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DAVID N. FRENCH, INC., METALLURGISTSONE LANCASTER ROADNORTHBOROUGH, M ASSACHUSETTS 01532TEL: (508) 393-36351997 VOL. XIV, NO.

A VIEW FROM THE PENTHOUBE: UBEFUL INFORMATION FOR THE WORLD OF BOILERB

M-BIDE PROBLEMB IN BH AND RHThere are four principal problemswith the steam path ind reheaters:Exfoliation of the steam-side

. The reaction of steam with thetube to form iron oxide whichof heat. The net effect is

h both exacerbates the fire-sidend leads to early creep

. Condensate that collects in them of pendants and in saggedtubes that leads to oxygen

. Weld backing rings or excessiveead penetration that leads to

. The exfoliation of steam-siderbine-blade erosiond loss of efficiency. When piecesf oxide spall, the larger piecesat the bottom of pendants;d the smaller pieces becomeained in the steam. At theom of the circuit, tumbling andd to more very finedes becomingained within the steam. Thesee particles then lead to turbine-erosion and loss of turbineThe large flakes ofthat are too big to be moved upendant with the steam flowat the bottom. When the unitdown, any fine oxidensate collect.n the unit re-starts, thecondensate and solidf scale sinter to form anmass. Locally, the scalekness build-up may approach 1/811more, and the deposit then acts aser to heatsfer. The thermal conductivity

of the deposit is perhaps 5% that othe steel and hence a pretty goodinsulator to heat flow from flue gato steam. The net effect is to raitube-metal temperatures, and creepfailures may occur.The formation of these localdeposits is exacerbated by cyclingservice. More frequent starts andstops increase moisture collectionwithin the pendant. As thecondensate evaporates, any dissolvesolids within the boiler water (forexample sodium phosphate used tocontrol pH or copper from thecondenser) sinters the loosecollection of oxide particles into hard, rigid mass.2. The reaction of steam with steeforms iron oxide. The rate at whicthe steam-side scale develops isrelated to both the composition ofthe steel and the temperature ofoperation. In general for thechromium-molybdenum steels similar T-11 and T-22, the average rate isabout 1 millyear. For the austenitstainless steels similar to 304H,321H, and 347H, the rate is less thhalf that, perhaps 1 mil every 3 oryears.The increase in metal temperaturas a result of the steam-side scaleformation depends on several factorthe heat flux, the tube diameter anwall thickness and, of course, thethickness of the steam-side scale.The temperature increase is somewhebetween 1 and 4 times the scalethickness. Thus for a superheaterwith a fairly high heat flux, athickness of 15 mils may raise thetube-metal temperature between 50and perhaps as much as 75OF. For areheater where the heat fluxes areusually much lower, the increase issomewhat less, perhaps 25O to 50F.In any case, the increase in tube-metal temperature will exacerbate t

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fire-side problems of fuel-ashcorrosion and will hasten the onsetof creep failures. A 50F increasein metal temperature reduces the timefor the onset of creep deformation by85%, and a hotter tube will suffermore serious fire-side corrosion.Both the exfoliation problemsassociated with turbine-blade erosionand the increase in tube-metaltemperature may be mitigated bychemical cleaning to remove thescale. Scale removal may prolong thelife of the superheater or reheatertube by 50 and 75% as the effectiveor average metal temperature iscycled between lower temperaturelimits.3. In cycling boilers, steamcondenses and collects either in thebottom of a pendant or in the bottomof a sagged horizontal superheater orreheater tube. When the unit is opento the atmosphere, the combination ofmoisture and air leads to oxygenpitting and corrosion. Theseproblems are also more prevalent incycling units, units frequentlyopened to the atmosphere.The second potential problem withcondensate blockage in pendant-stylesuperheaters and reheaters is that arapid start-up may lead to a short-term high-temperature failure.Failures occur before the condensateis evaporated and steam flow andsteam cooling are fully establishedthe circuit. These failuresare thin-lipped, wide-open bursts;and the microstructuraltransformations often indicate peaktemperatures well above 1500F.4. The fourth problem is associatedweld backing rings or excessiveroot penetration at tube-to-tube buttjoints. These irregularities of the. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ID surface lead to distortedsteamflow and poorer heat transferdownstream of the weld. Poor heattransfer leads to a local hot spotand, again, creep failures occurprematurely.Two other more uncommon problemoccur when excessive attemperationspray water enters the tubes. As water evaporates, the hydroxidewithin the boiler water used for pcontrol may concentrate to the poiwhere stress-corrosion cracking wiloccur in ferritic steels similar tT-11 or T-22. These events arealmost always associated with adamaged attemperation-spray nozzle.The attemperation spray should be fine mist that completely evaporat

within the header. A broken nozzleprojects a solid stream of water tdoes not evaporate within the headeThe liquid may then run down thesuperheater or reheater tubes befoit ultimately evaporates.Finally, when hydrochloric-acidsolutions are used for chemicalcleaning of the furnace, carryoverinto the superheater can occur. Whthe superheater contains austeniticstainless steel, eg. 304H, 321H,347H, stress-corrosion crackingfailures are inevitable. While thtype of damage is unusual, it is nunheard of, and care needs to beexercised to prevent back-filling tsuperheater with t he chemical-cleaning solution prescribed forferritic alloys within thewaterwalls.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FOR METALLURGICAL HELP, FAILU REANALYSIS 1 REMAINING LIF E ASSESSMENCALL 508-393-3635

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