Document MG2o9RZneLz8rm1dXDxJq3o9y

'PACKAGED^BOILER-TREATMENT tontlnueil i1 farthest away from the burner. The smaller-units are provided-with-;a 3- paas', S-sbaped gas path and an exit at the front end, left-hand drawing, p 101. ;'V Both; medium-size and'smaller hellers t-i .`have -watercooler!, ^baffles,- and all the tubes in their first pass are .placed more ,, widely apart than those of their second or third pass. Then, too,' designers reduce the volumetric site of those cor ridors from the first to the second, or on to the third, to maintain uniform veloc ity throughout the gas travel in face of the cooling of combustion products. ,ln>ekher design, .single- or -2-drum, the relatively:high ratio of furnace-wall y.re* ? .4-W y,j>a eooling to furnace volume offers rapid and efficient heat absorption--hence, lower gas temperatures entering the WATERTUBE PACKAGE wholely encases boiler proper with insulated steel jockst* suitable for either Indoor or outdoor installation; It requires no speclol foundation** (jWG-DftUM DESIGN, depending on steaming capacity, bos one, two or three qos SINOIE-DRUM unit's one gas pais splits uses Ilka the obove. Largest units havo only one post, with no convection baffling ot back ond returns to front duol exits convection bank. Water Circulation, Roth types have their feedwater entry in the ateam drum, and ft is here (hat (heir oval-shaped High Ratio of Wall-Cooling to Furnace ffijires Rapid Heat Absorption and Heightens Water-Treatment Needs water circulation begins and terminates. The single-drum boiler has two such circuits, one on either side of (he drum. Fresh water courses down the outer sets of tubes, through the sidewall headers, and-backup to (he drum bottom by way of the furnace-wall tubes.- The 2-drum boiler also has two sepa rate water-circuits. However, one is in side the other. All the incoming .water passes down the outer half "of the di rect drum-connecting tubes..In the mud drum much of the feedwater rises up through the Inner-half portion".of-these tubes, while the remainder travels- up through the waterwalls, and so on back to the upper drum. -i-` Front ends of the sidewatl headers of the.slngle-drum design prove the coolest sections of this type boiler. Their .loca tion in the tail-end of the gas travel, and, too, in the dead-pocket area sur rounding the burners mouth, produces this comparative -coolness. These two spots are, therefore, where the great est sludge' accumulation is expected. Surprisingly, though, absence of any aludge build-up here does not indicate a clean boiler, as you might suppose, but serves rather as a sign that scale forma tion has probably developed elsewhere In the unit. Since these sidewall headers act also os feeders for the highly sen sitive waterwalls, it is most vital to avoid excessive sludge plle^up here. Otherwise, fireside tubes theb become dangerously fouled. Hence, you need a close control over blowdown. In the 2-drum design, the relatively large mud drum ordinarily affords ample protection against any appreci able sludge recirculation. Sludge ac cumulation .is more apt to bo heaviest in the drum end beneath -the gas exit. The rapid water circulation in these boilers, however, makes it good opera tion m exercise careful blowdown. Scale Formation. The woterwall tubes, exposed to the radiant heat of th< furnace, constitute the principal hetu-sbsorbing elements of these boilers. As siuoh, they suffer most from the in sulating effect scale produces. Because of the*-greater' heat-transfer 1 rate, it takes far less deposit to ovorheat these tubes than any of the others. Moreover, the higher the pressuft, the thinner the scale need be, and the quicker such tubes blister. The silicate type of scale is denser than the 'sulfate, which Is, in tum, some what more eo than the carbonato. Their degree of - impedance to 'heal transfer follows this some.order. So, at the high er pressures, only a paper-thin silicate scale is enough to bring frequent tube failures in the waterwalls. But in the cooler se6;ion of the boiler and at modera!e-to-]ow operating pressures, as much as 1/16 in.'of carbonate deposit may be sustained for considerably long er periods. Any form of scale, though,' is most undesiroble. Once silicate de posits,' it proves extremely difficult to remove either mechanically or chemical ly. The overall heat loss may be only negligible- because of later recovery in (he convection stages. But the damage and maintenance that scale causes is sail entirely different story. They ctn bavj quite costly. ,'A. Corrosion. If makeup water is not properly deaerated, you can expect serious corrosion. The relatively cool iimniniu. viuiimiii; niui wgood\y all-year average of returns ond fclesr water supply of low-to-moderate ihsrdneis, no presoltening need be ap plied to the makeup water tor boilers Operating up to ISO psi. From ISO to feedwater, introduced into (he steam drum, comes in contact with the hot boiler -water at the end of its circuls- |250 psi, row-water hardness would [hive to be fairly low and volume of re covered condensate consistently high to tion cycle. Release of dissolved oxygen from the fresh water follows. This cor [unify use of internal treatment alone. `Above 250 pti, the desired internal boll- rosive gas may then pit the drum proper' er conditions can rarely be attained or along its water-level belt or proceed to corrode the down-comer tubes. Some oi: this gas escopes with the stesm and- ^maintained without the aid ot external treatment. ft A raw water having both its hardness later joins with carbon dioxide of (he find alkalinity quite high would be sub- condensate to attack (ha return lines. Carryover. The large upper drum of &eei to lime-soda softening. A combina tion oi moderately high hardness and a watertube packaged boiler affords adequate steam space and, in addition,. room for mechanical ateam-purifying jiirly low alkalinity renders the sodium fablite method of eoftening idee). In jwne instances, it may prove sound to facilities as well. Thqs, these units sre` [(1) couple sodium xeolite with hydro- not prone (a carryover problems unless pushed to fairly high ratings or made to' carry unfavorably high boiler-waier ;i*n seolitc or (2) supplement lime wftening alone with sodium zeolite. $illl other coses may call for either acid solids content. Embrittlement. The ell-welded drum seams of these units greatly minimise any possibility of iniercrystelline crack or.esimic follow-up to preaoftening. In ny event, the makeup water's physical roiid chemical characteriatics and the aWecific feedwater composition for a ing occurring from local caustic con centration in strained joints. But, V*MtnK'j]*r opening pressure dictate op roakea the more economical and with all boilers, you must (l) inefficient system. Don't forget, though, developing exceedingly strong esuitic *ith external conditioning, you always aikalinitics (2) always watch out ft soma measure of boiler internal ny embrittling tendencies, especially at the rolled-in tube ends. Where aW ^treatment as wellCorrosion Correction. Feedwater can't are suspected, run extensive and conclu tJyM just preheated, for good boiler opera sive tests and take corrective slept. tion. You must also deaerate it. The thing to do is to expel as much dis solved gas by mechanical means as prac ticable. Raw water always introduces both, oxygen and earbon dioxide gases. And, In addition, as iho water's natural bicarbonate alkalinity breaks down, more carbon dioxide builds up in the boiler cycle. Proper deaeration rids the feedwater of practically alt of these two gases. Oxygen, by far the more corrosive, can be completely eliminated with sodium sulfite, which absorbs oxygen to produce (he highly soluble and harm less sodium sulfate salt. By constantly carrying a slight excess of sulfite in the boiler water, you insure the absence of oxygen. Volatile organic amines fed into boiler water neutralize carbon dioxide gas in the steam. But you can't, recover these chemicals, and so justifying their relatively high cost is always a difficulty. If you use a steel economiser, injeot o little caustic soda. While caustic soda is somewhat hazardous to handle and difficult to regulate, it is non-gas-producing and oervei to restrain earbon di oxide action, which would otherwise -develop at this elevated temperature. Controlled recirculation of a small "onion of boiler water also gives a similar effect. Calcium Correction. As previously recommended for firetube-type pack aged boilers (Powkb, June, p 76), the phosphate-organic treatment has thus far proven the more effective of boiler internal corrective methods, ft precipi tates the boiler water's concentroting calcium hardness in the form of iricalcium phosphate. This product readily falls out os a fine non-sealing powder. By consistently -maintaining a slight residual of phosphate you are assured of a steady zero-hardness boiler water. Since both the phosphate and calcium can remain unreactive with each other below a pH of 10, be certain to (urniah enough alkalinity to the boiler water to drive this reaction to completion. Precalculated proportioning of the vorious forms of phosphates will give the desired alkalinity range under any given operating conditions. An appro priate organic coagulant added at the aame time combat* the packing and sticking tendencies of tricalclum phos phate. If this deposit ia kept fluid, blow down removes it easily. Magnesium Correction. Magnesium, generally the minor partner of the cal cium, magnesium hardness-contributing constituents, can, however, prove quite troublesome, especially if you let it come down in the phosphate form. To avoid producing this very sticky and uncoagubtive. deposit, you'll have to carry fairly high boiler-water olkallnitiea. By so doing, you Insure, instead, the precipitotion ol the gelatinous, and even helpful, magnesium hydroxide. Silica Retention. As silica undergoes concentration In the boiler water, it acquires a growing tendency to drop out of the solution, preferably in union (Continued on page 216) ENOINEERINO AND MANAGEMENT SECTION POWW % .PtCEMSER 1933