onl! used for stea" generatingtubes in industrial boilers. ts heat transfer and ther"al stabilit!

benefits ha e been recognized for "an! !ears. But failures of "ild-steel tubes can result if waterside deposits accu"ulate and cause"etal te"peratures to e#ceed 350 a process called long-ter"o erheat failure. See igure 6 for a detailed illustration ande#planation of the effects under heat transfer conditions.

6. 7ffect of waterside deposition and scaling on boiler tube heat


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transfer. 8nder clean heat transfer conditions on the internal tubesurface (left)' the tube "etal te"peratures are about 50 to 95abo e the saturation te"perature. (&he threshold for an o erheatingfailure is about 350 .) &he "iddle drawing shows what "ight happenwhen a waterside deposit or :scale la!er: is present. f the sa"ea"ount of heat is applied to a scaled tube to "aintain a constanttube "etal te"perature (&1)' the boiler water te"perature (&0) willbe lower as result of the heat transfer resistance caused b! thedeposit a pheno"enon not seen in utilit! boilers. n realit! (right)'at the sa"e stea" header pressure' the boiler water te"perature"ust be held at a constant to the clean tube condition' the &1 alueshown. &he resistance to heat transfer caused b! the depositre*uires that the boiler be fired harder' and as a result the "etalte"perature is higher than under clean-tube conditions. f the finaltube te"perature (&4) under the scaled condition e#ceeds 350 ' thepotential for an o erheating failure e#ists. Source: Nalco Co.

&here are two points worth "entioning here. ;ne is that the pri"ar!danger of a waterside deposit is an o erheat failure' not the loss ofheat transfer efficienc! and the associated fuel alue lost. 2an! largeindustrial boilers ha e sufficient heat transfer surface area so thatthe heat not collected in the radiant zone does get into the waterphase at so"e other part of the boiler. ccordingl!' it is thepossibilit! of tube failure that we are concerned with here' not fuelefficienc!.

lthough it is also nown that deposition "a! induce a nu"ber ofcorrosion failure "echanis"s' the pri"ar! issue is o erheatingfailures. 7 en a "oderate te"perature rise due to internal surfacedeposition can pro"ote fireside corrosion and tube wall thinning inheat-reco er! boilers. 95'000 lb@hr of stea". ccording to the "ill' the


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boiler has not recei ed a waterside che"ical cleaning in "ore than60 !ears. &he boiler water treat"ent progra" currentl! in use iscoordinated p,@phosphate control' with control "aintained withinthe target range about A3.5 of the ti"e. &he *ualit! of feedwater isconsidered e#cellent' with t!pical "etal o#ide concentrations of lessthan 5 ppb as both iron and copper.

&he repair proCect entailed re"o ing two tube wall panels' one fro"the right sidewall and the other fro" the rear wall. &he panel fro"the right sidewall included fi e adCacent tubes nu"bered &ube 3D to&ube A0. &he panel fro" the rear wall included 15 adCacent tubesnu"bered 9D to A0. &he furnace waterwalls consist of 6.5-inchoutside dia"eter tubes fusion-welded together with a >-in. spacing.&ube sections were re"o ed fro" two ele ations on each panel andidentified as top and botto". &he ele ations were separated b! about16 ft on the right sidewall panel and b! about 3 ft on the rear wallpanel. $%$ alues were "easured on each sa"ple using the glassbead blasting "ethod of deposit re"o al' a procedure detailed in/ 7 Standard &201AA-AA.

&ubular results

igure > shows the $%$ alues "easured on the hot side tubesurfaces of sa"ples re"o ed fro" the rear wall panel. &he alues aregenerall! between 6 and 10 g@ft6. slight trend to higher $%$

alues is apparent fro" &ube 30 to &ube 9D at both ele ations. &he$%$ alues fro" the right wall panel are shown in igure 4. &hese$%$ alues generall! range fro" D to 16 g@ft6. &he differencesbetween "easure"ents at different ele ations on the sa"e tube aregenerall! less than 4 g@ft6.

>. ,ea ! "etal. deposit weight densit! reading fro" the boiler+s


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rear wall panel shows a reasonabl! consistent deposition rate. Source: Nalco Co.

4. $eposition rates on the right wall panel were also consistent. &hattends to confir" that a single tube speci"en is indeed indicati e ofdeposition across the entire furnace. Source: Nalco Co.

&he deposit la!ers on the internal surfaces were generall! thin' hard'and dar -colored. thin' tenacious o#ide la!er was presente er!where beneath the deposit la!ers. So"e sa"ples were co eredb! friable' spott! red deposits in a few places. 2aterial on theinternal surfaces of so"e tube sa"ples was anal!zed using ascanning electron "icroscope e*uipped with an energ! dispersi espectro"eter for co"positional anal!sis and b! E-ra! diffraction forco"pound identification. &he results indicated that the depositspri"aril! consisted of iron as iron o#ides. So"e locations' especiall!on the cold sides of the tubes' had areas with appreciablecontributions of copper in the deposits' as ele"ental copper andcopper o#ide.

n addition to che"ical anal!sis on the internal surfaces of so"esa"ples' "etallographic speci"ens were re"o ed fro" thesesections and prepared for "icroscopic e#a"inations. &he"icrostructures of the tube "etal on all speci"ens did not showe idence of o erheating.

&he internal surfaces of so"e sa"ples were co ered in places b!"oderatel! thic la!ers of deposits that consisted of agglo"eratedele"ental copper and iron o#ide particles ( igure 5). ;ther sa"pleshad deposit la!ers containing "ottled o#ide particles' but no copperparticles ( igure D). &he surfaces of all sa"ples were co ered b! a


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er! thin' dense "agnetite la!er in "ost places. &he thic ness of theo#ide la!er was t!picall! less than 0.0005 inches.

5. Spread thic l!. "oderatel! thic la!er of spott! depositscontaining ele"ental copper particles on the hot side internal surfaceof the botto" ele ation of &ube 31 can be seen at a "agnification of600E. Courtesy: Nalco Co.

D. Spread thinl!. thin deposit la!er containing iron o#ide particleson the botto" ele ation of &ube 9D can be seen at a "agnification of600E. Courtesy: Nalco Co.

Su""ing up

&he pri"ar! conclusion of the /alco stud! is that watersidedeposition in the lower furnace of this particular boiler was *uiteconsistent' both across two tube panel sections and at "ultipleele ations along each tube length. 7#a"ination of 59 tube sectionsproduced the sa"e resultsF 7 en after an e#tended period of noche"ical cleaning' each tube speci"en indicated no need for a boiler

waterside cleaning. &he conclusion is that e en a single tube sa"pleand $%$ anal!sis will !ield sufficient infor"ation on boiler


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cleanliness' as related to a general area surrounding the sa"ple.

,owe er' deposition rates can ar! significantl! between distantlocations in a boiler' and Cudicious selection of sa"pling locations

such as high heat input areas that are "ost susceptible todeposition is still needed. %ithin these areas' the results should notbe e#pected to ar! considerabl!.

&here were so"e ariations in waterside deposit accu"ulations'e en in this unit. &hese were li el! due to ariances in flow elocitiesand fireside conditions' each of which can pla! a "aCor role inwaterside deposit build-up rates. $ifferences in "easure"ents "a!also arise due to surface irregularities on the sa"ples.

&he authors would li e to e#press their appreciation to the"anage"ent tea" and operating crews at the Pana"a it! "ill for

ta ing the ti"e and "a ing the effort to "a e this proCect a success.

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Boiler Waterside Deposition