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DAVID N. FRENCH, INC., METALLURGISTSONE LANCASTER ROAD

NORTHBOROUGH, MAS SACH USEllS 01532TEL: (508)393-3635

WINTER 1997 VOL* XIII, No*

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERS

DYNAMITE DAMAGE

In a coal-fired boiler, thecombustion characteristics (heatcontent, % volatiles, etc.) and theash content and composition(especially the ash-fusionemperature) determine the size ofhe furnace. The chemical make-up ofhe ash, often the amount of sodiumand potassium, defines the melting orsoftening temperature. Usually thehigher the sodium and potassium, theower the ash-fusion temperature. To

prevent fouling, fly ash should be"cold enoughff s it enters thesuperheater and reheater so it doesnot stick to the steam-cooled tubes.Thus, the height is adjusted so thathe temperature of the gas exitinghe furnace is less than the ash-usion temperature. Under theseemperature conditions, the fly ashs delivered to the convection pass

as fine solid particles. The depositbuild-up remains friable and easily

emoved. Periodic use of sootblowersan then maintain the heating surfaceree of ash deposits and keep heat-ransfer within the design range.The furnace is uniquely designed for

particular coal.A change in fuel, especially to

ne with a lower ash-fusionemperature can lead to slag andlinker formations within theuperheater and/or reheater. Whenhe temperature of the gas exitinghe furnace is higher than the

melting point of the ash, the fly ashs deposited as molten droplets ontohe tubes in the convection-pass.

When the droplets freeze on theteam-cooled surfaces, the resultantintered deposit is hard, glassy, andannot be removed by sootblower use.

Large clinkers may then develop.Control of clinker formation and

emoval of these slag masses becomesan on-going problem. The use of

shotguns is a frequent method fordestruction of clinkers. Under mostcircumstances, damage to the pressureparts by this slag-removal techniqueis minimal. Occasionally a leadpellet will lead to a steam leak,especially in the thinner tubes ofthe reheater. Thicker tubes ofwaterwalls and superheaters areseldom damaged beyond a slight dent.However, the use of deer slugs forthe removal of particularly stubbornslag accumulations can lead tosizeable holes if the slug scores adirect hit on a relatively cleantube. The development of these leaksthen forces the unit off-line for arepair. Damage from projectiles isoften neat, nearly round holes.Traces of lead from the slug can bedetected on the rim of the hole bycareful energy dispersive x-ray (EDX)analysis.

The use of dynamite to removeparticularly large or stubbornclinker formations is also a common

practice. Damage to the superheateror reheater is usually limited todistortion of the tube bundles. Anygross distortion will compound theclinker-formation problem by changingthe bundle spacing, however. Thechange in spacing may lead to lanesand fly-ash erosion; as the moreclosely spaced pendants fill withash. Occasionally steam leaksdevelop at broken alignment clips orother attachments.

Distortion of bundles from

dynamite damage is usually obvious,and leaks become obvious duringhydrostatic testing or shortly afterstart-up when the steam leaks arevisible. A more subtle, andcertainly less obvious, form ofdynamite damage occurs in themicrostructure of the ferritic steel.

A typical microstructure for acarbon or low-alloy steel after someyears of elevated-temperature service

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is ferrite and spher oidized carbides,similar t o that sho wn in Figure 1.

Figure 1

Here the microstructur e is a mixture- - -

of ferrite and sphe roidized carbideswith occasional large carbide-freeferrite grains. In older boilertubes where t he sulfur content tendsto be higher tha n th e modern steel-making practice, microstructur es willcontain the long, manganese-sulfidestringers, see Figur e 1.

The impact fro m the shock wave ofa dynamite blast leads t o two typesof microstructu ral damage. First,the formation of deformation twinswithin the ferrite; and secondly, thedevelopment of cracks at themanganese-sulfide stringers. Figure2 shows th e deformation twins, th enearly straight lines across theindividual ferrite grains. Thepresence of these deformation twinsis proof of th e impact onto theferritic tube. Deformation twins arefairly common in austenitic stainlesssteels and a re readily formed bysimple plastic deformation. Inferri t ic steels similar to carbon

twins do not form by simple plasticdeformation but r equir e a severe orharsh impact. The presence of thesedeformation twin s is proof of impactdamage.

Figure 2

The shock waves running throughthe steel form cracks at and aroundthe manganese-sulfide stringers, seeFigure 3. While th is damage may notlead to an immediate steam leak; thepresence of internal cracks canpropagate as fatigu e or creep-fatiguedamage later on and lead to a steamleak.

steel, T-11 or T-22, deformation Figure 3

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ASSESSMENT, AN D NEED FOR CHEMIC AL CLEANING, CALL DR. FREN CH AT (508) 393-3635