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DAVID N . FRENC H, INC . , METALLURGISTSSPRING 1989 O N E L A N C A S T E R R O A DN O R T H B O R O U G H . M A S S A C H U S E T T S 01532 V OL . V I , No.

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

Waterside deposi ts ar e the r es ul t of one of1) iron oxide formed by the reaction ofwater and stee l , usually referred to asscale,2) Water imp ur iti es improperly o rincompletely removed by treatment,3 ) Water treatment chemicals intentionallyadded as pa rt of t he water conditioning.4 ) Contaminatd condensate, o r5 ) Corrosion products.The detrimen tal e ff ec ts of watersideosit s come about f r m two fa ct or s.e thermal conductivity of th eseis subs tant ia l ly les s than tha t ofThe thermal conductivity of carbonusuall y used fo r waterwall constructio ns around 300 BTU/hr-sq. ft.-OF/in. Mostts have a thermal conductivity i n th ege of l e s s than 1 for a very porousi l i c a t e sca le, t o around 25 BTU/hrsq. ft . -The net ef fe ct of t h is insulati ngrr ie r on the waterside of the tube is t ois e th e tube-metal temperature. Excessiveal e and/or dep0sit.s can thus lead t o creeplur es within th e furnace tubes of aThe second effect is that porous depositse hydroxide ( o r aci d when th er es a pH upset a t the deposit / tube interface.i s dras t ic change i n pH leads t o un de rosi t. co rros ion and/or hydrogen damage. Theis: The depositsA t thesteam is formed asis hotter than the deposit. and water.that were i n the water

is now steam a r e l e f t behind with in th eThus the local concentration canange the pH t o very co rro sive le ve ls . Underal pH co nd it io ns (8.5-9.5) thew i l l have a pH i n t he 12-range, st ro ng ly ba sic . When ac id en te rsler during an upset , or l e f t behind

f t e r chemical cleaning, th e concentratedw i l l have a pH i n th e 2-3 range,The result is under-deposit

cor ros ion , tube wastage, and/or hydrogendamage.Both ef fe ct s, creep damage due t o sc al eformation and under-deposit cor ros ion , canoccur i n differ ent locat ions within th e sambo il er . Depending on th e heat fl ux, t.he r aof corr osion, th e amount of sca le , t he t-ubmetal temperature increase, etc .c d i n a t i o n s of creep fai l ure s and hydrogendamage have been observed i n di f fe ren tlocations of the same boiler.1. The most common waterside scale componis iron oxide. Magnetite scales are endema s t h e b oi l e r is bu il t of ferrous alloys; aeven pure water w i l l react with steel t o foir on oxide. The oxide can be formed in plaor formed elsewhere with in t he bo i l e r andtransported t o deposi t on t he high-heat-fluxregions of the furnace.2. Deposi ts formed from water impur iti es aassociated with calcium or magnesiumcarbonate, th e so-called "hardness sal ts tt ,s i l i c a , and magnesium ch lo ride . Calcium wanions of carbonat.e, sulfate, chloride,b icarbonate , or s i l ic a t es w i l l form theinsolu ble sa lt s: calcium carbonate. calciumsu lf at e, and calcium s i l i c a t e . Magnesiumwith hydroxide, s i li c at es , o r phosphates,w i l l al so form ins olub le s a l t s of magnesiumhydroxide, magnesium s i l i c a t e , and magnesiuphosphate. When the re ar e s i l i c a t e s presenth er e may al s o be iro n, sodium, o r aluminums i l i c a t e s formed. The formation of complexs i l i c a t e s with mixtures of aluminum andsodium cations is a ls o possib le. Under rarand more unusual conditions essentially pus i l i c a d ep os it s w i l l form.When magnesium chloride intrudes intohigh-temperature b o i l e r water as a resul t oa condenser lea k, f o r example, t hepr ec ip it at io n of i nso lub le magnesiumhydroxide lea ds t o th e formation ofhydrochloric acid . Hydrochloric acid w i l lconcentrate within porous deposits, asprevio usly mentioned, and lead t o rapid a cattack, wall thinning, and hydrogen damage3. Water treatment chemicals, i n th e

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w i l l form.Waterside deposits that form as the resultthe form of contamination. Under unusualumstances, o i l w i l l contaminate the boiler

w i l l carbonize on the hotterDepending on the level oftamination, t h i s leaves a dif fic ult -to -move, carbonaceous deposit.. Fu rthe rich would in j ec t raw water and/o r s ea water,

Products of th e corro sion of condensers o rw i l l also show up withine wat ers ide dep osi ts. Condensers and1 o r copper/nickel (monel1 al loys w i l l

nicke l, and metal lic copperthe waterside deposits. In fa ct ,ction of copper, z inc, or nicke l w i l lbe the f i r s t confirmation of corrosionas ferro us bicarbonate, on

is presen t i n make-up water. Ir oni s form may pr ec ip it at e when ei th e rr srposed t o oxygen. Iron oxidesw i l l precipitate from the ferrous

i t reacts with water to formThe 1i ber at. ion of carbonoxide gas i n t h i s react ion can a lso a l t e re pH and lead t o corros ion.Table I l ists a typical waterside depositOf the elements report ed , ir on isa r th e overwhelming element prese nt, andprese nt probably a s Fe304, (perhaps F e 2 9 1 .TABLE I. D ~ S I T A N & Y S I S . %

Ca.. . 0.79 Mg.. . 0.15.... .98 Mn.. . 0.40 A l . . . 0.29Mo. ... .02 N i . . . 1.25r . ... .07 Zn.. . 0.15 Cu.. . 2.10. < O . O l Sn.. . < O . O l

The nickel, copper and zinc presumably comefrom feedwater heaters or condensercorrosion. Manganese is present because thb o i le r s t e e l s c ont ain somewhere between 0.4and 0.8% manganese. I t would be expected tbe s imi l a r to i ron in i t s oxidation behavioThe ot he r elements detec ted , calcium,si li co n , magnesium, aluminum, a r e li k e l y t obe water impur it ies. Sodium and phosphorusa r e from pH contro l compounds, sodiumphosphates.When waterside deposits have reached somethickness whereby the heat transfer i simpeded and tube-metal tem peratu res may r i st o dangerous le ve ls , chemical cleaning isrecmended. The Atwood and Hale ana ly si sthe most widely used recommendation fo rdetermining th e need fo r chemical cle aning .Basic ally , t h i s technique removes th ewaterside deposit from the hot or fi re si de the tube and measures the amount. A t adep osi t loading above 40 gms/sq. f t . ,immediate chemical cleaning i s recme.nded.For deposit loadings between 15 and 40grns/sq. f t . , chemical cleani ng i s recommendat the next annual outage. For depositloadings less than 15 gms/sq. f t . , nochemical cleaning is needed.There can be variations in the actualmeasurements based on t he method of removalof thes e waterside depo sits. There can alsbe su bst ant ial varia tio ns i n th e amount ofdeposits, from the fi re si de and cold si de aw e l l as along the f i r e s ide i t s e l f . The mosevere deposits w i l l be at. th e highest mettemperatures which a r e mid-way between th emembranes. From a he at -t rans fe r viewpoint waterside deposit of 2-3 m i l s thick, canincrease t.ube metal temperatures by 50-100F depending on the heat flux. Thus a t deposit thickness of 3 m i l s a t t h e t h ic k esmeasured, i t is time f o r chemical cleaning.Whatever technique is used t o remove thesdeposits, the int.ent is to return the heat-tra nsf er surfaces t o the clean condit ion anprevent werheating and under-deposit.corrosion of the furnace tubes.

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